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Data Link Layer

Protocols

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  • Protocols for Noiseless Channel
  • Protocols for Noisy Channel
    • Stop-and-Wait ARQ
    • Go-Back-N ARQ
    • Selective Repeat ARQ

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What you’ll learn

Data link layer: Design issues, Framing: fixed size framing, variable size framing, flow control, error control, error detection and correction codes, CRC, Checksum: idea, one’s complement internet checksum, services provided to Network Layer, Elementary Data Link Layer protocols: simplex protocol, Simplex stop and wait, Simplex protocol for Noisy Channel. Sliding window protocol: One bit, Go back N, Selective repeat-Stop and wait protocol, Data link layer in HDLC, Point to point protocol (PPP).

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Unit-2

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Data Link Protocol

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Is a set of specifications used to implement the data link layer. Data link protocols differ by message delineation, frame length, and frame field structure.

  • Asynchronous transmission
    • Each character is sent independently
  • Synchronous transmission
    • Data is sent in a large block called frame

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Asynchronous Protocols

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It is sometimes called as start-stop transmission.

  • The transmission sequence begins with a start bit next the character is sent then the parity bit and finally a stop bit are sent.
    • The start bit is usually a 0 and the stop bit a 1.
  • Between transmissions (idle time), a series of stop bits are sent.
  • When a new character is sent, the start bit is used by the receiver for synchronization.
  • Asynchronous protocols
      • XMODEM, YMODEM, ZMODEM, BLAST, Kermit

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Synchronous Protocols

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  • Used on both point-to-point and multipoint circuits
    • In multipoint circuits, addressing information needs to be included in the frame.
  • Synchronous packets sometimes begin and end with a series of synchronization (SYN) characters that are used to help the receiver recognize incoming data.
  • Synchronous transmission protocols can be
    • Character-oriented (Byte-oriented): PPP
      • Interpret a transmission frame as a succession of characters
    • Bit-oriented: HDLC
      • Interpret a transmission frame as a succession of bits

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Bit-oriented Protocols

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  • Interpret a transmission frame as a succession of individual bits
  • Control information in a bit-oriented protocol can be one or multiple bits depending on the information embodied in the pattern

Bit-oriented Protocols

LANs

SDLC

HDLC

LAPs

Local Area Networks

Link Access Procedures

High-level Data Link Control

Synchronous Data Link Control

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Data Link Layer Protocols

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For Noiseless Channel

Simplest/ Unrestricted Simplex

Stop-and Wait

For Noisy Channel

Go-Back-N ARQ

Stop-and Wait ARQ

Selective Repeat ARQ

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Unrestricted Simplex Protocol

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An ideal channel in which no frames are lost, duplicated, or corrupted.

  • Data transmission in one direction only
  • Infinite buffer space
  • Processing time can be ignored
  • Both network layers are always ready

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Simplex Stop-and-Wait Protocol

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  • No infinite buffer space
  • Waiting for their respective turns
  • Communication channel is error free

Stop-and-Wait Protocol adds flow control to the previous protocol.

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However noiseless channels are nonexistent.

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Simplex Protocol for Noisy Channel

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  • Sender sends a frame, if data is correct the receiver sends acknowledgement

Automatic Repeat reQuest (ARQ): If errors in a transmitted message are detected by the receiving device, it requests the sending device to retransmit the message together with any other messages that may have been affected.

Continuous RQ: If a frame is corrupted during transmission, another copy of the frame is sent.

  • To improve the utilization of the data link, frames are transmitted continuously;
    • Retransmission requests for corrupted frames may be received after a number of other frames have been transmitted.

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Stop-and-Wait ARQ

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In Stop and Wait ARQ, at any point in time for a sender, there is only one frame, the outstanding frame, that is sent and waiting to be acknowledged

  • The receiver sends only positive acknowledgements.
    • The acknowledgment number always announces in modulo-2 arithmetic the sequence number of the next frame expected.
  • Numbering frames prevents the retaining of duplicate frames
  • There is no pipelining (A task begins before the previous task ends) in Stop and Wait ARQ.
    • Pipelining does apply to Go back N ARQ and Selective Repeat ARQ.

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Sliding Window Protocol

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Stop-and-Wait ARQ

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  • Sender keeps a copy of the frame until it receives an ACK.
  • Damaged or lost frames are treated as same by the receiver.
  • Sender starts a timer when it sends a frame. If an ACK is not received within an allotted time period, it resends the frame.

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Stop and Wait ARQ

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Stop-and-Wait ARQ

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Stop-and-Wait ARQ

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Stop-and-Wait ARQ

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Stop-and-Wait ARQ

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Stop and Wait Protocol

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FSM for the Stop and Wait Protocol

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Recap

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Video Links

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