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Digital Transmission Key Learning Points Fundamentals of Voice Digitization Pulse Code Modulation Quantification Nois

Digital Transmission Key Learning Points Fundamentals of Voice Digitization Pulse Code Modulation Quantification Noise Multiplexed Digital Lines. 2.5.2 Digital Leased Circuits ( from public carriers) - supports high level of inter-site traffic, generally more

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Digital Transmission Key Learning Points Fundamentals of Voice Digitization Pulse Code Modulation Quantification Nois

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  1. Digital Transmission • Key Learning Points • Fundamentals of Voice Digitization • Pulse Code Modulation • Quantification Noise • Multiplexed Digital Lines

  2. 2.5.2 Digital Leased Circuits (from public carriers) • - supports high level of inter-site traffic, generally more • expensive than modem based service • - provides direct digital connection between DTE’s • - basis of most private data & voice networks •  goal: understand organization & capacity of digital networks • Digital switching & transmission for voice & data used in most • public carrier networks • Eliminates Need for Modem – Voice data must be ‘digitized’ • ISDN: Network that allows Transmission of voice & data • Public Carriersleased digital circuitrates from kbps..100’sMbps • digital circuits must co-exist with other circuits for inter-change • traffic

  3. Amplitude 0 T 2T 3T 4T 5T • Voice Digitization • Voice signals are inherently analog • Spectral Content of Voice  4000Hz (except ) • Requires Analog To Digital Signal Conversion (ADC) • Nyquist Sampling Theorem: • Must Sample Twice Highest Frequency Component • - Sample Rate for Analog Voice Signal = 8000Hz • - Sampling Interval for Voice Signal = 0.125 ms

  4. (1) Pulse Amplitude Modulation (PAM): • Analog Voice Signal Sampled  converted to pulse stream • Pulse Amplitude: discrete analog signal, amplitude = • continuous analog signal • (2) Pulse Code Modulation(PCM): Quantize each Pulse into • Binary Form • - 8 bits used to quantize pulse  range = 0..256 levels • - R = 8 bits * 8000Hz = 64kbpsper voice channel • - minimum unit of capacity available for lease

  5. PAM signal PCM Signal sample clock Analog Voice Signal Digital Voice Signal Quantizer & Compander sampling circuit sample clock PAM signal PCM signal 10001010 10111010 125 us 8 bits

  6. analog voltage range binary codeword 6..8 volts 4..6 volts 2..4 volts 0..2 volts 0..-2volts -2..-4 volts -4..-6 volts -6..-8volts 0 11 0 10 0 01 0 00 1 00 1 01 1 10 1 11 • q = quantization interval:signal (voltage) difference between • adjacent discrete signal levels • - accuracy determined by number of bits in signal: n bits 2n • levels • - signals within a level are represented by same binary codeword • - one bit may be used for signal polarity (+ or - ) e.g.n = 3  8 levels range  8volts (16 volts) interval q = 16/8 = 2 volts

  7. V -V 0 11 0 10 0 01 0 00 1 00 1 01 1 10 1 11  q • Each codeword corresponds to nominal input voltage centered at q • actual input may differ by q/2 • quantization error, , actual signal amplitude – quantized signal • amplitude • quantization noise: random quantization error variance between • samples

  8. +q/2 0 -q/2 -  • actual amplitude •  = quantization error 

  9. e.g. analog voltage range =  8 volts, n = 3 • 8 signal levels  q = 2 volts • smaller amplitudes more sensitive to  • ear is sensitive to noise on quiet, low amplitude speech signals

  10. Compression and Expansion • continuous analog signal passed into compressor then into A/D • expander reverses the operation performed at output of D/A vi vo compressor expander Network D/A A/D • Practically non-linear PCM used to overcome quantization noise • 2 level digitization: segment level and quantization level • range of input signal amplitudes associated with each quantization • interval • input signal amplitude increases  corresponding code words • represent larger signal range

  11. At transmitter: analog voice non-linearly encoded into binary data • 1. compressor stage: analog input signal compressed • - encoded value depends on segment level • 2.ADC stage:compressed analog signal is digitized &linearly • quantized At receiver codewords converted to analog voice signal 1. DAC stage: compressed digital signal is linearly converted to analog signal 2. expander stage:analog output passed through expander – reverses compressor operation

  12. e.g. Let Signal Range ± 30 volts and n = 5 bits • 32 total levels divided into • 1 polarity level • 2 segment levels • 2 quantum levels

  13. ‘+’ signal encoding • similar for ‘-’signal

  14. segment levels 11 polarity bit 10 1 01 00 -16 -8 -4 -2 -V 2 4 8 16 V 0 quantum levels 00 01 10 11

  15. PCM codecs (coder/decoder) • older codecs operated as above • newer codecs use 2 digital compression/expansion techniques • u-law: (N. America, Japan) • A-law: (ITU-T) • - similar in principal to companding-expansion • - conversion needed when using leased & switched circuits that • span continents • - necessary only for voice

  16. Multiplexing (MUX) • Link Exchange Circuits: T1, T3, E1… • - carry multiple calls concurrently • - TDM Used: multiple digital signals assigned time slices • voice data: 8 bit sample @ 125us = 64kbps/ voice channel • control overhead: • (i) start of frame (frame synchronization) • (ii) call set-up (signaling)

  17. slot 23 slot 23 … slot 1 slot 0 frame bit 125 us • slots 6,12: 1 signal bit, 7 data bits  56 kbps • slots 7-11, 13-24: 8 data bits  64 kbps • DS1 or T1 Links: • 24 voice channels grouped  1.536Mbps (North America) • (1 frame/125us  24 slots) = 192 bits/125 us • 192 bits + 1 framing bit = 193 bits/125 us  1.544Mbps • Signaling Info: carried in 1st bit of time slots 6-12 • - leaves 7 bits for data • Frame synchronization: bit (framing bit) at start of ‘frame 1’ • - toggles from 1,0 for consecutive frames

  18. 64Kbps links digital links slots 0 1 .. 23 23 …. 1,0 23 …. 1,0 193 bits clock=8 KHz synch bits DS1 or T1 Link

  19. E1 Link: (ITU-T) • 30 voice channels at 64Kbps  1.920 Mbps • two additional slots for signaling and control • 32  (8/125us) = 2.048 Mbps • Signaling info: carried in time slot 16 • Frame synchronization: time slot 0 • - used for frame alignment • - allows receiver to interpret time slots in each frame on aligned • boundaries

  20. Higher Aggregate Link Rates: MUX several groups (DSxx, Ey) • Higher order mux circuits: known as • - plesiochronous (nearly synchronous) • - asynchronous • PDH: (plesiochronous digital hierarchy) results in higher-order • mux rates • higher bit rate links require additional bits for framing & control Fractional T1, E1: Lower Bit Rates on T1, E1 systems

  21. T1 = 2464Kbps links + control 64Kbps links T1 link 0 0 1 .. 23 clock=8 KHz 0 1 .. 23 T1 link 1 clock=8 KHz T3 link justification bit 0 1 .. 23 T3 = 28T1 links + control T1 link 27

  22. T3 lines hub hub Public Carrier Network CSU/DSU CSU/DSU CSU/DSU CSU/DSU router router Leased Line Interconnection • channel service unit (CSU): • electrical barrier • keep alive signal • loopback test • data service unit (DSU) translate data format between entities • T1 uses TDM DSX frames for Data • LAN serial data frame format (e.g. ethernet) • physical connector to LAN

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