CS601 - Data Communication - Lecture Handout 36

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Hamming Code

Redundancy Bits (r)

  • r must be able to indicate at least m+r+1 states
  • m+r+1 states must be discoverable by r bits
  • Therefore, 2r ≥ m+r+1
  • If m=7, r=4 as 24 ≥ 7+4+1

Redundancy Bits (r)

Hamming Code

  • Each r bit is the VRC bit for one combination of data bits
  • r1(r2) bit is calculated using all bit positions whose binary representation includes a 1 in the first(second) position, and so on

Hamming Code

Hamming Code 1

Hamming Code 2

Hamming Code 3

Data Link Layer

Introduction

  • Unless accurately received by a 2nd device, a signal TX over a wire is a waste of electricity
    • With TX alone, we can put a signal to the line, but we have
    • no way of controlling which of several devices attached to that line will receive it
    • no way of knowing if the intended receiver is ready and able to receive it
    • No way of keeping a second device from TX at the same time
  • Communication requires at least 2 devices working together:
    • Sender
    • Receiver
  • Even such a basic arrangement requires great deal of coordination
  • For Example, in Half Duplex TX, it is essential that only one device TX at a time
  • If both device TX, the signals will collide leaving nothing on the line but Noise
  • The coordination of half-duplex TX is a part of a procedure called Line Discipline, which is one of the functions included in the second layer of OSI Model, the data link layer
  • In addition to Line Discipline, the most important functions in the data link layer are Flow Control and Error Control
  • Collectively these functions are called Data Link Control

Data Link Layer & its Functions

Data Link Layer & its Functions

Data Link Control

Data Link Control

Line Discipline

  • Coordinates the link systems, which device can send and when it can send?

Flow Control:

  • The amount of data that can be sent before the receiving acknowledgement
  • It also provides the receiver’s acknowledgement for frames received intact and so is linked to error control

Error Control:

  • Means Error detection and Correction
  • It allows the receiver to inform the sender of any frames lost or damaged in TX and coordinates Retransmission of those frames by the sender

Line Discipline

  • How efficient the system is, no device in it should be allowed to transmit until that device has the evidence that the intended receiver is:
    • able to receive
    • is prepared to accept the TX
  • What if the Rx device does not expect a transmission or is busy
  • With no way of determining the status of the intended receiver, the transmitting device may waste its time sending data to a non-functioning receiver
  • The Line Discipline functions of the data link layer oversee:
    • the establishment of links and
    • The right of a particular device to transmit at a given time

Ways to do Line Discipline

Line Discipline is done in 2 ways:

  • Enquiry / Acknowledgement (ENQ/ACK)
    • Used in Peer-to-Peer Communication
  • Poll / Select
    • Primary-Secondary communication

Ways to do Line Discipline

Summary

  • Hamming Code
  • Data Link Control
  • Line Discipline
    • ENQ/ACK
    • POLL/SELECT
  • Flow Control

Reading Sections

  • Section 9.7, 10.1 “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan