PCM & PSTN Professor A. Manikas Imperial College London EE303 - Communication Systems Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 1 / 64
Table of Contents 1 Introduction 2 PCM: Bandwidth & Bandwidth Expansion Factor 3 The Quantization Process (output point-a2) Uniform Quantizers Comments on Uniform Quantiser Non-Uniform Quantizers max(snr) Non-Uniform Quantisers Companders (non-uniform Quantizers) Compression Rules (A and mu) The 6dB Law Di erential Quantizers 4 Noise E ects in a Binary PCM Threshold E ects in a Binary PCM Comments on Threshold E ects 5 CCITT Standards: Di erential PCM (DPCM) 6 Introduction to Telephone Network CCITT recommendations for PCM (24-channels and 30-channels) Single-Channel Path of 2nd CCITT rec. (30-channels PCM) Implementation of 2nd PCM CCITT Recomm Plesiochronous digital hierarchies (PDH) Synchronous digital hierarchies (SONET/SDH) Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 2 / 64
Introduction Introduction Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 3 / 64
Introduction PCM = sampled quantized values of an analogue signal are transmitted via a sequence of codewords. i.e. after sampling & quantization, a Source Encoder is used to map the quantized levels (i.e. o/p of quantizer) to codewords of γ bits i.e. quantized level 7! codeword of γ bits and, then a digital modulator is used to transmit the bits, i.e. PCM system There are three popular PCM source encoders (or, in other words, Quantization-levels Encoders). I I I Binary Coded Decimal (BCD) source encoder Folded BCD source encoder Gray Code (GC) source encoder Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 4 / 64
Introduction g(input) 7! g q (output) g q : occurs at a rate F s (N.B: F s # 2 F g ) Q = quantizer levels; γ = log 2 (Q) bits level samples sec Note: codeword rate (point B) " γ%bit codewords sec = quant. levels rate " levels sec = sampling rate " samples sec = F s = 2F g (1) Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 5 / 64
Introduction bit rate: r b = bits sec γ " bits level F s " levels sec e.g. for Q = 16 levels then r b = 4 " γ γ=4 # γ=4 # z} { z} { (e.g. transmitted sequ. = 10101100 {z} 1101...) " γ=4 F s versions of PCM: I I I I Di erential PCM (DPCM),PCM with di erential Quant. Delta Modulation (DM): PCM with di. quants having 2 levels i.e. + or % " are encoded using asinglebinarydigit Note: DM2DPCM Others Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 6 / 64
PCM: Bandwidth & Bandwidth Expansion Factor PCM: Bandwidth & Bandwidth Expansion Factor we transmit several digits for each quantizer s o/p level) B PCM > F g % BPCM denotes the channel bandwidth where represents the message bandwidth F g PCM Bandwidth baseband bandwidth: bandpass bandwidth: B PCM # B PCM # channel symbol rate 2 Hz (2) channel symbol rate 2 ) 2Hz (3) Note that, by default, the Lower bound of the baseband bandwidth is assumed and used in this course Bandwidth expansion factor β : β, channel bandwidth message bandwidth Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 7 / 64 (4)
PCM: Bandwidth & Bandwidth Expansion Factor Example - Binary PCM I Bandwidth: B PCM = channel symbol rate 2 = bit rate 2 = γf s 2 = γ " log 2 Q F g Hz ) B PCM = γf g (5) I Bandwidth Expansion Factor: B PCM = γf g ) B PCM F g = γ ) β = γ (6) Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 8 / 64
The Quantization Process (output point-a2) at point A2 : a signal discrete in amplitude and discrete in time. The blocks up to the point A2, combined, can be considered as a discrete information source where a discrete message at its output is a level selected from the output levels of the quantizer. Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 9 / 64
analogue samples 7! finite set of levels where the symbol 7! denotes a map In our case this mapping is called quantizing i.e. Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 10 / 64
quantizer parameters: 8 Q : number of levels b i : input levels of the quantizer, with i = 0, 1,..., Q >< (b 0 = lowest level): known as quantizer s end-points m i : outputs levels of the quantizer (sampled values after quantization) with i = 1,..., Q; known as output-levels >: rule: connects the input of the quantizer to m i RULE: the sampled values g(kt s ) of an analogue signal g(t) are converted to one of Q allowable output-levels m 1, m 2,..., m Q according to the rule: g(kt s ) 7! m i (or equivalently g q (kt s )=m i ) i b i%1 * g(kt s ) * b i with b 0 = %, b Q =+ Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 11 / 64
quantization noise at each sample instance: n q (kt s )=g q (kt s ) % g s (kt s ) (7) If the power of the quantization noise is small, i.e. P nq = E * nq(kt 2 s ) + = small, then the quantized signal (i.e. signal at the output of the quantizer) is a good approximation of the original signal. quality of approximation may be improved by the careful choice of b i s and m i s and such as a measure of performance is optimized. e.g. measure of performance: Signal to quantization Noise power Ratio (SNR q ) signal power SNR q = quant. noise power = P g P nq Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 12 / 64
8 >< Types of quantization: >: Transfer Function: uniform quantizer uniform non-uniform % di erential = uniform, or non-uniform plus a di erential circuit non-uniform quantizer for signals with CF = small for signals with CF = large Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 13 / 64
The following figure illustrates the main characteristics of di erent types of quantizers Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 14 / 64
Uniform Quantizers Uniform Quantizers Uniform quantizers are appropriate for uncorrelated samples let us change our notation: g q (kt s ) to g q and g(kt s ) to g the range of the continuous random variable g is divided into Q intervals of equal length (value of g) 7! (midpoint of the quantizing interval in which the value of g falls) or equivalently m i = b i%1 + b i 2 for i = 1, 2,..., Q (8) Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 15 / 64
Uniform Quantizers step size : rule: = b Q % b 0 Q % bi = b 0 + i rule: g q = m i i b i%1 < g * b i where m i = b i%1+b i 2 for i = 1, 2,..., Q "End-points" of the quantiser "Output-levels" of the quantiser (9) (10) Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 16 / 64
Comments on Uniform Quantiser Comments on Uniform Quantiser Since, in general, Q = large ) P gq ' P g,e * g 2+ Furthermore, large Q implies that Fidelity of Quantizer = " g q ' q Q = 8 % 16 are just su cient for good intelligibility of speech; (but quantizing noise can be easily heard at the background) voice telephony: minimum 128 levels; (i.e. SNR q ' 42dB) N.B.: 128 levels ) 7-bits to represent each level ) transmission bandwidth = " Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 17 / 64
Comments on Uniform Quantiser ( Quantizer = UNIFORM if pdf of the input signal = UNIFORM then SNR q = Q 2 = 2 2γ (11) Quantisation Noise Power P nq : rms value of Quant. Noise: Quantization Noise Power: P nq = 2 12 rms value of Quant. Noise = fixed = (12) p 12 6= f {g} (13) ) if g(t) =small for extended period of time ) SNR q < the design value " this phenomenon is obvious if the signal waveform has a large CREST FACTOR (14) Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 18 / 64
Comments on Uniform Quantiser SNRq as a function of the Crest Factor Remember: CREST FACTOR! peak rms (15) By using variable spacing {z } " small spacing near 0 and large spacing at the extremes ) CREST FACTOR e ects = # I =)this leads to NON-UNIFORM QUANTIZERS Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 19 / 64
Non-Uniform Quantizers Non-Uniform Quantizers Non-Uniform quantizers are (like unif. quants) appropriate for uncorrelated samples step size = variable = i if pdf i/p 6= uniform then non-uniform quants yield higher SNR q than uniform quants rms value of n q is not constant but depends on the sampled value g(kt s ) of g(t) Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 20 / 64
Non-Uniform Quantizers rule: g q = m i i b i%1 < g * b i where b 0 = %, b Q =+ i = b i % b i%1 = variable example: "End-points" of the quantiser "Output levels" of the quantiser Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 21 / 64
max(snr) Non-Uniform Quantisers max(snr) Non-Uniform Quantisers b i, m i are chosen to maximize SNR q as follows: I I since Q = large ) P gq ' P g,e * g 2+ ) SNR q = max if P nq = min where Q Z bi P nq = (g % m i ) 2 pdf g dg (16) i =1 b i%1 Therefore: min P nq m i,b i (17) 8 < dp nq db (17) () j = 0 dp : nq dm j = 0 (18) ( (bj % m j ) 2 pdf g (b j ) % (b j % m j+1 ) 2 pdf g (b j )=0 for j = 1, 2,..., Q ) %2 R b j b j%1 (g % m j ) pdf g (g) dg = 0 for j = 1, 2,..., Q (19) Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 22 / 64
max(snr) Non-Uniform Quantisers Note: the above set of equations (i.e. (19)) cannot be solved in closed form for a general pdf. Therefore for a specific pdf an appropriate method is given below in a step-form: METHOD: 1. choose a m 1 2. calculate b i s, m i s 3. check if m Q is the mean of the interval [b Q %1, b Q = ] if yes! STOP else! choose a new m 1 and then goto step-2 Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 23 / 64
max(snr) Non-Uniform Quantisers ASPECIALCASE max(snr) Non-Uniform Quantizer of a Gaussian Input Signal if the input signal has a Gaussian amplitude pdf, that is pdf q = N(0, σ g ) then it can be proved that: P nq = 2.2σ 2 g Q %1.96 " not easy to derive (12) In this case the Signal-to-quantization Noise Ratio becomes: SNR q = P gq P nq = σ 2 g 2.2σ 2 g Q %1.96 = 0.45Q 1.96 (13) Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 24 / 64
Companders (non-uniform Quantizers) Companders (non-uniform Quantizers) Their performance independent of CF Non-unif. Quant = SAMPLE COMPRESSION Compressor + Expander, Compander g + UNIFORM QUANTIZER + SAMPLE EXPANDER 7! f g c i.e. g c =f{g} : " pdf gc = uniform 7! f%1 g c means such that" Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 25 / 64
Companders (non-uniform Quantizers) Popular companders: use log compression Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 26 / 64
Companders (non-uniform Quantizers) Two compression rules (A-law and µ-law) which are used in PSTN and provide a SNR q independent of signal statistics are given below: µ-law (USA) A-law (Europe) In practice % A ' 87.6 µ ' 100 Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 27 / 64
Companders (non-uniform Quantizers) Compression-Rules (PCM systems) The µ and A laws µ-law 8 g c = ln(1+µ g >< gmax ) g ln(1+µ) max g c = >: A-law g / A gmax 1+ln(A) g max 0 * / g // g max < 1 A g / / 1+ln(A gmax ) 1 g 1+ln(A) max A * // g // g max < 1 where g c = compressor s output signal (i.e. input to uniform quantiser) g = compressor s input signal g max = maximum value of the signal g Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 28 / 64
Companders (non-uniform Quantizers) The 6dB LAW uniform quantizer: µ-law: A-law: SNR q = 4.77 + 6γ % 20 log(cf) db (20) remember CF = peak rms SNR q = 4.77 + 6γ % 20 log(ln(1 + µ)) db (21) SNR q = 4.77 + 6γ % 20 log(1 + ln A) db (22) Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 29 / 64
Companders (non-uniform Quantizers) REMEMBER the following figure (illustrates the main characteristics of di erent types of quantizers) Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 30 / 64
Companders (non-uniform Quantizers) COMMENTS uniform & non-uniform quantizers: use them when samples are uncorrelated with each other (i.e. the sequence is quantized independently of the values of the preceding samples) practical situation: the sequence {g(kt s )} consists of samples which are correlated with each other. In such a case use di erential quantizer. Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 31 / 64
Companders (non-uniform Quantizers) Examples PSTN F s = 8kHz, Q = 2 8 (A = 87.6 orµ = 100), γ = 8 bits/level i.e. bit rate: r b = F s ) γ = 8k ) 8 = 64 kbits/sec Mobile-GSM F s = 8kHz, Q = 2 13 uniform ) γ = 13 bits/level, i.e. bit rate: r b = F s ) γ = 8k ) 13 = 104 kbits/sec which, with a di erential circuit, is reduced to r b = 13 kbits/sec Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 32 / 64
Di erential Quantizers Di erential Quantizers Di erential quantizers are appropriate for correlated samples namely they take into account the sample to sample correlation in the quantizing process; e.g. Transmitter (Tx) Receiver (Rx) 0 @ input current message symbol 1 A The weights w are estimated based on autocorr. function of the input The Tx & Rx predictors should be identical. I Therefore, the Tx transmits also its weights to the Rx (i.e. weights w are transmitted together with the data) Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 33 / 64
Di erential Quantizers In practice, the variable being quantized is not g(kt s ) but the variable d(kt s ) i.e. where d(kt s )=g(kt s ) % ĝ(kt s ) (14) Because d(kt s ) has small variations, to achieve a certain level of performance, fewer bits are required. This implies that DPCM can achieve PCM performance levels with lower bit rates. 6dB law: SNR q = 4.77 + 6γ% a in db (15) where %10dB < a < 7.77dB Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 34 / 64
Di erential Quantizers ABetterDi erentialquantiser:msedi.quant. the largest error reduction occurs when the di erential quantizer operates on the di erences between g(kt s ) and the minimum mean square error (min-mse) estimator ĝ(kt s ) of g(kt s ) - (N.B.: but more hardware) Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 35 / 64
Di erential Quantizers ĝ(kt s )=w T g where ( g = [ g((k % 1)T s ), g((k % 2)T s ),..., g((k % L)T s )] T w = [w 1, w 2,..., w L ] T rule: % choose w to minimize E * (g(kts ) % ĝ(kt s )) 2+... for the Transmitter choose w to minimize E * (d q (kt s )+ĝ(kt s )) 2+... for the Receiver Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 36 / 64
Di erential Quantizers Di erential Quantisers: Examples The power of d(kt s ) can be found as follows: σ 2 d = E * d 2+ = E * g 2 (kt s ) + + E * g 2 ((k % 1)T s ) + % 2E {g(kt s )g((k % 1)T s )} {z } {z } {z } =σ 2 g =σ 2 g 2 R gg (T s ) + σ 2 d = 2 σ 2 g % 2 R gg (T s )=2 σ 2 g (1 % R gg (T s ) ) (23) σ 2 g Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 37 / 64
Di erential Quantizers e.g. disadvantages : unrecoverable degradation is introduced by the quantization process. I (Designer s task is to keep this to a subjective acceptable level) Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 38 / 64
Di erential Quantizers Remember 1 σ 2 g = R gg (0) 2 R gg (τ) σ 2 g = is known as the normalized autocorrelation function 3 DPCM with the same No of bits/sample! generally gives better results than PCM with the same number of bits. Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 39 / 64
Di erential Quantizers Example of mse DPCM assume a 4-level quantizer: I/P O/P +5 * input * +255 +7 0 * input * +4 +1 %4 * input *%1 %1 %255 * input *%5 %7 Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 40 / 64
Di erential Quantizers Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 41 / 64
Di erential Quantizers From the last 2 figures we can see that small variation to the i/p signal (25V () 26V) + large variations to o/p waveforms Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 42 / 64
Noise E ects in a Binary PCM Noise E ects in a Binary PCM It can be proved that the Signal-to-Noise Ratio at the output of a binary Pulse Code Modulation (PCM) system, which employs a BCD encoder/decoder and operates in the presence of noise, is given by the following expression Desired signal SNR out = E * g 0 (t) 2+ E {n 0 (t) 2 } + E {n q0 (t) 2 } = 2 2γ 1 + 4 p e 2 2γ (24) where Output noise (due to channel noise) Output quantisation noise p e = f(type of digital modulator) %q 5 p e = T (1 % ρ) EUE e.g. if the digital modulator is a PSK-mod. then np o p e = T 2 EUE Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 43 / 64
Noise E ects in a Binary PCM Threshold E ects in a Binary PCM Threshold E ects in a Binary PCM We have seen that: SNR out = 2 2γ 1+4 p e 2 2γ Let us examine the following two cases: SNR in = high and SNR in = low i) SNR in =HIGH ii) SNR in =LOW SNR in = high ) p e = small SNR in = low ) p e = large ) 1 + 4 p e 2 2γ ' 1 ) SNR out = 2 2γ ) 1 + 4 p e 2 2γ ' 4 p e 2 2γ ) SNR out ' 6γ db ) SNR out ' 1 4 p e Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 44 / 64
Noise E ects in a Binary PCM Threshold E ects in a Binary PCM Threshold Point - Definition Threshold point is arbitrarily defined as the SNR in at which the SNR out,i.e. 2 SNR out = 2γ 1 + 4 p e 2 2γ falls 1dB below the maximum SNR out (i.e. 1dB below the value 2 2γ ). By using the above definition it can be shown (... for you... ) that the threshold point occurs when p e = 1 16 2 2γ where γ is the number of bits per level. Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 45 / 64
Noise E ects in a Binary PCM Comments on Threshold E ects The onset of threshold in PCM will result in a sudden " in the output noise power. P signal = ")SNR in = ")SNR out reaches 6γ db and becomes independent of P signal ) above threshold: increasing signal power ) no further improvement in SNR out The limiting value of SNR out depends only on the number of bits γ per quantization levels Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 46 / 64 Threshold E ects Comments
CCITT Standards: Di erential PCM (DPCM) CCITT Standards: Di erential PCM (DPCM) CCITT = Consulting Committee for International Telegraphy & Telephony This is now known as ITU-T (Telecom. Standardization Sector of the International Telecommunications) DPCM = PCM which employs a di erential quantizer i.e. DPCM reduces the correlation that often exists between successive PCM samples The CCITT standards 32 kbits DPCM The CCITT standards 64 kbits sec sec speech signal - F g = 3.2kHz audio signal - F g = 7kHz F s = 8 ksamples sec Q = 16 levels (i.e. γ = 4 bits ) level F s = 16 ksamples sec bits Q = 16 levels (i.e. γ = 4 level ) DPCM Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 47 / 64
CCITT Standards: Di erential PCM (DPCM) Problems of DPCM: 1 slope overload noise: occurs when outer quantization level is too small for large input transitions and has to be used repeatedly 2 Oscillation or granular noise: occurs when the smallest Q-level is not zero. Then, for constant input, the coder output oscillates with amplitude equal to the smallest Q-level. 3 Edge Busyness noise: occurs when repetitive edge waveform is contaminated by noise which causes it to be coded by di erent sequences of Q-levels. Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 48 / 64
Introduction to Telephone Network Introduction to Telephone Network Note that, as calls are routed through the PSTN, they will be routed (multiplexed ) through a hierarchy of switching centers Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 49 / 64
Introduction to Telephone Network Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 50 / 64
Introduction to Telephone Network CCITT recommendations for PCM (24-channels and 30-channels) CCITT recommendations for PCM (24-channels and 30-channels) 1960 British Post O ce (BPO) (currently BT) had established a 24-ch PCM system with objective the system to be available in 1968. Some of this work become the basis to the formation of a number of CCITT recommendations. In Europe, the original 24-ch PCM systems, which were designed mainly for up to 32Km transmission routes, have been replaced by 30-ch PCM systems. Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 51 / 64
Introduction to Telephone Network CCITT recommendations for PCM (24-channels and 30-channels) There are two di erent CCITT recommendations for PCM. The main di erences between these two recommendations are shown in the following table: Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 52 / 64
Introduction to Telephone Network CCITT recommendations for PCM (24-channels and 30-channels) That is, 1st CCITT rec. (24-channels PCM) 2nd CCITT rec. (30-channels PCM) Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 53 / 64
Introduction to Telephone Network CCITT recommendations for PCM (24-channels and 30-channels) Note I I A-law = better than µ-law (cheaper to produce and easy equipment maintenance, smaller quantization error in particular within the most significant part of the dynamic range). in 24-ch PCM the signalling information is conveyed within each speech time-slot (technique known as bit stealing). Result: a slight reduction in speech-coding performance. Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 54 / 64
Introduction to Telephone Network Single-Channel Path of 2nd CCITT rec. (30-channels PCM) Single-Channel Path of 2nd CCITT rec. (30-channels PCM) Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 55 / 64
Introduction to Telephone Network Implementation of 2nd PCM CCITT Recomm Implementation of 2nd PCM CCITT Recomm. 1st Level Multiplexing Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 56 / 64
Introduction to Telephone Network Implementation of 2nd PCM CCITT Recomm Based on the 24-channels and 30-channels PCM CCITT recommendations (primary multiplex groups) the core telephone network evolved from using Frequency Division Multiplex (FDM) technology to digital transmission and switching I These two PCM CCITT recommendations have led to two PDH (Plesiochronous digital hierarchies) CCITT recommendations for assembling the TDM telephony data streams from di erent calls. I Plesiochronous means: almost synchronous because bits are stu ed into the frames as padding and the calls location varies slightly - jitters - from frame to frame Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 57 / 64
Introduction to Telephone Network Plesiochronous digital hierarchies (PDH) PDH Hierarchy Hierarchical Level American DS-x European CEPT-x 0 DS-0 64 kbits/s CEPT-0 64 kbits/s 1 DS-1 1, 544 kbits/s CEPT-1 2, 048 kbits/s 2 DS-2 6, 312 kbits/s CEPT-2 8, 448 kbits/s 3 DS-3 44, 736 kbits/s CEPT-3 34, 368 kbits/s 4 DS-4 274, 176 kbits/s CEPT-4 139, 264 kbits/s 5 CEPT-5 565, 148 kbits/s Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 58 / 64
Introduction to Telephone Network Plesiochronous digital hierarchies (PDH) The 24-channel PDH TDM CCITT recommendation (DS-x) The 30-channel PDH TDM CCITT recommendations (CEPT-x) Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 59 / 64
Introduction to Telephone Network Plesiochronous digital hierarchies (PDH) Main disadvantage of PDH Networks PDH multiplexing was designed for point-to-point communications and channels cannot be added to, or extracted from, a higher multiplexing level demultiplexing down and then multiplexing up again, through the entire PDH For instance, to isolate a particular call from DS4, say, it must be demultiplexed to DS1. i.e. this is a very complex procedure and needs very expensive equipment at every exchange to demultiplex and multiplex high speed lines American & European Telephone Systems are incompatible (therefore very expensive equipment required to translate one format to the other for transatlantic tra c) Solution: SONET/SDH Signal Hierarchy Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 60 / 64
Introduction to Telephone Network Synchronous digital hierarchies (SONET/SDH) Synchronous digital hierarchies (SONET/SDH) The traditional PDH standards are based on the DS (USA) and CEPT (Europe) PCM systems (24-channels and 30-channels PCM CCITT recommendation) PDH hierarchy is almost synchronous (extra bits are inserted into the digital stream to bring them to a common rate). In 1988 SDH (Synchronous Digital Hierarchy) was adopted by ITU and ETSI (European Telecommunications Standards Institute) based on SONET (synchronous optical Networks) SDH signals have a common external timing i.e. SDH is synchronous Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 61 / 64
Introduction to Telephone Network Synchronous digital hierarchies (SONET/SDH) The SDH standards used in Europe are STM-1 which provides 155 Mbits/sec STM-2 which provides 310 Mbits/sec STM-3 which provides 465 Mbits/sec STM-4 which provides 620 Mbits/sec etc. (increments of 155 Mbits/sec) The most important main standards are STM-1, STM-4 and STM-16. These are commercially available. Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 62 / 64
Introduction to Telephone Network Synchronous digital hierarchies (SONET/SDH) SONET/SDH Hierarchy Hierarchical Level American SONET STS -x European SDH STM -x 0 STS -3 = 3) DS-3 STM -1 = 1) CEPT-4 1 STS -12 = 12) DS-3 STM -4 = 4) CEPT-4 2 STS -48 = 48) DS-3 STM -16=16) CEPT-4 Key Advantages it is simple to add and drop channels to meet customer requirements more bandwidth is available for network management equipment is smaller and cheaper network flexibility integrate and manage various types of tra c on a single fiber. Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 63 / 64
Introduction to Telephone Network Synchronous digital hierarchies (SONET/SDH) Prof. A. Manikas (Imperial College) EE303: PCM & PSTN 7 Dec 2011 64 / 64