Medium Access Control Sublayer

Medium Access Control Sublayer
Telematics
Chapter 5: Medium Access Control Sublayer
User
watching
video clip
Server
with video
clips
Beispielbild
Univ.-Prof. Dr.-Ing. Jochen H. Schiller
Computer Systems and Telematics (CST)
Institute of Computer Science
Freie Universität Berlin
http://cst.mi.fu-berlin.de
Application Layer
Application Layer
Presentation Layer
Presentation Layer
Session Layer
Session Layer
Transport Layer
Transport Layer
Network Layer
Network Layer
Network Layer
Data Link Layer
Data Link Layer
Data Link Layer
Physical Layer
Physical Layer
Physical Layer
Contents
● Design Issues
● Network Topologies
● The Channel Allocation Problem
● Multiple Access Protocols
● Ethernet
● IEEE 802.2 – Logical Link Control
● Token Bus (historical)
● Token Ring (historical)
● Fiber Distributed Data Interface
● Structured Cabling
● Metropolitan Area Networks
●
●
●
●
●
●
(MAN)
Wide Area Networks (WAN)
Frame Relay (historical)
ATM
SDH
Network Infrastructure
Virtual LANs
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.2
Design Issues
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.3
Design Issues
● Two kinds of connections in networks
● Point-to-point connections
● Broadcast (Multi-access channel,
Random access channel)
● In a network with broadcast
connections
● Who gets the channel?
● Protocols used to determine who
gets next access to the channel
● Medium Access Control (MAC) sublayer
OSI Reference Model
Application Layer
Presentation Layer
Session Layer
Transport Layer
Network Layer
Data Link Layer
Physical Layer
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.4
Network Types for the Local Range
● LLC layer: uniform interface and same frame format to upper layers
SicherungsData Link
ebene
Layer
● MAC layer: defines medium access
LLC
802.3
MAC
ISO/OSI
...
IEEE 802.2 Logical Link Control
CSMA/CD
(Ethernet)
802.4
Token
Bus
802.5
Token
Ring
802.6
DQDB
ANSI
X3T9.5
ATM
Forum
FDDI
ATM LAN
Emulation
...
Reale
Netze Concepts
Classical
Network
Both concepts are implemented together in existing networks (as a device driver):
1. Packing of data into frames: error detection during frame transmission and
receipt
2. Media Access Control: this contains the frame transmission and the reaction to
transmission errors
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.5
Standardization: IEEE
● Institute of Electrical and Electronic Engineers (IEEE)
● Standardization of the IEEE 802.X-Standards for Local Area
Networks (www.ieee802.org) – many historical!
Overview and Architecture of LANs
Logical Link Control (LLC)
CSMA/CD (Ethernet)
Token Bus
Token Ring
DQDB (Distributed Queue Dual Bus)
Broadband Technical Advisory
Group (BBTAG)
 802.8 Fiber Optic Technical Advisory
Group (FOTAG)
 802.9 Integrated Services LAN
(ISLAN) Interface
 802.10 Standard for Interoperable
LAN Security (SILS)







802.1
802.2
802.3
802.4
802.5
802.6
802.7




802.11
802.12
802.14
802.15
 802.16
 802.17
 802.18
 802.19
 802.20
 802.21
www.ieee.org
Wireless LAN (WLAN)
Demand Priority (HP’s AnyLAN)
Cable modems
Personal Area Networks (PAN,
Bluetooth)
Wireless MAN
Resilient Packet Ring
Radio Regulatory Technical
Advisory Group (RRTAG)
Coexistence Technical
Advisory Group
Mobile Broadband Wireless
Access (MBWA)
Media Independent Handover
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.6
Network Categories
● Local Area Networks (LAN): 10m - few km, simple connection structure
● Ethernet/Fast Ethernet/Gigabit Ethernet/10Gigabit Ethernet
● Historical: Token Bus, Token Ring
● Historical: FDDI (up to 100 km, belongs rather to LANs)
● Wireless LAN (WLAN, up to a few 100 m)
LAN
● Metropolitan Area Network (MAN): 10 - 100 km, city range
● Historical
● DQDB
● FDDI II
● Resilient Packet Ring
MAN
● today: Gigabit Ethernet, SDH
● Wide Area Networks (WAN): 100 – 10,000 km, interconnection of
subnetworks
● Frame Relay
● ATM
● SDH
WAN
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.7
Network Topologies
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.8
Bus
B
Terminating
resistor
Example: Classical Ethernet


A
● Bus
● Broadcast Network: if station A intends to send data to station B, the message
reaches all connected stations. Only station B processes the data, all other
stations ignore it.
● Passive coupling of stations
● Restriction of the extension and number of stations to connected
● Simple, cheap, easy to connect new stations
● The breakdown of a station does not influence the rest of the network
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.9
Star
● Star
B
● Designated computer as central
station: a message of station A is
forwarded to station B via the
central station
● Broadcast network (Hub) or pointto-point connections (Switch)
● Expensive central station
A
● Vulnerability through central station
(Redundancy possible)
● N connections for N stations
● Easy connection of new stations
Example: Fast Ethernet
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.10
Tree
● Tree
● Topology: Connection of several busses or stars
● Branching elements can be active (Router) or passive (Repeater)
● Bridging of large distances
● Adaptation to given geographical structure
● Minimization of the cable length possible
Branch 1
Branch 2
A
B
C
D
Repeater
Router
Backbone
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.11
Ring
● Ring
● Broadcast Network
● Chain of point-to-point connections
B
● Active stations: messages are
regenerated by the stations
(Repeater)
● Breakdown of the whole network in
case of failure of one single station
or connection
● Large extent possible
A
● Easy connection of new stations
● Only N connections for N stations
● Variant: bidirectional ring
Example: Token Ring, FDDI
● stations are connected by two
opposed rings
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.12
Meshed Networks
● Fully Meshed Network
● Point-to-Point connections between
all stations
● For N stations
N ( N  1)
2
connections are needed
● Connecting a new station is a costly
process
● Redundant paths
● Maximal connection availability
through routing integration
Partly meshed network: cheaper, but routing, flow control,
and congestion control become necessary (Wide Area Networks)
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.13
Examples
● Ethernet (IEEE 802.3, 10 Mbps)
● originally the standard network
● available in an “immense number” of variants
● Token Ring (IEEE 802.5, 4/16/100 Mbps)
● for a long time the Ethernet competitor
● extended to FDDI (Fiber Distributed Data
Interface)
● Fast Ethernet (IEEE 802.3u, 100 Mbps)
● at the moment the most widely spread network
● extension of Ethernet for small distances
● Gigabit Ethernet (IEEE 802.3z, 1000 Mbps)
● very popular at the moment; 10 Gbps are
already in the planning phase at the moment
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.14
The Channel Allocation Problem
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.15
The Channel Allocation Problem
● The channel allocation problem
● Given N independent stations which want to communicate over a single channel
● Organize the sending order of the stations
Medium
 Wire or wireless
● Approaches
● Static channel allocation
● Simple procedures
● Dynamic channel allocation
● Complex procedure, that adapt to changes
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.16
Static Channel Allocation
● Time Division Multiple Access
(TDMA)
● Each user gets the entire
transmission capacity for a fixed
time interval
● Baseband transmission
● Frequency Division Multiple
Access (FDMA)
● Each user gets a portion of the
transmission capacity for the whole
time
● Frequency range
● Broadband transmission
Frequency
1 2 3 1 2 3 1 2 3
Frequency
Time
User 1
User 2
User 3
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
Time
5.17
Static Channel Allocation
● Problems with static channel allocation
● Works only for a fixed number of users
● When number of users change, the allocation scheme does not work
● Data traffic is very often bursty, i.e., long time no data and for a short time high
data (ok for classical voice communication!)
● Thus, users do not use their allocated channel capacity
Most of the channels will be idle most of the time
Dynamic Channel Allocation
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.18
Dynamic Channel Allocation
● Assumptions on dynamic channel allocation
● Station Model
● There are N independent stations (computers) that generate frames for transmission.
● Single channel
● A single channel is available for communication and all stations can transmit and receive
on it.
● Collisions
● If two frames are transmitted simultaneously, they overlap and the signals are garbled.
● Time
● Continuous time: No master clock, transmission of frames can begin at anytime.
● Slotted time: Time is divided into discrete intervals called slots. Frame transmissions
begin always at the start of a slot.
● Sensing of the medium
● Carrier sense: Stations can sense channel and tell whether it is busy. If so, stations do
not start with transmissions.
● No carrier sense: Stations can not sense the channel.
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.19
Multiple Access Protocols
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.20
Multiple Access Protocols: ALOHA
● Best known protocol: ALOHA
● Developed on the Hawaiian islands in 1970s: stations are connected by a satellite
● Very simple principle, no coordination:
● Stations are sending completely uncoordinated (random), all using the same frequency
● When two (or more) stations are sending at the same time, a collision occurs: both
messages are destroyed.
● Collisions occur even with very small overlaps!
● Vulnerability period: 2 times the length of a frame
● When a collision occurs, frames are repeated after a random time
● Problem: since traffic runs over a satellite a sender only hears after a very long time
whether the transmission was successful or not.
Collision
Sender A
Sender B
Sender C
t
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.21
Multiple Access Protocols: ALOHA
● Problem with ALOHA: even small overlaps result in transmission conflicts.
Therefore, often collisions result in many repetitions:
● No guaranteed response times
● Low throughput
● Improvement: Slotted ALOHA
● The time axis is divided into time slots (similar to TDMA, but time slots are not
firmly assigned to stations)
● The transmission of a block has to start at the beginning of a time slot
● Fewer collisions, vulnerability period of one frame length
● But: the stations must be synchronized!
Collision
Sender A
Sender B
Sender C
t
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.22
Multiple Access Protocols: ALOHA
● Performance of ALOHA
● Assumptions
● Infinite number of interactive users generating data
● Data is generated according to a Poisson distribution
● Poisson process
● Consider a time interval [0,t)
● Random variable X gives the number of events (packets, transmissions, …) in the
time interval of length t
● The probability that k events occur in the time t interval is given by
(  t ) k  t
P( X  k ) 
e
k!
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.23
Multiple Access Protocols: ALOHA
● Performance of ALOHA
● Assumptions
● Data is generated according to a Poisson distribution X with mean
G frames/s
● Collided frames are retransmitted
● Probability of k transmission trials per frame time is according to a Poisson distribution
with mean G
k
P( X  k ) 
G G
e
with G  t
k!
● Throughput (S) is given by the load (G) and the probability of a successful
transmission (P0)
S = G×P0
● What is a successful transmission?
● A frame is transmitted successful if no other frames are sent within one frame time
G 0 G
P0  P( X  0) 
e  e G
0!
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.24
Multiple Access Protocols: ALOHA
G 0 G
● Probability of zero frames is: P( X  0)  e  e G
0!
● Collision time
● ALOHA: tc=2t
● Slotted ALOHA: tc=t
● Throughput
● ALOHA: S = G P0 = G e-2G
● Slotted ALOHA: S = G P0 = G e-G
Maximum
 Slotted ALOHA ~36%
 ALOHA ~18%
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.25
Multiple Access Protocols: CSMA
● Variant of ALOHA for networks with small distances exists
● Similar to ALOHA: no coordination of the stations
● But: each station which wants to send first examines whether already another
station is sending
● If nobody sends, the station begins to send
Carrier Sense Multiple Access (CSMA)
● Notice
● This principle only works with networks having a short transmission delay
● Application of this principle for satellite systems is not possible, because there would be
no chance to know whether a conflict occurred before the end of the transmission
● Advantages: simple, because no master station and no tokens are needed;
nevertheless good utilization of the network capacity
● Disadvantage: no guaranteed medium access, a large delay up to beginning a
transmission is possible
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.26
Multiple Access Protocols: CSMA
● Persistent and Nonpersistent CSMA
● 1-persistent CSMA
● When a station has data to send, it first listens to the channel.
● If channel busy, the station waits until it becomes idle.
● When channel is idle, station transmits a frame.
● When a collision occurs, the station waits a random amount of time and starts all over
again.
● 1-persistent = station transmits with probability of one if channel idle
● Nonpersistent CSMA
● When channel is busy, station waits a random time, and repeats
● p-persistent CSMA
● Applied in slotted channels (slotted ALOHA)
● If channel idle, station transmits with probability p in current slot and with probability
(1-p) it defers until next slot
● If next slot is idle, the station again transmits with probability p and defers with (1-p)
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.27
Multiple Access Protocols: CSMA
● CSMA with Collision Detection: CSMA/CD
● Basis of classical Ethernet (not today’s versions with star topology!)
● A station who detects a collision stops immediately transmitting
● Afterwards it waits a random time and tries again
Frame
Transmission Contention
period
period
Frame
Frame
Frame
Idle period
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.28
Multiple Access Protocols
Collision-Free Protocols
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.29
Collision-Free Protocols: Reservation Protocols
● Communication follows in a two-phase scheme (alternating phases)
● Phase 1: Reservation
● In the reservation phase the sender makes a reservation by indicating the wish to send
data (or even the length of the data to be sent)
● Phase 2: Transmission
● In the transmission phase the data communication takes place (after successful
reservation)
● Advantage: very efficient use of the capacity
● Disadvantage:
● Delay by two-phase procedure
● Often a master station is needed, which cyclically queries all other stations whether they
have to send data. The master station assigns sending rights.
● Techniques for “easy” reservation without master station:
● Explicit reservation
● Implicit reservation
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.30
Collision-Free Protocols: Bit-Map Protocol
● Uses two frame types:
● reservation frame (very small) in the first phase
● data frame (constant length) in the second phase
● Variant 1: Without contention
● Only suitable for small number of users
● Each user i is assigned the i-th slot in the reservation frame. If it wants to send
data, it sets the i-th bit in the reservation frame to 1.
● After the reservation phase, all stations having set their reservation bit can send
their data in the order of their bits in the reservation frame.
reservation
frame
1
1
1
data frames of stations
having reserved
2
5
7
1
4
1
111
This procedure is called Bit-Map Protocol
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.31
Collision-Free Protocols: Bit-Map Protocol
● Variant 2: With contention
● For higher number of users
● The reservation frame consists of a limited number of contention slots (smaller
than the number of participating stations)
● Users try to get a contention slot (and by that make a reservation for a data slot)
by random choice, writing their station number into a slot
● If there is no collision in the reservation phase, a station may send.
reservation frame
data frames of stations
with contention slots
having reserved
17
45 4
17
45
4
22
11
31
5 22
11
31 34
25 12
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.32
Collision-Free Protocols: Binary Countdown
● Binary Countdown
● For large number of stations
● Binary station addresses, all
addresses to be the same length
Bit time
● A station wanting to use the
channel broadcasts its address as a
binary string starting with the highorder bit
Stations
0
0010
0
0100
0
● The bits from different stations are
ORed
● As soon as a station sees that a
high-order bit position that is 0 in
its address has been overwritten to
a 1, gives up
● Example: four stations with
addresses 0010, 0100, 1001, 1010
1001
1
2
1
0
0
1010
1
0
1
Result
1 0 1 0
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
3
0
5.35
Multiple Access Protocols
Limited Contention Protocols
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.36
Limited Contention Protocols: Adaptive Tree Walk
● Adaptive Tree Walk Protocol
● Stations are the leaves of a binary tree
● In the first contention slot following a successful frame, slot 0, all stations (A-H)
are permitted to try to acquire the channel
● If collision, during slot 1 only stations under node 2 (A-D) may compete
● If one gets the channel, next slot is reserved for stations under node 3 (E-H)
● If collision, during slot 2, only stations under node 4 (A, B)
1
2
3
4
A
5
B
C
6
D
E
7
F
G
H
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.37
Coordination by using a Token
● Introduction of a token (determined bit sequence)
● Only the owner of the token is allowed to send
● Token is cyclically passed on between all stations
● particularly suitable for ring topologies
1
5
2
● Token Ring (4/16/100 Mbps)
4
● Characteristics
3
Passing on of the token
● Guaranteed accesses, no collisions
● Very good utilization of the network capacity, high efficiency
● Fair, guaranteed response times
● Possible: multiple tokens
● But: complex and expensive
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.38
Ethernet
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.39
Ethernet
● Evolution of Ethernet
● 1970s on Hawaii ALOHANET (Abramson)
● Connecting computers on islands over radio
● Two channels
● Uplink shared by the clients (collision may occur)
● Downlink exclusively used by main computer
● Packets are acked by main computer
● Good performance under low traffic, but bad under heavy load
● 1970's: experimental network on the basis of coaxial cables, data rate of 3 Mbps.
Developed by the Xerox Corporation as a protocol for LANs with sporadic but
bursty traffic.
● 1976 Ethernet by Robert Metcalf at Xerox Parc
● Ether: luminiferous ether through which electromagnetic radiation was thought to
propagate
● Improvements to ALOHANET
● Listen to the medium before transmitting
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.40
Ethernet
● 1978: Development of 10 Mbps-variant by Digital Equipment Corporation (DEC),
Intel Corporation, and Xerox (DIX-standard)
● 1983: DIX-standard became
the IEEE 802.3 standard
● Metcalf founded 3Com
● Sold many, many million Ethernet adapters
● Original Ethernet structure:
● Bus topology with a maximum segment length of 500 meters, connection of a
maximum of 100 passive stations.
● Repeaters are used to connect several segments.
● Most common medium: Copper cable.
● In addition, optical fibers are used (the segment length increases).
● Early 90's: the bus topology is displaced more and more by a star topology, in
which a central hub or switch (based on Twisted Pair or Optical Fiber) realizes
connections to all stations.
● The switch offers the advantage that several connections can run in parallel.
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.41
Ethernet - historical
● Based on the standard IEEE 802.3 “CSMA/CD”
(Carrier Sense Multiple Access/Collision Detection)
● Several (passive) stations - one shared medium (random access)
● Originally, bus topology:
1. Is the medium available?
(Carrier Sense)
?
2. Data transmission
?
3. Check for collisions (Collision Detection)
If so: send jamming signal and stop
transmission. Go on with binary
exponential backoff algorithm
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.42
Carrier Sense Multiple Access
Principle:
 listen to the medium before sending
 send only if the medium is free
S1
1. Station S1 sends
Message from S1
Signal expansion
also in other
direction
2. Station S2 also wants
to send but notices that
a transmission already
takes place.
S2
Expansion of the signal on
the medium
 Advantages: simple, since no mechanisms are needed for the coordination;
with some extensions nevertheless a good utilization of the network capacity
 Disadvantage: no guaranteed access, a large delay before sending is possible
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.43
Problem with CSMA
Problem: the message which is sent by S1 spreads with finite speed on the
medium. Therefore, it can be that S2 only thinks that the medium would be free,
although S1 already has begun with the transmission. It comes to a collision:
both messages overlap on the medium and become useless.
Note: the signals from S1 and S2
also expand to the left direction,
not shown here for
simplification of the figure.
S1
1. Station S1 sends
Message from S1
2. Station S2 also
wants to send and
thinks the medium
would be free.
Message from S2
S2
Expansion of the signal on
the medium
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.44
Detection of Collisions
Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
● Principle:
● like CSMA
● additionally: stop the transmission if a collision occurs
Maximum distance in the network
S1
S2
S1 sends
S2 sends
S1 detects the conflict and
knows that the transmission
has to be repeated.
Time
S2 detects the
conflict and stops.
Transmission of a
jamming signal.
Only small overlapping, but nevertheless
both messages are destroyed
Note: with increasing expansion of the network the risk of a conflict also increases.
Therefore, this technology is suitable only for “small” networks (Ethernet)
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.45
Data transmission with CSMA/CD
● When does the collision detection in CSMA/CD work correctly?
● The maximum time for the detection of a collision is about twice as long as the
signal propagation delay on the medium.
● First compromise: one wants to create large networks, but although to have a
small probability of collisions …
● Result: the maximum expansion of the network is specified as 2,500m.
● At a signal speed of approximately 2,00,000 km/s (5 µs/km) the maximum signal
propagation delay (with consideration of the time in repeaters) is less than 25
µs.
● The maximum conflict duration thereby is less than 50 µs. To be sure to
recognize a collision, a sending station has to listen to the medium at least for
this time.
● Arrangement: a station only listens to the medium as long as it sends data.
● Based on a transmission rate of 10 Mbps a minimum frame length (64 byte) was
defined in order to make a collision detection possible.
● The 64 bytes need the maximum conflict duration of 50 µs
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.46
Performance of CSMA/CD
● The performance of Ethernet systems depends on the vulnerability part a:
Data still
to send
Data already sent
● a is the fraction of a frame which the sender has to transmit until the first bit
crossed the network
● If a station begins to send during the time a needs to cross the network, a conflict
arises
● The smaller a is, the better is the performance of the network
● a is small …
● when the network is small
● when frames are large
● when capacity is low
● Conclusion: the best network has nearly zero size, nearly zero capacity, and a station
should never stop sending.
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.47
Ethernet: Encoding on the Physical Layer
● No directly usage of binary encoding with 0 volts for a 0-bit and 5 volts for
a 1-bit
● Synchronization problems
● Manchester Encoding
● Transition in the middle of a bit
● The high signal is at +0.85 volts and the low signal at -0.85 volts
● Disadvantage: twice bandwidth, i.e., to send 10Mbps, 20MHz is required
+ 0.85 volt
0
1
0
1
1
0
0
1
0 volt
- 0.85 volt
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.48
The Ethernet Frame
Byte
7
1
Preamble SFD
1
2
6
6
2
0-1500
DA
SA
L/T
Data
3
4
5
1: 7 byte synchronization
Each byte contains 10101010
2: 1 byte start frame delimiter (SFD)
Marking of the begin of the frame by
the byte 10101011
3: 6 (2) byte destination address
MAC address of receiver
4: 6 (2) byte source address
MAC address of sender
6
0-46
4
Padding FCS
7
8
5: 2 byte length (IEEE 802.3)/type
(Ethernet)
• In 802.3: Indication of the length of the
data field (range: 0 - 1500 byte)
• In Ethernet: identification of the upper
layer protocol, e.g., IP, IPX, etc.
6: (0 – 1500) byte data
7: (0 – 46) byte padding
• Filling up of the frame to at least 64 byte
(smaller fragments in the network are
discarded, exception the jamming signal)
8: 4 byte Frame Check Sequence (FCS).
Use of a cyclic code (CRC).
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.49
The Ethernet Frame
● Preamble: marks a following transmission and synchronizes the receiver
●
●
with the sender.
The Start-of-Frame-Delimiter (resp. the two successive ones) indicates
that finally data are coming.
Destination address: the first bit determines the kind of receiver:
● First bit 0: an individual station
● First bit 1: a group address (multicast)
● Broadcast is given by 11…1
● Length(/Type): In IEEE 802.3 a value ≤1500 indicates the length of the
data part.
● In Ethernet, the meaning is changed, identifying the layer-3 protocol to which
the data have to be passed.
● For distinction from IEEE 802.3, only values from 1536 are permitted.
● FCS: Checksum, 32-bit (CRC).
● It covers the fields DA, SA, length/type, data/padding.
● Error detection
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.50
The Ethernet Frame: Addresses
● MAC address 6 byte
● Originally invented at Xerox PARC
● Unicast
● Multicast
● Broadcast
● Administrative
● Globally unique, assigned by IEEE
● Locally administered
1
2
3
4
5
6
or
Organizationally Unique
Identifier (OUI)
b1
b2
● Tools
● Windows: getmac, ipconfig /all, arp -a
● Linux: ifconfig, cat /proc/net/arp
b3
b4
b5
Network Interface
Controller (NIC) Specific
b6
b7
b8
0: unicast
1: multicast
0: globally unique
1: locally administered
● http://www.heise.de/netze/tools/mac-adressen
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.51
The Ethernet Frame: Network Analyzer
● Network packet analyzer:
Wireshark
● http://www.wireshark.org/
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.52
Ethernet
Resolving Transmission Conflicts
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.53
Resolving Transmission Conflicts
● What to do after a collision detection?
● Different categories of reaction methods
● Non-persistent (example: ALOHA):
● After a conflict, wait a random time afterwards start a new transmission
● Problem: possibly inefficient utilization of the medium
● 1-persistent
● Idea: it is very unlikely that during a current transmission two or more new
messages appear
● Start the next transmission attempt as soon as possible, thus as soon as the
channel is free or becomes free after having been busy / after a conflict
● Problem: Subsequent conflicts!
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.54
Resolving Transmission Conflicts
● p-persistence:
● In this variant conflicts between concurrently waiting messages should be
avoided
● In a free channel transmission takes place only with probability p
● In case of a conflict, a message needs on the average 1/p attempts
● But: how to select p?
● p large
 high risk for subsequent conflicts
● p small
●p = 0
 long waiting periods
 not possible
●p= 1
 1-persistent
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.55
Resolving Transmission Conflicts
● Performance of Ethernet
● Ethernet at 10 Mbps with 512-bit
slot times
● Assumptions
● T : Time to transmit a frame
●  : Propagation on cable
● A: Probability that a station gets the
channel
T
Channel efficieny 
T  2A
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.56
Resolving Transmission Conflicts
Compared to ALOHA, CSMA in any form has a good efficiency
(based on a mathematical modeling of network traffic)
Nevertheless for Ethernet a further procedure was developed: the
Binary Exponential Backoff mechanism
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.57
Resolving Collisions in Ethernet: Binary Exponential Backoff
● Binary Exponential Backoff (BEB)
● In order to avoid the simultaneous repetition of transmissions after a
collision (subsequent collision), a random waiting period is drawn from a
given interval.
● The interval is kept small, in order to avoid long waiting periods up to the repetition.
● Thus, the risk of a subsequent conflict is high.
● If it comes to a further collision, the interval before the next attempt is increased, in
order to create more clearance for all sending parties.
● The waiting period is determined as follows:
● After i collisions, a station throws a random number x from the interval [0, 2i-1]
● After 10 collisions, the interval remains fixed with [0, 210-1]
● After the 16-th collision a station aborts the transmission completely
● As soon as the medium is free, the sender waits for x time slots, whereby a time
slot corresponds to the minimum Ethernet frame length of 512 bits (for a 10
Mbps Ethernet this corresponds to the maximum conflict period of 51,2 µs).
● After the x-th time slot the station becomes active with carrier sense.
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.58
Resolving Collisions in Ethernet: Binary Exponential Backoff
● Advantage:
● Short waiting periods (by small interval) if not much traffic is present
● Distribution of repetitions (by large interval) if much traffic is present
● Disadvantage:
● Stations having a subsequent conflict during a repetition have to draw a random
waiting period from an interval twice as large. If they are having a further
conflict, the interval again is doubled, …
● Thus, single stations can be disadvantaged.
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.59
Ethernet
Types of Ethernet
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.60
Ethernet
Based on IEEE 802.3 “CSMA/CD”
http://www.ethernetalliance.org
4 classes of Ethernet variants:
Still partly in use
● Standard Ethernet
 10 Mbps
Today the most common used variant
● Fast Ethernet
 100 Mbps
Also used in MANs
● Gigabit Ethernet
 1,000 Mbps
Standardized not long ago
● 10Gigabit Ethernet
 10,000 Mbps
Ethernet became generally accepted within the LAN range.
It is used in most LANs as infrastructure:
● It is simple to understand, to build, and to maintain
● The network is cheap in the acquisition
● The topology allows high flexibility
● No compatibility problems, each manufacturer knows and complies
with the standard
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.61
Ethernet Parameters
Parameter
Maximum expansion
Ethernet
Fast
Ethernet
Gigabit
Ethernet
≤ 2500 meters
205 meters
200 meters
10 Mbps
100 Mbps
1000 Mbps
Minimum frame
length
64 byte
64 byte
520 byte
Maximum frame
length
1526 byte
1526 byte
1526 byte
Manchester
code
4B/5B code,
8B/6T code,
…
8B/10B code,…
Capacity
Signal representation
Max number of
5
2
repeaters Additionally, for the jamming a certain 4 byte pattern is sent.
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
1
5.62
Naming of Ethernet Variants
Indication of the used Ethernet variant by 3 name components:
1. Capacity in Mbps (10, 100, 1000, 10G)
2. Transmission technology (e.g. Base for baseband, Broad for broadband)
3. Maximum segment length in units of the medium used by 100 meters, resp.
type of medium
Examples:
● 10Base-5: 10 Mbps, baseband, 500 meters of segment length
● 100Base-T2: 100 Mbps, baseband, two Twisted Pair cables (i.e. two cores)
● 1000Base-X: 1000 Mbps, baseband, optical fiber
Some parameters depend on the variant, e.g., the minimum frame length
(because of different signal propagation delay):
● 1000Base-X: minimum frame length of 416 bytes
● 1000Base-T: minimum frame length of 520 bytes
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.63
Ethernet
Basic Ethernet (10Base) - historical
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.64
Ethernet - Configurations
 100
Stationen
to 100
Stations
up100
Stationen
 100 Stationen
 50 m  50 m
 50 m
Terminator
 2.5 m 2.5 mTerminator
 2.5 m
Terminator
 500 m 500 m
 500 m
Grundeinheit:
Segment
Grundeinheit:
Segment
Basic
configuration:
segment
Grundeinheit:
Segment
500 m
50 m 50 m
50 m 500 m
500 m
50 m 50 m
50 m
Segment1
Segment1
Segment1
Repeater
Repeater
Repeater
Segment2
Segment2
Segment2
Kopplung
zweier
Segmente
Connection
of
segments
through
a repeater
Kopplung
zweier
Segmente
Kopplung zweier Segmente
500 m 500 m
500 m
50
m
50 m
50 m
500 m 500 m
500 m
Glasfaserkabel
50
m
Optical fiber
50 m
50 m 50 m Glasfaserkabel
Glasfaserkabel
50 m
50 m
1000 m1000 m
1000 m
Ethernet
maximaler
Ethernet
maximaler
Länge Länge
Ethernet
maximaler
Länge
Ethernet with maximum
range
50 m 50 m
50 m
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.65
10Base-2 (Cheapernet)
● Cheap coaxial cable (flexible)
● Thin Ethernet
● Terminals are attached with BNC connectors
● Max. 5 segments (connected by repeaters)
● Max. 30 stations per segment
● At least 0.5 m distance between connections
● Max. 185 m segment length
● Maximum expansion 925 m
BNC plug
Coaxial cable
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.66
10Base-2 (Cheapernet)
Coax cable
Branch connection (T-Stück)
Terminator
Transceiver
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.67
10Base-T (Twisted Pair)
● Star topology using twisted pair: several devices are connected by a hub
● Devices are attached by a RJ-45 plug (Western plug),
●
●
●
however only 2 of the 4 pairs of the cables are used
Cable length to the hub max. 100 m
Total extension thereby max. 200 m
Long time the most commonly used variant
Hub
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.68
10Base-F
● Ethernet with Fiber optics
● Expensive
● Excellent noise immunity
● Used when distant buildings have to be connected
● Often used due to security issues, since wiretapping of fiber is difficult
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.69
Ethernet
Fast Ethernet
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.70
Fast Ethernet
● Principle: still use the Ethernet principles, but make it faster
● Compatibility with existing Ethernet networks
● 100 Mbps as data transmission rate, achieved by better technology, more
efficient codes, utilization of several pairs of cables, switches,…
● Result: IEEE 802.3u, 1995
● Problem
● The minimum frame length for collision detection with Ethernet is 64 byte.
● With 100 Mbps the frame is sent about 10 times faster, so that a collision
detection is not longer ensured.
● Result: for Fast Ethernet the expansion had to be reduced approx. by the factor
10 to somewhat more than 200 meters …
● Therefore, its concept is based on 10Base-T with a central hub/switch.
● Auto configuration
● Negotiation of speed
● Negotiation on communication mode (half-duplex, full-duplex)
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.71
100Base-T (Fast Ethernet)
● 100Base-T4
● Twisted pair cable (UTP) of category 3 (cheap)
● Uses all 4 cable pairs: one to the hub, one from the hub, the other two
depending upon the transmission direction
● Encoding uses 8B/6T (8 bits map to 6 trits)
● 100Base-TX
● Twisted pair cable (UTP) of category 5 (more expensive, but less absorption)
● Uses only 2 cable pairs, one for each direction
● Encoding uses 4B/5B
● The most used 100 Mbps version
● 100Base-FX
● Optical fiber, uses one fiber per direction
● Maximum cable length to the hub: 400 meters
● Variant: Cable length up to 2 km when using a switch. Hubs are not permitted
here, since with this length no collision detection is possible anymore. In the
case of using a good switch, no more collisions arise!
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.72
Ethernet
Gigabit Ethernet
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.73
Gigabit Ethernet
● 1998 the IEEE standardized the norm 802.3z, “Gigabit Ethernet”
● Again: compatibility to (Fast) Ethernet has to be maintained!
● Problem: for collision detection a reduction of the cable length to 20 meters
●
●
would be necessary … “Very Local Area Network”
Auto configuration as in Fast Ethernet (data, half-duplex, duplex, …)
Therefore, the expansion remained the same as for Fast Ethernet – instead a
new minimum frame length of 512 byte was specified by extending the
standard frame by a ‘nodata’ field (after the FCS, because of compatibility to
Ethernet). This procedure is called Carrier Extension.
● It is added by the hardware, the software part does not know
● When a frame is passed on from a Gigabit Ethernet to a Fast Ethernet, the ‘nodata’
part is simply removed and the frame can be used like a normal Ethernet frame.
PRE
SFD
Preamble
7 byte
Start Del.
1 byte
DA
SA
Length
/Type
DATA
Padding
FCS
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
nodata
5.74
Gigabit Ethernet
● With Gigabit Ethernet the sending of several successive frames is possible
●
(Frame Bursting) without using CSMA/CD repeatedly.
The sending MAC controller fills the gaps between the frames with
“Interframe-bits” (IFG), thus for other stations the medium is occupied.
MAC frame
(including nodata field)
IFG
MAC frame
IFG
….
MAC frame
● Under normal conditions, within Gigabit Ethernet no more hubs are used.
In the case of using a switch no more collisions occur, therefore the
maximum cable length is only determined by the signal absorption.
 usage for backbone connections in the MAN area
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.75
1000Base-T/X (Gigabit Ethernet)
● 1000Base-T
● Based on Fast Ethernet
● Twisted pair cable (Cat. 5/6/7, UTP); use of 4 pairs of cables
● Segment length: 100 m
● 1000Base-CX
● Shielded Twisted Pair cable (STP); use of 2 pairs of cables
● Segment length: 25 m
● Not often used
● 1000Base-SX
● Multimode fiber with 550 m segment length
● Transmission on the 850 nm band
● 1000Base-LX
● Single- or multimode over 5000 m
● Transmission on 1300 nm
Added later:
1000Base-LH
• Single mode on 1550 nm
• Range up to 70 km
• MAN!
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.76
Ethernet: 10-Gigabit Ethernet
● 10-Gigabit Ethernet, IEEE 802.3ae
● (First) only specified for optical fiber (LX or SX)
● Star topology using a switch
● CSMA/CD is no longer used since no collisions can occur (but nevertheless
implemented for compatibility with older Ethernet variants regarding frame
format and size …)
● It may also be used also in the MAN/WAN range: 10 - 40 km (Mono mode)
● Most important change: two specifications on physical layer (PHY)
● One PHY for LANs with 10 Gbps
● One PHY for WANs with 9,6215 Gbps (for compatibility with SDH/SONET, see Wide Area
Networks)
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.77
10G Ethernet: Variants
Type
Wavelength
[nm]
PHY
Coding
Fiber
10GBase-SR
serial
850
LAN
64B/66B
Multimode
26 – 65
10GBase-LR
serial
1310
LAN
64B/66B
Singlemode
10,000
10GBase-ER
serial
1550
LAN
64B/66B
Singlemode
40,000
10GBase-LX4
WWDM
1310
LAN
8B/10B
Singlemode
Multimode
10,000
300
10GBase-SW
serial
850
WAN
64B/66B
Multimode
26 – 65
10GBase-LW
serial
1310
WAN
64B/66B
Singlemode
10,000
10GBase-EW
serial
1550
WAN
64B/66B
Singlemode
40,000
Name
S: short
L: long
E: extended
Range
[m]
serial: “normal” transmission
WWDM: Wide Wavelength Division Multiplex
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.78
Wavelength Division Multiplexing (WDM)
Technical principle Wavelength Division Multiplexing: transmit data using four
different wavelengths in parallel:
2
2
3
1 + 2 + 3 +4
DEMUX
1
MUX
1
4
3
4
Data are distributed to four wavelengths – how to apply this concept to copper
cables?
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.79
Are Variants for Twisted Pair possible?
● Some years ago: no, impossible!
● But now e.g.:
● IEEE 802.3ak: 10GBASE-CX4 (Coax)
● Four pairs of cable for each direction
● Cable length of up to 15 meters …
● IEEE 802.3an: 10GBASE-T (Cat. 6/7 TP)
● Cat6 (50 meters) or Cat7 (100 meters) cabling
● Use of all 8 lines in the TP cable – in both directions in parallel!
● Filters for each cable to separate sending and receiving signal
● Layer 1: Variant of Pulse Amplitude Modulation (PAM) with 16 discrete levels
between -1 and +1 Volt (PAM16)
● MAC-Layer: keep old Ethernet-Formats …
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.80
And what’s next?
● Maybe combined with full optical networks?
● Optical multiplexers, optical switches
● But at the moment only tested in labs, expensive
● 100G-Ethernet under work (http://www.ethernetalliance.org)
● Data rates from 40G to 100G – currently under test (40GBASE, 100GBASE)
● E.g. IEEE 802.3bg: 40 Gbit/s optical, 802.3bj copper cable!
● Variants for 100 m and 10 km with duplex communication
● Ethernet is still developing (http://www.ieee802.org/3/)
We have a number of active projects as listed below:
IEEE P802.3 (IEEE 802.3bh) Revision to IEEE Std 802.3-2008 Task Force.
IEEE P802.3.1 (IEEE 802.3.1a) Revision to IEEE Std 802.3.1-2011 Ethernet MIBs TF.
IEEE P802.3bj 100 Gb/s Backplane and Copper Cable Task Force.
IEEE 802.3 Next Generation 100 Gb/s Optical Ethernet Study Group.
IEEE 802.3 Extended EPON Study Group.
IEEE 802.3 EPON Protocol over a Coax (EPoC) PHY Study Group.
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.81
IEEE 802.2: Logical Link Control
Revisited for Ethernet
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.82
IEEE 802.2: Logical Link Control
● Ethernet and IEEE 802.3 protocols offer only best effort
● Unreliable datagram service (No acks)
● What to do if error-control and flow-control is required?
● Logical Link Control (LLC)
● Runs on top of Ethernet and other IEEE 802.3 protocols
● Provides a single frame format and interface to the network layer
● Hides differences between the protocols
● Based on HDLC
● LLC provides
● Unreliable datagram service
● Acknowledged datagram service
● Reliable connection oriented service
● LLC header contains
● Destination access point  Which process to deliver?
● Source access point
● Control field  Seq- and ack-numbers
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.83
IEEE 802.2: Logical Link Control
● Relationship between Network Layer, LLC, and MAC
● Network layer passes packet to LLC
● LLC adds header with sequence number and ack number
 packet is inserted into the payload of a frame
Network Layer
Data
Link
Layer
Packet
LLC
LLC
Packet
MAC
MAC LLC
Packet
MAC
Physical Layer
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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IEEE 802.2: Logical Link Control
Preamble SFD
DA
SA
Byte
1
L/T
1
DSAP
Bit
DSAP
SSAP
I/G
C/R
Padding FCS
1 or 2
SSAP Control
7
I/G
Data
Information
7
DSAP Value C/R SSAP Value
Destination Service Access Point
Source Service Access Point
Individual/Group
Command/Response
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.85
Token Bus
Basic principle of interest – standard itself is historical
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.86
Token Bus
● Token Bus
● LAN with ring topology
● Token = Small frame, that circulates
● Only the node who possesses the token may send
● One example for a token network: IEEE 802.4 “Token Bus”
● All stations should be treated equally, i.e., they have to pass the token cyclically
● For this: logical ordering of all stations into a ring
● In a bus topology, the ordering is according the station addresses
49
62
12
42
33
17
21
5
15
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.87
Token Bus
● Application Area
● Mainly for industrial applications
● Forced by General Motors for their
Manufacturing Automation Protocol
standardization effort
● Usage e.g. as a field bus (Feldbus
in German) in industrial
environments with a high degree of
noise.
● Purpose: e.g. roboter control; a few
masters, many slaves (they only
listen).
● Data rate is not that important, but
guarantees in response times are
necessary (not possible with
Ethernet).
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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But… “Industrial Ethernet”
● The Token-Bus approach is more and more displaced by Ethernet variants,
●
e.g.:
EtherCAT (since 2003, http://www.ethercat.org/)
● Fast Ethernet based on a bus, star, or tree topology (very flexible)
● Uses TP or optical fiber as medium
● Synchronization necessary between all stations
● A master station polls the other stations with a single Ethernet frame – each
station has its one time slot to read out/write in data
● Ethernet Powerlink (http://www.ethernet-powerlink.org/)
● Introduction of time slots and a cyclic timing schedule
● Whole time axis is divided into isochronous and asynchronous phases
● Isochronous: for time-critical data transfer
● Asynchronous: for non-time-critical data transfer
● A managing node assigns time slots (in both phases!): in the isochronous phase
to all stations, in the asynchronous phase to one particular station
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.89
Token Ring
Basic principle of interest – standard itself is historical
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.90
Token Ring
● Token Ring
● LAN with ring topology
● Token = Small frame, that circulates
● Only the node who possesses the token may send
1
k
2
● Based on the standard IEEE 802.5 “Token Ring”
● The stations share a ring of point-to-point connections
● The token is cyclically passed on
● particularly suitable for rings
● Token Ring (4/16/100 Mbps)
…
3
Passing on the token
● Mainly supported by IBM
● Characteristics:
● Guaranteed access, no collisions
● Fair, guaranteed response times
● Possible: multiple tokens
● However: complex and expensive
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Token Ring
● Performance
● Under light load: inefficient, since a station has to wait for the token
● Under heavy load: efficient and fair
● Round robin fashion transmission of stations
● Disadvantage
● Token maintenance
● Lost token can block the network
● Duplication of token
● Monitor station observes the ring
● Central entity
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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CSMA/CD vs. Token Bus vs. Token Ring
● CSMA/CD
● Advantages
● Token Bus
● Advantages
● Widely deployed, high
expertise and experience
● More deterministic than
CSMA/CD
● Simple protocol
● Short frames possible
● Installation of stations
during operation (plug-andplay)
● Provides priorities
● Provides guarantees
● Passive cable
● Low delay by low traffic
● Disadvantages
● Token Ring
● Advantages
● full digital
● Automatic recognition and
elimination of cable
problems by wiring-centers
● Provides priorities
● Short frames possible,
frame length restricted by
token hold time
● Good performance by high
load
● Disadvantages
● Disadvantages
● Analogous components, min.
frame length 64 byte, max.
frame length 1500 byte
● Protocol is complicated
● Central monitor
● Delay by low load
● Probabilistic, no priorities
● Lost tokens may cause big
problems
● Analog components
● Limited cable length
● Poor performance by high
load
● Problems at the monitor
may affect the whole ring
● Long delay due to token
exchange
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Token Ring vs. CSMA/CD
Mean
Delay 10
[ms]
Data rate: 10 Mbps
Frame length: 1500 byte
Cable length: 2.5 km
Number of stations: 100
8
CSMA/CD
6
(unlimited Delay)
Token Ring
(Delay is limited)
4
2
0
0.0
0.2
0.4
0.6
0.8
1.0
(normalized)
Throughput
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Fiber Distributed Data Interface (FDDI)
Basic principle of interest – standard itself is historical
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Fiber Distributed Data Interface (FDDI)
● FDDI is a high performance token ring LAN based on optical fibers
● ANSI standard X3T9.5
● Data rates of 100 Mbps
● Range of up to 200 km (MAN?)
● Support of up to 1000 stations, with distances of maximally 2 km
● Often used as Backbone for small LANs
802.5
LAN
FDDI
802.3 LAN
● Successor: FDDI-II, supports besides normal data also synchronous circuit
●
switched PCM data (speech) and ISDN traffic
Variant: CDDI (Copper Distributed Data Interface), with 100 Mbps over
Twisted Pair
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Structure of FDDI
Wiring within FDDI: 2 optical fiber rings with opposite transmission direction
● During normal operation, only the primary ring is
used, the secondary ring remains in readiness
● If the ring breaks, the other one (also called
protection ring) can be used.
● If both rings break or if a station fails, the rings
can be combined into only one, which has double
length:
Two classes of stations exist: DAS (Dual Attachment Station) can be attached to
both rings, the cheaper SAS (Single Attachment Station) are only attached to one
ring.
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Structured Cabling
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Structured Cabling
● Why do we need a structured cabling?
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Structured Cabling
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5.100
Structured Cabling
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5.101
Structured Cabling
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5.102
Structured Cabling
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.103
Structured Cabling
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Structured Cabling
● Structured cabling: Partitioning of
a network, i.e., cabling
infrastructure, which is connected
to a backbone or a central switch
● Each user outlet is cabled to a
communications closet using
individual cables
● In the communications closet the
user outlets terminate on patch
panels
● Patch panels are mounted usually
on 19“ racks
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Structured Cabling
● Advantages of structured cabling
● Consistency
● Usage of the same cabling systems for data, voice, and video
● Support for multi-vendor equipment
● A standard based cable system will support equipment from different vendors
● Simplify modifications
● Supports the changes in within the system, e.g., adding, changing, and moving of
equipment
● Simplify troubleshooting
● Problems are less likely to down the entire network and simplifies the isolation and
fixing of problems
● Support for future applications
● Support for fault isolation
● By dividing the entire infrastructure into simple manageable blocks, it is easy to test and
isolate specific points of fault and correct them
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Structured Cabling
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Structured Cabling
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.108
Structured Cabling
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Structured Cabling
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.110
Structured Cabling
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.111
Structured Cabling
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.112
Structured Cabling
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Wide Area Networks (WAN)
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Wide Area Networks
● Characteristics of Wide Area Networks
● Bridging of any distance
● Usually for covering of a country or a continent
● Topology is normally irregular due to orientation to current needs.
● Therefore, not the shared access to a medium is the core idea, but the thought “how to
achieve the fast and reliable transmission of as much data as possible over a long
distance”.
● Usually quite complex interconnections of sub-networks which are owned by
different operators
● No broadcast, but point-to-point connections
● Range: several 1000 km
● Examples:
● Frame Relay
● Asynchronous Transfer Mode (ATM)
WAN
● Synchronous Digital Hierarchy (SDH)
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Transmission Technologies for WANs
● Point-to-Point Links
● Provision of a single WAN connection from a customer to a remote network
● Example: telephone lines. Usually communication resources are leased from the
provider.
● Accounting is based on the leased capacity and the distance to the receiver.
● Circuit Switching
● A connection is established when required, communication resources are
reserved exclusively. After the communication process, the resources are
released.
● Example: Integrated Services Digital Network (ISDN)
● Packet Switching
● “Enhancement” of the “Circuit Switching” and the Point-to-Point links.
● Shared usage of the resources of one provider by several users, i.e., one physical
connection is used by several virtual resources.
● Shared usage reduces costs
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Transmission Technologies for WANs
● Circuit Switching
● Reservation of resources for the
time of the connection
● Packet Switching
● Sharing of the resources
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Packet Switching
● Packet Switching is the most common communication technology in WANs
today
● The provider of communication resources provides virtual connections (virtual
circuits, circuit switching) between remote stations/networks, the data are
transferred in the form of packets.
● Examples: Frame Relay, ATM, OSI X.25
● Two types of Virtual Circuits:
● Switched Virtual Circuits (SVCs)
● Useful for senders with sporadic transmission wishes.
● A virtual connection is established, data are transferred, after the transmission the
connection is terminated and the resources are released.
● Permanent Virtual Circuits (PVCs)
● Useful for senders which need to transfer data permanently.
● The connection is established permanently, there exists only the phase of the data
transfer.
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Frame Relay
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Frame Relay Network Implementation
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Frame Relay
● Based on Packet Switching, i.e., the transmission of data packets
● Originally designed for the use between ISDN devices, usage has spread further
● The packets can have variable length
● Statistical Multiplexing (i.e. “mixing” of different data streams) for controlling
the network access.
● This enables a flexible, efficient use of the available bandwidth
● A first standardization took place 1984 by the CCITT. However, it did not result
in a complete specification.
● Therefore, in 1990 Northern Telecom, StrataCom, Cisco, and DEC formed a
consortium that build up upon the incomplete specification and developed some
extensions to Frame Relay which should make a usage in the complex Internet
environment possible.
These extensions were called Local Management Interface (LMI)
Due to their success, ANSI and CCITT standardized own LMI variants
● Frame Relay finally became internationally standardized by the ITU-T, in the
USA by ANSI.
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Structure of Frame Relay
● Purpose: simple, connection-oriented technology for economic transmission
of data with acceptable speed
● Data transmission rates of 56 Kbps up to 45 Mbps can be leased
● Mostly used for permanent virtual connections for which no signaling for the
connection establishment is necessary
● Two device categories can be distinguished:
● Data Terminal Equipment (DTE)
typically in the possession of the end user, for example PC, router, bridges, …
● Data Circuit-Terminating Equipment (DCE)
in the possession of a provider. DCEs realize the transmission process. Usually they
are implemented as packet switches.
DTE
DTE
DCE
DTE
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Communication within Frame Relay
● Frame Relay offers connection-oriented communication on the LLC layer:
● Between two DTEs a virtual connection is established. It is identified by a unique
connection identifier (Data-Link Connection Identifier, DLCI).
● Note: DLCIs only refer to one hop, not to the entire connection; in addition they are
only unique in a LAN, not globally:
DTE
DLCI
DLCI
12
22
62
89
27
45
36
DTE
62
● The virtual connection offers a bidirectional communication path.
● Several virtual connections can be multiplexed to a single physical
connection (reduction of equipment and network complexity).
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Communication within Frame Relay
● Frame Relay offers two types of connections
● Switched Virtual Circuits (SVC)
● Temporary connections used when sporadic data transfer between DTEs is
required
● Four states
● Call setup: Establish virtual circuit between two DTEs
● Data transfer: Transmit data
● Idle: Connection is active, but no data to transfer
● Call termination: Bring down the virtual circuit
● Permanent Virtual Circuits (PVC)
● Permanent established connections for consistent data transfers between DTEs
● Do not require a call setup, two states
● Data transfer: Transmit data
● Idle: No data to transfer
 Small protocol overhead, high data transmission rates
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Flow Control within Frame Relay
● Flow Control in Frame Relay
● Frame Relay does not possess an own flow control mechanism for controlling the
traffic of each virtual connection.
● Frame Relay is used typically on reliable network media, therefore flow control
can be left over to higher layers.
● Instead: Notification mechanism (Congestion Notification) to report bottlenecks
to higher protocol layers, if a control mechanism on a higher layer is
implemented.
● There are two mechanisms for Congestion Notification:
● Forward-Explicit Congestion Notification (FECN)
● Initiated, when a DTE sends frames into the network
● In case of overload, the DCEs (switches) in the network set a special FECN bit to 1
● If the frame arrives at the receiver with set FECN bit, it recognizes that an overload on
the virtual connection is present. The information is relayed to higher layers.
● Backward-Explicit Congestion Notification (BECN)
● Similarly to FECN, but the BECN bit is set in frames which are transmitted in the
opposite direction from frames with set FECN bit
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Asynchronous Transfer Mode (ATM)
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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ATM for the Integration of Data and
Telecommunication
Telecommunication
● Primary goal: Telephony
Data communication
● Primary goal: Data transfer
● Connection-oriented
● Firm dispatching of resources
● Connectionless
● Flexible dispatching of resources
● Performance guarantees
● Unused resources are lost
● No performance guarantees
● Efficient use of resources
● Small end-to-end delay
● Variable end-to-end delay
Time Division Multiplexing
Statistical Multiplexing
bandwidth allocation
bandwidth allocation
t
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
t
5.127
Characteristics of ATM
● Characteristics of ATM
● ITU-T standard (resp. ATM forum) for cell transmission
● Integration of data, speech, and video transmissions
● Combines advantages of
● Circuit Switching (granted capacity and constant delay)
● Packet Switching (flexible and efficient transmission)
● Cell-based Multiplexing and Switching technology
● Connection-oriented communication: virtual connections are established
● Guarantee of quality criteria for the desired connection (bandwidth, delay, …)
● For doing so, resources are being reserved in the switches.
● No flow control and error handling
● Supports PVCs, SVCs, and connection-less transmission
● Data rates: 34, 155, or 622 Mbps (optical fiber)
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ATM Cells
● No packet switching, but
cell switching: like time
division multiplexing, but
without reserved time slots
● Fix cell size: 53 byte
Cell
header
Payload
48 byte
5 byte
Cell multiplexing on an ATM connection:
● Asynchronous time multiplexing of several virtual connections
● Continuous cell stream
● Unused cells are sent empty
● In overload situations, cells are discarded
1
2
3
2
3
1
3
2
3
2
empty cell
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Cell Size: Transmission of Speech
Coding audio: Pulse-code modulation (PCM)
Example (simplification: Quantization with 3 bits)
● Transformation of analogous
into digital signals
● Regular scanning of the
analogous signal
● Each value is quantized with 8
bits (i.e. a little bit rounded).
● A speech data stream therefore
has a data rate of
8 bits × 8000 1/s = 64 kbps
Quantization range
● Scanning theorem
(Nyquist):
Scanning rate  2×cutoff
frequency of the original signal
Cutoff frequency of a
telephone: 3.4 kHz
 scanning rate of 8000 Hz
Origin signal
Reconstructed signal
Interval
number
Binary
code
+4
111
+3
110
+2
Scanning error
101
+1
-1
-2
100
000
T
001
-3
011
-4
010
Scanning
Intervals
Time
1 0 0 1 0 1 1 1 1 1 1 1 1 0 0 0 1 0 0 1 1 0 0 1
produced pulse code
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Cell Size within ATM
Problem: Delay of the cell stream for speech is 6 ms
48 samples with 8 bits each
= 48 byte
= Payload for an ATM cell
Larger cells would cause too large delays during
speech transmission
Smaller cells produce too much overhead for
“normal” data (relationship Header/Payload)
i.e. 48 byte is a compromise.
t=125 ms
Continuous data stream with
scanning rate 1/125 ms
TD = 6 ms
header
overhead
64+5
32+4
packetisation
delay
100%
10ms
50%
5ms
48+5
0
20
40
60
80
cell size [bytes]
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ATM Network
● Two types of components:
● ATM Switch
● Dispatching of cells through the network by switches. The cell headers of incoming cells
are read and information is updated. Afterwards, the cells are switched to the
destination.
● ATM Endpoint
● Contains an ATM network interface adapter to connect different networks with the ATM
network.
ATM Endpoints
Router
ATM network
LAN switch
ATM switch
Workstation
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Structure of ATM cells
● Two header formats:
● Communication between switches and endpoints: User-Network Interface (UNI)
● Communication between ATM switches in private networks
● Communication between two switches: Network-Network Interface (NNI)
7
6
5
4
3
GFC/VPI
2
1
0
VPI
VPI
VCI
PTI
CLP
HEC
Payload
(48 bytes)
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Structure of ATM cells
● Header Fields
7
● Generic Flow Control (GFC)
6
5
4
3
2
GFC
● Only with UNI, for local control of the
transmission of data into the network.
1
0
VPI
VPI
VCI
● Virtual Path Identifier (VPI)/
Virtual Channel Identifier (VCI)
PTI
● Identification of the next destination of
the cell
CLP
HEC
● Payload Type Identifier (PTI)
● Describes content of the data part,
e.g., user data or different control data
● Cell Loss Priority (CLP)
● If the bit is 1, the cell can be discarded in
overload situations.
● Header Error Control (HEC)
● CRC for the first 4 bytes; single bit errors
can be corrected.
7
6
5
4
3
2
1
0
VPI
VCI
PTI
CLP
HEC
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Connection Establishment in ATM
● The sender sends a connection establishment request to its ATM switch, containing the
ATM address of the receiver and demands about the quality of the transmission.
● The ATM switch decides on the route, establishes a virtual connection (assigning a
connection identifier) to the next ATM switch and forwards (using cells) the request to this
next switch.
● When the request reaches the receiver, it sends back the established path and
acknowledgement.
● After establishment, ATM addresses are no longer needed, only virtual connection
identifiers are used.
EC
Establish connection to
23.0074.4792.783c.7782.7845.0092.428c.c00c.1102.01
EC
OK
OK
EC
EC
OK
OK
ATM address
23.0074.4792.783c.
7782.7845.0092.428
c.c00c.1102.01
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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ATM Switching
● Before the start of the communication a virtual connection has to be established.
The switches are responsible for the forwarding of arriving cells on the correct
outgoing lines. For this purpose a switch has a switching table.
2
2
...
...
n
Old
Header
Out
New
Header
1
a
n
a
2
c
n
d
...
1
In
...
1
Switching Table
...
Outgoing lines
Switch
...
Incoming lines
n
b
2
e
n
● The header information, which are used in the switching table are VPI (Virtual
Path Identifier) and VCI (Virtual Channel Identifier).
● If a connection is being established via ATM, VPI, and VCI are assigned to the
sender. Each switch on the route fills in to where it should forward cells with this
information.
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.137
Path and Channel Concept of ATM
● Physical connections “contain” Virtual Paths (VPs, a group of connections)
● VPs “contain” Virtual Channels (VCs, logical channels)
● VPI and VCI only have local significance and can be changed by the switches.
● Distinction between VPI and VCI introduces a hierarchy on the path identifiers.
Thus: Reduction of the size of the switching tables.
There are 2 types of switches in the ATM network:
Virtual Path Switching
Virtual Channel Switching
VP Switch
VCI 1
VCI 2
VPI 1
VC Switch
VPI 4
VCI 3
VCI 4
VCI 1
VCI 3
VCI 4
VCI 5
VCI 6
VPI 5
VPI 2
VPI 6
VPI 3
VP-SWITCH
VCI 3
VCI 4
VCI 2
VCI 5
VCI 6
VCI 1
VCI 2
VPI 1
VPI 2
VCI 2
VPI 3
VCI 4
VP/VC-SWITCH
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.138
Layers within ATM
● Physical Layer
● Transfers ATM cells over the medium
● Generates checksum (sender) and verifies it (receiver); discarding of cells
● ATM Layer
● Generates header (sender) and extract contents (receiver), except checksum
● Responsible for connection identifiers (Virtual Path and Virtual Channel Identifier)
● ATM Adaptation Layer (AAL)
● Adapts different requirements of higher layer applications to the ATM Layer
● Segments larger messages and reassembles them on the side of the receiver
Station
Station
Higher Layers
Higher Layers
ATM Adaptation
Layer
Switch
ATM Layer
ATM Layer
ATM Layer
ATM Layer
Physical Layer
Physical Layer
Physical Layer
Physical Layer
Switch
ATM Adaptation
Layer
~DLL of OSI
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.139
Service Classes of ATM
Service Class
Criterion
A
Data rate
Negotiated
maximum
cell rate
Synchronization
(source - destination)
Bit rate
Adaptation Layer (AAL):
C
Maximum and
average
cell rate
Dynamic
rate adjustment
to free
resources
constant
“Take what
you can
get”
variable
Connection-oriented
 Moving pictures
 Telephony
 Video conferences
AAL 1
D
No
Yes
Connection
Mode
Applications:
B
AAL 2
Connectionless
 Data communication
 File transfer
 Mail
AAL 3
AAL 4
AAL 5
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.140
AALs
Load
AAL 1: Constant Bit Rate (CBR)
● Deterministic service
● Characterized by guaranteed fixed bit rate
● Parameter: Peak Cell Rate (PCR)
AAL 2: Variable Bit Rate (VBR)
● Real time/non real time, statistical service
● Characterized by guaranteed average bit rate. Thus also
suited for bursty traffic.
● Parameter: Peak Cell Rate (PCR), Sustainable Cell Rate
(SCR), Maximum Burst Size
AAL 3: Available Bit Rate (ABR)
● Load-sensitive service
● Characterized by guaranteed minimum bit rate and loadsensitive, additional bit rate (adaptive adjustment)
● Parameter: Peak Cell Rate, Minimum Cell Rate
AAL 4: Unspecified Bit Rate (UBR)
● Best Effort service
● Characterized by no guaranteed bit rate
● Parameter: Peak Cell Rate
PCR
Load
Time
PCR
SCR
Load
Time
ABR/
UBR
Other
connections
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
Time
5.141
Services Classes of ATM
Line Capacity
100%
Available Bit Rate (ABR)
Unspecified Bit Rate (UBR)
Variable Bit Rate (VBR)
Constant Bit Rate (CBR)
0%
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.142
Traffic Management
● Connection Admission Control (CAC)
● Reservation of resources during the connection establishment (signaling)
● Comparison between connection parameters and available resources
● Traffic contract between users and ATM network
● Usage Parameter Control/Network Parameter Control
● Test on conformity of the cell stream in accordance with the parameters of the
traffic contract at the user-network interface (UNI) or network-network interface
(NNI)
● Generic Cell Rate Algorithm/Leaky Bucket Algorithm
● Switch Congestion Control (primary for UBR)
● Selective discarding of cells for the maintenance of performance guarantees in
the case of overload
● Flow Control for ABR
● Feedback of the network status by resource management cells to the ABR
source, for the adjustment of transmission rate and fair dispatching of the
capacity
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.143
Integration of ATM into Existing Networks
● What does ATM provide?
● ATM offers an interface to higher layers (similar to TCP in the Internet protocols)
● ATM additionally offers QoS guarantees (Quality of Service)
● ATM had problems during its introduction
● Very few applications which build directly upon ATM
● In the interworking of networks TCP/IP was standard
● Without TCP/IP binding, ATM could not be sold!
● Therefore different solutions for ATM were suggested, e.g.
● IP over ATM (IETF)
● LAN emulation (LANE, ATM forum)
● Today: ATM still is in use in some regions, but SDH (as a technology
coming from the telecommunication sector) took over the leading role in
WAN technology
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.144
Synchronous Digital Hierarchy (SDH)
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.145
Synchronous Digital Hierarchy (SDH)
● All modern networks in the public
“Dark Fiber”
area are using the SDH technology
rented wavelength
● Example: the German B-WIN (ATM)
GRE
was replaced by the G-WIN
KIE
ROS
HAM
AWI
(Gigabit-Wissenschaftsnetz)
DES
EWE
on basis of SDH
FFO
BRE
TUB
POT
HAN
HUB
● Since 2006: X-WIN – complete
BIE
MUE
Surfnet
MAG
ADH
ZIB
BRA
redesign of topology, additionally
DUI
GOE
KAS
integration of DWDM (dense
LEI
DRE
FZJ
MAR
wavelength division multiplexing):
JEN
BIR
AAC
CHE
GIE
up to 160 parallel transmissions
ILM
FRA
Geant2
over a fiber, giving 1.6 Tbps
BAY
GSI
capacity!
ESF
WUE
● Also used within the MAN range
(Replaced by Gigabit Ethernet?)
● Analogous technology in the USA:
Synchronous Optical Network
(SONET)
HEI
SAA
Renater
ERL
REG
FZK
AUG
KEH
STU
GAR
Switch/GARR
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.146
Synchronous Digital Hierarchy (SDH)
● Introduction of PCM in the 1960s
● Digital telephone system
● Before SDH was introduced Plesiochronous Digital Hierarchy (PDH) was used
● Europe: Combination of 30 channels of 64kbps
● USA, Canada, Japan: Combination of 24 channels of 64kbps
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.147
SDH Structure
● SDH achieves higher data rates than ATM (at the moment up to about 40 Gbps)
● Flexible capacity utilization and high reliability
● Structure: arbitrary topology, meshed networks with a switching hierarchy
(exemplarily 3 levels):
SDH Cross Connect
155 Mbps
Regional switching centers
155 Mbps
Add/Drop Multiplexer
34 Mbps
Local networks
2 Mbps
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
Synchronous Digital Hierarchy (SDH)
2.5 Gbps
Supraregional switching
5.148
Multiplexing within SDH
2 Mbps,
34 Mbps,…
155 Mbps
622 Mbps
2.5 Gbps
10 Gbps
+ control information for signaling
Switching center
34 Mbps
622 Mbps
2 Mbps
Switching
center
2 Mbps
622 Mbps
155 Mbps
SDH Cross Connect
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.149
Characteristics of SDH
● World-wide standardized bit rates on the hierarchy levels
● Synchronized, centrally clocked network
● Multiplexing of data streams is made byte-by-byte, simple multiplex pattern
● Suitable for speech transmission:
● Since on each hierarchy level four data streams are mixed byte-by-byte and a hierarchy
level has four times the data rate of the lower level, everyone of these mixed data
streams has the same data rate as on the lower level. Thus the data experience a
constant delay.
● Direct access to signals by cross connects without repeated demultiplexing
● Short delays in inserting and extracting signals
● Additional control bytes for network management, service and quality control,…
● Substantial characteristic: Container for the transport of information
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.150
SDH Architecture
● Physical Layer
● Transmission medium, typically
fiber optics
● Radio and Satellite links
● Regenerator Section
PSTN/ISDN
ATM
IP
VC-12 Layer


VC-4 Layer
● Path between regenerators
Multiplex Section
● Multiplex Section
Regenerator Section
● Link between multiplexers
Physical Layer
● VC Layer (Virtual Container)
● Part of the mapping procedure, i.e.,
packing of ATM and PDH signals
into SDH
2Mbps
140Mbps
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.151
Components of a SDH Network
● Four different types of network
elements
● Regenerators
● Regenerate incoming signal (clock
and amplitude)
● Clock signal is derived from incoming
signal
● Terminal multiplexer
● Combine PDH and SDH signals into
higher bit rate STM signals
● Add/drop multiplexer
● Insert or extract PDH and SDH lower
bit rate signals
● Digital cross connects
● Mapping of PDH tributary signals into
virtual containers
● Switching of various containers
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.152
Components of a SDH Network
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.153
Synchronization in SDH
● All network elements have to be synchronized
● Central clock with high accuracy, i.e., 1 x 10-11
● Primary Reference Clock (PRC)
● Clock signal is distributed in the network
● Hierarchical structure to distribute clock signals
● Subordinate synchronization supply units (SSU)
● Synchronous equipment clocks (SEC)
● Synchronization path can be the same as for data
PRC
G.811
SSU
SSU
G.812
G.812
SEC
SEC
SEC
SEC
G.813
G.813
G.813
G.813
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.154
SDH Transport Module (Frame)
270 columns (bytes)
Synchronous Transport Module
(STM-N, N=1,4,16, 64)
STM-1 structure:
 9 lines with 270 bytes each
 Each byte in the payload
represents a 64 kbps channel
 Basis data rate of 155 Mbps
9x270x8x8000 bps = 155.52 Mbps
9 columns (bytes)
1
3
Regenerator Section
Overhead (RSOH)
4
Administrative Unit Pointers
5
Multiplex Section
Overhead (MSOH)
9
261 columns (bytes)
Payload
9 lines
(125 µs)
Administrative Unit Pointers
 Permit the direct access to components of the Payload
Section Overhead
 RSOH: Contains information concerning the route between two repeaters or a repeater
and a multiplexer
 MSOH: Contains information concerning the route between two multiplexers without
consideration of the repeaters in between.
Payload
 Contains the utilizable data as well as further control data
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.155
Creation of a STM
● Creation of a Synchronous Transport Module (STM)
● Payload is packed into a container
● A distinction of the containers is made by size: C-1 to C-4
● Payload data are adapted if necessary by padding to the container size
● Some additional information to the payload are added for controlling the data
flow of a container over several multiplexers
● Path Overhead (POH)
● Control of single sections of the transmission path
● Change over to alternative routes in case of an error
● Monitoring and recording of the transmission quality
● Realization of communication channels for maintenance
● By adding the POH bytes, a container becomes a Virtual Container (VC)
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.156
Creation of a STM
● If several containers are transferred in a STM payload, these are
●
●
multiplexed byte-by-byte in Tributary Unit Groups (TUG).
By adding an Administrative Unit Pointer, the Tributary Unit Group
becomes an Administrative Unit (AU).
Then the SOH bytes are supplemented, the SDH frame is complete. RSOH
and MSOH contain for example bits for
● Frame synchronization
● Error detection (parity bit)
● STM-1 identifications in larger transportation modules
● Control of alternative paths
● Service channels
● … and some bits for future use
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.157
SDH Hierarchy
155 Mbps
622 Mbps
2.5 Gbps
STM-4
STM-1
261
STM-16
4x261=1044
9
4x9=36
Assembled from
Assembled from
Basis transportation module for
155 Mbps, e.g. contains:
 a continuous ATM cell stream
(C-4 container)
 a transportation group (TUG-3)
for three 34 Mbps PCM systems
 a transportation group (TUG-3)
for three containers, which
again contain TUGs
4x1044=4176
4x36=144
4 x STM-1
4 x STM-4
Assembled from
4 x STM-1
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.158
SDH Hierarchy
● Higher hierarchy levels assembling STM-1 modules
● Higher data rates are assembled by multiplexing the contained signals byte-bybyte
● Each byte has a data rate suitable of 64 kbps for the transmission of voice
(telephony)
● Except STM-1, only transmission over optical fiber is specified
9 columns
261 byte
4 * 261 byte
4 * 9 columns
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.159
Types of SDH Containers
C-n
VC-n
TU-n
TUG-n
Container n
Virtual Container n
Tributary Unit n
Tributary Unit Group n
Payload
Tributary Unit, n (n=1 to 3)
 Contains VC-n and Tributary Unit
Pointer
C-4
TUG-3
VC-4
3
H4
or
2
1
VC-3
VC-4 Path Overhead (POH)
Container, C-n (n=1 to 4)
 Defined unit for payload capacity (e.g. C-4
for ATM or IP, C-12 for ISDN or 2 Mbps)
 Transfers all SDH bit rates
 Capacity can be made available for transport
from broadband signals not yet specified
Virtual Container, VC-n (n=1 to 4)
 Consists of container and POH
 Lower VC (n=1,2): single C-n plus basis
Virtual Container Path Overhead (POH)
 Higher VC (n=3,4): single C-n, union
of TUG-2s/TU-3s, plus basis Virtual
Container POH
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.160
Types of SDH Containers
● Virtual Containers (VC)
SDH
Bit Rate [Mbps]
Size of VC
Rows x Columns
VC-11
1,728
9x3
VC-12
2,304
9x4
VC-2
6,912
9 x 12
VC-3
48,960
9 x 85
VC-4
150,336
9 x 261
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.161
Types of SDH Containers
VC-3
TU-3
7
6
or
5
4
C-3
3
2
C-n
VC-n
TU-n
TUG-n
AU-n
STM-N
Container n
Virtual Container n
Tributary Unit n
Tributary Unit Group n
Administrative Unit n
Synchronous
Transport Module N
1
VC-2
TUG-2
Administrative Unit n (AU-n)
3
TUG-12
2
1
 Adaptation between higher order
path layer and multiplex unit
VC-12
C-12
 Consists of payload and
Administrative Unit Pointers
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.162
SDH Multiplex Structure
STM-N
xN
AUG
AU-4
VC-4
x3
TUG-3
x3
AU-3
C-4
TU-3
VC-3
C-3
VC-3
x7
x7
TUG-2
TU-2
Pointer Processing
x3
Multiplexing
TU-12
C-n
Container n
x4
VC-n
Virtual container n
TU-11
TU-N
Tributary Unit n
TUG-n
Tributary Unit Group n
AU-n
Administrative Unit n
AUG
STM-N
Administrative Unit Group
VC-2
C-2
VC-12
C-12
VC-11
C-11
Synchronous Transport Module N
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.163
SDH Multiplexing
PTR
Container-1
Logical association
Physical association
Pointer
VC-1 POH
TU-1 PTR
(1) PTR
(2) PTR
(3) PTR
VC-3 POH
(4) PTR
VC-1
VC-1 (1)
VC-1 (2)
TUG-2
AU-3 PTR
VC-1 (3)
AUG
AU-3 PTR
VC-1 (4)
TUG-2
VC-3
VC-1
TU-1
TUG-2
VC-3
AU-3
VC-3
AU-3 PTR
SOH
Container-1
VC-3
AUG
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
AUG
STM-N
5.164
What can SDH achieve?
SONET
Electrical
Optical
SDH
Optical
Data rate (Mbps)
Gross
Net
STS-1
OC-1
STM-0
51.84
50.112
STS-3
OC-3
STM-1
155.51
150.336
STS-9
OC-9
(STM-3)
466.56
451.008
STS-12
OC-12
STM-4
622.08
601.344
STS-18
OC-18
(STM-6)
933.12
902.016
STS-24
OC-24
(STM-8)
1,244.16
1,202.688
STS-36
OC-36
(STM-12)
1,866.24
1,804.032
STS-48
OC-48
STM-16
2,488.32
2,405.376
STS-96
OC-96
STM-32
4,976.64
4,810.752
STS-192
OC-192
STM-64
9,953.28
9,621.504
STS-768
OC-758
STM-256
39,813.12
38,486.016
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.165
Network Infrastructure
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.166
Network Infrastructure
● For building computer networks more complex than a short bus, some
additional components are needed:
● Repeater
● Physically increases the range of a
local area network
● Hub
● Connects several computers or local
area networks of the same type (to a broadcast network)
● Bridge
● Connects several local area networks
(possibly of different types) to a large LAN
● Switch
● Like a hub, but without broadcast
● Router
● Connects several LANs with the same
network protocol over large distances
● Gateway
● Understands two different technologies and can convert
the contents from one to the other and vice versa
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.167
Network Infrastructure
Application layer
Application gateway
Transport layer
Transport gateway
Network layer
Router
Data link layer
Bridge, Switch
Physical layer
Repeater, Hub
Packet from network layer
Frame
header
Packet
header
TCP
header
User
data
CRC
Frame in Data link layer
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.168
Infrastructure Components: Hub & Repeater
● Transmission of data on the physical layer
● Reception and refreshment of the signal, i.e., the signals received
on one port are newly produced on the other(s)
● Do not understand frames, packets, or headers
● Increase of the network range
● Stations cannot send and receive at the same time
● One shared channel (Broadcast)
● Low security, because all stations can monitor the whole traffic
● Low costs
Layer 1
Hub
Segment 1
Repeater
Hub: “one to all”
Repeater:
Linking of 2
networks
Segment 2
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.169
Infrastructure Components: Hub & Repeater
Hub
Hub
Device1
Device2
Device3
Devicen
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
...
5.170
Infrastructure Components: Bridge
● Bridge
Layer 2
● Bridge connects 2 or more LANs
● Operates on frame addresses
● Can support different network type
Upper Layer
(Bridge protocol, Bridge management)
Data path
Control path
LLC
LLC
MAC1
MAC-Relay
MAC2
PHY
PHY
Network1
Network 2
MAC1 Data LLC
MAC1
MAC2 Data LLC
MAC2
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.171
Bridging
● Typically a LAN comes rarely alone
● What to do if many LANs exist?
● Connect them by bridges
● A bridge examines the data link layer address for routing
● Reasons why one organization could have multiple LANs
● Autonomy of the owner
● Several buildings with each having a LAN
● Machines are too distant
● Ethernet supports only up to 2.5 km
● Load
● Security
● Reliability
● Requirements:
● Bridges should be transparent
● Moving of machines from one segment to another must not require the change
of software or hardware
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.172
Network Infrastructure
● Bridges from 802.x to 802.y
● Problems when moving frames between LANs
● Different frame formats
● Different data rates
● Different max. frame length
● Security: Some support encryption others do not
● Quality of Service
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
5.173
Infrastructure Components: Bridges
● With bridges, several LANs are connected on the link layer – possibly LANs
●
of different types, i.e., having different header formats
Major tasks:
● Appropriate forwarding of the data
● Adaptation to different LAN types
● Reduction of the traffic in a LAN segment, i.e., packets which are sent from A to
C are not forwarded by the bridge to LAN2. Thus, station D can communicate
with E in parallel.
● Increases physical length of a network
● Increased reliability through demarcation of the LAN segments
A
C
LAN1
B1
D
E
LAN2
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Infrastructure Components: Bridges
● Transparent bridges (e.g. for CSMA and Token Bus networks)
A
C
B1
LAN1
D
LAN2
E
B
²
LAN3
● Characteristics
● Coupling of LANs is transparent for the stations, i.e., not visible
● Hash tables contain the destination addresses
● Routing Procedure
● Source and destination LAN are identical
 frame is rejected by bridge, e.g., B1 in case of a transmission from A to C
● Source and destination LAN are different
 forward frames, e.g., in case of a transmission from D to E
● Destination LAN unknown
 flood frame
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Transparent Bridges
 To realize transparency, bridges have to learn in which LAN a host is located
 Each bridge maintains a forwarding database with entries
<MAC address, port, age>
 MAC address: host name
 port:
port number of bridge used to send data to the host
 age:
aging time of entry
 Assume a MAC frame arrives on port x:
Port x
Bridge 2
Is MAC address of
destination in forwarding
database for ports A, B, or C?
Found and  x?
Forward the frame on the
appropriate port
Port A
Found and = x?
Ignore frame
Port C
Port B
Not found ?
Flood the frame, i.e., send the
frame on all ports except port x.
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Transparent Bridges: Address Learning
● Database entries are set automatically with a simple heuristic
● the source field of a frame that arrives on a port tells which hosts are reachable
from this port.
● Algorithm:
● For each frame received, the source stores the source field in the forwarding
database together with the port where the frame was received.
● All entries are deleted after some time (default is 15 seconds).
Src=x,
Dest=y
Src=x,
Dest=y
Src=x,
Src=x,
Src=y,
Dest=y
Dest=y
Dest=x
Port 1
Port 2
Port 3
x is at Port 3
y is at Port 4
Port 4
Src=y,
Src=x,
Dest=x
Dest=y
Port 5
Src=x,
Dest=y
Port 6
Src=x,
Dest=y
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Loops
● Consider two LANs that are
connected by two bridges.
● Assume host n is transmitting a
frame F with unknown destination.
● Bridges A and B flood the frame
to LAN 2.
● Bridge B sees F on LAN 2 (with
unknown destination), and copies
the frame back to LAN 1
● Bridge A does the same.
● The copying continues
LAN 2
F
F
Bridge B
Bridge A
F
F
LAN 1
F
● Solution: Spanning Tree Algorithm
host n
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Spanning Tree Bridges
● Preventing loops: compute a
spanning tree from all connected
bridges
LAN 2
d
● Spanning Tree Algorithm:
● Determine one root bridge
● The bridge with the smallest ID
Bridge 4
Bridge 3
Bridge 1
LAN 5
● Determine a designated bridge for
each LAN
Bridge 5
● The bridge which is nearest to the
root bridge
LAN 1
● Determine root ports
● Port for the best path to root bridge
considering costs for using a path,
e.g., the number of hops.
Bridge 2
LAN 3
LAN 4
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Spanning Tree Algorithm
● At the beginning, all bridges assume to be root bridge and send out a
packet containing their own ID and current costs (initialized with zero)
over all of their ports:
root ID
costs
bridge ID
port ID
e.g. for station B on port P1:
B
0
B
P1
● A bridge receiving such a packet checks the root ID and compares it with its own
one. Root ID and costs are updated for received packets with smaller ID in the
root bridge field and forwarded. Updating the costs is made by adding the own
costs for the station from which the packet was received to the current costs
value.
● When the (updated) packets of all bridges have passed all other bridges, all
bridges have agreed on the root bridge. The received packets containing the
smallest costs value to the root bridge determine the designated bridge for a
LAN and designated ports for the bridges to send out data.
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Spanning Tree Algorithm: Example
Network:
LAN 1
5
B2
ID=27
8
LAN 4
Spanning Tree:
LAN 1
B2
ID=27
12
B1
ID=93
20
10
LAN 3
5
6
B5
ID=9
B1
ID=93
LAN 3
5
10
B3
ID=18
B4
ID=3
10
7
LAN 2
ports
ID: bridge ID
: designated port
LAN 5
B3
ID=18
B4
ID=3
LAN 2
designated bridge
for LAN 2
root bridge
LAN 4
B5
ID=9
LAN 5
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Infrastructure Components: Bridges
● Source Routing Bridges (e.g. for ring networks)
C
A
B1
Ethernet
FDDI
B
²
D
Ethernet
● Characteristics:
● Sources must know (or learn), in which network segment the receivers are
located
● Large expenditure for determining the optimal route, e.g., via using a Spanning
Tree algorithms or sending out Route Discovery Frames using broadcast
● All LANs and Bridges on the path must be addressed explicitly
● Connection-oriented, without transparency for the hosts
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Infrastructure Components: Switch
● Like a bridge, but:
● Point-to-point communication, no broadcast
● Switch learns the addresses of the connected computers
Layer 2/3/4
● Stations can send and receive at the same time
● No carrier control necessary
● Buffer for each individual
station/each port
Switch
● Higher costs
 “Layer 3-Switch”: also has
functionalities of level 3, i.e., it
can e.g. take over the routing.
 “Layer 4-Switch”: looks up
additionally in the TCP-header,
can therefore be used e.g. for
load balancing.
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Infrastructure Components: Switch – Realization
● Mostly used: buffered crossbar
● For each input port, provide buffers for the output ports
● At any time, only one input port can be connected to an output line
● Additional speedup possible with small buffers at each cross-point
● With a buffered switch, nearly no more collisions are possible!
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Switched LANs – Mechanismen
● Cut Through
● Adresstabelle wird angesprochen, sobald die Zieladresse eingelesen ist
● Weiterleitung des Datenpakets, sobald der Weg geschaltet ist
● Geringe Latenzzeit
● Store and Forward
● Datenpaket wird zunächst vollständig eingelesen und zwischengespeichert
● Kontrolle der CRC-Prüfsumme und Ausführen von Filterfunktionen
● Hybrides Switching
● Kombination von Cut Through / Store and Forward
● Auswahl abhängig von Fehlerrate
● Predictive Switching
● Pfad in Schaltmatrix wird hergestellt, bevor Zieladresse vollständig eingelesen
● Basierend auf den vorher geschalteten Pfaden
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Infrastructure Components: Router
● What are the limitations of bridges?
● Even though bridges are suitable to connect computers in several networks,
there are also some disadvantages, e.g.:
● Bridges can support only some thousand stations, which especially has the reason that
addresses are used which do not have any geographical reference.
● LANs coupled with bridges already form a “large LAN”, although a separation often
would be desirable (e.g. regarding administration or errors).
● Bridges pass broadcast frames on to all attached LANs.
This can result in “Broadcast Storms”.
● Bridges do not communicate with hosts, i.e., they do not hand over information about
overload situations or reasons for rejected frames.
Router overcome these weaknesses
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Infrastructure Components: Router
● Principal task of routers
● Incoming packets are being forwarded on the best path
possible to the destination on the basis of a global address
Layer 3
● In principle no restriction concerning the number of hosts (hierarchical
addressing)
● Local administration of the networks (ends at the router), Firewalls are possible
● Broadcasts are not let through by the routers, Multicast depending on the router
● Communication between host and router improves performance
A
LAN 1
Network1
B
R3
R2
R1
Network2
LAN 2
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Infrastructure Components: Gateway
● Transport Layer Gateways
● Connection of computers using different transport protocols, e.g., a computer
using TCP/IP and one using ATM transport protocol
● Copies packets from one connection to another
● Application Layer Gateways
● Understand the format and contents of the data and translate messages from
one format to another format, e.g., email to SMS
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Virtual LANs
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Virtual LANs
● Organization of LANs
● In early Ethernet days all computers were on one LAN
● With 10Base-T came new cabling in buildings
● Configuration of LAN logically rather than physically
● Requirement: Decoupling of the logical topology from the physical topology
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Virtual LANs
● Management often requires structuring of LANs due to
● Different departments want different LANs
● Security
● Load
● Broadcast (broadcast storm)
● What happens if users move from one department to another?
● Rewire in hub/switch
● VLANs with VLAN-aware switches
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Virtual LANs
A
B
C
D
A
B
C
D
I
M
I
M
J
N
J
N
K
O
K
O
L
L
E
F
G
H
E
F
G
H
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Virtual LANs
● Virtual LANs require VLAN-aware switches
● VLANs are often named by colors (VLAN ID)
● Allows colored diagrams which show logical and physical topology at the same
time
● VLAN-aware devices have to know about the VLANs
● Switch has a table which tells which VLAN is accessible via which port
● A port may have access to multiple VLANs
● How do a VLAN-switch know the VLANs?
● Assign every port of the device a VLAN ID
● Only machines belonging to the same VLAN can be attached
● Every MAC address is assigned to a VLAN
● Device needs tables of the 48-bit MAC addresses assigned to VLANs
● Every Layer 3 protocol (IP address) is assigned to a VLAN
● Violates the independency of layers
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Virtual LANs
● IEEE 802.1Q
● Special field in frame header telling the VLAN assignment
● Problems:
● What happens with existing Ethernet cards?
● Who generates the new field?
● What happens with full frames (maximum length)?
● Solution:
● The first VLAN-aware device adds a VLAN-tag
● The last VLAN-aware device removes the VLAN-tag
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Virtual LANs
● IEEE 802.1Q Frame Format
● Additional pair of 2-byte fields
● TPID: Tag Protocol Identifier (0x8100)
● Tag comprises three fields
● Pri: 3-bit priority field, does not have anything to do with VLANs
● CFI: Canonical Format Indicator
● Indicates that payload has a IEEE 802.5 frame
● VLAN ID: 12-bit VLAN identifier
● The only relevant field
Preamble SFD
DA
SA
Preamble SFD
DA
SA
L/T
TPID
Tag
Data
L/T
Padding FCS
Data
Padding FCS
Pri CFI VLAN ID
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Virtual LANs
● Who inserts the VLAN-tag?
● New cards (Gigabit Ethernet) support 802.1Q
● Otherwise
● First VLAN-aware switch adds the tag
● Last VLAN-aware switch removes the tag
● How does the switch know which frame belongs to which VLAN?
● First device has to decide based on the port or MAC address
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Summary
● Layer 1 and 2: “How to physically transport data reliably from one
computer to a neighbored one”?
● Layer 1 defines transmission medium and bit representation on this medium
● Layer 1 additionally specifies transmission mode, data rate, pin usage of
connectors, …
● Layer 2 protects against transmission errors (mostly CRC) and receiver overload
(flow control, sliding window)
● Layer 2 also defines medium access coordination for broadcast networks
● Both layers together define how to transfer data from one computer to a
directly connected one (maybe over a hub/switch) – on that reason both are
implemented in one piece of software: the network interface card driver.
● Bridges in principle allow to connect lots of LANs over long distances – is that the
Internet?
Univ.-Prof. Dr.-Ing. Jochen H. Schiller ▪ cst.mi.fu-berlin.de ▪ Telematics ▪ Chapter 5: Medium Access Control Sublayer
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Summary
● LANs
● Ethernet as standard for local networks
● 10G-Ethernet also possible for use in MANs
● WANs
● SDH/Sonet as standard for wide area networks
● 10G-Ethernet as access technology to the core network
● Integration of DWDM – transmission on 160 wavelengths in parallel dramatically
increases the capacity
● Also possible: SDH with 40 Gbps, DWDM with 4096 channels – 164 Tbps!
● Dream of “all optical network”: switch/route data streams with optical
components (think of a prism)
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