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Physical Layer Issues and Methods

CS 640. 2. Physical Layer Data Transfer. Signals are placed on wire via transceiversProblem is how to do transmit 0's and 1's (signal encoding) in a robust fashionBinary voltage encodingMap 1 to high voltageMap 0 to low voltageHow are consecutive 0's or 1's detected at node?Clock synchronization problemTransmitted signals have a variety of problemsAttenuationNoise Dispersion.

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Physical Layer Issues and Methods

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    1. Physical Layer Issues and Methods Outline Physical Layer Overview Non-Return to Zero Manchester 4B/5B

    2. CS 640 2 Physical Layer Data Transfer Signals are placed on wire via transceivers Problem is how to do transmit 0s and 1s (signal encoding) in a robust fashion Binary voltage encoding Map 1 to high voltage Map 0 to low voltage How are consecutive 0s or 1s detected at node? Clock synchronization problem Transmitted signals have a variety of problems Attenuation Noise Dispersion

    3. CS 640 3 Encoding Taxonomy Digital data, digital signal Codes which represent bits Our focus Many options! Analog data, digital signal Sampling to represent voltages Digital data, analog signal Modulation to represent bits Analog data, analog signal Modulation to represent voltages

    4. CS 640 4 Encoding Requirements Small bandwidth Enables more efficient use of signaling capability Low DC level Increases transmission distance Frequent changes in the voltage Enables synchronization between the transmitter and the receiver without the addition of extra signal Non-polarized signal Enables use of 2-wire cable to not be affected by the physical connection of the wires.

    5. CS 640 5 Non-Return to Zero (NRZ) High voltage = 1 and low voltage = 0 Voltage does not return to 0 between bits Receiver keeps average of signal seen to distinguish 0 from 1

    6. CS 640 6 NRZ Benefits Easy to engineer most basic encoding Efficient use of bandwidth not many transitions Drawbacks Long strings of 0s can be confused with no signal Long strings of 1s can cause signal average to wander Clock synchronization can be poor High DC average of V

    7. CS 640 7 NRZ-Inverted (NRZI) NRZI addresses clock synchronization problem Encodes 1 by transitioning from current signal Encodes 0 by staying at current signal So were still out of luck on consecutive strings of 0s

    8. CS 640 8 Manchester Data Encoding Explicit merging of clock and bit stream Each bit contains a transition High-low = 1 Low-high = 0 Enables effective clock signal recovery at receiver Clocks are still needed to differentiate between bit boundaries Poor bandwidth utilization Effective sending rate is cut in half Used by 802.3 10Mbps Ethernet

    9. CS 640 9 Manchester Encoding contd.

    10. CS 640 10 4B/5B Encoding Tries to address inefficiencies in Manchester Idea is to insert extra bits in bit stream to break up long sequences of 0s or 1s Every 4 bits of data are encoded in a 5 bit code Encodings selections At most one leading 0 At most two trailing 0s Never more than three consecutive 0s Uses NRZI to put bits on the wire This is why code is focused on zeros 80% efficiency See text for details of codes

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