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The Logical Link Control (LLC) sublayer
– Framing
– Flow Control
– Error Control
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The Media Access Control (MAC) sublayer
– Random Access (CSMA)
– Token Passing
K. Salah 1

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Framing is partitioning a bit stream into discrete units or blocks of data.
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To distinguish between same bit pattern of real user data and the start-stop, bit stuffing is employed.
– E.g., stuff a bit after every 5 th consecutive ones.
K. Salah 2

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Preamble is for synchronization
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Source and destination are 48 bits each and have the MAC address, e.g, 08:00:20:01:D6:2A. The left 3 bytes are vendor specific, and the others are serial number of device assigned by
Sun. 00:00:0C is CISCO.
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This MAC address is different than the IP address !!!
K. Salah 3

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Flow Control is the process that controls the rate at which data is exchanged between two nodes. It involves a feedback mechanism.
– Ensuring the sending entity does not overwhelm the receiving entity, and thus preventing buffer overflow.
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Transmission time: Time taken to emit all bits into medium
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Propagation time: Time for a bit to traverse the link
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Two Protocols:
– Stop-and-Wait
– Sliding Window
K. Salah 4

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Source transmits frame
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Destination receives frame and replies with acknowledgement
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Source waits for ACK before sending next frame
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Destination can stop flow by not send ACK
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Works well for a few large frames
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Only one frame at a time can be in transit.
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Providing multiple ACKs for one big message is inadequate.
K. Salah 5

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Allow multiple frames to be in transit
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Receiver has buffer W long
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Transmitter can send up to
W frames without ACK
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Each frame is numbered
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Sequence number bounded by size of field (k)
– Frames are numbered modulo 2 k
K. Salah 6

K. Salah 7

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If the maximum window size is one, then we have stop-and-wait protocol.
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If duplex, use piggybacking
– Attach ACK with data frames.
– If no data to send, use acknowledgement frame
– If data but no acknowledgement to send, send last acknowledgement number again.
K. Salah 8

Error correction implies error detection.
– Parity bits
– CRC
Two forms of error correction:
– Autonomous error correction (e.g. CRC)

Transmitted frame contains redundancy bits (or check bits) correct bit errors without retransmission.
 Using parity bits
 Using Hamming code
Expensive operation, especially when having multiple bits.

– Error correction through retransmission


Use of ACKs for lost frames
Use of timers for lost ACKs


Use of sequence number for duplicate frames. Few LAN DL do this sequencing.
The concept of error detection, ACK, and retransmission is called
automatic repeat request (ARQ).
 Stop-and-wait ARQ
 Sliding window ARQ
» Go-back-n ARQ
» Selective reject ARQ
K. Salah 9

1. Damaged Frames 2. Lost Frames
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For identification, both data and ACK frames are numbered alternately 0 and 1.
– Data 0 is acknowledged by ACK 1, meaning Receiver got data 0 and expecting Data 1.
– Numbering is important in case of duplicate transmission of data and acks when lost or delayed.
K. Salah 10
3. Lost Ack

1. Damaged Frames 2. Lost Frames
Go-back-n Sliding Window ARQ
– ACK 3 means frame 0,1, & 2 are received correctly. NAK 3 means frame 3 is lost, in error, and frames 0-2 were received correctly.
K. Salah 11
3. Lost Ack

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Uses ACK as in go-back-n algorithm.
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Only the specific damaged or lost frame is retransmitted.
The frame is resent out of sequence when NAK is received.
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Better performance
– Retransmit only damaged or lost frames.
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But more complex
– Requires storage and search at the receiver.
– Extra logic also needed at sender

Need to buffer

Must be able to send frames out of sequence
K. Salah 12

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For a block of k bits transmitter generates n bit sequence
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Transmit k+n bits which is exactly divisible by some number
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Receiver divides frame by that number
– If no remainder, assume no error
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Nice animation at: http://www.mhhe.com/engcs/compsci/forouzan/dcn/graphics/animations/09_09.swf
K. Salah 13

K. Salah 14

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CRC-16 used for various file transfers
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CRC-CCITT serves as an international standard
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CRC-32 used in Ethernet and token ring LANs. The chances of bad data being received and not detected is 1 in 4.3 billion (2 32 -1)
K. Salah 15