1. CS4470 Computer Networking Protocols
5/20/2018
CS Computer Networking Protocols
4. Data Link Layer 2
Huiping Guo
Department of Computer Science
California State University, Los Angeles

2. Outline LANs Topologies Ethernet Extended LANs 4. LAN 2 CS4470_F17
5/20/2018
Outline
LANs
Topologies
Ethernet
Extended LANs
4. LAN CS4470_F17

3. Point-to-Point communication
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Point-to-Point communication
Each communication channel connects exactly two computers
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4. Point-to-Point communication (cont.)
Disadvantages:
The number of connections grows quickly as the size of the network increases
The number of connections needed for N computers is proportional to N2
Direct connections required = (N2-N)/2
The expense is especially high
4. LAN CS4470_F17

5. Shared communication channels
Local Area Networks
Devised as alternatives to expensive, dedicated point-to-point connections.
Rely on the shared medium
A set of computers attach to a cable
The computers take turns sending data
Sharing reduces cost!
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6. Comparison Networks based on the shared medium
Used for LOCAL communication.
Point-to-point connections
Used for long-distance networks
Why not shared medium?
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7. Shared medium not suitable for long distance networks
Computers attached to a shared network must coordinate use of the network
Coordination requires communication, introduces longer delays
Shared networks with long delays are inefficient
they spend more time coordinating the use of the shared medium and less time sending data
4. LAN CS4470_F17

8. LAN: A definition LAN --- Local Area Network
5/20/2018
LAN: A definition
LAN --- Local Area Network
A LAN is computer network in which devices (computers, printers) are connect to a shared medium (wire or cable) to exchange data or share resources within a small geographic area
An office, the floor of a building, a building itself, or a small campus
Not always clear where a “local area” ends and a “wide area” begins
Some say when you need to involve a 3rd party such as a phone company or other telecom provider that’s where the “local area” ends
4. LAN CS4470_F17

9. LANs are popular LANs are the most popular form of computer network
5/20/2018
LANs are popular
LANs are the most popular form of computer network
They are relatively cheap
They are typically fast
Deployed today in not just workplace and offices, but also in the home
4. LAN CS4470_F17

10. Features Limited to short distance Rely on shared media Topologies
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Features
Limited to short distance
Rely on shared media
Topologies
Many LAN technologies exist
Determined by the link layer protocol
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11. Network Topology Specify general “shape” of a network
Often applied to LAN
LAN Topologies
Bus
Star
Tree
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12. Star topology All computers attach to a central point. hub or switch
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13. Bus topology The computers are attached to a single , long cable (bus)
Any attached computer can send a signal down the cable and all computers receive the signal.
4. LAN CS4470_F17

14. Ring topology All computers are connected in a closed loop
The ring refers to logical connection, not physical connection
Data flow in one direction
4. LAN CS4470_F17

15. Choice of Topology Reliability Expandability Performance
Needs considering in context of:
Medium
Wiring layout
Access control
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16. Example ring network: IBM Token Ring
5/20/2018
Example ring network: IBM Token Ring
Access the shared medium -- ring
Use token passing
token passing ring networks
Media access control
A computer must wait for permission before it can send data
Once it obtains permission, the sending computer has complete control of the network
The data pass from the sender to the next computer, then to the next computer and so on until the data pass completely around the ring and arrive back at the sender
4. LAN CS4470_F17

17. Example ring network: IBM Token Ring (cont.)
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Example ring network: IBM Token Ring (cont.)
Token
A special reserved bit pattern which gives a computer permission to send data
The token passes around the ring
How to make sure data doesn’t contain the token?
A computer cannot hold a token forever!
Receive a token
Remove it from the ring
Send a frame
Release the token
4. LAN CS4470_F17

18. Example ring network: IBM Token Ring (cont.)
Station waits for token before sending
Signal travels around entire ring
Sender receives its own transmission
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19. Pros and Cons Advantages Disadvantages Easy detection of
Broking ring
Hardware failures
Interference
Disadvantages
Each computer attached to a ring must pass bits of a frame to the next computer
Failure of a single machine or link can disable the entire network
Point-to-Point wiring: difficult to add/remove a satation
4. LAN CS4470_F17

20. Failure recovery in ring networks
Automatic failure recovery
Introduced by FDDI
Uses two rings
Terminology
Dual-attached
Counter rotating
Self healing
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21. Illustration of failure recovery
Normal operation uses one of two rings
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22. Illustration of failure recovery
Normal operation uses one of two rings
Second ring used for loopback during failure
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23. Outline
LANs
Topologies
Ethernet
Extended LANs
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24. Ethernet Ethernet uses bus topologies “dominant” wired LAN technology:
First widely used LAN technology
Simpler, cheaper than token ring, FDDI and ATM
Kept up with speed race: 10 Mbps – 10 Gbps
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25. Shared medium in a LAN Shared medium used for all transmissions
Only one station transmits at any time
Stations “take turns” using medium
Media Access Control (MAC) policy ensures fairness
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26. Illustration Of Ethernet Transmission
Only one station transmits at any time
Signal propagates across entire cable
All stations receive transmission
CSMA/CD media access scheme
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27. CSMA/CD Paradigm Multiple Access (MA) Carrier Sense (CS)
Multiple computers attach to shared media
Each uses same access algorithm
Carrier Sense (CS)
Wait until medium idle
Begin to transmit frame
CSMA plus Collision Detection (CD)
Listen to medium during transmission
Detect whether another station’s signal interferes
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28. Transmitter algorithm
When a station has a frame to send and the line is idle, it transmits the frame immediately
There is no negotiation with other adaptors
The frame size is up to 1,500 bytes which means that the station can occupy the line for only a fixed length of time
When an adaptor has a frame to send and the line is busy, it waits for the line to go idle and then transmits immediately
The Ethernet is said to be 1-persistent protocol because a station with a frame to send transmits with prob. 1 whenever a busy line goes idle
In general, a p-persistent algorithm transmits with prob. P after a line becomes idle
4. LAN CS4470_F17

29. Transmitter algorithm
Collisions
Two stations find the line idle and transmit at the same time
While a station on one end starts to transmit a frame, the other station on the opposite end doesn’t sense the line busy and transmits
While a station is transmitting, it keeps listing to the line
If there is a collision, the station stops transmission and sends a jam signal
Back off from the interference and try again
4. LAN CS4470_F17

30. Backoff after collision
Exponential backoff
after the mth collision, chooses a K at random from {0,1,2,…,2m-1}
waits K·512 bit times
Bit time:
1 microsec for 10 Mbps Ethernet ;
For K=1023, wait time is about 50 msec
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31. Backoff after collision (cont.)
Goal
adapt retransmission attempts to estimated current load
heavy load: random wait will be longer
first collision
choose K from {0,1}; delay is K· 512 bit transmission times
after second collision
choose K from {0,1,2,3}…
after ten collisions, choose K from {0,1,2,3,4,…,1023}
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32. Addressing: Identifying A Destination
A pair of computers communicate across a LAN
All other computer in the LAN receive and process each copy of the message
How to avoid this?
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33. Addressing: Identifying A Destination (cont.)
Allow sender to specify destination
Each station assigned unique 48-bit address: MAC address
Address assigned when network interface card (NIC) manufactured
Each frame contains address of intended recipient
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34. Ethernet Address Recognition
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35. Broadcast On Ethernet All 1s address specifies broadcast Sender
Places broadcast address in frame
Transmits one copy on shared network
All stations receive copy
Receiver always accepts frame that contains
Station’s address
The broadcast address
4. LAN CS4470_F17

36. Frame header and frame format
Each LAN defines the exact frame format used with the technology
General format:
Frame header+DATA
All frames with the same LAN technology have the same header size
4. LAN CS4470_F17

37. Ethernet Frame Structure
Preamble:
7 bytes with pattern followed by one byte with pattern
used for synchronization 8 6 6 2 4
4. LAN CS4470_F17

38. Ethernet Frame Structure (more)
Addresses: 6 bytes
if adapter receives frame with matching destination address, or with broadcast address it passes data in frame to net-layer protocol
otherwise, adapter discards frame
Type: indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC: checked at receiver, if error is detected, the frame is simply dropped
4. LAN CS4470_F17