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    Physical

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    Sureshkumar Alagarsamy
    This document summarizes various techniques for encoding digital data onto an analog transmission medium at the physical layer, including: 1) Non-Return to Zero (NRZ) encoding where 1=high signal and 0=low signal, but this can cause desynchronization during long runs of 1s or 0s. 2) Non-Return to Zero Inverted (NRZI) solves the problem for long runs of 1s but not 0s. 3) 4b/5b encoding which encodes every 4 bits as a 5-bit sequence to guarantee transitions and avoid long runs of 0s or 1s, at the cost of reduced efficiency. 4) Manchester encoding where each bit is represented

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    Physical

    Uploaded by

    Sureshkumar Alagarsamy
    16 views10 pages
    This document summarizes various techniques for encoding digital data onto an analog transmission medium at the physical layer, including: 1) Non-Return to Zero (NRZ) encoding where 1=high signal and 0=low signal, but this can cause desynchronization during long runs of 1s or 0s. 2) Non-Return to Zero Inverted (NRZI) solves the problem for long runs of 1s but not 0s. 3) 4b/5b encoding which encodes every 4 bits as a 5-bit sequence to guarantee transitions and avoid long runs of 0s or 1s, at the cost of reduced efficiency. 4) Manchester encoding where each bit is represented

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    This document summarizes various techniques for encoding digital data onto an analog transmission medium at the physical layer, including: 1) Non-Return to Zero (NRZ) encoding where 1=high signal and 0=low signal, but this can cause desynchronization during long runs of 1s or 0s. 2) Non-Return to Zero Inverted (NRZI) solves the problem for long runs of 1s but not 0s. 3) 4b/5b encoding which encodes every 4 bits as a 5-bit sequence to guarantee transitions and avoid long runs of 0s or 1s, at the cost of reduced efficiency. 4) Manchester encoding where each bit is represented

    Copyright:

    © All Rights Reserved
    Download as PPTX, PDF, TXT or read online from Scribd
    Download as pptx, pdf, or txt
    16 views10 pages

    Physical

    Uploaded by

    Sureshkumar Alagarsamy
    This document summarizes various techniques for encoding digital data onto an analog transmission medium at the physical layer, including: 1) Non-Return to Zero (NRZ) encoding where 1=high signal and 0=low signal, but this can cause desynchronization during long runs of 1s or 0s. 2) Non-Return to Zero Inverted (NRZI) solves the problem for long runs of 1s but not 0s. 3) 4b/5b encoding which encodes every 4 bits as a 5-bit sequence to guarantee transitions and avoid long runs of 0s or 1s, at the cost of reduced efficiency. 4) Manchester encoding where each bit is represented

    Copyright:

    © All Rights Reserved
    Download as PPTX, PDF, TXT or read online from Scribd
    Download as pptx, pdf, or txt
    of 10

    CSE390 Advanced

    Computer Networks
    Lecture 5: Physical Layer
    (The layer for EE majors…)

    Based on slides from D. Choffnes Northeastern U. Revised Fall


    2014 by P. Gill
    Physical Layer
    2

     Function:
     Get bits across a physical medium
    Application
     Key challenge:
    Presentation  How to represent bits in analog
    Session  Ideally, want high-bit rate
    Transport  But, must avoid desynchronization
    Network
    Data Link
    Physical
    Key challenge
    3

     Digital computers
     0s and 1s
     Analog world
     Amplitudes and frequencies
    Assumptions
    4

     We have two discrete signals, high and low, to encode 1 and 0


     Transmission is synchronous, i.e. there is a clock that controls signal
    sampling
    Sample

    Time
     Amplitude and duration of signal must be significant
    Non-Return to Zero (NRZ)
    5

     1  high signal, 0  low signal


    0 0 1 0 1 0 1 1 0 0

    NRZ

    Clock

     Problem: long strings of 0 or 1 cause desynchronization


     How to distinguish lots of 0s from no signal?
     How to recover the clock during lots of 1s?
    Desynchronization
    6

     Problem: how to recover the clock during sequences of


    0’s or 1’s?
    0 1 1 1 1 1 1 1 1 0

    NRZ

    0 1 1 1 1 1 1 1 0

    Transitions Receiver
    signify clock misses a 1
    ticks due to skew
    Non-Return to Zero Inverted (NRZI)
    7

     1  make transition, 0  remain the same


    0 0 1 0 1 0 1 1 0 0

    NRZI

    Clock

     Solves the problem for sequences of 1s, but not 0s


    4-bit/5-bit (100 Mbps Ethernet)
    8

     Observation: NRZI works as long as no sequences of 0


     8-bit
    Idea: / 10-bit
    encode usedsequences
    all 4-bit in Gigabit
    as Ethernet
    5-bit sequences with no
    more than one leading 0 and two trailing 0
    4-bit 5-bit 4-bit 5-bit
    0000 11110 1000 10010
    0001 01001 1001 10011
    0010 10100 1010 10110
    0011 10101 1011 10111
    0100 01010 1100 11010
    0101 01011 1101 11011
    0110 01110 1110 11100
    0111 01111 1111 11101
     Tradeoff: efficiency drops to 80%
    Manchester
    9

     1  high-to-low, 0  low-to-high
    0 0 1 1 0

    NRZI

    Clock

     Good: Solves clock skew (every bit is a transition)


     Bad: Halves throughput (two clock cycles per bit)
    General comment
    10

     Physical layer is the lowest, so…


     We tend not to worry about where to place functionality
     There aren’t other layers that could interfere
     We tend to care about it only when things go wrong
     http://blog.level3.com/level-3-network/the-10-most-bizarre-and-
    annoying-causes-of-fiber-cuts/
     Physical layer characteristics are still fundamentally
    important to building reliable Internet systems
     Insulated media vs wireless
     Packet vs. circuit switched media

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