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CS601 - Data Communication - Lecture Handout 32

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T Lines

T Lines for Analog Lines

  • T Lines are digital lines designed for digital data
  • However they can also be used for analog transmission (Telephone connections)
  • Analog signals are first sampled and the Time Multiplexed

T Lines for Analog Lines

T 1 Frame (Figure)

  • DS-1 requires 8 Kbps of overhead
  • To understand this overhead, lets examine format of a 24-voice channel frame
  • Frame used on T-1 line is usually 193 bits divided into 24 slots of 8 bits each + 1 bit for synchronization (24*8+1=193)
  • 24 segments are interleaved in one frame
  • If a T-1 carries 8000 frames, the data rate is 1.544 Mbps (193 * 8000=1.544 Mbps) which is capacity of the line

T 1 Frame (Figure)

Fractional T Lines(Figures)

  • Many subscribers do not need the entire capacity of the T line
  • For example, a small business may need only one-fourth of the capacity of a T-1 line
  • If four business of same size lie in same building, they can share a T-1 line
  • DSU/CSU allow the capacity of T-1 line to be interleaved into 4 channels

Fractional T Lines(Figures)

E-Lines

  • Europeans use a version of T-lines called E-lines
  • Two are conceptually identical but capacities vary

E-Lines

Fiber to the Curb (FTTC)

  • Optical fiber has many advantages:
    • Noise Resistance
    • High BW Capacity
  • Very Expensive
  • Telephone & Cable companies have devise FTTC to employ optical fiber while keeping the expense down
  • Optical Fiber is the medium from the Central Office of the telephone company or from main office of a cable company to the Curb
  • The medium from the Curb to the subscriber’s premises if the less expensive twisted pair or coaxial cable

FTTC in Telephone Network

  • Optical Fiber is used to connect and multiplex different voice channels
  • Copper twisted pair coming from individual premises is multiplexed in junction boxes and converted to optical signals. Optical signals are multiplexed using WDM at switching office to create wider BW optical

FTTC in Telephone Network

FTTC in Cable TV Network

  • Cable TV uses optical fibers to connect and multiplex different cable channels
  • Coaxial cables coming from individual premises are multiplexed in the junction box and converted to the optical signals
  • Optical signals are multiplexed using WDM at switching office to create wider BW optical signals

FTTC in Cable TV Network

ERROR DETECTION AND CORRECTION

Introduction

  • Networks must be able to transfer data from one device to another with complete accuracy
  • A system that cannot guarantee that the data received by one device is identical to the data transmitted by another device is essentially useless
  • Anytime the data is TX from source to destination, it gets corrupted on the way
  • Many factors including NOISE can alter or wipe out one or more bits of a given data unit
  • Reliable systems must have a mechanism for detecting and correcting such ERRORS
  • Error detection and Correction is implemented either at the data link layer or at the Transport layer

Types of Errors

Types of Errors

  • Whenever an EM signal flows from one point to the other, it is subject to unpredictable interference from heat, magnetism and other forms of electricity
  • This interference can change the shape or timing of the signal
  • If the signal is carrying encoded binary data, such changes can alter the meaning of the data

Summary

  • T-Lines in Analog Transmission
  • Fractional T-Lines
  • E-Lines
  • Fiber To The Curb (FTTC)
  • Error Detection and Correction

Reading Sections

  • Section 8.5, 8.7, 9.1, “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan