Wireless Local Area Networks (WLAN)

Department of Computer Science Institute for System Architecture, Chair for Computer Networks
Wireless Local Area Networks
(WLAN)
Mobile Communication and Mobile Computing
Prof. Dr. Alexander Schill
http://www.rn.inf.tu-dresden.de
Network Types
• classification of networks
via dimension:
− Private Area Network (PAN)
− Local Area Network (LAN)
− Metropolitan Area Network
(MAN)
− Wide Area Network (WAN)
− Global Area Network (GAN)
• wireless versions:
− WPAN, e.g. Bluetooth
− WLAN, e.g. 802.11
− WMAN, WWAN,
e.g. 802.16 and 802.20,
and UMTS / LTE
2
Wireless LANs:
Motivation / Application areas
Advantages
• flexibility
• widely available Internet
access at low cost
• ad-hoc-networks
• no problems with cables
Problems
• higher error rate on the
transmission link in
comparison to Standard-LANs
• security aspects
• shared medium
• Some national restrictions
concerning the used
frequency bands
(Industrial Scientific Medical
(ISM)- Band)
Applications
• Internet access
• networks in exhibition
halls
• hospitals
• warehouses
• airports
• structure of networks
in historic buildings
• extension of existing
wired local area
networks in offices,
universities etc.
3
Wireless Standards in general
Common
Name:
Standard:
Wi-Fi
(Wireless
Fidelity)
WiMAX
IEEE 802.11
IEEE 802.16 /
802.16a
Bluetooth,
ZigBee
IEEE 802.15
Frequency
2,4 and 5
Band: [GHz]
2..66
2,4
Data rate:
[MBit/s]
300 and more
Up to 134
1-20
Reach:
about 100 m
coverage up to
50 km (cellular)
about 5 m,
sometimes up to
50 m
Specifics
cheap and
flexible,
improved
security
wireless
stationary
internet access
voice channel,
peripheral
devices,
sensor networks
Used for
WLAN
WMAN
Remote Devices
4
802.11 – Frequency Bands
• 2,4 GHz Band
− 2,4 to 2,4835 GHz
− ISM-Band
− License-free
− transmitted power max.
100 mW
• 5 GHz Band
− 5,15 - 5,725 GHz in
Europe
− License-free
− transmitted power
max. 1000 mW with
special power control
5
802.11 - Network Topologies (1)
• infrastructure mode
− like a star-network
− Access-Point (AP) is a central point
− AP coordinates the network nodes and communicates
with other networks
AP
AP
Three infrastructure APs
in one fixed network
Network
AP
6
802.11– Network Topologies (2)
• Ad-hoc Mode
− Like Peer-to-Peer Network
− no central Station or
higher-level infrastructure
available
− All network nodes are
equivalent
Direct connection
− the nodes see each other and can
communicate one with each other
Beaconing-Mechanism
− every node sends a “Beacon”Signal in certain intervals. Via this
signal every node knows its direct
neighbors. ad-hoc-nets appear
spontaneously and organize and
administrate themselves
Indirect connection
− no direct communication possible
− special routing methods for
transmission of the data (e.g.
OLSR Optimized Link State
Routing)
7
802.11 – System Architecture
• Station (STA)
− device with 802.11-interface
• Access Point (AP)
BSS1
802.x
LAN
STA1
AP
Portal
Distribution System
ESS
STA2
AP
− Allow access for registered stations
to the distribution system
• Basic Service Set (BSS)
− AP and associated stations
• Independent BSS (IBSS)
− in Ad-hoc-Mode
• Distribution System
STA3
BSS2
− Connects more than one BSS via the
access points to form a larger logical
network
• Extended Service Set (ESS)
− Multiple connected wireless networks
• Portal
− Allows entering of other networks
8
802.11 - WLAN standards
Standard
Frequency Max. Normal Trensmission
Bandwidth data rate Data rate DRmax
DR
Range R (indoor/
outdoor)
Remarks
802.11
2,4 GHz
2 MBit/s
1,2 MBit/s
DSSS (FHSS, Infrared)
30/300
outdated
802.11a
5 GHz
54 MBit/s
32 MBit/s
OFDM
10/100
high data rate, but incompatible to other standards, low range
802.11b
2,4 GHz
11 MBit/s
7 MBit/s
DSSS
30/300
higher range, but lower data rate
802.11g
2,4 GHz
54 MBit/s
32 MBit/s
OFDM
30/300
higher data rate and range, but sensitive to noise
802.11n
802.11ac
2,4 GHz and 5 GHz 300 MBit/s
(and more)
~ 100
MBit/s
OFDM
10/100
very high data rate, but also sensitive to noise
DSSS ... Direct Sequence Spread Spectrum
FHSS … Frequency Hopping Spread Spectrum
OFDM ... Orthogonal Frequency Division Multiplexing
9
802.11 Sub-Standards
Standard Characteristics 802.11a
Enhancement of the physical layer
802.11b
Enhancement of the physical layer
802.11c
Define Wireless Bridging between Access Points
802.11d
Country specific regulations for 802.11b
802.11e
Enhanced WLAN for QoS (Quality of Service)
802.11f
Roaming and inter-­operability between base station different vendors
802.11g
Enhancement of the physical layer
802.11h
Optional for 802.11a for Europe
802.11i
Enhancement of security and authentication
802.11j
Model of 802.11a in Japan 802.11n, 802.11ac
Enhancements for higher data rates
802.11o
Prioritizing of voice data in WLAN in opposite to the data traffic
802.11p
Enhancement to 802.11a for use in vehicular networks
802.11r
Fast Roaming at the swapping between Access Points
802.11u
Inter-­operability with other non 802 networks
10
OFDM: Example 802.11a
• 64 sub-bearer signals per bearer, use of 64QAM
(Quadrature Amplitude Modulation)
• 48 sub-bearers for data modulation, 4 as phase
reference, and 12 as distance to next bearer
11
OFDM with 802.11b
• ISM frequency band of 2,4 GHz
• Bandwidth per channel: 22 MHz, 14 overlapping channels
• Channel allocation slightly different in various countries
12
802.11n, 802.11ac (1)
• Focus: Higher end applications in WLANs
− Wireless Streaming Media
− Videoconferencing
• Technical aspects
− Actual data rate should reach in the area of 100 MBit/s
− Possible gross data rate up to 300 MBit/s
− MIMO-techniques (Multiple Input/Multiple Output)
− use of several sender-/receiver channels (max. 4)
SISO
Transmitter
Receiver
MIMO
Transmitter
Receiver
13
802.11n, 802.11ac (2)
• OFDM, adaptive modulation with BPSK, QPSK, 16QAM, 64QAM,
256QAM
• 2,4 GHz-band with downward-compatibility to 802.11b/g, 5 GHz
band also a possible option
• optional extension of the radio channel from 20 MHz up to 40 MHz
• radio interface
•
regular sending
•
packet aggregation
(more efficient)
Backoff: Jam signal
BPSK: Binary Phase Shift Keying
SIFS:
Short Inter Frame Spacing for ACK and response to polling
ACK:
Acknowledgement signal
DIFS: Distributed Coordination Function Inter Frame Spacing for
asynchronous services
-> Overhead due to packet aggregation reduced
14
Specific Problems and Solutions
• Physical problems
− interference: spread spectrum techniques, frequency hopping
− hidden Terminal problem: use of CSMA/CA (see later)
• Data security
− Wired Equivalent Privacy (WEP) service
− Current improvements
− WPA (WiFi Protected Access), WPA2
− 802.11i
15
802.11 – Medium Access (1)
• Carrier Sense Multiple Access with collision avoidance
(CSMA/CA)
− CSMA/CD in wireless networks not possible
− No collision detection
− principle: „listen before talking“ (only CSMA without CD)
• Procedure:
− 1. a station intends to send, is listening before sending
− 2. if medium is free then send
− 3. if medium is busy, wait until the „back off“ interval is over
and begin at 1 (repeat until maximum number of attempts)
16
802.11 – Medium Access (2)
• Problem of CSMA in the wireless case: hidden terminal
− A and C cannot communicate directly
− A sends to B and occupies the medium
− C also wants to send to B, but does not recognize the
occupied medium
− Collision at B, so B cannot reconstruct data
− A also does not detect the collision
Ú A is hidden for C and C is hidden for A
A
B
C
17
802.11 – Medium Access (3)
• Further problem: exposed terminal
− B sends to A, and C wants to send to an other station (not A
and not B)
− C recognizes the signals from B and is waiting until B will
finish its transmission
− unnecessary waiting, because signals from C cause no
collision at A
Ú exposed terminal: C is exposed to other stations
A
B
C
18
802.11 – Medium Access (4)
• solution: RTS/CTS-Mechanism (Request To Send/Clear To Send)
• solution: hidden terminal
− A sends a RTS-Signal to B and B sends CTS after that (ready for transmission)
− All other possible senders (C) also get the CTS-signal and will wait and reschedule
their transmission
RTS
A
CTS
B
CTS
C
• solution: exposed terminal
− C sends RTS to the receiver and gets CTS, so it can transmit
− B hears RTS, too. But B does not receive any CTS (too far away).
− So B assumes the channel to be free (which is ok) and can also send.
A
B
C
RTS
CTS
19
802.11: Data Security in WLANs
• WEP (Wired Equivalent Privacy)
- symmetrical cryptography,
e.g. using RC4
However: Small key lengths
(40-128 bit), same key for
multiple clients, low security!
clear text (in frame body)
integrityalgorithm
secret
key
+
Integrity check
value (ICV)
clear text
of frame
body +
ICV
Generator for
Pseudo-random number key sequence
+
Encryption
encoded
text in
frame
body
20
802.11-Security – WPA / WPA2
WiFi protected access, subset of 802.11i, resolves the WEP
problems
• authentication
– Pre-Shared-Key (PSK), 8-64
characters password, used for
generation of the session key
– Extensible Authentication
Protocol based on 802.1x (e.g.
RADIUS-Server – Remote
Access Dial-in User Service)
• encryption
– Integrity Check
– TKIP (Temporal Key
Integrity Protocol)
generates dynamic key per
packet (WPA)
– RC4 (WPA) or AES (WPA2)
for encryption
• remaining security problems
– Simple PSK allows “brute force” or dictionary
attack
21
802.11 Security – Summary
WEP
WPA
WPA2/
IEEE802.11i
RC4
RC4
AES
Key length [Bit]
40, 104
128 or more
128 or more
Data integrity
CRC-­32
“Michael”
CCM
Header integrity non
“Michael”
CCM
Key management
non
EAP-­based
EAP-­based Features Encryption
• RC4 – R.Rivest Encryption symmetrical method
• AES – Advanced Encryption Standard, a symmetrical
cryptosystem, modern DES, RC4 successor
• CCM – Counter Mode with Cipher Block Chaining Message
Authentication Code Protocol
• EAP – Extensible Authentication Protocol, used on data link layer,
frequently with PPP and SSL/TLS
22
WPAN (Wireless Personal Area Networks):
Bluetooth
• Harald Bluetooth was the King of Denmark in the 10th century
• Initiated by Ericsson, Intel, IBM, Nokia, Toshiba;
Open Standard: IEEE 802.15.1
• Generally for wireless Ad-hoc-piconets (range < 10m);
single-chip solution
• Frequency band in 2,4 GHz area
• Integrated security (128 bit encryption)
• Data rates: − 433,9 kBit/s asynchronous-symmetrical
−
−
−
−
723,2 kBit/s / 57,6 kbit/s asynchronous-asymmetrical
64 kBit/s synchronous, voice service
Extensions up to 20 Mbit/s (IEEE 802.15.3a
UWB (Ultra Wide Band))
Basic setup
2,4-GhzHF
BluetoothBasebandController
HostSystem
23
Bluetooth - Functionality
Standby
Not connected;
Standby
connection-setup
status
t =2 s
Inquiry with
known
Address
Page for
Unknown
Address
t =0,6 s
active
states
Low-Powerstates
Send data
connected
PARK
HOLD
SNIFF
Adress
resigned
(paused) (periodic)
Address available
24
Possible Configurations
Master
Slave
Piconet
(up to 7 “slaves” per “master”)
Scatternet
25
WPAN: ZigBee
• Low-energy network for ad-hoc connectivity of sensors and
other small devices
• Standard: IEEE 802.15.4 Low Rate WPAN
• Technical data
− Frequency range: 2,4-GHz-Band (16 channels),
− Transmission power: only 1 to 10 mW
− Reach: up to 50 m
− Data rate: 250 kBit/s per channel
ZED
ZED
ZED
• Components
ZED
− ZigBee End Device (ZED)
− ZigBee Router (ZR)
ZR
− ZigBee Coordinator (ZC)
ZED
• Network topology
ZR
ZC
− Star topology with ZEDs
ZED
− P2P-Topology with ZRs
− Up to 65.000 devices
ZED
ZED
per cluster (ZC)
ZED
ZED
ZED
ZED
ZED
WPAN: RFID – Radio Frequency
Identification
Tag
Tag
RFIDReader
Tag
Reader signal
Reflected
signal
Tag
• Tags:
- Antenna and RFID-Chip
− 96-Bit-Identifier, small memory, passive
− very cheap, universal usage
• Reader:
- active, powerful, own protocols
- Sends bearer signal, reflected by Tag
• Backscatter: Tag creates overlay of bearer signal with own
transmission bits à Reader can filter and detect these bits
• Multiple access handled by collision detection protocol
WPAN: NFC – Near Field Communication
• Contactless transmission via very small distance (4 cm)
(smartcard is being directly attached to reader)
• Transmission
− connectionless: passive RFID-Tags
− Connection oriented: active transmitters (e.g. smartphone)
− Data rate up to 424 kBit/s
• Applications
− Payment by smartphone or smartcard
− Smartphone as door key
• Problems
− Not really secure (controlled by distance only)
WPAN: Applications
• May replace infrared for peripherals
• Wireless headsets (e.g. hands-free interaction with
mobile phones)
• Digital image transmission between cameras and
archives
• Control of home appliances by networked sensors
(examples: heating control, smart electricity metering,
alarm systems)
• Digital payment systems
29
Some further readings
• IEEE 802.11:
standards.ieee.org/getieee802/802.11.htm
• WiFi Alliance (configuration, security):
www.wi-fi.org
• Bluetooth special interest group:
www.bluetooth.org
30
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