PULSE CODE MODULATION 57 Distorted received signal Regenerated signal Regenerator Received bit Pattern Transmitted bit pattern Figure 5.2 The principle of regeneration Errors at the
Trang 15.1 DIGITAL TRANSMISSION
In investigating analogue transmission we found a relationship between bandwidth and
overall information carrying capacity, and we described frequency division multiplexing
( F D M ) This was a method of reducing the number of physical wires needed to carry a
multitude of individual channels between two points, and it worked by sharing out the overall bandwidth of a single set of four-wires (transmit and receive pairs) between all
and the equivalents of analogue bandwidth and channel multiplexing In contrast with
analogue networks, digital networks are ideal for the direct carriage of data, because as
individual digits Not just any type of digits, but binary digits (bits) in particular
electrically stable in one of two states, equivalent to ‘on’ (binary value ‘l’) or ‘off’ (binary value ‘0’) Thus a simple form of digital line system might use an electrical current as the
and ‘current off’ A more recent digital transmission medium using optical fibre (which
medium capable of displaying distinct ‘on/off’ states could also be used
55
Networks and Telecommunications: Design and Operation, Second Edition.
Martin P Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)
Trang 256 DIGITAL TRANSMISSION AND PULSE CODE MODULATION
For simultaneous two-way (or duplex) digital transmission a four-wire (or equivalent)
transmission medium is always required Just as with four-wire analogue transmission
(as discussed in Chapter 3) one pair of wires (or its equivalent, for example an optical
fibre) is used for the transmit (Tx) direction whereas the other pair is used for the receive
( R x ) direction These allow the digital pulses to pass in both directions simultaneously
which can be made to work adequately in a two-way mode over only two wires (recall
quality of connection With only two ‘allowed’ states on the line (‘off’ and ‘on’) it is not
all that easy to confuse them even when the signal is distorted slightly along the line by
needs to detect whether the received signal is above or below a given threshold value
If the pulse shape is not a ‘clean’ square shape, it does not matter Allow the same
electrical disturbance to interfere with an analogue signal, and the result would be a low
incomprehensible
regenerate the signals at intervals along the line A regenerator reduces the risk of
counteracting the effects of attenuation and distortion, which show up in digital signals
as pulse shape distortions In this corrective function a digital regenerator may be
regarded as the equivalent of an analogue repeater
The process of regeneration involves detecting the received signal and recreating a
The regeneration of digital signals is all that is needed to restore the signal to its
original form; there is no need to amplify, equalise or process it in any other way The
fact that the signal can be regenerated exactly is the reason why digital transmission
produces signals of such high quality
Trang 3PULSE CODE MODULATION 57
Distorted received signal Regenerated signal
Regenerator
Received bit Pattern Transmitted bit pattern
Figure 5.2 The principle of regeneration
Errors at the detection stage can be caused by noise, giving the impression of a pulse
when there is none Their likelihood can, however, be reduced by stepping up the electrical power (which effectively increases the overall pulse size or height), and a probability equivalent to one error in several hours or even days of transmission can be obtained
(a so-called bit error rute or more correctly bit error ratio ( B E R ) of 1 errored bit in
1 million bits is noted as BER = 1 X 1OP6) This is good enough for speech, but if the circuit is to be used for data transmission it will not be adequate; the error rate
checking techniques this can be improved to lO-I3
A digital line system may be designed to run at almost any bit speed, but on a single
digital circuit it is usually 64 kbit/s This is equivalent to a 4 kHz analogue telephone
equivalent to the bandwidth of an analogue line system; the more information there is
to be carried, the greater the required bit speed Later in the chapter we also discuss
physical circuit, by a method known as time division multiplex (or T D M ) TDM has the
same multiplying effect on the circuit carrying capacity of digital line systems as FDM has for analogue systems
5.2 PULSE CODE MODULATION
You may well ask, how is a speech signal, a TV signal or any other analogue signal to
be converted into a form that can be conveyed digitally? The answer lies in a method of
analogue to digital signal conversion known as pulse code modulation
Pulse code modulation ( P C M ) functions by converting analogue signals into a format compatible with digital transmission, and it consists of four stages First, there is the translation of analogue electrical signals into digital pulses Second, these pulses are coded into a sequence suitable for transmission Third, they are transmitted over the
approximation of it) at the receiving end PCM was invented as early as 1939, but it was only in the 1960s that it began to be widely applied This was mainly because before the
principles of PCM effectively
Trang 458 DIGITAL TRANSMISSION AND PULSE CODE MODULATION
Speech or any other analogue signals are converted into a sequence of binary digits
by sampling the signal waveform at regular intervals At each sampling instant the
transport medium At the receiving end, the original electrical signal is reconstructed by translating it back from the incoming digitalized signal The technique is illustrated in Figure 5.3, which shows a typical speech signal, with amplitude plotted against time
Sampling is pre-determined to occur at intervals of time t (usually measured in
microseconds) The numerical values of the sampled amplitudes, and their 8-bit binary translations, are shown in Table 5.1
only When the waveform amplitude does not correspond to an exact integer value, as occurs at time 4t in Figure 5.3, an approximation is made Hence at 4t, value -2 is used instead of the exact value of -2.4 This reduces the total number of digits that need to be sent The signal is reconstituted at the receiving end by generating a stepped waveform,
each step of duration t , with amplitude according to the digit value received The signal
of Figure 5.3 is therefore reconstituted as shown in Figure 5.4
In the example, the reconstituted signal has a square waveform rather than the
similarity of the reconstituted signal to the original may be improved by
are taken, and/or
Trang 5PULSE CODE MODULATION 59
Table 5.1 Waveform samples from Figure 4.1
quantization levels are the points on the vertical scale of Figure 5.3
However, without an infinite sampling rate and an infinite range of quantum values, it
irrecoverable element of quantization noise is introduced in the course of translating the original analogue signal into its digital equivalent The sampling rate and the
number of quantization levels need to be carefully chosen to keep this noise down to levels at which the received signal is comprehensible to the listener The snag is that the greater the sampling rate and the greater the number of quantization levels, the greater
is the digital bit rate required to carry the signal Here again a parallel can be found
required3delity of an analogue signal, the greater is the bandwidth required
The minimum acceptable sampling rate for carrying a given analogue signal using
Nyquist criterion, (after the man who discovered it) The criterion states that the sample rate must be at least double the frequency of the analogue signal being sampled For a
I Reconstituted signal
Figure 5.4 Reconstruction of the waveform of Figure 4.1 from transmitted samples
Trang 660 DIGITAL TRANSMISSION AND PULSE CODE MODULATION
bandwidth of a speech channel being 4 kHz
second, times 8 bits, or 64 kbit/s In other words a digital channel of 64 kbit/s capacity is
why the basic digital channel is designed to run at 64 kbit/s
quantization noise (also quantizing noise) Now, if the 256 quantization levels were equally spaced over the amplitude range of the analogue signal, then the low amplitude signals would incur far greater percentage quantization errors (and thus distortion) than higher amplitude signals For this reason, the quantization levels are not linearly spaced, but instead are more densely packed around the zero amplitude level, as shown
in Figure 5.5 This gives better signal quality in the low amplitude range and a more
Trang 7QUANTIZATION NOISE 61
quantization levels are in common use for speech signal quantization They are called
quantization and the Mu-law code is used in North America Unfortunately, because
comprising both A-law and Mu-law digital transmission plant
compromise between the different quantization levels An 8-bit binary number in one of the codes corresponds to a particular quantization value at a particular sample instant
in the other quantization code The conversion is therefore a relatively simple matter of mapping (i.e converting) between one eight bit value and another
5.4 QUANTIZATION NOISE
along the line, and it is minimized by applying the special A-law and p-law codes as
received signal is usually quoted in terms of the number of quantization levels by which the signal differs from the original
This value is quoted as a number of quantization distortion units (or qdus) Typically
processing undertaken on the connection) Another possible type of speech processing
is the technique of speech compression, and we shall see in Chapter 38 that the overall bit rate can be reduced by speech compression, at the cost of some increase in quantiza- tion noise
Quantization noise only occurs in the presence of a signal Thus, the quiet periods
view of the quality of digital transmission
first channel is followed by a sequence of eight from the second channel, and so on The principle is illustrated in Figure 5.6, in which the TDM equipment could be imagined to
Trang 862 DIGITAL TRANSMISSION AND PULSE CODE MODULATION
m
c
Trang 9TIME-DIVISION MULTIPLEXING 63
comprise, in turn, byte A1 (from channel A), byte B1 (from channel B), byte C1 (from
bit rate is required on the output channel, to ensure that all the incoming data from all
6 X 8 = 48 bits in 250 microseconds, i.e 192 kbit/s (Unsurprisingly, the result is equal to
3 X 64 kbit/s) Thus, in effect the various channels ‘time-share’ the outgoing transmission
TDM can either be carried out by interleaving a byte (i.e 8 bits) from each tribu-
tary channel in turn, or it can be done by single bit interleaving Figure 5.6 shows the
plexing function Figure 5.7 shows a typical digital line terminating equipment, used to
individual physical circuits) and a single digital bit stream carried on a single physical
( P M U X ) contains an analogue to digital conversion facility for individual telephone
channel conversion to 64 kbit/s, and additionally a time division multiplex facility In
Trang 1064 DIGITAL TRANSMISSION AND PULSE CODE MODULATION
generally used for purposes other than carriage of information.)
The transmitting equipment of a digital line system has the job of multiplexing the
operation of transmitter and receiver, and to this end particular patterns of pulses are
the extra 8 kbit/s of the North American 1.5 Mbit/s system
transmission on the line Given that each channel must be transmitted at 64 kbits/s, the overall bit speed is usually related to an integer multiple of 64 kbit/s There are three basic hierarchies of transmission rates which have been standardized for international use, but these extend to higher bit rates than the 2.048 Mbit/s and 1.544 Mbit/s versions
so far discussed
synchronization and signalling, more of which we shall discuss later in the chapter
interleaving a number of 2 Mbit/s systems as illustrated in Figure 5.8 The standardized rates are:
shows
In the second ITU-T standard (which currently predominates in North America), a
block of 24 X 64 kbit/s channels plus 8 kbit/s forfrarning, giving a bitrate of 1.544 Mbit/s (T1 line system) p-Law encoding is used for pulse code modulation of speech signals The principles of multiplexing, however, are largely the same, and diagrams similar to Figure 5.8 could have been drawn,
Trang 11DIGITAL FRAME FORMATTING AND ‘JUSTIFICATION’ 65
Figure 5.8 European digital multiplex hierarchy
DSO = 64 kbit/s, the basic channel
T2 or DS2 = 6.3 12 Mbit/s (4 X 1.5 Mbit/s)
there is some overlap with the North American system p-Law encoding is applied to
speech pulse code modulation
DSO = 64 kbit/s, the basic channel
53 = 32.064 Mbit/s (5 X 6 Mbit/s)
54 = 97.728 Mbit/s (3 X 32 Mbit/s)
undertaken in the country which uses the 1.544 Mbit/s standard
to 6 X 64 kbit/s, 24 X 64 kbit/s and 30 X 64 kbit/s All three bitrates may be supported by
an El line system, only the first two from a T1 or J1 system
5.7 DIGITAL FRAME FORMATTING AND ‘JUSTIFICATION’
As we noted earlier in the chapter, it is common in a 2.048 Mbit/s system to use only
Trang 1266 DIGITAL TRANSMISSION AND PULSE CODE MODULATION
1.544 Mbit/s system, the bit rate required to carry twenty-four 64 kbit/s channels is only
spare capacity? The answer: it is used for synchronization and signalling functions Consider a 2.048 Mbit/s bit stream, and in particular the bits carried during a single time interval of 125 microseconds During a period of 125 microseconds a single sample
of 8 bits will have been taken from each of the 30 constituent or tributary channels making up the 2.048 Mbit/s bit stream These are structured into an imaginary frame,
frame is structured in the same way, so that the first timeslot of eight bits holds the eight-bit sample from tributary channel 1, the second timeslot the sample from channel
2, and so on The principle is shown in Figure 5.6 It is very like a single frame of a movie film; the only thing missing is the equivalent of the film perforations which allow
function is in fact performed by the first timeslot in the frame It is given the name
of 32 timeslots
signalling
We cannot leave timeslot zero, without briefly discussing its synchronization function
which serves to keep the line system bit rate running at precisely the right speed
end users connected to exchanges A and C
Each of the exchanges A, B and C in Figure 5.10 will be designed to input and receive data from the digital transmission links A-B and B-C at 2.048 Mbit/s What happens if
synchronization steps to prevent it In the circumstances shown, the bit stream received
link from B to C correctly, and a slip of 1 ‘wasted’ bit will occur once per second Conversely, in the direction from C to A via B, unsent bits will gradually be stored up
are lost when the store in exchange B overflows Neither slip nor overflow of bits is
level, in other words are controlled to run at exactly the same speed Actually, they run
used to try to maintain the synchronization, but systems are not firmly locked in-step
of the clock (faster or slower) to keep it in step with other systems, but as there is more than one clock in the network, there is still a discrepancy in the synchronization of the