Module 4.0: Data Link Layer

1
Module 4.0 Data Link Layer
Ch 10- 11

• The Logical Link Control (LLC) sublayer
• Framing
• Flow Control
• Error Control
• The Media Access Control (MAC) sublayer
• Random Access (CSMA)
• Token Passing

2
LLC Framing

• Framing is partitioning a bit stream into
  discrete units or blocks of data.
• 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 5th consecutive
  ones.

3
Ethernet/802.3 Frame

• Preamble is for synchronization
• Source and destination are 48 bits each and have
  the MAC address, e.g, 08002001D62A. The
  left 3 bytes are vendor specific, and the others
  are serial number of device assigned by Sun.
  00000C is CISCO.
• This MAC address is different than the IP address
  !!!

4
LLC Flow Control

• 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.
• Transmission time Time taken to emit all bits
  into medium
• Propagation time Time for a bit to traverse the
  link
• Two Protocols
• Stop-and-Wait
• Sliding Window

5
Stop-and-Wait

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

6
Sliding Window

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

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8
Sliding Window Comments

• If the maximum window size is one, then we have
  stop-and-wait protocol.
• 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.

9
LLC Error Control

• 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

10
Stop-and-Wait ARQ
2. Lost Frames
1. Damaged Frames
3. Lost Ack

• 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.

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1. Damaged Frames
2. Lost Frames
3. Lost Ack

• 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.

12
Selective-Reject Sliding Window ARQ

• Uses ACK as in go-back-n algorithm.
• Only the specific damaged or lost frame is
  retransmitted. The frame is resent out of
  sequence when NAK is received.
• Better performance
• Retransmit only damaged or lost frames.
• 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

13
Cyclic Redundancy Check (CRC)

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

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CRC Example
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Standard Polynomials (Divisors)

• CRC-16 used for various file transfers
• CRC-CCITT serves as an international standard
• 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 (232-1)