CIS 267 Data Link Control Lecture Chapter 9

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CIS 267 Data Link Control Lecture Chapter 9

• Vaughn L. Lucas

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Chapter 9 Overview

• Topics covered
• Data Link Control Protocol
• A set of techniques that, collectively, is
  embodied in a control mechanism
• Coordinates the presence of more than two devices
  on a line
• Provides for error control
• HDLC (high-level data link control)

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Flow Control

• Necessary when data is being sent faster than it
  can be processed by receiver
• Computer to printer is typical setting
• Can be from computer to computer when a
  processing program is limited in capacity
• Usually a buffer is filled, and transfer is
  stopped until buffer is emptied

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Stop-and-Wait Flow Control

• Simplest form of Frame-Oriented Control
• Source may not send new frame until receiver
  acknowledges previous one
• Very inefficient, especially when a single
  message is broken into many small frames
• buffer size of receiver is limited
• if error, detected sooner and less data need be
  retransmitted

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Stop-and-Wait Example

• 40 km link, 10 Mbps rate, 1000 bit frame
• Signal in copper propagates at 2 x 108 m/sec
• Takes 0.1 msec to transmit frame
• 0.2 msec delay to begin arriving
• See Figure 9-1
• Total 0.3 msec to receive, 0.2 msec to ACK
• Line busy for 0.5 msec for 0.1 msec frame
• 20 efficiency

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Sliding-Window Flow Control

• Allows multiple frames to be in transit
• Receiver sends acknowledgement with sequence
  number of anticipated frame
• Sender maintains list of sequence numbers it can
  send, receiver maintains list of sequence numbers
  it can receive
• ACK (acknowledgement) supplemented with RNR
  (receiver not ready)

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Sliding Window Sequence

• Sequence number included in frame
• 3-bit number allows 8 values
• See Figure 9-2
• http//www.humboldt.edu/aeb3/telecom/SlidingWindo
  w.html
• Windows not necessarily in synch if many frames
  in transit
• See Figure 9-3

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Error Control Process

• All transmission media have potential for
  introduction of errors
• All data link layer protocols must provide method
  for controlling errors
• Error control process has two components
• Error detection
• Error correction

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Error Detection Parity Bits

• Bit added to each character to make all bits add
  up to an even number (even parity) or odd number
  (odd parity)
• Good for detecting single-bit errors only
• High overhead (one extra bit per 7-bit
  character12.5)

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Error Detection CRC

• Cyclic Redundancy Check (CRC)
• Data in frame treated as a single binary number,
  divided by a unique prime binary, and remainder
  is attached to frame
• 17-bit divisor leaves 16-bit remainder, 33-bit
  divisor leaves 32-bit remainder
• For a CRC of length N, errors undetected are 2-N
• Overhead is low (1-3)

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Error Correction

• Two types of errors
• Lost frame - never arrives
• Damaged frame - error in bits
• Automatic Repeat reQuest (ARQ)
• Error detection
• Positive acknowledgment if received
• Retransmission after time-out if not ACK
• Negative ACK and retransmission if error

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Stop-and-Wait ARQ

• One frame received and handled at a time
• If frame is damaged, receiver discards it and
  sends no acknowledgment
• Sender uses timer to determine whether or not to
  retransmit
• Sender must keep a copy of transmitted frame
  until acknowledgment is received
• If acknowledgment is damaged, sender will know it
  because of numbering

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Go-Back-N ARQ

• Uses sliding-window flow control
• When receiver detects error, it sends negative
  acknowledgment (REJ)
• Sender must begin transmitting again from
  rejected frame
• Transmitter must keep a copy of all transmitted
  frames

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Data Link Control

• Specified flow and error control for synchronous
  communication
• Data link module arranges data into frames,
  supplemented by control bits
• Receiver checks control bits, if data is intact,
  it strips them

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High-Level Data Link Control

• On transmitting side, HDLC receives data from an
  application, and delivers it to the receiver on
  the other side of the link
• On the receiving side, HDLC accepts the data and
  delivers it to the higher level application layer
• Both modules exchange control information,
  encoded into a frame

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HDLC Frame Structure

• Flag 01111110, at start and end
• Address secondary station (for multidrop
  configurations)
• Information the data to be transmitted
• Frame check sequence 16- or 32-bit CRC

• Control purpose or function of frame
• Information frames contain user data
• Supervisory frames flow/error control (ACK/ARQ)
• Unnumbered frames variety of control functions
  (see p.220)

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HDLC Frame Structure
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HDLC Operation

• Initialization S-frames specify mode and
  sequence numbers, U-frames acknowledge
• Data Transfer I-frames exchange user data,
  S-frames acknowledge and provide flow/error
  control
• Disconnect U-frames initiate and acknowledge

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Protocols Preview

• OSI Seven-Layer Model
• TCP/IP Layers
• IBMs System Network Architecture
• While OSI model is increasingly out of favor in
  application development, it is still very useful
  in understanding networking in a conceptual
  context

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ISOs OSIOpen Systems Interconnection

• Application Layer
• Presentation Layer
• Session Layer
• Transport Layer
• Network Layer
• Data Link Layer
• Physical Layer

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Physical Layer

• Refers to transmission of unstructured bits over
  physical medium
• Deals with characteristics of and access to the
  physical medium

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Data Link Layer

• Provides for reliable transfer of information
  across physical link
• Includes
• transmission of blocks of data (frames)
• synchronization
• error control
• flow control