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Analog and Digital Transmission Interfaces and Multiplexing (Physical Layer)

Analog and Digital Transmission Interfaces and Multiplexing (Physical Layer). Lita Lidyawati 2012. Multiplexing. Multiplexing (“ muxing ”) allows multiple flows to share a channel , within the limits of the overall capacity. Multiplexing (‘cont).

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Analog and Digital Transmission Interfaces and Multiplexing (Physical Layer)

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  1. Analog and Digital Transmission Interfaces and Multiplexing (Physical Layer) Lita Lidyawati 2012

  2. Multiplexing • Multiplexing (“ muxing”) allows multiple flows to share achannel, within the limits of the overall capacity.

  3. Multiplexing (‘cont) • Frequency division (FDM) - analogous to radio spectrumwithin a cable; not a good environment for data due to noise from “baseband loading”. • Time division (TDM) - interleaves bits from slower datastreams onto a single, faster data stream.

  4. Multiplexing (‘cont)

  5. Multiplexing (‘cont)

  6. Multiplexing (‘cont)

  7. Converting Digital Information to AnalogInformation

  8. Modulation • Modulation means varying some property of a signalto impress information on the signal

  9. Amplitude Modulation • Assuming amplitude 1 = binary 0, and amplitude 2 = binary 1,this signal would represent 0011010

  10. Phase Modulation + =

  11. Quadrature Amplitude Modulation

  12. Quadrature Amplitude Modulation • first and second bit taken as a binary number are the multiple o f 90o • third bit indicates the amplitude

  13. Quadrature Amplitude ModulationExample • Let's encode a big bit stream: 001010100011101000011110 • We break it up into 3-bit triads:001-010-100-011-101-000-011-110

  14. Digital Transmission • The foregoing discussion assumes the signal is modulatedaccording to some continuous input that behaves in a wayanalogous to the information, for example, the output current from a microphone. • Such a sample can be represented as binary numbers, or a “digital” signal

  15. Encoding a Digital Signal • An encoder samples, or measures the amplitude of the incominganalog signal 8,000 times a second • The amplitude of each sample is given a pre-established 8-digitbinary code, which is determined by the height of the sample. • Each 8-digit binary code is transmitted behind the 8-digit binarycode of the previously encoded sample in the conversation,creating a signal of 64,000 b/s (8,000 samples a second at 8 bitsper sample.

  16. Encoding a Digital Signal

  17. Multiple Bits per Baud • QAM is an example of the way modern modems can pack a lotof information into a sample. • Depending on the quality of the analog channel, it is possibleto encode several bits into every sample taken form thechannel: multiple bits per baud • Given n levels of signal that can be discriminated in eachsample based on amplitude frequency or phase, the bit rate is:

  18. Multiple Bits per Baud • where C is the channel capacity as before and b is thesignallingrate (also called sampling rate or baud rate) • Shannon’s law defines the absolute limit for C

  19. Multiple Bits per Baud • Sample analog voice signal at the Nyquist rate = 2 fH (twicethe highest frequency if fL= 0), or2 X 4000 Hz = 8000 samples per second • Convert each sample to an 8-bit binary number (called quantizing) using Pulse Code Modulation (PCM) • Send this digital data as 8 (bit samples) X 8000 (samples per second), or 64,000 bps

  20. Digital Transmission of Voice • A group of 24 voice channels requires • 24 X 64 kbps = 1,536,000 bps • which can fit on a T1 carrier channel

  21. Digital Audio Fidelity • 8-bit PCM is very adequate for telephone use but is not “highfidelity” with regard to either noise or bandwidth. • When a digitally encoded signal is converted back to analog,there is an added “noise of quantization”: • thus for 8-bit coding S/N=216=65,536=48.2dB so the noise willbe no better than 48.2 dB below maximum possible signal level

  22. Digital Audio Fidelity • For CDROM quality fH = 20kHz and fL= 0; and the sample isencoded in 16 bits; thus

  23. Digital Pulse Codes • Purpose: Make efficient use of available bandwidth while avoiding errors (also may be designed to eliminate DC component, as required by some media) • Non-Return to Zero-L (NRZ-L): straight binary data • Manchester: 01 = 1; 10 = 0 (two baud per bit); guaranteesan equal number of ones and zeros; requires 2x bandwidth in medium • Bipolar alternate mark inversion (AMI): a pulse for each one; every pulse changes polarity

  24. Digital Pulse Codes

  25. Physical Interfaces • EIA-232-D (“ RS-232”) • Most common serial interface • If used for asynchronous transmission, the interface can work with as few as five wires. • Many more pins are defined

  26. Physical Interfaces • EIA-449 (“ RS-449”) • Higher data rate (up to 2 Mbps) • Balanced line capable • Common on 56/64 kbps and T1/E1 links • Variations include RS-422, V.35 • Built-in loopback capability

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