CH. 7 Data Link Control Protocols

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CH. 7 Data Link Control Protocols

• Data and Computer Communications by Stallings

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Requirements Objectives of Data Link Control

• Frame Synchronization
• Flow Control
• Error Control
• Addressing
• Control and Data on Same Link
• Link Management

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7.1 Flow Control

• Definition
• A technique for assuring that a transmitting
  station does not overwhelm a receiving station
  with data.
• Stop-and Wait Flow Control
• The sender sends a frame and then waits until the
  receiver acknowledges the frame.
• Works well for large blocks of data--but large
  blocks are usually broken into smaller blocks.
• Does not always perform well for small blocks.

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7.1 Flow Control (p.2)

• Characterization of Stop-and-Wait
• Let B length of the link in bits--the number of
  bits present on the link when a stream of bits
  fully occupies the link.
• Let R data rate of the link, in bps.
• Let d length, or distance, of the link in
  meters.
• Let V velocity of propagation, in
  meters/second.
• Then B R x d/V (Equation 7.1).
• Variable a B/L (Equation 7.2)
• Utilization--Fig. 7.2

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7.1 Flow Control (p.3)

• Example 7.1 Stop and Wait
• a. Fiber Optic Link (d 200 meters R 1 G bps
  V 2 x 108 meters/second 1,000 bytes/frame)
• B 1,000 bits L 8,000 bits tframe 8 ?s.
• a 1,000/8,000 .125 lt 1 (see Fig. 7.2b)
• Total Normalized Time--1 2a 1.25 (10?s)
• b. Satellite Relay Link ( d 2 x 36,000 km R
  1 M bps V 3 x 108 meters/second 1,000
  bytes/frame.)
• B 240,000 bits L 8,000 bits tframe 8ms.
• a 30 gt 1 (see Fig. 7.2a)
• Total Normalized Time--1 2a 61 (488 ms).

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7.1 Flow Control (p.4)

• Sliding Window Flow Control (Fig.7.3 and 7.4)
• Allows more than one frame to be sent at a time.
• More than one frame may be acknowledged at a
  time.
• Source A keeps a list of sequence numbers that
  are allowed to be sent.
• Destination B maintains a list of sequence
  numbers that it is prepared to receive.
• If k bit sequence numbers are used, frames are
  numbered modulo(2k) and maximum window size is 2k
  -1.

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7.1 Flow Control (p.5)

• Example 7.3 Sliding Window
• a. Fiber Optic Link
• Time until ACK for 1st frame was 10??s.
• tframe was 8?s.
• A. window size of 2 would be all that is needed
  for transmission to be continuous.
• Note W ?Time until ACK/tframe? will be the
  maximum window needed for continuous
  transmission.
• b. Satellite Link
• W ?488ms/8ms?61 for continuous transmission.
• If W 7 (3-bit window), sends 7 frames and must
  then wait for an ACK.

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7.2 Error Control

• Two Types of Errors lost and damaged frames.
• Elements of Error Control automatic repeat
  request (ARQ)
• Error Detection use a CRC
• Positive ACK
• Destination returns a positive ACK for error-free
  frames.
• Retransmission after Timeout
• Source retransmits a frame that has not been
  acknowledged after a predetermined amount of
  time.
• Negative ACK and Retransmission
• Destination returns a negative ACK for frames in
  which an error is detected the source
  retransmits the frames.

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7.3 Error Control (p.2)

• Stop-and Wait ARQ (Fig. 7.5)
• Based on stop-and-wait flow control.
• Source transmits a single frame and then must
  wait for an ACK.
• If an error is detected a NAK could be sent.
• If there is no response, then source times-out
  and resends the message.
• To avoid duplications, 1 bit sequence number
  could be added to frame and or ACKs.
• It is simple but sometimes inefficient.

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7.3 Error Control (p.3)

• Go-back-N ARQ (Fig. 7.6a)
• Source transmits multiple frames (sliding window)
  and then waits for ACK (RR).
• Destination sends ACKs up to the last correct
  frame received.
• One ACK can acknowledge several frames.
• The error frame is retransmitted along with all
  subsequent frames.
• Selective-reject ARQ (Fig. 7.6b)
• Only error frames are retransmitted.

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7.3 High Level Data Link Control (HDLC)

• ISO 3309, ISO 4335.
• Three Types of Stations
• Primary--has the responsibility for controlling
  the operation of the link.
• Secondary--controlled by primary station.
• Combination--combines features of primary and
  secondary

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7.3 HDLC (p.2)

• Two Link Configurations
• Unbalanced (Primaries and Secondaries)
• Used in point to point and multipoint operation.
• Balanced (Combinations)
• Used only in point to point operation.

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7.3 HDLC (p.3)

• Three Data Transfer Modes
• Normal response mode (NRM) unbalanced
  configuration poll and selection (used on
  multidrop lines).
• Asynchronous balanced mode (ABM) balanced
  configuration either combined station may
  initiate transmission (used on pt-to-pt lines).
• Asynchronous response mode (ARM) unbalanced
  configuration secondary may initiate
  transmission( rarely used).

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7.3 HDLC (p.4)

• Frame Structure(Fig. 7.7)
• Flag Field (8 bits)
• 01111110 (Ox7E)
• Transparency is handled using bit- stuffing.
• Transmitter will stuff a 0 after 5 1's--see Fig.
  7.8.
• Address Field (One or more Octets)
• Used to identify secondary stations

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7.4 HDLC (p.5)

• Frame Structure (cont.)
• Control Field (8 or 16 bits)
• Information frames carry data and ACKs.
• Supervisory frames ACKs.
• Unnumbered frames supplemental control.
• Information Field (variable number of bits)
• Frame Check Sequence Field (16 or 32 bits)

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7.4 HDLC (p.6)

• HDLC Operations (Fig. 7.9)
• Initialization, Data Transfer, Disconnect.
• Table 7.1 Commands and Responses
• Reject supports "Go-Back-N".
• Selective Reject supports "Selective Reject".
• Receiver Not Ready, Receiver Ready
• Used as ACKs and for flow control.
• Mode setting commands.
• Information transfer commands.
• Recovery commands.
• Error types can be reported.