8/16/20021 Digital Transmission Key Learning Points Fundamentals of Voice Digitization Pulse Code Modulation Quantification Noise Multiplexed Digital Lines

8/16/ 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 Carriers leased digital circuit rates from kbps..100’sMbps - digital circuits must co-exist with other circuits for inter-change traffic

8/16/20023 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 = ms 0 T 2T 3T 4T 5T Amplitude

8/16/20024 (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 = levels - R = 8 bits * 8000Hz = 64kbps per voice channel - minimum unit of capacity available for lease

8/16/ bits 125 us sample clock PAM signal PCM signal Digital Voice Signal sampling circuit Quantizer & Compander sample clock PCM Signal PAM signal Analog Voice Signal

8/16/20026 q = quantization interval: signal (voltage) difference between adjacent discrete signal levels - accuracy determined by number of bits in signal: n bits  2 n 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 binary codeword analog voltage range 6..8 volts 4..6 volts 2..4 volts 0..2 volts 0..-2volts volts volts volts

8/16/20027 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  q V -V

8/16/20028 actual amplitude  = quantization error +q/2 0 -q/2 -  

8/16/20029 analog voltage discrete voltage  % maximum error binary codeword % % % % % % % %111 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

8/16/ 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 vovo vivi compressor expander A/D D/A Network continuous analog signal passed into compressor then into A/D expander reverses the operation performed at output of D/A Compression and Expansion

8/16/ 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

8/16/ 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 signalpolaritysegment code segment size quantam code q 7 < S  V < S  V < S  V  S  V  % 

8/16/ polarity segment level quantam level input range output range ‘+’ signal encoding similar for ‘-’signal

8/16/ polarity bit segment levels quantum levels V V

8/16/ 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

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

8/16/ 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 leaves 7 bits for data Frame synchronization: bit (framing bit) at start of ‘frame 1’ - toggles from 1,0 for consecutive frames slots 6,12: 1 signal bit, 7 data bits  56 kbps slots 7-11, 13-24: 8 data bits  64 kbps slot 23 slot 23 … slot 1 0 frame bit 125 us

8/16/ clock=8 KHz 64Kbps links digital links …. 1,0 193 bits 23 …. 1,0 synch bits slots DS1 or T1 Link

8/16/ E1 Link: (ITU-T) 30 voice channels at 64Kbps  Mbps two additional slots for signaling and control 32  (8/125us) = 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

8/16/ Fractional T1, E1: Lower Bit Rates on T1, E1 systems 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 Link Type64Kbps channels payload ratetotal rate including control data E3 = 16  E Mbps34.368Mbps T3 = 28  T Mbps Mbps

8/16/ clock=8 KHz 64Kbps links T1 = 24  64Kbps links + control T1 link T1 link T1 link 27 T3 = 28  T1 links + control clock=8 KHz T3 link justification bit

8/16/ 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 Public Carrier Network CSU/DSU router hub CSU/DSU router hub T3 lines