mobile computing - Thesis Scientist

mobile computing - Thesis Scientist
A Course Material on
MOBILE COMPUTING
By
Mr. D.PRABHAKARAN
ASSISTANT PROFESSOR
DEPARTMENT OF INFORMATION TECHNOLOGY & COMPUTER APPLICATIONS
SASURIE COLLEGE OF ENGINEERING
VIJAYAMANGALAM – 638 056
QUALITY CERTIFICATE
This is to certify that the e-course material
Subject Code
: YCS012
Scubject
: MOBILE COMPUTING
Class
: III B.sc CT (COMPUTER TECHNOLOGY)
being prepared by me and it meets the knowledge requirement of the university curriculum.
Signature of the Author
Name:
Designation:
This is to certify that the course material being prepared by Mr.D.Prabhakaran is of adequate quality. He has referred more
than five books amont them minimum one is from aborad author.
Signature of HD
Name:
SEAL
YCS012
MOBILE COMPUTING
UNIT I WIRELESS COMMUNICATION FUNDAMENTALS 9
LTPC
3003
Introduction – Wireless transmission – Frequencies for radio transmission – Signals – Antennas
– Signal Propagation – Multiplexing – Modulations – Spread spectrum – MAC – SDMA – FDMA
– TDMA – CDMA – Cellular Wireless Networks.
UNIT II TELECOMMUNICATION NETWORKS 9
Telecommunication systems – GSM – GPRS – DECT – UMTS – IMT-2000 – Satellite Networks
- Basics – Parameters and Configurations – Capacity Allocation – FAMA and DAMA –
Broadcast Systems – DAB - DVB.
UNIT III WIRLESS LAN 9
Wireless LAN – IEEE 802.11 - Architecture – services – MAC – Physical layer – IEEE 802.11a 802.11b standards – HIPERLAN – Blue Tooth.
UNIT IV MOBILE NETWORK LAYER 9
Mobile IP – Dynamic Host Configuration Protocol - Routing – DSDV – DSR – Alternative
Metrics.
UNIT V TRANSPORT AND APPLICATION LAYERS 7
Traditional TCP – Classical TCP improvements – WAP, WAP 2.0.
TOTAL : 45
REFERENCE BOOKS:
1. Jochen Schiller, “Mobile Communications”, PHI/Pearson Education, Second Edition, 2003.
(Unit I Chap 1,2 &3- Unit II chap 4,5 &6-Unit III Chap 7.Unit IV Chap 8- Unit V Chap 9&10.)
2. William Stallings, “Wireless Communications and Networks”, PHI/Pearson Education, 2002.
(Unit I Chapter – 7&10-Unit II Chap 9)
3. Kaveh Pahlavan, Prasanth Krishnamoorthy, “Principles of Wireless Networks”, PHI/Pearson
Education, 2003.
4. Uwe Hansmann, Lothar Merk, Martin S. Nicklons and Thomas Stober, “Principles of Mobile
Computing”, Springer, New York, 2003.
YCS012 -MOBILE COMPUTING
UNIT I
WIRELESS COMMUNICATION FUNDAMENTALS
Introduction – Wireless transmission – Frequencies for radio transmission – Signals – Antennas – Signal
Propagation – Multiplexing – Modulations – Spread spectrum – MAC – SDMA – FDMA – TDMA – CDMA –
Cellular Wireless Networks.
INTRODUCTION
Mobile computing means different things to different people. Ubiquitous, wireless and remote computing
Wireless and mobile computing are not synonymous. Wireless is a transmission or information transport method
that enables mobile computing.
Aspects of mobility:
user mobility: users communicate (wireless) “anytime, anywhere, with anyone”
device portability: devices can be connected anytime, anywhere to the network
Mobility Issues
•
•
•
•
•
Bandwidth restrictions and variability
Location-aware network operation
o User may wake up in a new environment
o Dynamic replication of data
Querying wireless data & location-based responses
Busty network activity during connections & handling disconnections
Disconnection
o OS and File System Issues - allow for disconnected operation
o Database System Issues - when disconnected, based on local data
Portability Issues
• Battery power restrictions
• Risks to data
- Physical damage, loss, theft
- Unauthorized access
- encrypt data stored on mobiles
- Backup critical data to fixed (reliable) hosts
• Small user interface
- Small displays due to battery power and aspect ratio constraints
- Cannot open too many windows
- Difficult to click on miniature icons
- Input - Graffiti, (Dictionary-based) Expectation
- Gesture or handwriting recognition with Stylus Pen Voice matching or voice recognition
2
APPLICATIONS
Vehicles
transmission of news, road condition, weather, music via DAB
personal communication using GSM
position via GPS
local ad-hoc network with vehicles close-by to prevent accidents, guidance system, redundancy
vehicle data (e.g., from busses, high-speed trains) can be transmitted in advance for maintenance
Emergencies
early transmission of patient data to the hospital, current status, first diagnosis
Replacement of a fixed infrastructure in case of earthquakes, hurricanes, fire etc.
crisis, war, ...
Travelling salesmen
direct access to customer files stored in a central location
consistent databases for all agents
mobile office
Replacement of fixed networks
remote sensors, e.g., weather, earth activities
flexibility for trade shows
LANs in historic buildings
Entertainment, education,
outdoor Internet access
intelligent travel guide with up-to-date location dependent information
ad-hoc networks for multi user games
Location dependent services
Location aware services
what services, e.g., printer, fax, phone, server etc. exist in the local environment
Follow-on services
automatic call-forwarding, transmission of the actual workspace to the current location
Information services
„push“: e.g., current special offers in the supermarket
„pull“: e.g., where is the Black Forrest Cherry Cake?
Support services
caches, intermediate results, state information etc. „follow“ the mobile device through the fixed
network Privacy
who should gain knowledge about the location
Effects of device portability
Power consumption
limited computing power, low quality displays, small disks due to limited battery capacity
CPU: power consumption ~ CV2f
3
Loss of data
•
•
•
C: internal capacity, reduced by integration
V: supply voltage, can be reduced to a certain limit
f: clock frequency, can be reduced temporally
higher probability, has to be included in advance into the design (e.g., defects, theft)
Limited user interfaces
compromise between size of fingers and portability
integration of character/voice recognition, abstract symbols
Limited memory
limited value of mass memories with moving parts
Flash-memory or? as alternative
Wireless networks in comparison to fixed networks
Higher loss-rates due to interference
emissions of, e.g., engines, lightning
Restrictive regulations of frequencies
frequencies have to be coordinated, useful frequencies are almost all occupied Low transmission
rates
local some Mbit/s, regional currently, e.g., 9.6kbit/s with GSM .Higher delays, higher jitter
connection setup time with GSM in the second range, several hundred milliseconds for other
wireless systems
Lower security, simpler active attacking
radio interface accessible for everyone, base station can be simulated, thus attracting calls from
mobile phones
Always shared medium
secure access mechanisms important
Early history of wireless communication
Many people in history used light for communication
heliographs, flags („semaphore“), ...
150 BC smoke signals for communication;
(Polybius, Greece)
1794, optical telegraph, Claude Chappe
Here electromagnetic waves are of special importance:
1831 Faraday demonstrates electromagnetic induction
J. Maxwell (1831-79): theory of electromagnetic Fields, wave equations (1864)
H. Hertz (1857-94): demonstrateswith an experiment the wave character of electrical transmission
through space(1886, in Karlsruhe, Germany, at the location of today’s University of Karlsruhe)
4
Wireless systems: overview of the development
cellular phones
1981:
NMT 450
1982:
Inmarsat
1983:
AMPS
1986:
NMT 900
1988:
Inmarsat
1991:
CDMA
1987:
CT1+
-C
1992:
Inmarsat
Inmarsat
1994:
DCS 1800
1998:
Iridium
analog
2005?:
UMTS/IMT
-B
-M
199x:
proprietary
1991:
DECT
1995/96/97:
IEEE 802.11,
HIPERLAN
2005?:
MBS, WATM
- 2000
Areas of research in mobile communication
Wireless Communication
transmission quality (bandwidth, error rate, delay)
modulation, coding, interference
media access, regulations
Mobility
location dependent services
location transparency
quality of service support (delay, jitter, security)
Portability
power consumption
limited computing power, sizes of display, ...
usability
5
CT1
1989:
CT 2
1993:
PDC
wireless
LAN
1984:
-A
1991:
D - AMPS
1992:
GSM
digital
cordless
phones
1980:
CT0
satellites
Simple reference model used here
Application
Application
Transport
Transport
Network
Network
Data Link
Data Link
Data Link
Data Link
Physical
Physical
Physical
Physical
Radio
Influence of mobile communication to the LAYER MODEL
Application layer
service location
new applications, multimedia
adaptive applications
Transport layer
congestion and flow control
quality of service
Network layer
addressing, routing, device location
hand-over
Data link layer
authentication
media access
multiplexing
media access control
6
Network
Network
Medium
Physical layer
encryption
modulation
interference
attenuation
frequency
WIRELESS TRANSMISSION - FREQUENCIES FOR RADIO TRANSMISSION
Frequencies for communication
twisted
pair
coax cable
1 Mm
10 km
30 kHz
300 Hz
VL
F
optical transmission
100 m
3 MHz
LF
MF
•
•
•
•
•
VLF = Very Low Frequency
LF = Low Frequency
MF = Medium Frequency
HF = High Frequency
VHF = Very High Frequency
•
Frequency and wave length:
•
wave length
 = c/f
1m
300 MHz
HF
VHF
100
m
3 THz
10 mm
30 GHz
UHF
SHF
EHF
1 m
300 THz
infrared
visible
light
UV
UHF = Ultra High Frequency
SHF = Super High Frequency
EHF = Extra High Frequency
UV = Ultraviolet Light
, speed of light c
 3x10 m/s, frequency f
8
Frequencies for mobile communication
•
VHF-/UHF-ranges for mobile radio
• simple, small antenna for cars
• deterministic propagation characteristics, reliable connections
•
SHF and higher for directed radio links, satellite communication
• small antenna, focusing
• large bandwidth available
•
Wireless LANs use frequencies in UHF to SHF spectrum
• some systems planned up to EHF
• limitations due to absorption by water and oxygen molecules (resonance frequencies)
• Weather dependent fading, signal loss caused by heavy rainfall etc.
Frequencies and regulations
ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio
USA
Europe
Japan
Conferences)
Mobile
phones
7
Cordless
telephones
NMT 453 - 457MHz,
463 -467 MHz;
GSM 890 -915 MHz,
935 -960 MHz;
1710 - 1785 MHz,
1805 - 1880 MHz
CT1+ 885 - 887 MHz,
930 -932 MHz;
CT2
864 868
MHz
DECT
AMPS , TDMA , CDMA
824 -849 MHz,
869 -894 MHz;
TDMA , CDMA , GSM
1850 - 1910 MHz,
1930 - 1990 MHz;
PACS 1850 - 1910 MHz,
1930 - 1990 MHz
PACS - UB 1910 - 1930 MHz
W
ireless
PDC
810 -826 MHz,
940 -956 MHz;
1429 - 1465 MHz,
1477 - 1513 MHz
PHS
1895 - 1918 MHz
JCT
254 -380 MHz
LANs
1880 - 1900 MHz
IEEE 802.11
2400 - 2483 MHz
IEEE 802.11
2400 - 2483 MHz
IEEE 802.11
2471 - 2497 MHz
SIGNALS
physical representation of data
function of time and location
signal parameters: parameters representing the value of data
classification
o
o
o
o
continuous time/discrete time
continuous values/discrete values
analog signal = continuous time and continuous values
digital signal = discrete time and discrete values
signal parameters of periodic signals:
period T, frequency f=1/T, amplitude A, phase shift j
sine wave as special periodic signal for a carrier:
s(t) = At sin(2 p ft t + jt)
Fourier representation of periodic signals



g t 
( )
8
1
2

c  a
n 1




n
sin( 2 nft )

 b
n 1




n
cos( 2 nft )
1
1
0
0
t
Ideal
periodic
signal
t
Real composition (
harmonics
)
based on
Different representations of signals
amplitude (amplitude domain)
frequency spectrum (frequency domain)
phase state diagram (amplitude M and phase j in polar coordinates)
Composed signals transferred into frequency domain using Fourier transformation
Digital signals need
infinite frequencies for perfect transmission
Modulation with a carrier frequency for transmission (analog signal!)
ANTENNAS
Isotropic radiator
Radiation and reception of electromagnetic waves, coupling of wires to space for radio transmission
Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna
Real antennas always have directive effects (vertically and/or horizontally)
Radiation pattern: measurement of radiation around an antenna
Ideal isotropic radiator
9
y
z
z
y
x
x
Simple dipoles
Real antennas are not isotropic radiators but, e.g., dipoles with lengths l/4 on car roofs or l/2 as Hertzian dipole,
shape of antenna proportional to wavelength
/4
Example: Radiation pattern of a simple Hertzian dipole
10
/2
•
Real antennas are not isotropic radiators but, e.g., dipoles wit
h lengths
shape of antenna proportional to wavelength
roofs or /2 as Hertzian dipole
/4
/4 on car
/2
• Example: Radiation pattern of a simple Hertzian dipole
y
z
y
x
side view (
z
xy - plane)
side view (
yz - plane )
x
top view (
Simple dipole
xz - plane)
• Gain: maximum power in the direction of the main lobe compared t
the power of an isotropic radiator (with the same average power)
Directed and Sectorized
Often used for microwave connections or base stations for mobile phones (e.g., radio coverage of a valley)
y
z
y
Directed antenna
z
x
side view ( xy-plane)
x
top view ( xz-plane)
side view ( yz-plane)
z
z
x
x
top view, 3 sector
11
top view, 6 sector
Sectorized antenna
o
Antennas: diversity
Grouping of 2 or more antennas
o multi-element antenna arrays
Antenna diversity
o switched diversity, selection diversity
receiver chooses antenna with largest output
diversity combining
combine output power to produce gain
cophasing needed to avoid cancellation
/2
/2
/4
/4
/2
+
+
SIGNAL PROPAGATION
Transmission range
communication possible
low error rate
Detection range
detection of the signal possible
no communication possible
Interference range
signal may not be detected
signal adds to the background noise
12
Sende
r
Transmission
Distance
Detection
Interferenc
e
Signal propagation
Propagation in free space always like light (straight line)
Receiving power proportional to 1/d²
(d = distance between sender and receiver)
Receiving power additionally influenced by
fading (frequency dependent)
shadowing
reflection at large obstacles
scattering at small obstacles
diffraction at edges
13
Shadowing
Reflection
Scattering
Diffraction
Multipath propagation
Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction
Time dispersion: signal is dispersed over time
è Interference with “neighbor” symbols, Inter Symbol Interference (ISI)
The signal reaches a receiver directly and phase shifted
è Distorted signal depending on the phases of the different parts
Effects of mobility
Channel characteristics change over time and location
signal paths change
different delay variations of different signal parts
different phases of signal parts
èQuick changes in the power received (short term fading)
Additional changes in
distance to sender
obstacles further away
è Slow changes in the average power received (long term fading)
MULTIPLEXING
Multiplexing in 4 dimensions
space (si)
time (t)
frequency (f)
code (c)
Frequency Division Multiplexing - FDM
The oldest used technique used for multiplexing. Possible when the useful bandwidth of the medium exceeds that
of the signals it has to carry. Each signal is modulated on a different carrier frequency. This results in shifting the
spectrum of the signal around the carrier frequency. Sufficient guard-band is given so those neighboring signals
do not overlap in the frequency domain.
At the receiving end each signal is extracted by first passing it through a band-pass filter and then demodulating
with the same carrier frequency that was used to modulate the signal. The signals carried using FDM may be
analog signals or may be analog signals representing digital data. However FDM is mostly a technique from the
14
era of analog communications. In FDM a device uses some of the channel all of the time. FDM is used in radio
and television broadcasting. FDM is also used in high capacity long distance links in the telephone network.
Frequency division multiplexing (FDM) achieves multiplexing by using different carrier frequencies .Receiver
can "tune" to specific frequency and extract modulation for that one channel .Frequencies must be separated to
avoid interference - “Wastes” potential signal bandwidth for guard channels.Only useful in media that can carry
multiple signals with different frequencies - high-bandwidth required .
Used in:
The standard of the analog telephone network
The standard in radio broadcasting
The standard for video
1. Broadcast
2. Cable
3. Satellite
Frequency Division Multiplexing Diagram
Time Division Multiplexing - TDM
Time division multiplexing is more suitable for digital data. TDM can be used when the data rate available on
a communication link exceeds the data rate required by any one of the sources. In TDM each source that is to
use the link fills up a buffer with data. A TDM multiplexer scans the buffers in some predetermined order and
transmits bits from each source one after the other.
Requires digital signaling & transmission
Requires data rate = sum of inputs + framing
Data rate much higher than equivalent analog bandwidth uses
Separates data streams in time not frequency
The standard of the modern digital telephone system
15
Code Division Multiplexing - CDM
Each channel has a unique code. All channels use the same spectrum at the same time.
Advantages:
bandwidth efficient
no coordination and synchronization necessary
good protection against interference and tapping
Disadvantages:
lower user data rates
more complex signal regeneration
16
k1
k
k3
k4
k5
2
k
C
6
F
T
MODULATIONS
Digital modulation
o
o
o
digital data is translated into an analog signal (baseband)
ASK, FSK, PSK - main focus in this chapter
differences in spectral efficiency, power efficiency, robustness
Analog modulation
o
17
shifts center frequency of baseband signal up to the radio carrier Motivation
o
o
o
smaller antennas (e.g., l/4)
Frequency Division Multiplexing
medium characteristics
Basic schemes
o
o
o
Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase Modulation (PM)
Modulation and demodulation
digital
data
101101001
digital
modulation
analog
baseband
signal
analog
modulation
Radio transmitter
radio
carrier
analog
demodulation
analog
baseband
signal
synchronization
decision
radio
carrier
Digital modulation
Modulation of digital signals known as Shift Keying.
Amplitude Shift Keying (ASK):
very simple
low bandwidth requirements
very susceptible to interference
Frequency Shift Keying (FSK):
needs larger bandwidth
Phase Shift Keying (PSK):
18
digital
data
101101001
Radio receiver
more complex
robust against interference
1
0
1
t
1
0
ASK
1
t
1
0
FSK
1
t
PSK
Advanced Frequency Shift Keying
bandwidth needed for FSK depends on the distance between the carrier frequencies
special pre-computation avoids sudden phase shifts
è MSK (Minimum Shift Keying)
bit separated into even and odd bits, the duration of each bit is doubled
depending on the bit values (even, odd) the higher or lower frequency, original or inverted is chosen
the frequency of one carrier is twice the frequency of the other
even higher bandwidth efficiency using a Gaussian low-pass filter
è GMSK (Gaussian MSK), used in GSM.
Advanced Phase Shift Keying
BPSK (Binary Phase Shift Keying):
bit value 0: sine wave
bit value 1: inverted sine wave
very simple PSK
low spectral efficiency
robust, used e.g. in satellite systems
19
QPSK (Quadrature Phase Shift Keying):
2 bits coded as one symbol
symbol determines shift of sine wave
needs less bandwidth compared to BPSK
more complex
Often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK (IS-136, PACS, PHS
BPSK (Binary Phase Shift Keying):
Q
1
0
I
QPSK (Quadrature Phase Shift Keying):
1
0
Q
1
1
I
0
0
0
1
Quadrature Amplitude Modulation
Quadrature Amplitude Modulation (QAM): combines amplitude and phase modulation
•
•
•
20
it is possible to code n bits using one symbol
2n discrete levels, n=2 identical to QPSK
bit error rate increases with n, but less errors compared to comparable PSK schemes
SPREAD SPECTRUM
Effects of spreading and interference
P
P
i)
user signal
f
ii )
broadband interference
narrowband interference
f
sender
P
iii )
P
P
f
iv )
receiver
f
v)
f
DSSS (Direct Sequence Spread Spectrum)
XOR of the signal with pseudo-random number (chipping sequence)
• many chips per bit (e.g., 128) result in higher bandwidth of the signal
Advantages
•
•
reduces frequency selective fading
in cellular networks
o base station scan use the same frequency range several base stations can detect and recover the
signal
o soft handover
Disadvantages
•
21
precise power control necessary
user data
X
spread
spectrum
signal
transmit
signal
modulator
radio
carrier
chipping
sequence
transmitter
correlator
lowpass
received
signal
filtered
signal
demodulator
sampled
sums
products
X
integrator
data
decision
chipping
sequence
radio
carrier
receiver
FHSS (Frequency Hopping Spread Spectrum)
Discrete changes of carrier frequency
sequence of frequency changes determined via pseudo random number sequence
Two versions
Fast Hopping:
several frequencies per user bit
Slow Hopping:
several user bits per frequency
Advantages
frequency selective fading and interference limited to short period
simple implementation
uses only small portion of spectrum at any time
Disadvantages
not as robust as DSSS
simpler to detect
22
FHSS (Frequency Hopping Spread Spectrum)
tb
user data
0
1
f
0
1
1
t
td
f3
slow
hopping
(3 bits/hop)
f2
f1
f
t
td
f3
fast
hopping
(3 hops/bit)
f2
f1
t
td: dwell time
tb: bit period
Frequency Hopping Spread Spectrum
narrowband
signal
user data
modulator
transmitter
received
signal
hopping
sequence
23
modulator
frequency
synthesizer
narrowband
signal
demodulator
frequency
synthesizer
spread
transmit
signal
hopping
sequence
data
demodulator
receiver
MAC
Medium Access Control (MAC)
MAC protocol which were developed for nodes at short distance did not show good performance for
nodes at longer distance so another protocol has to be developed Known as 2p MAC Protocol.
802.11 protocols were good for devices which had no power supply issue frequent charging were
available to them etc.
1. This protocol based devices were not good for certain operation like monitoring the natural
habitat of wildlife.
2. Sampling the water level of dam.
These applications do not require frequent human intervention and are required to run for a longer
duration.
To fulfill the requirement other protocol was developed sensor network (802.15.4)
• Energy Budgets:-Main points which were discussed in this were how its protocol helps in saving
power by cleverly managing the time when device should sleep when to wake up.
• MAC protocol used in 802.15.4.
• Routing and tree formation in ZigBee: - Routing protocol was developed by Zigbee firm.
Wireless MAC Issues
Wireless medium makes the MAC design more challenging than the wireline networks.
The three important issues are:
1.
2.
3.
Half Duplex operation –> either send or receive but not both at a given time
Time varying channel
Burst channel errors
1. Half Duplex Operation
In wireless, it’s difficult to receive data when the transmitter is sending the data, because:
When node is transmitting, a large fraction of the signal energy leaks into the receiver path
The transmitted and received power levels can differ by orders of magnitude
The leakage signal typically has much higher power than the received signal ->“Impossible
to detect a received signal, while transmitting data”
Collision detection is not possible, while sending data
As collision cannot be detected by the sender, all proposed protocols attempt to minimize the probability of
collision -> Focus on collision avoidance
2. Time Varying Channel
Three mechanisms for radio signal propagation
•
24
Reflection – occurs when a propagating wave impinges upon an object that has very large dimensions
than the wavelength of the radio wave e.g. reflection occurs from the surface of the earth and from
buildings and walls
•
Diffraction – occurs when the radio path between the transmitter and the receiver is obstructed by a
surface with sharp edges
• Scattering – occurs when the medium through which the wave travels consists of objects with
The received signal by a node is a superposition of time-shifted and attenuated versions of the ransmitted
signals the received signal varies with time .The time varying signals (time varying channel) phenomenon also
known as multipath propagation. The rate of variation of channel is determined by the coherence time of the
hannel Coherence time is defined as time within which When a node’s received signal strength drops below a
certain threshold the node is said to be in fade .Handshaking is widely used strategy to ensure the link quality
is good enough for data communication. A successful handshake between a sender and a receiver (small
message) indicates a good communication link.
3. Burst Channel Errors
As a consequence of time varying channel and varying signals strengths errors are introduced in the
transmission (Very likely) for wire line networks the bit error rate (BER) is the probability of packet error is
small .For wire line networks the errors are due to random For wireless networks the BER is as high.For wireless
networks the errors are due to node being in fade as a result errors occur in a long burst. Packet loss due to burst
errors - mitigation techniques
•
»
Smaller packets
•
»
Forward Error Correcting Codes
•
»
Retransmissions (Acks)
•
Location Dependent Carrier Sensing
Location Dependent Carrier Sensing results in three types of nodes that protocols need to deal with:
Hidden Nodes
Even if the medium is free near the transmitter, it may not be free near the intended receiver
Exposed Nodes
Even if the medium is busy near the transmitter, it may be free near the intended receiver
Capture
Capture occurs when a receiver can cleanly receive a transmission from one of two simultaneous
transmissions
Hidden Node/Terminal Problem
A hidden node is one that is within the range of the intended destination but out of range of sender Node
B can communicate with A and C both A and C cannot hear each other When A transmits to B, C cannot
detect the transmission using the carrier sense mechanism C falsely thinks that the channel is idle
Exposed Nodes
An exposed node is one that is within the range of the sender but out of range of destination .when a node’s
received signal strength drops below a certain threshold the node is said to be in fade .Handshaking is widely used
strategy to ensure the link quality is good enough for data communication. A successful handshake between a
sender and a receiver (small message) indicates a good communication link.
25
In theory C can therefore have a parallel transmission with any node that cannot hear the transmission from
B, i.e. out of range of B. But C will not transmit to any node because its an exposed node. Exposed nodes
waste bandwidth.
Capture
Capture is said to occur when a receiver can cleanly receive a transmission from one of two simultaneous
transmissions both within its range Assume node A and D transmit simultaneously to B. The signal strength
received from D is much higher than that from A, and D’s transmission can be decoded without errors in presence
of transmissions from A.D has captured A. Capture is unfair because it gives preference to nodes that are closer to
the receiver. It may improve protocol performance
MULTIPLE ACCESS
FDMA
It is an ANALOQUE technique in time. Synchronization the transmission bandwidth is partitioned to frequency
slots different users has different RF carrier frequencies, i.e. Each user is assigned a particular frequency slot.
users/signals are at the receiver by separated out FILTERING if all frequency slots are occupied then the
system has reached its.
TDMA
It is a DIGITAL technique requires between users synchronization each user/signal is assigned a particular
(within a time-frame) time slot.
CELLULAR WIRELESS NETWORKS
Implements space division multiplex: base station covers a certain transmission area (cell).Mobile stations
communicate only via the base station
Advantages of cell structures:
higher capacity, higher number of users
less transmission power needed
more robust, decentralized
base station deals with interference, transmission area etc. locally
Problems:
fixed network needed for the base stations
handover (changing from one cell to another) necessary
interference with other cells
Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies
Frequency reuse only with a certain distance between the base stations
Standard model using 7 frequencies:
f5
f4
f3
f2
f6
f1
f4
f7
f3
f5
f1
f2
Fixed frequency assignment:
certain frequencies are assigned to a certain cell
problem: different traffic load in different cells
Dynamic frequency assignment:
base station chooses frequencies depending on the frequencies already used in neighbor cells
more capacity in cells with more traffic
assignment can also be based on interference measurements
f3
f2
f3
f1
f1
f3
f3
f1
f3
f2
f2
f1
f3
f2
f2
f1
f3
3 cell cluster
f2
f2
f2
f1 f
f
f
1
1
f3
h2
h2
f3
3
h1
h1
g2
g
h3
g2
h3
g1
g1
g1
g3
g3
g3
2
f3
3 cell cluster with 3 sector antennas
Cell : Why Hexagon?
• In reality the cell is an irregular shaped circle, for design convenience and as a first order approximation,
it is assumed to be regular polygons
•
The hexagon is used for two reasons:
– A hexagonal layout requires fewer cells, therefore, fewer transmission site
– Less expensive compared to square and triangular cells
•
Irregular cell shape leads to inefficient use of the spectrum because of inability to reuse frequency on
account of co channel interference uneconomical deployment of equipment, requiring relocation from
one cell site to another
YCS012 -MOBILE COMPUTING
UNIT II
TELECOMMUNICATION NETWORKS
Telecommunication systems – GSM – GPRS – DECT – UMTS – IMT-2000 – Satellite Networks- Basics –
Parameters and Configurations – Capacity Allocation – FAMA and DAMA – Broadcast Systems – DAB - DVB.
Telecommunication systems -GSM – GPRS – DECT – UMTS – IMT-2000
Building Blocks
•
AMPS – Advanced Mobile Phone System
•
TACS – Total Access Communication System
•
NMT – Nordic Mobile Telephone System
AMPS – Advanced Mobile Phone System
•
analog technology
•
used in North and South America and approximately 35 other countries
•
operates in the 800 MHz band using FDMA technology
TACS – Total Access Communication System
•
variant of AMPS
•
deployed in a number of countries
•
primarily in the UK
NMT – Nordic Mobile Telephone System
•
analog technology
•
deployed in the Benelux countries and Russia
•
operates in the 450 and 900 MHz band
•
first technology to offer international roaming – only within the Nordic countries
System Architecture
Mobile Stations
(MS
)
A interface
Base Transceiver
Station (BTS)
Base
Station
(BSC)
Base
Transceiver
(BTS)
VLR
HLR
Abis interface
Base Station
(BS)
Um interface
Mobil
e
gCentr
e
)
G
M
S
C
P
S
T
N
Base
Transceiver
(BTS)
Base
Station
(BSC)
Base
Transceiver
(BTS)
Abis interface
Base Station
(BS)
CCITT
Signalling
System No. 7
(SS7)
interface
Mobile Station (MS)
Mobile Equipment (ME)
Subscriber Identity Module (SIM)
Base Station Subsystem (BBS)
Base Transceiver Station (BTS)
Base Station Controller (BSC)
Network Subsystem
Mobile Switching Center (MSC)
Home Location Register (HLR)
Visitor Location Register (VLR)
Authentication Center (AUC)
Equipment Identity Register (EIR)
•
Mobile Station: is a subscriber unit intended for use while on the move at unspecified locations. It could be a
hand-held or a portable terminal.
•
Base Station: a fixed radio station used for communication with MS. It is located at the centre of a cell and
consist of Transmitters and Receivers.
•
Mobile Switching Centre: it coordinates the routing of calls, do the billing, etc.
Mobile Station (MS)
The Mobile Station is made up of two entities:
1. Mobile Equipment (ME)
2. Subscriber Identity Module (SIM)
Mobile Equipment
•
•
•
Produced by many different manufacturers
Must obtain approval from the standardization body
Uniquely identified by an IMEI (International Mobile Equipment Identity)
Subscriber Identity Module (SIM)
•
•
•
•
•
Smart card containing the International Mobile Subscriber Identity (IMSI)
Allows user to send and receive calls and receive other subscribed services
Encoded network identification details
Protected by a password or PIN
Can be moved from phone to phone – contains key information to activate the phone
Base Station Subsystem (BBS)
Base Station Subsystem is composed of two parts that communicate across the standardized Abis interface allowing
operation between components made by different suppliers
1. Base Transceiver Station (BTS)
1. Base Station Controller (BSC)
Base Transceiver Station (BTS)
•
•
•
•
Houses the radio transceivers that define a cell
Handles radio-link protocols with the Mobile Station
Speech and data transmissions from the MS are recoded
Requirements for BTS:
o ruggedness
o reliability
o portability
o minimum costs
Base Station Controller (BSC)
•
•
•
Manages Resources for BTS
Handles call set up
Location update
•
Handover for each MS
Network Subsystem
Mobile Switching Center (MSC)
•
•
•
Switch speech and data connections between:
Base Station Controllers
Mobile Switching Centers
GSM-networks
Other external networks
Heart of the network
Three main jobs:
1) Connects calls from sender to receiver
2) Collects details of the calls made and received
3) Supervises operation of the rest of the network components
Home Location Registers (HLR)
- contains administrative information of each subscriber
- Current location of the mobile
Visitor Location Registers (VLR)
-
- contains selected administrative information from the HLR
authenticates the user
tracks which customers have the phone on and ready to receive a call
periodically updates the database on which phones are turned on and ready to receive calls
Authentication Center (AUC)
-
mainly used for security
data storage location and functional part of the network
Ki is the primary element
Equipment Identity Register (EIR)
-
Database that is used to track handsets using the IMEI (International Mobile
Equipment Identity)
Made up of three sub-classes: The White List, The Black List and the Gray List
Optional database
Basic Features Provided by GSM
• Call Waiting
- Notification of an incoming call while on the handset
• Call Hold
- Put a caller on hold to take another call
• Call Barring
- All calls, outgoing calls, or incoming calls
• Call Forwarding
- Calls can be sent to various numbers defined by the user
• Multi Party Call Conferencing
- Link multiple calls together
Advanced Features Provided by GSM
•
•
•
•
•
•
Calling Line ID
- incoming telephone number displayed
Alternate Line Service
- one for personal calls
- one for business calls
Closed User Group
- call by dialing last for numbers
Advice of Charge
- tally of actual costs of phone calls
Fax & Data
- Virtual Office / Professional Office
Roaming
- services and features can follow customer from market to market
Advantages of GSM
•
•
•
•
•
•
•
•
•
•
Crisper, cleaner quieter calls
Security against fraud and eavesdropping
International roaming capability in over 100 countries
Improved battery life
Efficient network design for less expensive system expansion
Efficient use of spectrum
Advanced features such as short messaging and caller ID
A wide variety of handsets and accessories
High stability mobile fax and data at up to 9600 baud
Ease of use with over the air activation, and all account information is held in a smart card which can be
moved from handset to handset
UMTS (Universal Mobile Telephone System
•
Reasons for innovations
- new service requirements
- availability of new radio bands
•
User demands
-
seamless Internet-Intranet access
wide range of available services
compact, lightweight and affordable terminals
simple terminal operation
-
open, understandable pricing structures for the whole spectrum of available services
UMTS Basic Parameter
•
•
•
•
•
Frequency Bands (FDD : 2x60 MHz):
– 1920 to 1980 MHz (Uplink)
– 2110 to 2170 MHz (Downlink)
Frequency Bands (TDD: 20 + 15 MHz):
– 1900 – 1920 MHz and 2010 – 2025 MHz
RF Carrier Spacing:
– 4.4 - 5 MHz
RF Channel Raster:
– 200 KHz
Power Control Rate:
– 1500 Cycles per Second
UMTS W-CDMA Architecture
7
SATELLITE NETWORKS
History of satellite communication
1945 Arthur C. Clarke publishes an essay about „Extra
Terrestrial Relays“
1957 first satellite SPUTNIK
1960 first reflecting communication satellite ECHO
1963 first geostationary satellite SYNCOM
1965 first commercial geostationary satellite Satellit „Early Bird“
(INTELSAT I): 240 duplex telephone channels or 1 TV
channel, 1.5 years lifetime
1976 three MARISAT satellites for maritime communication
1982 first mobile satellite telephone system INMARSAT-A
1988 first satellite system for mobile phones and data
communication INMARSAT-C
1993 first digital satellite telephone system
1998 global satellite systems for small mobile phones
Applications
Traditionally
weather satellites
radio and TV broadcast satellites
military satellites
satellites for navigation and localization (e.g., GPS)
Telecommunication
global telephone connections
backbone for global networks
connections for communication in remote places or underdeveloped areas
global mobile communication
satellite systems to extend cellular phone systems (e.g., GSM orAMPS)
Classical satellite systems
Basics
Satellites in circular orbits
attractive force Fg = m g (R/r)²
centrifugal force Fc = m r ω²
m: mass of the satellite
R: radius of the earth (R = 6370 km)
r: distance to the center of the earth
g: acceleration of gravity (g = 9.81 m/s²)
ω: angular velocity (ω = 2 π f, f: rotation frequency)
Stable orbit
Fg = Fc
Basics
o
o
o
o
o
o
o
o
Elliptical or circular orbits
Complete rotation time depends on distance satellite-earth
Inclination: angle between orbit and equator
Elevation: angle between satellite and horizon
LOS (Line of Sight) to the satellite necessary for connection
1. High elevation needed, less absorption due to e.g. buildings
Uplink: connection base station - satellite
Downlink: connection satellite - base station
Typically separated frequencies for uplink and downlink
1. Transponder used for sending/receiving and shifting of frequencies
2. Transparent transponder: only shift of frequencies
3. Regenerative transponder: additionally signal regeneration
I
Elevation
Link budget of satellites
Parameters like attenuation or received power determined by four parameters:
Sending power
Gain of sending antenna
Distance between sender and receiver
Gain of receiving antenna Problems
Varying strength of received signal due to multipath propagation
Interruptions due to shadowing of signal (no LOS) possible solutions
Link Margin to eliminate variations in signal strength
Satellite diversity (usage of several visible satellites at the same time) helps to use less sending power
L: Loss
f: carrier frequency
r: distance
c: speed of light
ORBITS
Four different types of satellite orbits can be identified depending on the shape and diameter of the orbit:
GEO: geostationary orbit, ca. 36000 km above earth surface
LEO (Low Earth Orbit): ca. 500 - 1500 km
MEO (Medium Earth Orbit) or ICO (Intermediate Circular Orbit):
ca. 6000 - 20000 km
HEO (Highly Elliptical Orbit) elliptical orbits
Geostationary satellites
Orbit 35,786 km distance to earth surface, orbit in equatorial plane (inclination 0°)
Complete rotation exactly one day, satellite is synchronous to earth rotation
Fix antenna positions, no adjusting necessary
Satellites typically have a large footprint (up to 34% of earth surface!), therefore difficult to reuse frequencies
Bad elevations in areas with latitude above 60° due to fixed position above the equator
High transmit power needed
High latency due to long distance (ca. 275 ms)
Not useful for global coverage for small mobile phones and data transmission, typically used for radio and TV
transmission
LEO systems
Orbit ca. 500 - 1500 km above earth surface
Visibility of a satellite ca. 10 - 40 minutes
Global radio coverage possible
Latency comparable with terrestrial long distance
Connections, ca. 5 - 10 ms
Smaller footprints, better frequency reuse
But now handover necessary from one satellite to another
Many satellites necessary for global coverage
More complex systems due to moving satellites
Examples:
Iridium (start 1998, 66 satellites)
Global star (start 1999, 48 satellites)
MEO systems
Orbit ca. 5000 - 12000 km above earth surface
Comparison with LEO systems:
Slower moving satellites
Less satellites needed
Simpler system design
For many connections no hand-over needed
Higher latency, ca. 70 - 80 ms
Higher sending power needed
Special antennas for small footprints needed
Example:
ICO (Intermediate Circular Orbit, Inmarsat) start ca. 2000
Routing
One solution: inter satellite links (ISL)
Reduced number of gateways needed
Forward connections or data packets within the satellite network as long as possible
Only one uplink and one downlink per direction needed for the connection of two mobile phones
Problems:
More complex focusing of antennas between satellites
High system complexity due to moving routers
Higher fuel consumption
Thus shorter lifetime
Iridium and Teledesic planned with ISL
Other systems use gateways and additionally terrestrial networks
Localization of mobile stations
Mechanisms similar to GSM
Gateways maintain registers with user data
HLR (Home Location Register): static user data
VLR (Visitor Location Register): (last known) location of the mobile station
SUMR (Satellite User Mapping Register):
Satellite assigned to a mobile station
Positions of all satellites
Registration of mobile stations
Localization of the mobile station via the satellite’s position
Requesting user data from HLR
Updating VLR and SUMR
Calling a mobile station
Localization using HLR/VLR similar to GSM
Connection setup using the appropriate satellite
Handover in satellite systems
Several additional situations for handover in satellite systems compared to cellular terrestrial mobile phone networks
caused by the movement of the satellites
Intra satellite handover
Handover from one spot beam to another
Mobile station still in the footprint of the satellite, but in another cell
Inter satellite handover
Handover from one satellite to another satellite
Mobile station leaves the footprint of one satellite
Gateway handover
Handover from one gateway to another
Mobile station still in the
footprint of a satellite, but
gateway leaves the footprint
Inter system handover
Handover from the satellite
network to a terrestrial cellular
network
Mobile station can reach a
terrestrial network again which
might be cheaper, has a lower
latency etc.
Overview of LEO/MEO systems
YCS012 -MOBILE COMPUTING
UNIT III
Wireless LAN – IEEE 802.11 - Architecture – services – MAC – Physical layer – IEEE 802.11a - 802.11b standards –
HIPERLAN – Blue Tooth.
WIRELESS LAN
Characteristics of wireless LANs
Advantages
o
o
o
o
Very flexible within the reception area
Ad-hoc networks without previous planning possible
(almost) no wiring difficulties (e.g. historic buildings, firewalls)
More robust against disasters like, e.g., earthquakes, fire - or users pulling a plug...
Disadvantages
o
o
o
Typically very low bandwidth compared to wired networks (1-10 Mbit/s)
Many proprietary solutions, especially for higher bit-rates, standards take their time (e.g. IEEE
802.11)
Products have to follow many national restrictions if working wireless, it takes a vary long time
to establish global solutions like, e.g., IMT-2000
Design goals for wireless LANs
o
global, seamless operation
o
o
o
o
o
o
o
o
low power for battery use
no special permissions or licenses needed to use the LAN
robust transmission technology
simplified spontaneous cooperation at meetings
easy to use for everyone, simple management
protection of investment in wired networks
security (no one should be able to read my data), privacy (no one should be able to collect user profiles),
safety (low radiation)
transparency concerning applications and higher layer protocols, but also location awareness if necessary
Comparison: infrared vs. radio transmission
•
Infrared
– uses IR diodes, diffuse light,
multiple reflections (walls,
furniture etc.)
•
Advantages
•
Disadvantages
•
•
Comparison: infrastructure vs. ad-hoc networks
Advantages
– experience from wireless
WAN and mobile phones
can be used
– coverage of larger areas
possible (radio can
penetrate walls, furniture
etc.)
Disadvantages
– very limited license free
frequency bands
– shielding more difficult,
interference with other
electrical devices
Example
– IrDA (Infrared Data
Association) interface
available everywhere
Radio
– typically using the license
free ISM band at 2.4 GHz
– simple, cheap, available in
many mobile devices
– no licenses needed
– simple shielding possible
– interference by sunlight,
heat sources etc.
– many things shield or
absorb IR light
– low bandwidth
•
•
•
Example
– WaveLAN , HIPERLAN,
Bluetooth
infrastructure
network
AP
AP
AP: Access Point
wired network
AP
ad-hoc network
IEEE 802.11 - ARCHITECTURE – SERVICES - ARCHITECTURE – SERVICES – MAC – PHYSICAL LAYER – IEEE
802.11A - 802.11B STANDARDS
802.11 - Architecture of an infrastructure network
Station (STA)
o
terminal with access mechanisms to the wireless medium and radio contact to the access point
Basic Service Set (BSS)
o group of stations using the same radio frequency
Access Point
o
station integrated into the wireless LAN and the distribution system
o
bridge to other (wired) networks
Portal
Distribution System
o interconnection network to form one logical network (EES: Extended Service Set) based
on several BSS
802.11 LAN
STA1
802. x LAN
BSS1
Portal
Access
Point
Distribution System
Access
Point
ESS
BSS2
STA2
STA3
802.11 LAN
802.11 - Architecture of an ad-hoc network
Direct communication within a limited range
o
Station (STA):
terminal with access mechanisms to the wireless medium
o
Basic Service Set (BSS):
group of stations using the same radio frequency
802.11
STA
LAN
1
STA
BSS
3
1
STA
2
BSS
2
STA
5
STA
4
802.11
LAN
7.6.1
IEEE standard 802.11
Application
Application
Transport
Transport
Network
Network
Data Link
Data Link
Data Link
Data Link
Physical
Physical
Physical
Physical
Radio
802.11 - Layers and functions
MAC
Access mechanisms, fragmentation, encryption
MAC Management
Synchronization, roaming, MIB, power management
PLCP Physical Layer Convergence Protocol
Clear channel assessment signal (carrier sense)
PMD Physical Medium Dependent
Modulation, coding
PHY Management
Channel selection, MIB
Station Management
Coordination of all management functions
802.11 - Layers
Network
Network
Medium
LLC
DL
C
MAC
MAC Management
PLCP
PHY Management
PMD
PH
Y
802.11 - Physical layer
3 versions: 2 radio (typ. 2.4 GHz), 1 IR
o
data rates 1 or 2 Mbit/s
FHSS (Frequency Hopping Spread Spectrum)
o
o
spreading, despreading, signal strength, typ. 1 Mbit/s
min. 2.5 frequency hops/s (USA), two-level GFSK modulation
DSSS (Direct Sequence Spread Spectrum)
o
o
o
o
Infrared
o
o
DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying), DQPSK for 2 Mbit/s
(Differential Quadrature PSK)
preamble and header of a frame is always transmitted with 1 Mbit/s, rest of transmission 1 or 2
Mbit/s
chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code)
max. radiated power 1 W (USA), 100 mW (EU), min. 1mW
850-950 nm, diffuse light, typ. 10 m range
carrier detection, energy detection, synchronization
802.11 - MAC layer I - DFWMAC
Traffic services
Asynchronous Data Service (mandatory)
exchange of data packets based on “best-effort”
support of broadcast and multicast
Time-Bounded Service (optional)
implemented using PCF (Point Coordination Function)
Access methods
DFWMAC-DCF CSMA/CA (mandatory)
collision avoidance via randomized „back-off“ mechanism
minimum distance between consecutive packets
ACK packet for acknowledgements (not for broadcasts)
DFWMAC-DCF w/ RTS/CTS (optional)
Distributed Foundation Wireless MAC
avoids hidden terminal problem
DFWMAC- PCF (optional)
access point polls terminals according to a list
Priorities
defined through different inter frame spaces
no guaranteed, hard priorities
SIFS (Short Inter Frame Spacing)
highest priority, for ACK, CTS, polling response
PIFS (PCF IFS)
medium priority, for time-bounded service using PCF
DIFS (DCF, Distributed Coordination Function IFS)
lowest priority, for asynchronous data service
802.11 - MAC layer
DIFS
medium
busy
DIFS
PIFS
SIFS
contention
next
frame
t
direct access
if
medium
is free
 DIFS
MAC address format
scenario
8
to DS from address 1 address 2 address 3 address 4
DS
0
0
DA
SA
BSSID
0
1
DA
BSSID
SA
ad-hoc network
infrastructure
network, from AP
infrastructure
1
network, to AP
infrastructure
1
network, within DS
0
BSSID
SA
Vasantha Kumar .VRA Lecturer CSE
1
TA
DA
-
DA
SA
DS: Distribution System
AP: Access Point
DA: Destination Address
SA: Source Address
BSSID: Basic Service Set Identifier
RA: Receiver Address
TA: Transmitter Address
MAC management
Synchronization
try to find a LAN, try to stay within a LAN
timer etc.
Power management
sleep-mode without missing a message
periodic sleep, frame buffering, traffic measurements
Association/Reassociation
integration into a LAN
roaming, i.e. change networks by changing access points
scanning, i.e. active search for a network
MIB - Management Information Base
managing, read, write
HIPERLAN
•
ETSI standard
– European standard, cf. GSM, DECT, ...
– Enhancement of local Networks and
interworking with fixed networks
– integration of time
-sensitive services from the early beginning
•
HIPERLAN family
– one standard cannot satisfy all requirements
• range, bandwidth,
QoS support
• commercial constraints
– HIPERLAN 1 standardized since 1996
higher layers
medium access
control layer
channel
access
control layer
physical layer
HIPERLAN
layers
network layer
data link layer
physical layer
OSI layers
logical link
control layer
medium access
control layer
physical layer
IEEE 802.x
layers
7.31.1
Original HIPERLAN protocol family
HIPERLAN 1 HIPERLAN 2HIPERLAN 3 HIPERLAN 4
Application wireless LAN access to ATM wireless localpoint-to-point
fixed networks
loop
wireless ATM
connections
Frequency
17.2-17.3GHz
5.1-5.3GHz
Topology decentralized ad
cellular,
point-topoint-to-point
hoc/infrastructure centralized
multipoint
omni-directional
directional
Antenna
Range
50 m
50-100 m
5000 m
150 m
QoS
statistical
ATM traffic classes (VBR, CBR, ABR, UBR)
Mobility
<10m/s
stationary
Interface
conventional LAN
ATM networks
Data rate
Power
conservation
>20 Mbit/s
23.5 Mbit/s
yes
155 Mbit/s
not necessary
HIPERLAN 1 - Characteristics
Data transmission
point-to-point, point-to-multipoint, connectionless
23.5 Mbit/s, 1 W power, 2383 byte max. packet size
Services
asynchronous and time-bounded services with hierarchical priorities
compatible with ISO MAC
Topology
infrastructure or ad-hoc networks
transmission range can be larger then coverage of a single node („forwarding“ integrated in mobile
terminals)
Further mechanisms
power saving, encryption, checksums
Services and protocols
CAC service
definition of communication services over a shared medium
specification of access priorities
abstraction of media characteristics
MAC protocol
MAC service, compatible with ISO MAC and ISO MAC bridges
uses HIPERLAN CAC
CAC protocol
provides a CAC service, uses the PHY layer, specifies hierarchical access mechanisms for one or
several channels
Physical protocol
send and receive mechanisms, synchronization, FEC, modulation, signal strength
HIPERLAN 1 - Physical layer
Scope
modulation, demodulation, bit and frame synchronization
forward error correction mechanisms
measurements of signal strength
channel sensing
Channels
3 mandatory and 2 optional channels (with their carrier frequencies)
mandatory
channel 0: 5.1764680 GHz
channel 1: 5.1999974 GHz
channel 2: 5.2235268 GHz
optional (not allowed in all countries)
channel 3: 5.2470562 GHz
channel 4: 5.2705856 GHz
BLUETOOTH
Consortium: Ericsson, Intel, IBM, Nokia, Toshiba - many members
Scenarios
connection of peripheral devices
loudspeaker, joystick, headset
support of ad-hoc networking
small devices, low-cost
bridging of networks
e.g., GSM via mobile phone - Bluetooth - laptop
Simple, cheap, replacement of IrDA, low range, lower data rates
2.4 GHz, FHSS, TDD, CDMA
Bluetooth MAC layer
• Synchronous Connection
-Oriented link (SCO)
– symmetrical, circuit switched, point
-to -point
• Asynchronous Connectionless Link (ACL)
– packet switched, point
-to -multipoint, master polls
• Access code
– synchronization, derived from master, unique per channel
• Packet header
– 1/3 -FEC, MAC address (1 master, 7 slaves), link type,
alternating bit ARQ/SEQ, checksum
72
54
access code
3
0-2745
packet header
4
1
bits
payload
1
1
8
bits
YCS012 -MOBILE COMPUTING
UNIT IV
MOBILE NETWORK LAYER
Mobile IP – Dynamic Host Configuration Protocol - Routing – DSDV – DSR – Alternative Metrics
Mobile IP
A standard for mobile computing and networking
Computers doesn’t stay put.
Change location without restart its application or terminating any ongoing communication
IP Networking
Protocol layer
Network Layer
Transport Layer
What does IP do
moving packets from source to destination
No ’end-to-end’ guarantees
IP addresses
Network-prefix
Host portion
IP Routing
Packet Header
Network-prefix
Every node on the same link has the same network-prefix
Mobile IP Solves the following problems
f a node moves from one link to another without chnging its IP address, it will be unable to receive
packets at the new link; and
If a node moves from one link to another without chnging its IP address, it will be unable to receive
packets at the new link; and
Mobile IP Overview
Solution for Internet
Scalable, robust, secure, maintain communication
Use their permanent IP address
Routing protocol
Route packets to nodes that could potentially change location very rapidly
Layer 4-7, outside Mobile IP, but will be of major interest
Mobile IP: Terminology
•
Mobile Node (MN)
– node that moves across networks without changing its IP address
•
Correspondent Node (CN)
–
•
Home Agent (HA)
–
–
•
host in the home network of the MN, typically a router
registers the location of the MN, tunnels IP packets to the COA
Foreign Agent (FA)
–
–
•
ost with which MN is “corresponding” (TCP)
host in the current foreign network of the MN, typically a router
forwards tunneled packets to the MN, typically the default router for MN
Care-of Address (COA)
–
–
address of the current tunnel end-point for the MN (at FA or MN)
actual location of the MN from an IP point of view
Tunneling
An encapsulating IP packet including a path and an original IP packet
IP-in-IP encapsulation
IP-in-IP encapsulation
• IP-in-IP-encapsulation (mandatory in RFC 2003)
– tunnel between HA and COA
ver.
IHL
TOS
length
IP identification
flags
fragment offset
TTL
IP-in-IP
IP checksum
IP address of HA
Care-of address
length
ver.
IHL
COA
TOS
IP identification
flags
fragment offset
TTL
lay. 4 prot.
IP checksum
IP address of CN
IP address of MN
TCP/UDP/ ... payload
Mobile IP and IPv6
Mobile IP was developed for IPv4, but IPv6 simplifies the protocols
•
Security is integrated and not an add-on, authentication of registration is included
•
•
COA can be assigned via auto-configuration (DHCPv6 is one candidate), every node has address
auto configuration
No need for a separate FA, all routers perform router advertisement which can be used instead of
the special agent advertisement;
Addresses are always co-located
MN can signal a sender directly the COA, sending via HA not needed in this case (automatic
path optimization)
soft“hand-over, i.e. without packet loss, between two subnets is supported
•
•
MN sends the new COA to its old router
the old router encapsulates all incoming packets for the MN and forwards them to the new COA
•
•
•
•
Authentication is always granted
ROUTING
Motivation for Mobile IP
Routing
•
•
based on IP destination address, network prefix (e.g. 129.13.42)
determines physical subnet
•
change of physical subnet implies change of IP address to have a topological correct address
(standard IP) or needs special entries in the routing tables
Specific routes to end-systems?
•
change of all routing table entries to forward packets to the right destination
•
does not scale with the number of mobile hosts and frequent changes in the location, security
problems
Changing the IP-address?
•
adjust the host IP address depending on the current location
•
almost impossible to find a mobile system, DNS updates take to long time
Requirements to Mobile IP
Transparency
•
mobile end-systems keep their IP address
•
continuation of communication after interruption of link possible
•
point of connection to the fixed network can be changed
Compatibility
•
support of the same layer 2 protocols as IP
•
no changes to current end-systems and routers required
•
mobile end-systems can communicate with fixed systems
Security
•
authentication of all registration messages
Efficiency and scalability
•
•
•
only little additional messages to the mobile system required (connection typically via a low
bandwidth radio link)
world-wide support of a large number of mobile systems in the whole
Internet
IPv6 availability
•
•
•
•
Generally available with (new) versions of most operating systems.
• BSD, Linux 2.2 Solaris 8
An option with Windows 2000/NT
Most routers can support IPV6
Supported in J2SDK/JRE 1.4
IPv6 Design Issues
•
•
•
•
•
•
Overcome IPv4 scaling problem
• Lack of address space.
Flexible transition mechanism.
New routing capabilities.
Quality of service.
Security.
Ability to add features in the future.
Mobile ad hoc networks
Standard Mobile IP needs an infrastructure
•
•
Home Agent/Foreign Agent in the fixed network
DNS, routing etc. are not designed for mobility
Sometimes there is no infrastructure!
•
•
•
•
remote areas, ad-hoc meetings, disaster areas
Cost can also be an argument against an infrastructure!
no default router available
every node should be able to forward
Traditional routing algorithms
Traditional algorithms are pro-active – i.e. operate independent of user-message demands. Suitable for
wired networks.
Distance Vector
•
periodic exchange of messages with all physical neighbors that contain information about who
can be reached at what distance
•
selection of the shortest path if several paths available Link State
•
periodic notification of all routers about the current state of all physical links
•
routers get a complete picture of the network Example
•
•
•
•
ARPA packet radio network (1973), DV-Routing, up to 138 nodes
every 7.5s exchange of routing tables including link quality
updating of tables also by reception of packets
routing problems solved with limited flooding
Problems of traditional routing algorithms
Dynamics of the topology
•
•
Frequent changes of connections, connection quality, participants
Limited performance of mobile systems
•
periodic updates of routing tables need energy without contributing to the transmission of user
data; sleep modes difficult to realize
•
Limited bandwidth of the system is reduced even more due to the exchange of routing
information
•
Links can be asymmetric, i.e., they can have a direction dependent transmission quality
•
Uncontrolled redundancy in links
•
Interference – ‘unplanned links’ (advantage?)
DSDV
DSDV (Destination Sequenced Distance Vector)
Early work
•
on demand version: AODV (Ad-hoc On-demand Distance Vector
Expansion of distance vector routing (but still pro-active)
Sequence numbers for all routing updates
• assures in-order execution of all updates
• avoids loops and inconsistencies
Decrease of update frequency (‘damping’)
•
•
store time between first and best announcement of a path
inhibit update if it seems to be unstable (based on the stored time values)
DYNAMIC HOST CONFIGURATION PROTOCOL
Dynamic Host Configuration Protocol (DHCP) is a network protocol for automatically assigning TCP/IP
information to client machines. Each DHCP client connects to the centrally-located DHCP server which
returns that client's network configuration, including the IP address, gateway, and DNS servers
DHCP is useful for automatic configuration of client network interfaces. When configuring the client
system, the administrator can choose DHCP and instead of entering an IP address, netmask, gateway,
or DNS servers. The client retrieves this information from the DHCP server. DHCP is also useful if an
administrator wants to change the IP addresses of a large number of systems. Instead of reconfiguring all
the systems, he can just edit one DHCP configuration file on the server for the new set of IP addresses. If
the DNS servers for an organization changes, the changes are made on the DHCP server, not on the DHCP
clients. Once the network is restarted on the clients (or the clients are rebooted), the changes take effect.
Furthermore, if a laptop or any type of mobile computer is configured for DHCP, it can be moved from
office to office without being reconfigured as long as each office has a DHCP server that allows it to
connect to the network.
Configuration File
The first step in configuring a DHCP server is to create the configuration file that stores the network
information for the clients. Global options can be declared for all clients, while other options can be
declared for individual client systems.
The configuration file can contain extra tabs or blank lines for easier formatting. Keywords are caseinsensitive and lines beginning with a hash mark (#) are considered comments.
Two DNS update schemes are currently implemented — the ad-hoc DNS update mode and the interim
DHCP-DNS interaction draft update mode. If and when these two are accepted as part of the Internet
Engineering Task Force (IETF) standards process, there will be a third mode — the standard DNS update
method. The DHCP server must be configured to use one of the two current schemes. Version 3.0b2pl11
and previous versions used the ad-hoc mode; however, it has been deprecated.
There are two types of statements in the configuration file:
Parameters — State how to perform a task, whether to perform a task, or what network
configuration options to send to the client.
Declarations — Describe the topology of the network, describe the clients, provide addresses for
the clients, or apply a group of parameters to a group of declarations.
Some parameters must start with the option keyword and are referred to as options. Options configure
DHCP options; whereas, parameters configure values that are not optional or control how the DHCP
server behaves.
8
In Example the routers, subnet-mask, domain-name, domain-name-servers, and time-offset options are used
for any host statements declared below it.
Additionally, a subnet can be declared, a subnet declaration must be included for every subnet in the
network. If it is not, the DHCP server fails to start.
In this example, there are global options for every DHCP client in the subnet and a range declared.
Clients are assigned an IP address within the range.
subnet 192.168.1.0 netmask 255.255.255.0 {
option routers
192.168.1.254;
option subnet-mask
255.255.255.0;
option domain-name
"example.com";
option domain-name-servers
192.168.1.1;
option time-offset
-18000;
# Eastern Standard Time
range 192.168.1.10 192.168.1.100;
}
DSR
Dynamic source routing
Split routing into discovering a path and maintaining a path
Discovering a path
Only if a path for sending packets to a certain destination is needed and no path is currently available (reactive
algorithm)
Maintaining a path
Only while the path is in use: make sure that it can be used continuously
Path discovery
Broadcast a packet (Route Request) with destination address and unique ID
•
•
if a station receives a broadcast packet
if the station is the receiver (i.e., has the correct destination address) then return the packet to the sender
(path was collected in the packet)
if the packet has already been received earlier (identified via ID)
then
Discard the packet
•
•
otherwise, append own address and broadcast packet
sender receives packet with the current path (address list)
Maintaining paths
•
After sending a packet
•
•
•
wait for a layer 2 acknowledgement (if applicable)
listen into the medium to detect if other stations forward the packet (if possible)
request an explicit acknowledgement
•
if a station encounters problems it can inform the sender of a packet or look-up a new path locally
ALTERNATIVE METRICS.
Mobile IP with reverse tunneling
Router accepts often only “topological correct“addresses (firewall!)
• a packet from the MN encapsulated by the FA is now topological correct
• furthermore multicast and TTL problems solved (TTL in the home network correct, but MN is to
far away from the receiver)
Reverse tunneling does not solve
• problems with firewalls, the reverse tunnel can be abused to circumvent security mechanisms
(tunnel hijacking)
• optimization of data paths, i.e. packets will be forwarded through the tunnel via the HA to a sender
(double triangular routing)
• The standard is backwards compatible
• the extensions can be implemented easily and cooperate with current implementations without
these extensions
Agent Advertisements can carry requests for reverse tunneling
World Wide Web and mobility
Protocol (HTTP, Hypertext Transfer Protocol) and language
(HTML, Hypertext Markup Language) of the Web have not been designed for mobile applications and
mobile devices, thus creating many problems!
Typical transfer sizes
•
HTTP request: 100-350 byte
•
•
responses avg. <10 kbyte, header 160 byte, GIF 4.1kByte, JPEG
12.8 kbyte, HTML 5.6 kbyte
•
but also many large files that cannot be ignored
•
The Web is no file system
•
Web pages are not simple files to download
•
static and dynamic content, interaction with servers via forms, content transformation, push
technologies etc.
•
many hyperlinks, automatic loading and reloading, redirecting
•
a single click might have big consequences!
YCS012 -MOBILE COMPUTING
UNIT V
TRANSPORT AND APPLICATION LAYERS
Traditional TCP – Classical TCP improvements – WAP, WAP 2.0.
TRADITIONAL TCP
TCP is an alternative transport layer protocol over IP.
•
TCP provides:
• Connection-oriented
• Reliable
• Full-duplex
• Byte-Stream
Connection-Oriented
•
•
•
Connection oriented means that a virtual connection is established before any user data is transferred.
If the connection cannot be established - the user program is notified.
If the connection is ever interrupted - the user program(s) is notified.
Reliable
•
•
Reliable means that every transmission of data is acknowledged by the receiver.
If the sender does not receive acknowledgement within a specified amount of time, the sender retransmits
the data
Byte Stream
•
Stream means that the connection is treated as a stream of bytes.
•
The user application does not need to package data in individual datagrams (as with UDP).
Buffering
•
TCP is responsible for buffering data and determining when it is time to send a datagram.
•
It is possible for an application to tell TCP to send the data it has buffered without waiting for a buffer to fill
up.
Full Duplex
•
TCP provides transfer in both directions.
•
To the application program these appear as 2 unrelated data streams, although TCP can piggyback control
and data communication by providing control information (such as an ACK) along with user data.
TCP Ports
•
Interprocess communication via TCP is achieved with the use of ports (just like UDP).
• UDP ports have no relation to TCP ports (different name spaces).
TCP Segments
•
The chunk of data that TCP asks IP to deliver is called a TCP segment.
•
Each segment contains:
• data bytes from the byte stream
• control information that identifies the data bytes
TCP Lingo
•
•
•
•
•
•
When a client requests a connection it sends a “SYN” segment (a special TCP segment) to the server port.
SYN stands for synchronize. The SYN message includes the client’s ISN.
ISN is Initial Sequence Number.
Every TCP segment includes a Sequence Number that refers to the first byte of data included in the
segment.
Every TCP segment includes an Acknowledgement Number that indicates the byte number of the next data
that is expected to be received.
• All bytes up through this number have already been received.
There are a bunch of control flags:
•
•
•
•
•
•
•
URG: urgent data included.
ACK: this segment is (among other things) an acknowledgement.
RST: error – connection must be reset.
SYN: synchronize Sequence Numbers (setup)
FIN: polite connection termination
MSS: Maximum segment size (A TCP option)
Window: Every ACK includes a Window field that tells the sender how many bytes it can send before the
receiver will have to toss it away (due to fixed buffer size).
CLASSICAL TCP IMPROVEMENTS
TCP Connection Creation
•
•
•
Programming details later - for now we are concerned with the actual communication.
A server accepts a connection.
• Must be looking for new connections!
A client requests a connection.
• Must know where the server is!
Client Starts
•
A client starts by sending a SYN segment with the following information:
• Client’s ISN (generated pseudo-randomly)
• Maximum Receive Window for client.
• Optionally (but usually) MSS (largest datagram accepted).
• No payload! (Only TCP headers)
Server Response
•
•
When a waiting server sees a new connection request, the server sends back a SYN segment with:
• Server’s ISN (generated pseudo-randomly)
• Request Number is Client ISN+1
• Maximum Receive Window for server.
• Optionally (but usually) MSS
• No payload! (Only TCP headers)
When the Server’s SYN is received, the client sends back an ACK with:
• Acknowledgment Number is Server’s ISN+1
TCP 3-way handshake
Client: “I want to talk, and I’m starting with byte number X”.
Server: “OK, I’m here and I’ll talk. My first byte will be called number Y, and I know your first byte will be
number X+1”.
Client: “Got it - you start at byte number Y+1”.
Bill: “Monica, I’m afraid I’ll syn and byte your ack”
TCP Data and ACK
•
•
•
Once the connection is established, data can be sent.
Each data segment includes a sequence number identifying the first byte in the segment.
Each segment (data or empty) includes a request number indicating what data has been received
Buffering
•
•
•
Keep in mind that TCP is part of the Operating System. The O.S. takes care of all these details
asynchronously.
The TCP layer doesn’t know when the application will ask for any received data.
TCP buffers incoming data so it’s ready when we ask for it.
TCP Buffers
•
•
Both the client and server allocate buffers to hold incoming and outgoing data
• The TCP layer does this.
Both the client and server announce with every ACK how much buffer space remains (the Window field in
a TCP segment).
Send Buffers
•
•
•
The application gives the TCP layer some data to send.
The data is put in a send buffer, where it stays until the data is ACK’d.
The TCP layer won’t accept data from the application unless (or until) there is buffer space.
ACKs
•
•
•
A receiver doesn’t have to ACK every segment (it can ACK many segments with a single ACK segment).
Each ACK can also contain outgoing data (piggybacking).
If a sender doesn’t get an ACK after some time limit, it resends the data.
TCP Segment Order
•
•
•
Most TCP implementations will accept out-of-order segments (if there is room in the buffer).
Once the missing segments arrive, a single ACK can be sent for the whole thing.
Remember: IP delivers TCP segments, and IP is not reliable - IP datagrams can be lost or arrive out of
order.
Termination
•
•
The TCP layer can send a RST segment that terminates a connection if something is wrong.
Usually the application tells TCP to terminate the connection politely with a FIN segment.
TCP Sockets Programming
•
•
•
•
Creating a passive mode (server) socket.
Establishing an application-level connection.
Sending/receiving data.
Terminating a connection.
Establishing a passive mode TCP socket
Passive mode:
• Address already determined.
•
Tell the kernel to accept incoming connection requests directed at the socket address.
• 3-way handshake
•
Tell the kernel to queue incoming connections for us.
Accepting an incoming connection
•
Once we start listening on a socket, the O.S. will queue incoming connections
• Handles the 3-way handshake
• Queues up multiple connections.
•
When our application is ready to handle a new connection, we need to ask the O.S. for the next connection.
Terminating a TCP connection
•
•
Either end of the connection can call the close() system call.
If the other end has closed the connection, and there is no buffered data, reading from a TCP socket returns
0 to indicate EOF
Client Code
•
TCP clients can connect to a server, which:
• takes care of establishing an endpoint address for the client socket.
•
don’t need to call bind first, the O.S. will take care of assigning the local endpoint address
(TCP port number, IP address).
• Attempts to establish a connection to the specified server.
• 3-way handshake
Reading from a TCP socket
•
•
•
By default read() will block until data is available.
Reading from a TCP socket may return less than max bytes (whatever is available).
You must be prepared to read data 1 byte at a time!
WIRELESS APPLICATION PROTOCOL (WAP)
Empowers mobile users with wireless devices to easily access and interact with information and services.
A “standard” created by wireless and Internet companies to enable Internet access from a cellular phone
WAP: Main Features
Browser
– “Micro browser”, similar to existing web browsers
Markup language
– Similar to HTML, adapted to mobile devices
Script language
– Similar to Javascript, adapted to mobile devices
Gateway
– Transition from wireless to wired world
Server
– “Wap/Origin server”, similar to existing web servers
Protocol layers
– Transport layer, security layer, session layer etc.
Telephony application interface
– Access to telephony functions
I
Internet Model
HTML
HTTP
TLS/SSL
TCP/IP
WAP Architecture
Clien
t
WML
WMLScript
Web
Server
WAP
Gateway
WML Encoder
WSP/WTP
WTAI
HTTP
WMLScript
Compiler
Protocol Adapters
Etc.
Content
W
ML
Dec
ks
with
WM
LScri
pt
WML
WML Script
WTAI
Etc.
WML Encoder
WSP/WTP
WMLScript
Compiler
Protocol Adapters
Application
Logic
WML Decks
with WML
-Script
WAP Application Server
Client
CGI
Script
s
etc.
Content
WAP Application Server
WAP: Network Elements
fixed network
wireless network
WAP Specifies
Wireless Application Environment
–
–
–
–
–
WML Microbrowser
WMLScript Virtual Machine
WMLScript Standard Library
Wireless Telephony Application Interface (WTAI)
WAP content types
Wireless Protocol Stack
–
–
–
–
–
WAP Stack
Wireless Session Protocol (WSP)
Wireless Transport Layer Security (WTLS)
Wireless Transaction Protocol (WTP)
Wireless Datagram Protocol (WDP)
Wireless network interface definitions
WAE (Wireless Application Environment):
– Architecture: application model, browser, gateway, server
– WML: XML-Syntax, based on card stacks, variables, ...
– WTA: telephone services, such as call control, phone book etc.
WSP (Wireless Session Protocol):
– Provides HTTP 1.1 functionality
– Supports session management, security, etc.
WTP (Wireless Transaction Protocol):
– Provides reliable message transfer mechanisms
– Based on ideas from TCP/RPC
WTLS (Wireless Transport Layer Security):
– Provides data integrity, privacy, authentication functions
– Based on ideas from TLS/SSL
WDP (Wireless Datagram Protocol):
– Provides transport layer functions
– Based on ideas from UDP
WHY WAP?
Wireless networks and phones
– have specific needs and requirements
– not addressed by existing Internet technologies
WAP
– Enables any data transport
• TCP/IP, UDP/IP, GUTS (IS-135/6), SMS, or USSD.
– Optimizes the content and air-link protocols
– Utilizes plain Web HTTP 1.1 servers
• leverages existing development methodologies
• utilizes standard Internet markup language technology (XML)
• all WML content is accessed via HTTP 1.1 requests
– WML UI components map well onto existing mobile phone user interfaces
• no re-education of the end-users
• leveraging market penetration of mobile devices
– Several modular entities together form a fully compliant Internet entity
WAP: “Killer” Applications
Location-based services
– Real-time traffic reporting, Event/restaurant recommendation
Enterprise solutions
– Email access, Database access, “global” intranet access
– Information updates “pushed” to WAP devices
Financial services
– Banking, Bill-paying, Stock trading, Funds transfers
Travel services
– Schedules and rescheduling, Reservations
Gaming and Entertainment
– Online, real-time, multi-player games
– Downloadable horoscopes, cartoons, quotes, advice
M-Commerce
– Shopping on the go
– Instant comparison shopping
– Location-based special offers and sales
Wireless Application Environment (WAE)
Goals
– device and network independent application environment
– for low-bandwidth, wireless devices
– considerations of slow links, limited memory, low computing power, small display, simple user
interface (compared to desktops)
– integrated Internet/WWW programming model
– high interoperability
WAE Components
Architecture
– Application model, Microbrowser, Gateway, Server
User Agents
– WML/WTA/Others
– content formats: vCard, vCalendar, Wireless Bitmap, WML, ...
WML
– XML-Syntax, based on card stacks, variables, ...
WMLScript
– procedural, loops, conditions, ... (similar to JavaScript)
WTA
– telephone services, such as call control, text messages, phone book, ... (accessible from WML/
WMLScript)
Proxy (Method/Push)
WAE: Logical Model
Origin Servers
web
server
other content
server
Client
Gateway
response
with
content
Method proxy
encoded
response
with
content
Push proxy
push
content
request
encoders
&
decoders
encoded
push
content
encoded
request
WTA
user agent
WML
user agent
other
WAE
user agents
WML: Wireless Markup Language
Tag-based browsing language:
– Screen management (text, images)
– Data input (text, selection lists, etc.)
– Hyperlinks & navigation support
Takes into account limited display, navigation capabilities of devices
XML-based language
– describes only intent of interaction in an abstract manner
– presentation depends upon device capabilities
Cards and Decks
– document consists of many cards
– User interactions are split into cards
– Explicit navigation between cards
– cards are grouped to decks
– deck is similar to HTML page, unit of content transmission
Events, variables and state mgmt
The basic unit is a card. Cards are grouped together into Decks Document ~ Deck (unit of transfer)
All decks must contain
– Document prologue
XML & document type declaration
– <WML> element
Must contain one or more cards
WML Example
WML>
<CARD>
<DO TYPE=“ACCEPT”>
<GO URL=“#eCard”/>
</DO
Welcome!
</CARD>
<CARD NAME=“eCard”>
<DO TYPE=“ACCEPT”>
<GO URL=“/submit?N=$(N)&S=$(S)”/>
</DO>
Enter name: <INPUT KEY=“N”/>
Choose speed:
<SELECT KEY=“S”>
<OPTION VALUE=“0”>Fast</OPTION>
<OPTION VALUE=“1”>Slow</OPTION>
<SELECT>
</CARD>
</WML>
WMLScript
Complement to WML
– Derived from JavaScript™
Provides general scripting capabilities
– Procedural logic, loops, conditionals, etc.
– Optimized for small-memory, small-cpu devices
Features
– local user interaction, validity check of user input
– access to device facilities (phone call, address book etc.)
– extensions to the device software
• configure device, download new functionality after deployment
Bytecode-based virtual machine
– Stack-oriented design, ROM-able
– Designed for simple, low-impact implementation
WMLScript compiler resides in the network
WMLScript Libraries
Lang - VM constants, general-purpose math functionality, etc.
String - string processing functions
URL - URL processing
Browser - WML browser interface
Dialog - simple user interface
Float - floating point functions
Wireless Telephony Application (WTA)
Collection of telephony specific extensions
– designed primarily for network operators
Example
– calling a number (WML)
wtai://wp/mc;07216086415
– calling a number (WMLScript)
WTAPublic.makeCall("07216086415");
Implementation
– Extension of basic WAE application model
– Extensions added to standard WML/WMLScript browser
– Exposes additional API (WTAI)
WTA Features
Extension of basic WAE application model
– network model for interaction
• client requests to server
• event signaling: server can push content to the client
– event handling
• table indicating how to react on certain events from the network
• client may now be able to handle unknown events
– telephony functions
• some application on the client may access telephony functions
WTAI includes:
– Call control
– Network text messaging
– Phone book interface
– Event processing
Security model: segregation
– Separate WTA browser
– Separate WTA port
WAP Push Services
Web push
– Scheduled pull by client (browser)
• example: Active Channels
– no real-time alerting/response
• example: stock quotes
Wireless push
– accomplished by using the network itself
• example: SMS
– limited to simple text, cannot be used as starting point for service
• example: if SMS contains news, user cannot request specific news item
WAP push
– Network supported push of WML content
• example: Alerts or service indications
– Pre-caching of data (channels/resources)
Push Access Protocol
Based on request/response model
Push initiator is the client
Push proxy is the server
Initiator uses HTTP POST to send push message to proxy
Initiator sends control information as an XML document, and content for mobile (as WML)
Proxy sends XML entity in response indicating submission status
Initiator can
– cancel previous push
– query status of push
– query status/capabilities of device
Push Proxy Gateway
WAP stack (communication with mobile device)
TCP/IP stack (communication with Internet push initiator)
Proxy layer does
– control information parsing
– content transformation
– session management
– client capabilities
– store and forward
– prioritization
– address resolution
– management function
WTP Services and Protocols
WTP (Transaction)
– provides reliable data transfer based on request/reply paradigm
• no explicit connection setup or tear down
• optimized setup (data carried in first packet of protocol exchange)
• seeks to reduce 3-way handshake on initial request
– supports
• header compression
• segmentation /re-assembly
• retransmission of lost packets
• selective-retransmission
• port number addressing (UDP ports numbers)
• flow control
– message oriented (not stream)
– supports an Abort function for outstanding requests
– supports concatenation of PDUs
– supports User acknowledgement or Stack acknowledgement option
• acks may be forced from the WTP user (upper layer)
• default is stack ack
WAP 2.0.
WSP - Wireless Session Protocol
Goals
– HTTP 1.1 functionality
• Request/reply, content type negotiation, ...
– support of client/server transactions, push technology
– key management, authentication, Internet security services
WSP Services
– provides shared state between client and server, optimizes content transfer
– session management (establish, release, suspend, resume)
– efficient capability negotiation
– content encoding
– push
WSP/B (Browsing)
– HTTP/1.1 functionality - but binary encoded
– exchange of session headers
– push and pull data transfer
– asynchronous requests
WSP Overview
Header Encoding
– compact binary encoding of headers, content type identifiers and other well-known textual or
structured values
– reduces the data actually sent over the network
Capabilities (are defined for):
– message size, client and server
– protocol options: Confirmed Push Facility, Push Facility, Session Suspend Facility,
Acknowledgement headers
– maximum outstanding requests
– extended methods
– header code pages
Suspend and Resume
– server knows when client can accept a push
– multi-bearer devices
– dynamic addressing
– allows the release of underlying bearer resources
Session Context and Push
– push can take advantage of session headers
– server knows when client can accept a push
Connection-mode
– long-lived communication, benefits of the session state, reliability
Connectionless-mode
– stateless applications, no session creation overhead, no reliability overhead
WAP: Ongoing Work
WDP
– Tunnel to support WAP where no (end-to-end) IP bearer available
WTLS
–
–
–
WTP
–
–
–
WSP
–
–
WAE
–
–
–
support for end-to-end security (extending WTLS endpoint beyond WAP Gateway)
interoperable between WAP and Internet (public key infrastructure)
integrating Smart Cards for security functions
efficient transport over wireless links (wireless TCP)
bearer selection/switching
quality of service definitions
quality of service parameters
multicast data, multimedia support
User agent profiles: personalize for device characteristics, preferences etc
Push architecture, asynchronous applications
Billing
UNIT-I
WIRELESS COMMUNICATION FUNDAMENTALS
1. What is mobile computing?
Mobile computing is a technology that allows transmission of data, via a
computer, without having to be connected to a fixed physical link.
2. What are two different kinds of mobility?
User Mobility: It refers to a user who has access to the same or similar
telecommunication services at different places.
Device Portability: many mechanisms in the network and inside the device
have to make sure that communication is still possible while the device is moving.
3. Find out the characteristics while device can thus exhibit during
communication.




Fixed and Wired
Mobile and Wired
Fixed and Wireless
Mobile and Wireless
4. What are applications of Mobile Computing?






Vehicles
Emergencies
Business
Replacement of wired networks
Infotainment
Location dependent services
Follow-on services
Location aware services
Privacy
MOBILE COMPUTING
Information services
Support services
 Mobile and wireless devices
Sensor
Embedded controllers
Pager
Mobile phones
Personal digital assistant
Pocket computer
Notebook/laptop
5. What are the obstacles in mobile communications?







Interference
Regulations and spectrum
Low Bandwidth
High delays, large delay variation
Lower security, simpler to attack
Shared Medium
Adhoc-networks.
6. What is TETRA?
TETRA (Terrestrial Trunked Radio) systems use different radio carrier
frequencies, but they assign a specific carrier frequency for a short period of time
according to demand. TETRA’s are highly reliable and extremely cheap.
7. Which elements of the network perform the data transfer?
Physical medium
8. Compare the different types of transmission errors that can occur in wireless
and wired networks.
Types of Error:
 Data loss
 Noise
 Low power
9. Define Signal.
A signal is defined as any physical quantity carrying information that varies
with time. The value of signal may be real or complex. The types of signal are
continuous signal and discrete time signal.
10. Define antenna.
An antenna or aerial is one or more electrical conductors of a specific length
that radiate radio waves generated by a transmitter or that collect radio waves at the
receiver.
MOBILE COMPUTING
11. State the relation between wavelength and frequency.
Wavelength is the length or distance of one cycle of an ac wave. It is also the
distance that an ac wave travels in the time required for one cycle of that signal.
Wavelength is expressed as the ratio of the speed of light to the frequency of the
signal
12. What are the main problems of signal propagation?
Power additionally influenced by
 fading (frequency dependent)
 shadowing
 Reflection at large obstacles
 Refraction depending on the density of a medium
 scattering at small obstacles
 Diffraction at edges
13. How a receiver adopts for Multi-path propagation effects during wireless
reception?
Time dispersion: signal is dispersed over time Interference with .neighbor.
Symbols, Inter Symbol Interference (ISI),The signal reaches a receiver directly and
phase shifted distorted signal depending on the phases of the different parts.
14. What is multipath propagation?
Multipath propagation is the direct from a sender to a receiver the propagation
effects mentioned in the previous section lead to one of the most severe radio channel
impairments.
15. What are the types of Frequency Modulation?
Based on the modulation index FM can be divided into types. They are
Narrow band FM and Wide band FM. If the modulation index is greater than one then
it is wide band FM and if the modulation index is less than one then it is Narrow band
FM.
16. What is the basic difference between an AM signal and a narrowband FM
signal?
In the case of sinusoidal modulation, the basic difference between an AM
signal and a narrowband FM signal is that the algebraic sign of the lower side
frequency in the narrow band FM is reversed.
17. How will you generate message from frequency-modulated signals?
First the frequency-modulated signals are converted into corresponding
amplitude-modulated signal using frequency dependent circuits.Then the original
signal is recovered from this AM signal.
MOBILE COMPUTING
18. What is called multipath Interference?
The interference caused by the interfacing of the signal form the indirect path
with the signal of direct path is called multipath interference.
19. Define reflection loss
Reflection loss is defined as the number of nippers or decibels by which the
current in the load under image matched conditions would exceed the current actually
flowing in the load
20. Define multiplexing.
Multiplexing is defined as the process of transmitting several message signals
simultaneously over a single channel.
21. List out the various Multiplexing Schemes?




Space division multiplexing
Frequency division multiplexing
Time division multiplexing
Code division multiplexing
22. Give the use of SDMA./ What is SDMA?
Space Division Multiple Access (SDMA) is used for allocating separated
spaces to users in wireless networks. The basis for the SDMA algorithm is formed by
cells and sectorized antennas which constitute the infrastructure implementing space
division multiplexing (SDM).
23. Define CDMA.
Code Division Multiple Access systems use codes with certain characteristics
to separate different users. To enable access to the shared medium without
interference. The users use the same frequency and time to transmit data. The main
problem is to find good codes and to separate this signal from noise. The good code
can be found the following 2 characteristic
1. Orthogonal.
2. Auto Correlation.
24. What the features are of Code Division multiple Accesses?
 It does not require external synchronization networks.
 CDMA offers gradual degradation in performance when the no. of
users is increased But it is easy to add new user to the system.
 If offers an external interference rejection capability.
MOBILE COMPUTING
25. How are guard spaces realized between users in CDMA?
The guard space between a pair of users in CDMA systems is the
orthogonality between their spreading codes. The lower the correlation between any
pair of spreading codes is, the better is the user separation.
26. What is hopping sequence?
Transmitter and receiver stay on one of the channels like
and TDM. The
pattern of channel usage is called the hopping sequence,
27. What is the need for modulation?
Needs for modulation:
 Ease of transmission
 Multiplexing
 Reduced noise
 Narrow bandwidth
 Frequency assignment
 Reduce the equipments limitations.
28. Give the classification of modulation.
There are two types of modulation. They are
 Analog modulation
 Digital modulation
29. Give the classification of Digital modulation.
 Amplitude shift keying
 Phase shift keying
 Frequency shift keying
30. Define demodulation.
Demodulation or detection is the process by which modulating voltage is
recovered from the modulated signal. It is the reverse process of modulation.
31. Define stability.
It is the ability of the receiver to deliver a constant amount of output for a
given a given period of time.
32. What are the 3 different basic schemes analog modulations?
1. Amplitude modulation
2. Frequency modulation
3. Phase modulation
MOBILE COMPUTING
33. Define frequency modulation.
Frequency modulation is defined as the process by which the frequency of the
carrier wave is varied in accordance with the instantaneous amplitude of the
modulating or message signal.
34. Define phase modulation.
Phase modulation is defined as the process of changing the phase of the carrier
signal in accordance with the instantaneous amplitude of the message signal.
35. What is the advantage of a spread spectrum technique?
The main advantage of spread spectrum technique is its ability to reject
interference whether it be the unintentional interference of another user
simultaneously attempting to transmit through the channel (or) the intentional
interference of a hostile transmitter to jam the transmission.
36. What is called frequency hop spread spectrum?
In frequency hop spread spectrum, the frequency of the carrier hops randomly
from one frequency to another frequency.
37. What is the function of Medium Access Control Layer?
The functions of Medium Access Control Layer are responsible for
establishes, maintains, and releases channels for higher layers by activating and
deactivating physical channels.
38. What are the several versions in CSMA?
There are several versions in CSMA, they are as follows
a) Non-persistent CSMA
b) p-persistent CSMA
c) 1-persistent CSMA
39. What is meant by non-persistent CSMA?
In, non-persistent CSMA, stations sense the carrier and start sending
immediately if the medium is idle, if the medium is busy, the station pauses a random
amount of time before sensing the medium again and repeating this pattern.
40. What is meant by p-persistent CSMA?
In p-persistent CSMA system nodes also sense the medium, but only
transmit with a probability of p. With the station deferring to the next slot with the
probability1-p, i.e. access is slotted in addition.
MOBILE COMPUTING
41. What is FDD?
In FDMA, the base station and the mobile station establish a duplex channel.
The two directions, mobile station to base station and vice versa are separated using
different frequencies. This Scheme is called Frequency Division Duplex (FDD)
42. What is dwell time?
The time spend on a channel with a certain frequency is called the dwell time
43. What is fast frequency hopping?
If the hop rate is an integer multiple of symbol rate (multiple hops per symbol)
then it is called fast frequency hopping.
44. What is slow frequency hopping?
If the symbol rate of MFSK is an integer multiple of hop rate (multiple
symbols per hop) then it is called slow frequency hopping.
45. What is a burst error?
A burst error is when two or more consecutive bits within a given data string
are in error. These errors can affect one or more characters within a message.
46. What are the 2 sub layers in DLC?
o Logical Link Control(LLC)
o Media Access Control(MAC)
47. Define traffic multiframe and control multiframe?
1. The periodic pattern of 26 slots occurs in all TDMA frames with a
TCH.
2. The combination of these frames is called traffic multiframe
3. TDMA frames containing data for the other logical channels are
combined to a control multiframe.
48. Explain about transparent mode?
The transparent mode transfer simply forwards MAC data without any
further processing. The system then has to rely on the FEC which is always used in
the radio layer.
49. List out the advantage of cellular wireless networks.




Higher capacity, higher number of users
Less transmission power needed
More robust, decentralized
Base station deals with interference, transmission area etc.
MOBILE COMPUTING
UNIT II TELECOMMUNICATION NETWORKS
1. What are the disadvantages of cellular systems?
The advantages of cellular systems are,
 Infrastructure needed
 Hand over needed
 Frequency planning
2. What are the basic groups of logical channels?
GSM specifies 2 basic groups of logical channels,
 Traffic channels
 Control channels
3. What are the categories of Mobile services?
 Bearer services
 Tele services
 Supplementary services
4. What are subsystems in GSM system?
 Radio subsystem (RSS)
 Network & Switching subsystem (NSS)
 Operation subsystem (OSS)
5. What are the control channel groups in GSM?
The control channel groups in GSM are:
 Broadcast control channel (BCCH)
 Common control channel (CCCH)
 Dedicated control channel (DCCH)
6. What are the four types of handover available in GSM?
1. Intra cell Handover
2. Inter cell Intra BSC Handover
3. Inter BSC Intra MSC handover
4. Inter MSC Handover
7. Give the information’s in SIM?




card type, serial no, list of subscribed services
Personal Identity Number(PIN)
Pin Unlocking Key(PUK)
An Authentication Key(KI)
8. What is the frequency range of uplink and downlink in GSM network?
 The frequency range of uplink in GSM network is 890-960 MHz
 The frequency range of downlink in GSM network is 935-960 MHz
MOBILE COMPUTING
9. What are the security services offered by GSM?
The security services offered by GSM are:
Access control and authentication.
Confidentiality.
Anonymity.
10. What are the control channel groups in GSM?
The control channel groups in GSM are:
Broadcast control channel (BCCH).
Common control channel (CCCH).
Dedicated control channel (DCCH).
11. What is authentication centre (AuC)?
As the radio interface and mobile stations are particularly vulnerable a
separate AuC has been defined to protect user identity and data transmission. The
AuC contains the algorithms for authentication as well as the keys for encryption and
generates the values needed for user authentication in the HLR. The AuC may, in fact,
be situated in a special protected part of the HLR.
12. What is Network and Switching subsystem?
The heart of the GSM is formed by the Network and Switching System (NSS).
NSS consists of the following switches and databases:
• Mobile Services switching Center (MSC)
• Home Location register (HLR)
• Visitor Location Register (VLR)
13. What are the services provided by supplementary services?
• User identification
• Call redirection
• Call forwarding
• Closed user groups
• Multiparty Communication
14. What are types of Handover?
 Intra-cell handover
 Inter-cell, intra- BSC handover
 Inter-BSC, intra-MSC handover
 Inter MSC handover
15. What are the reasons for delays in GSM for packet data traffic?
Collisions only are possible in GSM with a connection establishment. A
slotted ALOHA mechanism is used to get access to the control channel by which the
base station is told about the connection establishment attempt. After connection
establishment, a designated channel is installed for the transmission.
MOBILE COMPUTING
16. If 8 speech channels are supported on a single radio channel, and if no guard
band is assumed, what is the number of simultaneous users that can be
accommodated in GSM?
1000 users.
17. What is meant by beacon?
A beacon contains a timestamp and other management information used for
power management and roaming. e.g., identification of the base station subsystem
(BSS)
18. List out the numbers needed to locate an MS and to address the MS.
The numbers needed to locate an MS and to address the MS are:
Mobile station international ISDN number (MSISDN)
International mobile subscriber identity (IMSI)
Temporary mobile subscriber identity (TMSI)
Mobile station roaming number (MSRN)
19. What is meant by GPRS?
The General Packet Radio Service provides packet mode transfer for
applications that exhibit traffic patterns such as frequent transmission of small
volumes.
20. What is meant by GGSN?
GGSN is Gateway GPRS Support Node. It is the inter-working unit between
the GPRS network and external packet data networks. The GGSN is connected to
external networks via the Gi interface and transfers packets to the SGSN via an IP-
based GPRS backbone network.
21. What is meant by SGSN?
SGSN is Serving GPRS Support Node. It supports the MS via the Gb
interface. The GSN is connected to a BSC via frame relay.
22. What is meant by BSSGP?
BSSGP is Base Station Subsystem GPRS Protocol.
It is used to convey
routing and QoS- related information between the BSS and SGSN.BSSGP does not
perform error correction and works on top of a frame relay network.
23. Define the protocol architecture of DECT.
The protocol architecture of DECT consists of three layers. They are:
1. Physical Layer.
2. Medium Access Layer.
3. Data Link Control Layer.
4. Network Layer.
MOBILE COMPUTING
24. What are the steps perform during the search for a cell after power on?
The steps perform during the search for a cell after power on is:
 Primary Synchronization.
 Secondary Synchronization.
 Identification of the scrambling code.
25. What are the applications in satellites?





Weather forecasting satellites
Radio & TV broadcast satellites
Military satellites
Satellites for navigation
Mobile communication
26. Define the terms:
(i). Earth Station,(ii). Uplink,(iii). Downlink.
Earth Station:-The antenna systems on or near the earth are referred to as Earth
Station.
Uplink:-A transmission from an earth station to the satellite is referred to as Uplink.
Downlink:-A transmission from the satellite to the earth station is referred to as
Downlink.
27. What are the factors limited the number of sub channels provided within the
satellite channel?
There are three factors limited the number of sub channels provided within the
satellite channel. They are:
 Thermal Noise.
 Inter modulation Noise.
 Cross talk.
28. What is meant by GEO?
GEO means Geostationary or Geosynchronous earth orbit.GEO satellites have
a distance of almost 36000 km to the earth. Examples are almost all TV and radio
broadcast satellites, many weather satellites and satellites operating as backbone for
the telephone network.
29. What is communication satellite?
Communications satellite
is an artificial satellite stationed in space for the
purposes of telecommunications. Modern communications satellites use a variety of
orbits including geostationary orbits, Molniya orbits, other elliptical orbits and low
(polar and non-polar) Earth orbits.
MOBILE COMPUTING
30. What are the registers maintained by the gateway of satellite?
1. Home Location Register (HLR)
2. Visitor Location Register (VLR)
3. Satellite User Mapping Register (SUMR)
31. What are the advantages of LEO?
 Data rate is 2400 bit/s
 Packet delay is relatively low
 Smaller footprints of LEO allows frequency reuse
 Provide high elevations
32. Define the inclination angle and perigee.
The inclination angle is defined as the angle between the equatorial plane
and the lane
described by the satellite orbit.
An inclination angle of 0 degrees
means that the satellite is exactly above the equator. If the satellite does not have a
circular orbit, the closest point to the earth is called the perigee.
33. Define the elevation angle and footprint.
The elevation angle is defined as the angle between the centre of satellite
beam
and the plane
tangential
to
the
earth's surface.
The foot-print
can
be
defined as the area on earth where the signals of the satellite can be received.
34. What are the advantages of GEO?
Three GEO satellites are enough for a complete coverage of almost any spot
on earth, senders and receivers can use fixed antennas positions, and no adjusting is
needed. Therefore GEO’s are ideal for T.V and radio broadcasting
35. What is Handover?
The satellite is the base station in satellite communication systems and that it
is moving. So, additional instance of handover are necessary due to the movement of
the satellite
1. Intra Satellite handover:
2. Inter Satellite handover.
3. Gateway handover.
4. Inter System handover.
36. Advantages of MEO.
Using Orbits around 10,000Km, the system only requires a dozen satellites
which is more than the GEO system, but much less than a LEO system. Furthermore
these satellites move slower relative to the earth’s rotation allowing a simpler system
design. Depending on the inclination a MEO can cover larger populations, thus
requiring less handovers.
37. What is browsing channel allocation and fixed channel allocation?
Cells with more traffic are dynamically allotted more frequencies. This
scheme is known as browsing channel allocation, while the first fixed scheme is
called fixed channel allocation.
MOBILE COMPUTING
38. Write short notes on DAB.
 MSC




FIC
DAB Frame Structure
Components of DAB sender
Multimedia Object Transfer Protocol
39. What are the two basic transport mechanisms used by DAB?
The two basic transport mechanisms used by DAB are:
1. Main Service Channel (MSC).
2. Fast Information Channel (FIC).
40. What are different interleaving and repetition schemes applied by DAB to
objects and segments?
1. Object Repetition.
2. Interleaved Objects.
3. Segment repetition.
4. Header repetition.
41. What are the advantages of DAB?
1. DAB can offer sound in CD like quality.
2. DAB can use single frequency network where all senders transmitting the
same radio program can operate at the same frequency.
3. DAB use VHF and UHF frequency bands.
4. DAB uses DQPSK modulation scheme.
5. DAB user COFDM and FEC.
6. DAB can transmit up to six stereo audio programmes with a data rate of
192kbit/s each.
42. What is object repetition?
DAB can repeat objects several times. If an object A consists of four segments
(A1,A2,A3,A4)
a
single
repetition
A1A2A3A4A1A2A3A4A1A2A3A4……..
pattern
would
be
43. State the different types of transport modes and the channel used to carry
packets in Digital Audio Broadcasting.
 Two transport modes are possible in main service channel, namely,
stream mode and packet mode.
 Each frame has three parts, namely synchronization channel, fast
information channel and main service channel.
MOBILE COMPUTING
44. What is FIC?
The Fast Information Channel (FIC) contains Fast Information Block(FIB)
with 256bits each(16 bit checksum). An FIC carries all control information which is
required for interpreting the configuration and content of the MSC.
45. What is MSC?
Main Service Channel (MSC) carries all user data.eg. audio, multimedia data.
46. What are the two transport modes defined for MSC?
The two transport modes defined for MSC are:
o Stream Mode
o Packet Mode.
47. What are the goals of DVB?
The goal of DVB is to
introduce digital TV broadcasting using satellite
transmission (DVB-5)cable technology(DVB-c)and terrestrial transmission (DVB-7)
48. What is EIT?
Event Information Table (EIT) contains status information about the current
transmission and some additional information for set-top boxes.
49. What is the service information sent by DVB?
Digital Video Broadcast Containers are basically MPEG-2 frames. DVB sends
service information. This information is,
1. Network information Table (NIT).
2. Service Description Table (SDT).
3. Event Information Table (EIT).
4. Time and Date Table (TDT)
50. What are the advantages of DVB?
 Data rates planned for users are 6-38mbit/s for the downlink and 33100kbit/s for the uplink.
 Transmitted along with TV programmes and doesn’t require additional
lines or hardware per customer.
 Can be used in remote areas and developing countries where there is
no high band width wired network.
51. What is EY-NMPA?
Elimination yield -Non Pre-emptive Multiple Access (EY-NMPA) is a
scheme which uses several phases to sense the medium. Access the medium
and for contention resolution. Priority schemes can also be included. This is actually
used in HIPERLAN1 specification.
MOBILE COMPUTING
UNIT III WIRLESS LAN
1. What are the Advantages of wireless LAN?
 Flexibility,
 Planning,
 Design,
 Robustness,
 Quality Service,
 Cost,
 Proprietary Solution,
 Restriction,
 Safety and Security
2. What are the Design Goals of Wireless LAN?
 Global Operation
 Low Power
 License-free Operation
 Robust transmission technology
 Simplified spontaneous co-operation
 Easy to use
 protection of investment
 Safety and Security
 Transparency for application
3. Mention some of the disadvantages of WLANS?
• Quality of service
• Proprietary solutions.
• Restrictions
• Safety and Security
4. Mention the features of radio transmission?
• Cover large areas.
• Can penetrate walls, furniture’s.
• Does not need a LOS.
• Higher transmission rates.
5. What are the disadvantages of radio transmission?
• Shielding is not so simple.
• Can interfere with other senders.
• Limited ranges of license-free bands.
MOBILE COMPUTING
6. Mention the features of infrared transmission?
• Simple
• Extremely cheap
• Licenses are not needed
• Electrical devices do not interfere
7. What are Advantages and Disadvantages of Infrared?
Advantages:
 Simple and extremely cheap senders and receivers which integrated in almost
all mobile devices
 No licenses are needed for infrared technology and shielding is very simple.
 Electrical devices do not interfere with infrared transmission.
Disadvantages:
i. Low bandwidth
ii. Quite easily shielded
iii. Cannot Penetrate
8. What is the difference between infrastructure and ad-hoc networks?
Infrastructure-based wireless networks:
Communication takes place only between the wireless nodes and the access
point, but not directly between the wireless nodes.
Ad-hoc wireless networks:
Communication takes place directly with other nodes, so no access point
Controlling medium access is necessary.
9. Define frequency hopping spread spectrum?
FHSS allows for the coexistence of multiple networks in the same area by
separating different networks using different hopping sequences.
10. Define random back off time?
If the medium is busy, nodes have to wait for the duration of DIFS, entering a
contention phase afterwards. Each node now chooses a random back off time within a
contention window and delays medium access for this random amount of time.
11. What is the primary goal of IEEE 802.11?
The primary goal of the standard was the specification of a simple, robust,
WLAN which offers time bounded and asynchronous services also it should be able
to operate with multiple physical layers.
12. Is IEEE 802.11 and Wi-Fi same/ State the purpose of Wi-Fi.
Ans: No
It is wireless internet. Your laptop has an internal wireless card so you can
connect to wireless routers. If you goto a hotel that advertises free wireless internet,
you should be able to connect to it. You don't have to have an Ethernet cable to
connect to the web at home either.
MOBILE COMPUTING
13. Why the PHY layer of IEEE 802.11 is subdivided? What about HiperLAN2
and Bluetooth?
PLCP Physical Layer Convergence Protocol
Clear channel assessment signal (carrier sense)
PMD Physical Medium Dependent
Modulation, coding
PHY Management channel selection,
o MIB Station Management coordination of all management functions
o
o
o
o
o
14. What are the various versions of a physical layer defined in IEEE 802.11
standards?




IEEE 802.11-83.5 MHz
IEEE 802.11a -300 MHz
IEEE 802.11b. 83.5 MHz
IEEE 802.11g - 83.5 MHz
15. What are the system integration functions of MAC management?




Synchronization
Power management
Roaming
Management information base (MIB)
16. What is the main problem for WATM during handover?
The main problem for WATM during the hand over is rerouting of all
connections and maintaining connection quality.
17. What are the different segments in ATM end-to-end connection?
 An ATM end-to-end connection is separated into different segments.
 A fixed segment is a part of the connection that is not affected by the handover
 Hand over segment is affected by the hand over and is located completely
within a hand over domain.
18. What is meant by SIFS?
SIFS means Short Inter Frame Spacing. The shortest waiting time defined for
short control message such as acknowledgements or polling response.
19. What is SCO?
SCO-stands for Synchronous Connection Oriented Link Standard telephone
(voice) connection require symmetrical, circuit-switched, point-to-point connections.
For this type of link, the master reserves two consecutive slots at fixed intervals.
20. What are the three phases in EY-NPMA?
i. Prioritization: Determine the highest priority of a data packet ready to be
sent on competing nodes.
MOBILE COMPUTING
ii. Contention: Eliminate all but one of the contenders, if more than one
sender has the highest current priority.
iii. Transmission: Finally, transmit the packet of the remaining node.
21. What do you meant by roaming?
Moving between access points is called roaming. Even wireless networks may
require more than one access point to cover all rooms. In order to provide
uninterrupted service, we require roaming when the user moves from one access point
to another.
22. What is mobile routing?
Even if the location of a terminal is known to the system, it still has to route the traffic
through the network to the access point currently responsible for the wireless
terminal. Each time a user moves to a new access point, the system must reroute
traffic. This is known as mobile routing.
23. What are the functions which support service and connection control?





Access point control function
Call control and connection control function
Network security agent
Service control function
Mobility management function
24. What are the examples for service scenarios identified in WATM?






Office environments
Universities, schools, training, centers
Industry
Hospitals
Home
Networked vehicles
25. What is BRAN?
The broadband radio access networks (BRAN) which have been standardized
by European Telecommunications Standard Institute(ETSI) are a possible choice for
an RAL for WATM. Although BRAN has been standardized independently from
WATM, there is co-operation between the two to concentrate the common efforts on
one goal. The main motivation behind BRAN is the deregulation and privatization of
the telecommunication sector in Europe.
26. What are the different network types of BRAN?




Hyperlan1
Hyperlan2
Hyper access
Hyperlink
MOBILE COMPUTING
27. What is the main problem for WATM during handover?
The main problem for WATM during the handover is rerouting of all
connections and maintaining connection quality.
28. What are the different segments in ATM end-to-end connection?
An ATM end-to-end connection is separated into different segments.
 A fixed segment is a part of the connection that is not affected by the
handover
 Handover segment is affected by the handover and is located
completely within a handover domain.
29. What is anchor point?
The Anchor point is the boundary between a handover segment and a fixed
segment.
30. What are different types of handover?







Hard handover
Terminal initiated
Network initiated
Network initiated, terminal assisted
Network controlled
Backward handover
Forward handover
31. What is mobile terminal and wireless terminal?
 Mobile terminal is a standard ATM terminal with the additional capability of
reconnecting after access point change. The terminal can be moved between
different access points within a certain domain.
 Wireless terminal is accessed via a wireless link, but the terminal itself is
fixed, i.e., the terminal keeps its access point to the network.
32. What are the three Low Power States provided by Bluetooth?
PARK state
HOLD state
SNIFF state
33. Mention the elements of Bluetooth core protocols?
• Radio
• Baseband
• Link manager protocol
• Logical link control and adaptation protocol
• Service discovery protocol
MOBILE COMPUTING
34. What is the purpose of sniff state?
The sniff state has the highest power consumption. The device listens to the
piconet at a reduced rate.
35. What is the use of hold state?
The device does not release its AMA but stops ACL transmission. A slave
may still exchange SCO packets.
36. What is the purpose of park state?
In this state the device has the lowest duty cycle and the lowest power
consumption. The device releases its AMA and receives a parked member address.
The device is still a member of the piconet, but gives room for another device to
become active.
37. How does registration on layer 3 of a mobile node work?
In the real system, a mobile node can connect to the network by using multiple
interfaces with different access technologies such as Wi-Fi, CDMA. At the same time
it can perform multiple connections for multiple services such as video, voice, or just
chatting.
38. What are the advantages and problems of forwarding mechanisms in
Bluetooth networks regarding security and power saving?
 Advantage: Bluetooth network enables setting up of the network without
much preparation. It sets itself automatically.
 Problems: Security and power are major constraints. Security may be
compromised and power may be spent on traffic not meant for a particular
device.
39. Why Bluetooth specification comprises so many protocols and components?
The Bluetooth protocol stack, in common with all such standards, is specified
as several separate layers
MOBILE COMPUTING
UNIT IV MOBILE NETWORK LAYER
1. What are the requirements of mobile IP?
• Compatibility
• Transparency
• Scalability and efficiency
• Security
2. Mention the different entities in a mobile IP.
• Mobile Node
• Correspondent Node
• Home Network
• Foreign Network
• Foreign Agent
• Home Agent
• Care-Of address
 Foreign agent COA
 Co-located COA
3. Define Mobile node:
A mobile node is an end-system or router that can change its point of
attachment to the Internet using mobile IP. The MN keeps its IP address and can
continuously with any other system in the Internet as long as link layer connectivity is
given.
4. Explain Cellular IP.
CellularIP provides local handovers without renewed registration by installing
a single cellularIP gateway for each domain, which acts to the outside world as a
foreign agent.
5. What do you mean by mobility binding?
The Mobile Node sends its registration request to the Home Agent. The HA
now sets up a mobility binding containing the mobile node’s home IP address and the
current COA.
6. Define COA.
The COA (care of address) defines the current location of the MN from an IP
point of view. All IP packets sent to the MN are delivered to the COA, not directly to
the IP address of the MN. Packet delivery toward the MN is done using the tunnel.
DHCP is a good candidate for supporting the acquisition of Care Of Addresses.
MOBILE COMPUTING
7. Define a tunnel.
A tunnel establishes a virtual pipe for data packets between a tunnel entry and
a tunnel endpoint. Packets entering a tunnel are forwarded inside the tunnel and leave
the tunnel unchanged.
8. What is encapsulation?
Encapsulation is the mechanism of taking a packet consisting of packet header
and data putting it into the data part of a new packet.
9. What is decapsulation?
The reverse operation, taking a packet out of the data part of another packet, is
called decapsulation.
10. Define an outer header.
The HA takes the original packet with the MN as destination, puts it into the
data part of a new packet and sets the new IP header in such a way that the packet is
routed to the COA. The new header is called the outer header.
11. Define an inner header.
There is an inner header which can be identical to the original header as this
case for IP-in-IP encapsulation, or the inner header can be computed during
encapsulation.
12. What is meant by generic routing encapsulation?
Generic routing encapsulation allows the encapsulation of packets of one
protocol suite into the payload portion of a packet of another protocol suite.
13. Why is need of routing?
Routing is to find the path between source and destination and to forward the
packets appropriately.
14. What is the use of network address translation?
The network address translation is used by many companies to hide internal
resources and to use only some globally available addresses.
15. Define triangular routing.
The inefficient behavior of a non-optimized mobile IP is called triangular
routing. The triangle is made up of three segments, CN to HA, HA to COA\MN, and
MN back to CN.
MOBILE COMPUTING
16. What is meant by a binding cache?
One way to optimize the route is to inform the CN of the current location by
caching it in a binding cache which is a part of the local routing table for the CN.
17. Define binding request.
Any node that wants to know the current location of an MN can send a
binding request to the HA. The HA can check if the MN has allowed dissemination of
its current location. If the HA is allowed to reveal the location it sends back a binding
update.
18. What is known as Binding update?
This message sent by the HA to CNs reveals the current location of the MN.
The message contains the fixed IP address of the MN and the COA. The binding
update can request an acknowledgement.
19. Explain binding acknowledgement.
If requested, a node returns this acknowledgement receiving a binding update
message.
20. Define binding warning.
If a node decapsulates a packet for a MN, but it is not the current FA for this
MN, this node sends a binding warning. The warning contains MN’s home address
and a target node address.
21. What are the advantages of cellular IP?
 Manageability:
 Cellular IP is mostly self-configuring, and integration of the CIPGW
into a firewall would facilitate administration of mobility-related
functionality.
 Efficiency
 Transparency
 Security
22. What is known as mobility anchor point?
HMIPv6 provides micro-mobility support by installing a mobility anchor
point, which is responsible for a certain domain and acts as a local HA within this
domain for visiting MNs.
MOBILE COMPUTING
23. Explain destination sequence distance vector routing.
Destination sequence distance vector routing is an enhancement to distance
vector routing for ad-hoc networks and is used as routing information protocol in
wired networks.
24. What are the two things added to the distance vector algorithm?
• Sequence Numbers
• Damping
25. How the dynamic source routing does divide the task of routing into two
separate problems?
1. Route discovery
2. Route Maintenance
26. How can DHCP be used for mobility and support of mobile IP?
Normally, a mobile node uses a care-of-address. In some cases, the mobile
node may have to act as its own foreign agent by using co-located care of address.
The means by which a mobile node acquires a co-located address is beyond the scope
of mobile IP. One means is to dynamically acquire temporary IP address an the move
using services such as DHCP.
27. List out the some of the popular Routing protocols.
 DSDV(Destination Sequence Distance Vector)
 DSR(Dynamic Source Routing)
 AODV(Ad-Hoc On Demand Vector Routing)
28. What is meant by Transparency?
Mobility should remain invisible for many higher layer Protocols and
applications. The only affects of mobility should be a higher delay and lower
bandwidth which are natural in the case of mobile networks.
29. Specify the field of minimal encapsulation method in mobile network layer.
o Minimal encapsulation doing,
o Avoids repetition of identical fields e.g. TTL, IHL, version, TOS
o Only applicable for unfragmented packets, no space left for fragment
identification
MOBILE COMPUTING
30. What do you meant by roaming?
Moving between access points is called roaming. Even wireless networks may
require more than one access point to coverall rooms. In order to provide
uninterrupted service, we
point to another.
require roaming when the user moves from one access
31. What is mobile routing?
Even if the location of a terminal is known to the system, it still has to route the
traffic through the network to the access point currently responsible for the wireless
terminal. Each time a user moves to a new access point, the system must reroute
traffic. This is known as mobile routing.
MOBILE COMPUTING
UNIT V TRANSPORT AND APPLICATION LAYERS
1. What is slow start?
TCP’s reaction to a missing acknowledgement is necessary to get rid of
congestion quickly. The behavior TCP shows after the detection of congestion is
called slow start.
2. What is the use of congestion threshold?
The exponential growth of the congestion window in the slow start mechanism
is dangerous as it doubles the congestion window at each step. So a congestion
threshold is set at which the exponential growth stops.
3. What led to the development of Indirect TCP?
• TCP performs poorly together with wireless links
• TCP within the fixed network cannot be changed.
This led to the development of I-TCP which segments a TCP connection into a
fixed part and a wireless part.
4. What is the goal of M-TCP?
The goal of M-TCP is to prevent the sender window from shrinking if bit
errors or disconnection but not congestion cause current problems. It wants • To
provide overall throughput
• To lower the delay
• To maintain end-to-end semantics of TCP
• To provide a more efficient handover.
5. What do you mean by persistent mode?
Persistent mode is the state of the sender will not change no matter how long
the receiver is disconnected. This means that the sender will not try to retransmit the
data.
6. What are the characteristics of 2.5G/3.5G wireless networks?
• Data rates
• Latency
• Jitter
• Packet loss
7. What are the configuration parameters to adapt TCP to wireless
environments?
• Large Windows
• Limited Transmit
• Large MTU
MOBILE COMPUTING
• Selective Acknowledgement
• Explicit Congestion Notification
• Timestamp
• No header compression
8. State the requirements of WAP.
• Interoperable
• Scalable
• Efficient
• Reliable
• Secure
9. Name the layers of WAP.
• Transport layer
• Security layer
• Transaction layer
• Session layer
• Application layer
10. Name some ICMP messages.
• Destination unreachable
• Parameter problem
• Message too big
• Reassembly failure
• Echo request/reply
11. What is WTP? What are its classes?
WTP stands for Wireless Transaction Protocol. It has been designed to run on
very thin clients such as mobile phones. It has three classes.
• Class 0: provides unreliable message transfer without any result message.
• Class 1: provides reliable message transfer without exactly one reliable result
message.
• Class 2: provides reliable message transfer with exactly one reliable result
message.
12. What is WSP?
The Wireless Session Protocol has been designed to operate on top of the
datagram service WDP or the transaction service WTP. It provides a shared state
between a client and a server to optimize content transfer.
13. Name some features of WSP adapted to web browsing.
• HTTP/1.1 functionality
• Exchange of session headers
• Push and pull data transfer
• Asynchronous request
MOBILE COMPUTING
14. What is WML?
The Wireless Markup Language is based on the standard HTML known from
the www and on HDML. WML is specified as an XML document type.
15. What are the features of WML?
• Text and Images
• User interaction
• Navigation
• Context Management
16. What are the advantages of WML Script over WML?
WML Script offers several capabilities not supported by WML:
• Validity check of user input
• Access to device facilities
• Local user interaction
• Extension to the device software
17. Name the libraries specified by WML Script.
• Lang
• Float
• String
• URL
• WML Browser
• Dialogs
18. What are the classes of libraries?
• Common network services
• Network specific services
• Public services
19. Name the operations performed by PAP.
Push access Protocol performs the following operations:
• Push submission
• Result notification
• Push cancellation
• Status query
• Client capabilities query
20. What are the components of WAP2.0?
The protocol framework of WAP2.0 consists of four components:
• Bearer networks
• Transport services
• Transfer services
• Session services
MOBILE COMPUTING
21. What is the use of congestion threshold?
The exponential growth of the congestion window in the slow start mechanism
is dangerous as it doubles the congestion window at each step. So a congestion
threshold is set at which the exponential growth stops.
22. What is image scaling?
If a page contains a true color, high-resolution picture, this picture can be
called down to fewer colors, lower resolution, or finally to only the title of the picture.
The user can decide to download the picture separately. Further one can offer
clipping, zooming, or detail Studies to users if they are interested in a part of the
picture.
23. Define WAP
WAP is Wireless Application Protocol. It is the basic Objective of the WAP
forum are to bring diverse Internet content and others data service to digital cellular
phones and other wireless, mobile terminals. More ever a protocol suite should enable
global wireless communication across different wireless network technologies. All
WAP forum solution must be: interoperable, scalable, efficient, and reliable.
24. What is WML Browser?
WML Browser is a library that provides several functions typical for a
browser, such as per to go back one card or refresh to update the context of the user
interface.
25. What are the features of WML?
WML includes several basic features.
i) Text and Images
ii)User Interaction
iii)Navigation
iv)Context Management
26. What are the two functions of the transport layer in the internet?
The two functions of the transport layer in the internet are check summing
over user data and multiplexing/ demultiplexing of data from applications.
27. What is called the exponential growth of the congestion window?
The senders always calculate congestion window for a window start size of the
congestion window is one segment. Sender sends one packet and waits for
acknowledgement. If acknowledgement arises it raises the level of congestion
window by one. If sender sends two packets if acknowledgement arises it raises the
level of congestion window by two. This scheme raises the level of congestion
window every time the acknowledges come back, which takes roundtrip time
(RTT).This is called the exponential growth of the congestion window.
MOBILE COMPUTING
28. Advantages of I-TCP:
 I-TCP does not require any changes in the TCP protocol as used by the
hosts in the fixed network or other hosts in a wireless network that do
not use this optimization.
 Without partitioning retransmission of lost packets would take place
between mobile host and correspondent host across the whole network.
 Optimization of new mechanisms is quite simple to be done in I-TCP
as they only cover a single hop.
 The short delay between the mobile host and foreign agent can be
determined and is independent of other traffic streams. Therefore an
optimized TCP can use precise time-outs to guarantee retransmission
as fast as possible.
 Partitioning into two connections also allows the use of a different
transport layer protocol between the foreign agent and the mobile host
or the use of compressed headers etc. The foreign agent can act as a
gateway to translate between different protocols.
29. Disadvantages of I-TCP:
 The loss of the end to end semantics of TCP cause problems if the
foreign agent portioning the TCP connection crashes.
 An increased handover latency is more problematic in practical use
 The foreign agent must be a trusted entity because the TCP
connections end at this point.
30. How does data transmission takes place?
Data transmission takes place using network adapters, fiber optics, copper
wires, special hardware for routers etc.
31. Mention two WAP service provides. Find two cell phones supporting WAP
and identify which WAP version they support.
Wireless application protocol (WAP) is a common effort of many companies
and organizations to set up a framework for wireless and mobile web access using
many different transport systems. Eg. GSM, GPRS, UMTS
32. How and why does I-TCP isolate problems on the wireless link? What are the
main drawbacks of this solution?
The loss of the end to end semantics of TCP causes problems if the foreign
agent portioning the TCP connection crashes. Increased handover latency is more
problematic in practical use . The foreign agent must be a trusted entity because the TCP
connections end at this point.
MOBILE COMPUTING
33. Can the problems using TCP for mobile communication be solved by replacing
TCP with snooping TCP? Justify your answer.
Ans: yes
 buffering of packets sent to the mobile host lost packets on the wireless link
(both retransmitted immediately by the mobile host ordirections!) will be
foreign agent, respectively (so called .local. retransmission)
 the foreign agent therefore .snoops. the packet flow and recognizes
acknowledgements in both directions, it also filters ACKs
 changes of TCP only within the foreign agent
34. What are the key elements of the WAP specification?
 Networks and Network Bearers
 TCP/IP as Transport Protocol
 Processors
35. What are the goals of WTLS layer?
It provides the upper-level layer of WAP with a secure transport service
interface that preserves the transport service interface below it. In addition, WTLS
provides an interface for managing (e.g., creating and terminating) secure
connections. It provides functionality similar to TLS 1.0 and incorporates additional
features such as datagram support, optimized handshake and dynamic key refreshing.
36. What is mean by SCPS-TP?
The set of protocols developed for space communication is known as space
communications protocol standards (SCPS),the extended TCP is called SCPStransport protocols.(SCPS-TP).
37. What are Advantage and Disadvantage of Mobile TCP?
Advantage:
i. M-TCP maintains the TCP end-to-end semantic. The SH does not send any
ACK itself but forwards the ACKs from the MH.
ii. If the MH is disconnected, M_TCP avoids useless retransmissions, slow
starts or breaking connections by simply shrinking the sender’s window to 0;
iii. Since M-TCP does not buffer data in the SH as I-TCP does, it is not
necessary to forward buffers to a new SH. Lost packets will be automatically
retransmitted to the new SH.
Disadvantage:
i. As the SH does not act as proxy as in I-TCP, packet loss on the wireless link
due to
bit errors is propagated to the sender. M-TCP assumes low bit error rates,
which is not always a valid assumption.
ii. A modified TCP on the wireless link not only requires modification to the
MH, protocol software but also new network elements like the bandwidth
manager.
MOBILE COMPUTING
38. What is fast retransmit?
The gap in the packet stream is not due to severe congestion, but a simple
packet loss due to a transmission error. The sender can now retransmit the missing
packet before the timer expires. This behavior is called fast retransmit.
39. What is fast recovery?
The receipt of acknowledgement shows that there is no congestion justifying a
slow start. The sender can continue with the current congestion window. The sender
performs a fast recovery from the packet loss. This mechanism can improve the
efficiency of TCP dramatically.
40. What is HTTP?
The Hypertext transfer protocol is a stateless, lightweight, application level
protocol for data transfer between servers and clients. An HTTP transaction consists
of an HTTP request issued by a client and an HTTP response from the server.
Stateless means that all HTTP transactions independent of each other.
41. Define Damping.
Transient changes in topology that are short duration should not destabilize the
routing mechanism .Advertisements containing changes in topology currently stored
are therefore not disseminated further .A node waits with dissemination if these
changes are most likely not yet stable. Waiting time depends on the time between the
first and the best announcement.
42. Define WDP.
WDP is Wireless Datagram Protocol operates on top of many different bearer
services capable of carry in data. At the T-SAP WDP offers a consistent datagram
transport service independent of the underlying bearer.WDP offers source and
destination port numbers used for multiplexing and demultiplexing of data
respectively.
43. What are the three ways of WTA extends the WAE application model?
i) Content push: A WTA organ server can push the content.
ii) Handling of network events: A device can have a table indicating how to
react to certain events from the mobile network.
iii) Access to telephony function: Application running on the client can access
telephony functions from WML or WML script is very simple.
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