Elementary Data Link Protocols

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Elementary Data Link Protocols
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Elementary Data Link Protocols

• Assumptions
• Each layer (physical, data link and network) are
  independent of each other
• Machine A wants to send a long data stream to
  machine B using a reliable connection-oriented
  service.
• The data link layer encapsulates the packet in a
  frame by adding a data link header and trailer.

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Elementary Data Link Protocols

• Assumptions
• The software (and hardware) is dedicated full
  time to handling just one channel
• Checksums are computed and used to manage
  erroneous frames.
• A frame header and tail will never be given to
  the network layer

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Elementary Data Link Protocols

• Five data structures that describe a frame
  boolean, integer, packet, frame kind and frame
• A frame is composed of four fields (frame
  header) kind, seq, ack, and info
• Frame sequence numbers are always in the range of
  0 and a predefined maximum constant and are
  incremented in a circular manner

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Elementary Data Link Protocols

• Timers are used to manage lost or missing frames
  or acknowledgements
• Network layer can be disabled to control the
  number of packets to prevent the network layer
  from swamping it with packets for which it has no
  buffer space

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An Unrestricted Simplex Protocol

• UTOPIA
• Data transmitted in one direction only
• Transmitting and receiving layers are always
  ready
• Processing time is ignored
• Infinite buffer space
• Ideal communication channel - no loss, no errors

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A Simplex Stop-and-Wait Protocol

• Consider a slow receiver
• no buffer space
• slow processor
• but channel is still error free and traffic is
  still simplex
• Possible solutions
• introduction of delays
• send information very slowly

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A Simplex Stop-and-Wait Protocol

• Stop and wait
• Sender only sends out the succeeding packet after
  receiving an acknowledgement
• Data traffic is simplex
• Communication channel needs to be bidirectional
• Communication channel can be half-duplex

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A Simplex Protocol for a Noisy Channel

• Noisy Channel
• Lost or damaged frames
• Damage must be detectable through checksums
• Possible solution
• Receiver only acknowledges when there is an
  undamaged frame received.

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A Simplex Protocol for a Noisy Channel

• What happens if the acknowledgement frame gets
  lost?
• The protocol would keep passing wrong information
  up to the network protocol
• The proper solution requires the receiver to
  distinguish a frame that it is seeing for the
  first time from a retransmission
• Use sequence numbers

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A Simplex Protocol for a Noisy Channel

• What is the minimum number of bits needed for the
  sequence number?
• to ensure that the frame header is also small
• 1-bit sequence number is sufficient
• Protocols in which the sender waits for a
  positive acknowledgement before advancing to the
  next data item
• PAR (Positive Acknowledgement with
  Retransmission)
• ARQ (Automatic Repeat reQuest

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A Simplex Protocol for a Noisy Channel

• Timeout interval must be long enough to prevent
  premature timeouts
• It cannot be set too short
• allow time for the frame to reach the receiver,
  to get processed and to receive an acknowledgement

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A Simplex Protocol for a Noisy Channel

• Sender can wait for three things
• an acknowledgement frame arrives undamaged
• an acknowledgement frame arrives damaged
• timer goes off
• Valid frames are passed on to the network layer
• Duplicate frames and damaged frames are not
  passed on to the network layer.

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Sliding Window Protocols

• Bidirectional traffic
• Interleaving of data and acknowledgements
• Piggybacking the technique of temporarily
  delaying outgoing acknowledgements so that they
  can be hooked onto the next outgoing data frame

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Sliding Window Protocols

• Advantage of piggybacking
• use of less bits to transmit an acknowledgement,
• fewer interrupts for the acknowledgement receiver
• fewer required buffer space in the receiver
• Complication How long to timeout?

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Sliding Window Protocols

• Characteristics of Sliding Window Protocols
• sequence numbers (with a maximum and minimum)
• sending window
• receiving window

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Sliding Window Protocols

• Maintain the requirement of a wire-like
  protocol
• Sender window - frames sent but not yet
  acknowledged
• Sender must have n buffers if it has a windows
  size of n, in order to retransmit frames upon
  timeout or error

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Protocol Using Go Back n

• What happens when there is a long roundtrip time?
• Pipelining At all times n unacknowledged frames
  are outstanding the senders maximum window size
  is n.

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Protocol Using Go Back n

• Go back n strategy - discard all subsequent
  frames after an error

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Protocol Using Go Back n

• Selective Repeat - store all correct frames
  following a bad one

• Tradeoff between bandwidth and data link layer
  buffer space

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Protocol Using Selective Repeat

• How to simulate a sequential receive given a
  non-sequential receive?

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Protocol Using Selective Repeat

• Making sure that the windows do not overlap
• What is the required buffer size?
• What is the required number of timers?