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To get us anywhere with long haul we need to address ourselves to the following inescapably pertinent topics: 0 attenuation loss of signal strength over distance 0 line loading a way

3 Long-haul

Communication

None of the circuits that we have so far discussed are suitable as they stand for long haul

communication To get us anywhere with long haul we need to address ourselves to the following inescapably pertinent topics:

0 attenuation (loss of signal strength over distance)

0 line loading (a way of reducing attenuation on medium length links)

0 amplification (how to boost signals on long haul links)

0 equalization (how to correct tonal distortion)

0 multiplexing (how to increase the number of ‘circuits’ that may be obtained from one physical cable)

In this chapter we discuss predominantly how these line issues affect analogue transmission systems and how they may be countered The effects o n digital transmission are discussed in

later chapters

3.1 ATTENUATION AND REPEATERS

Sound waves diminish the further they travel and electrical signals become weaker as they pass along electromagnetic transmission lines With electrical signals the attenuation (as this type of loss in signal strength is called) is caused by the various electrical properties

of the line itself These properties are known as the resistance, the capacitance, the leakance and the inductance The attenuation becomes more severe as the line gets longer On very long haul links the received signals become so weak as to be imperceptible, and something needs to be done about it Usually the attenuation in analogue transmission lines is

countered by devices called repeaters, which are located at intervals along the line, their

function being to restore the signal to its original wave shape and strength

29

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)

Signal attenuation occurs in simple wireline systems, in radio and in optical fibre

systems The effect of attenuation on a line follows the function shown in Figure 3.1,

where the signal amplitude (the technical term for signal strength) can be seen to fade

with distance travelled according to a negative exponential function, the rate of decay

of the exponential function along the length of the line being governed by the attenua-

tion constant, alpha ( a )

Complex mathematics, which we will not go into here, reveal that the value of the

attenuation constant for any particular signal frequency on an electrical wire line

transmission system is given by the following formula:

cli = J ( i { J [ ( R 2 + 47r2f2L2)(G2 + 47r2f2C2)] + ( R G - 47r2f2LC)})

The larger the value of (, the greater the attenuation, the exact value depending on the

following line characteristics:

R : the electrical resistance per kilometre of the line, in ohms

G: the electrical leakance per kilometre of the line, in mhos

L: the inductance per kilometre of the line, in henries

C: the capacitance per kilometre of the line, in farads

f : the frequency of the particular component of the signal

The resistance of the line causes direct power loss by impeding the onward passage of the

signal The leakance is the power lost by conduction through the insulation of the line

The inductance and Capacitance are more complex and current-impeding phenomena

caused by the magnetic effects of alternating electric currents

It is important to realize that because the amount of attenuation depends on the

frequency of the signal, then the attenuation may differ for different frequency compo-

nents of the signal For example, it is common for high frequencies (treble tones) to be

disproportionately attenuated, leaving the low frequency (bass tones) to dominate This

leads to distortion, also called frequency attenuation distortion or simply attenuation

distortion

Amplltude at any pomt = Ta X e-&

U

3

Dosronce d

1\ c -Transmitted signol omplitude = Ta

Figure 3.1 The effect of distance on attenuation cr = attenuation constant

LINE LOADING 31

3.2 LINE LOADING

minimizing the effect of unwanted attenuation and distortion is to increase the inductance of the line to counteract some or all of the line capacitance

Taking a closer look at the equation given in the last section, we find that attenuation

is zero when both resistance and leakance are zero, but will have a real value when either resistance or leakance is non-zero The lesson is that both the resistance and the leakance of the line should be designed to be as low as is practically and economically

possible This is done by using large gauge (or diameter) wire and good quality insulating sheath

A second conclusion from the formula for the attenuation constant is that its value is minimized when the inductance has a value given by the expression

L = C R / G henrieslkm

This, in effect, reduces the electrical properties of the line to a simple resistance, mini-

mizing both the attenuation and the distortion simultaneously In practice, the attenua-

tion and distortion of a line can be reduced artificially by increasing its inductance

L ideally in a continuous manner along the line’s length The technique is called line loading It can be achieved by winding iron tape or some other magnetic material

directly around the conductor, but it is cheaper and easier to provide a lumped loading

coil a t intervals (say 1-2 km) along the line The attenuation characteristics of unloaded and loaded lines are shown in Figure 3.2

a -

Lump loaded line

Continuously loaded line

- S p e e c h b a n d

Signal frequency

Figure 3.2 Attenuation characteristics of loaded line

The use of line loading has the disadvantage that it acts as a high frequency filter,

tending to suppress the high frequencies It is therefore important when lump loading is

used, to make sure that the wanted speech band frequencies suffer only minimal

attenuation If the steep part of lump loaded line curve (marked by an asterisk in

Figure 3.2) were to occur in the middle of the speech band, then the higher frequencies

in the conversation would be disproportionately attenuated, resulting in heavy and

unacceptable distortion of the signal at the receiving end

Although line loading reduces speech-band attenuation, there is still a loss of signal

which accumulates with distance, and at some stage it becomes necessary to boost the

signal strength This is done by the use of an electrical amplifier The reader may well

ask what precise distance line loading is good for There is, alas, no simple answer as it

depends on the gauge of the wire and on the transmission bandwidth required by the

user In general, the higher the bandwidth, the shorter the length limit of loaded lines

(10-15 km is a practical limit)

Devices called repeaters are spaced equally along the length of a long transmission

line, radio system or other transmission medium Repeaters consist of amplifiers and

other equipment, the purpose of which is to boost the basic signal strength

Normally a repeater comprises two amplifiers, one for each direction of signal

transmission A splitting device is also required to separate transmit and receive signals

This is so that each signal can be fed to a relevant transmit or receive amplifier, as

Figure 3 3 illustrates The splitting device is called a hybrid or hybrid transformer

Essentially it converts a two-way communication over two wires into two one-way,

two-wire connections, and it is then usually referred to as a four-wire communication

(one direction of signal transmission on each pair of a two-pair set) So, while a single

two-wire line is adequate for two-way telephone communication over a short distance,

as soon as the distance is great enough to require amplification then a conversion to

Repeater

r - - - - _ _ _ _ - - - - - - l

I A m p l i f i e r I

Figure 3.3 A simple telephone repeater system

AMPLIFICATION 33

four-wire communication is called for Figure 3.3 shows a line between two telephones with a simple telephone repeater in the line Each repeater consists of two hybrid transformers and two amplifiers (one for each signal direction)

The amplijication introduced at each repeater has to be carefully controlled to overcome the effects of attenuation, without adversely affecting what is called the

stability of the circuit, and without interfering with other circuits in the same cable

Repeaters too far apart or with too little amplification would allow the signal current to fade to such an extent as to be subsumed in the electrical noise present on the line

Conversely, repeaters that are too close together or have too much amplification, can lead to circuit instability, and to yet another problem known as crosstalk

A circuit is said to be unstable when the signal that it is carrying is over-amplified, causing feedback and even more amplification This in turn leads to even greater feedback, and so on and on, until the signal is so strong that it reaches the maximum power that the circuit can carry The signal is now distorted beyond cure and all the

listener hears is a very loud singing noise For the causes of this distressing situation let

us look at the simple circuit of Figure 3.4

The diagram of Figure 3.4 shows a poorly engineered circuit which is electrically unstable At first sight, the diagram is identical to Figure 3.3 The only difference is that various signal attenuation values (indicated as negative) and amplification values (indicated as positive) have been marked using the standard unit of measurement, the decibel (dB) The problem is that the net gain around the loop is greater than the net loss Let us look more closely The hybrid transformer H1 receives the incoming signal from telephone Q and transmits it to telephone P, separating this signal from the one that will be transmitted on the outgoing pair of wires towards Q Both signals suffer a

3 dB attenuation during this ‘line-splitting’ process Adding the attenuation of 1 dB which is suffered on the local access line by the outgoing signal coming from telephone

P, the total attenuation of the signal by the time it reaches the output of hybrid H1 is therefore 4 dB The signal is further attenuated by 5 dB as a result of line loss Thus the input to amplifier A1 is 9 dB below the strength of the original signal Amplifier A1 is set to more than make up for this attenuation by boosting the signal by 13 dB, so that at

+ 13 dB

Line loss Arnplifler

+ l 3 dB

Figure 3.4 An unstable circuit

its output the signal is actually 4 d B louder than it was at the outset However, by the

time the second hybrid loss (in H2) and Q’s access line attenuation have been taken into

account, the signal is back to its original volume This might suggest a happy ending,

but unfortunately the circuit is unstable Its instability arises from the fact that neither

of the hybrid transformers can actually carry out their line-splitting function to

perfection; a certain amount of the signal received by hybrid H2 from the output of

amplifier A1 is finding its way back onto the return circuit (Q-to-P)

A well-designed and installed hybrid would give at least 30 dB separation of receive

and transmit channels However, in our example, the hybrid has either been poorly

installed or become faulty, and the unwanted retransmitted signal (originated by P but

returned by hybrid H2) is only 7 dB weaker at hybrid Hs’s point of output than it was at

the output from Al, and is then attenuated by 5 dB and amplified 13 dB before finding

its way back to hybrid H l , where it goes through another undesired retransmission,

albeit at a cost of 7 dB in signal strength The strength of this signal, which has now

entirely ‘lapped’ the four-wire section of the circuit, is 2 dB less than that of the original

signal emanating from telephone P However, on its first ‘lap’ it had a strength 4 d B

lower than the original In other words, the re-circulated signal is actually louder than it

was on the first lap! What is more, if it goes around again it will gain 2 dB in strength for

each lap, quickly getting louder and louder and out of control This phenomenon is

called instability The primary cause is the feedback path available across both hybrids,

which is allowing incoming signals to be retransmitted (or ‘fed back’) on their output

The path results from the non-ideal performance of the hybrid In practice it is

impossible to exactly balance the hybrid’s resistance with that of the end telephone

handset

One way to correct circuit instability is to change the hybrids for more efficient (and

probably more expensive) ones This solution requires careful balancing of each

telephone handset and corresponding hybrid, and is not possible if the customer lines

and the hybrids are on opposite sides of the switch matrix (as would be the case for a

two-wire customer local line connected via a local exchange to a four-wire trunk or

junction) A cheaper and quicker alternative to the instability problem is simply to

reduce the amplification in the feedback loop This is done simply by adding an

attenuating device Indeed most variable amplifiers are in fact fixed gain amplifiers

(around 30 dB) followed immediately be variable attenuators or pads For example, in

the case illustrated in Figure 3.4 a reduction in the gain of both amplifiers A1 and A2 to

12 dB will mean that the feedback signal is exactly equal in amplitude to the original In

this state the circuit may just be stable, but it is normal to design circuits with a much

greater margin of stability, typically at least 10dB For the Figure 3.4 example this

would restrict amplifier gain to no more than 7 dB Under these conditions the volume

of the signal heard in telephone Q will be 6 d B quieter than that transmitted by

telephone P, but this is unlikely to trouble the listener

Crosstalk is the name given to an overheard signal on an adjacent circuit It is

brought about by electromagnetic induction of an over-amplified signal from one

circuit onto its neighbour, and Figure 3.5 gives a simplified diagram of how it happens

Because the transit amplifier on the circuit from telephone A to telephone B in

Figure 3.5 is over-amplifying the signal, it is creating a strong electromagnetic field

around the circuit and the same signal is induced into the circuit from P to Q As a

result the user of telephone Q annoyingly overhears the user of telephone A as well as

TWO- AND FOUR-WIRE CIRCUITS 35

Repeaters

Figure 3.5 Crosstalk Q hears A

the conversation from P It is resolved either by turning down the amplifier or by increasing the separation of the circuits If neither of these solutions is possible then a third, more expensive, option is available, involving the use of specially screened or

transverse-screened cable In such cable a foil screen wrapped around the individual pairs of wires makes it relatively immune to electromagnetic interference

The diagram in Figure 3.3 illustrates a single repeater, used for boosting signals on a two-wire line system If the wire is a long one, a number of individual repeaters may

be required Figure 3.6 shows an example of a long line in which three amplifiers have been deployed

Such a system may work quite well but it has a number of drawbacks, the most important of which is the difficulty in maintaining circuit stability and acceptable

received signal volume simultaneously; this difficulty arises from the interaction of the various repeaters An economic consideration is the high cost of the many hybrid transformers that need to be provided All but the first and last hybrids could be dispensed with if the circuit were wired instead as a four-wire system along the total length of its repeatered section, as shown in Figure 3.7 This arrangement reduces the problem of achieving circuit stability when a large number of repeaters are needed, and

Repeater 1 Repeater 2 Repeater 3

2 - w i r e 2 - w i r e

Figure 3.6 A repeatered 2-wire circuit

Repeater 1 Repeater 2 Repeater 3

L - w i r e

l i n e

L- wire Line

Figure 3.7 A repeatered 4-wire circuit

it eases the maintenance burden This is why most amplified long haul (i.e trunk)

circuits are set up on four-wire transmission lines Shorter circuits, typically requiring

only 1-2 repeaters (i.e junction circuits), can however make do with two-wire systems

We have mentioned the need for equalization on long haul circuits, to minimize the

signal distortion Speech and data signals comprise a complex mixture of pure single

frequency components, each of which is affected differently by transmission lines The

result of different attenuation of the various frequencies is tonal degradation of the

received signal; at worst, the entire high or low-frequency range could be lost Figure 3.8

shows the relative amplitudes of individual signal frequencies of a distorted and an

undistorted signal

Amp1 i t u d e

(signal strength)

I- -r- / c - \

listorted signal

I

p - S p e e c h b a n d

D Signal frequency

Figure 3.8 Amplitude spectrum of distorted and undistorted signals (attenuation distortion)

FREQUENCY DIVISION MULTIPLEXING (FDM) 37

In Figure 3.8 the amplitude of the frequencies in the undistorted (received) signal is the same across the entire speech bandwidth, but the high and low frequency signals (high and low notes) have been disproportionately attenuated (lost) in the distorted signal

The effect is known as attenuation distortion or frequency attenuation distortion

T o counteract this effect, equalizers are used, which are circuits designed to amplify

or attenuate different frequencies by different amounts The aim is to ‘flatten’ the frequency response diagram to bring it in line with the undistorted frequency response diagram In our example above, the equalizer would need to amplify the low and high frequencies more than it would amplify the intermediate frequencies

Equalizers are normally included in repeaters, so that the effects of distortion can be corrected all the way along the line, in the same way that amplifiers counteract the effect

of attenuation

3.6 FREQUENCY DIVISION MULTIPLEXING (FDM)

When a large number of individual communication channels are required between two points a long distance apart, providing a large number of individual physical wire circuits, one for each channel, can be a very expensive business For this reason, what is known as multiplexing was developed as a way of making better use of lineplant Multiplexing allows many transmission channels to share the same physical pair of wires

or other transmission medium It requires sophisticated and expensive transmission line-

terminating equipment ( L T E ) , but has the potential for overall saving in cost because the number of wire pairs required between the end points can be reduced

Table 3.1 Frequency division multiplex (FDM) hierarchy Bandwidth Consists Usual Number of name of Bandwidth bandwidth channels

Channel

(1 telephone channel)

Group

Supergroup

Basic hypergroup

(also called a ‘super

mastergroup’)

Basic hypergroup

(alternative)

Mastergroup

Hypergroup

(12 MHz)

Hypergroup

(60 MHz)

24 telegraph subchannels

120 Hz spacing

12 channels

5 groups

15 supergroups

(3 mastergroups)

16 supergroups

5 supergroups

9 mastergroups

36 mastergroups

4 kHz

48 kHz

240 kHz

3.7 MHz (3.6 MHz used)

(240 kHz per supergroup

with 8 kHz spacing

normally between each)

4 MHz 1.2 MHz

12 MHz

60 MHz

0-4 kHz

60-108 kHz

3 12-552 kHz

(4 MHz line)

312-4082 kHz

60-4028 kHz

3 12-1 548 kHz 312-12336kHz 4404-59 580 kHz

1

12

60

900

960

300

2 700

10 800

The method of multiplexing used in analogue networks is called frequency division

multiplex ( F D M ) FDM calls for a single, high grade, four-wire transmission line (or

equivalent), and both pairs must be capable of supporting a very large bandwidth Some

FDM cables have a bandwidth as high as 12 MHz (million cycles per second), or even

60 MHz This compares with the modest 3 l kHz (thousand cycles per second) required

for a single telephone channel The large bandwidth is the key to the technique, as it sub-

divides readily into a much larger number of individual small bandwidth channels

The lowest constituent bandwidth that makes up an FDM system is a single channel

bandwidth of 4 kHz This comprises the 3 l kHz needed for a normal speech channel,

together with some spare bandwidth to create separation between channels on the

system as a whole Various other standard bandwidths are then integral multiples of

a single channel Table 3.1 illustrates this and gives the names of these standard

bandwidths

The overall bandwidth of the FDM transmission line is equal to one of the standard

bandwidths (e.g supergroup, group) named in Table 3.1, and is then broken down into a

number of sub-bandwidths, called tributaries in the manner shown in Figure 3.9 The

equipment which performs this segregation of bandwidth is called translating equipment

Thus supergroup translating equipment ( S T E ) subdivides a supergroup into its five

component groups, and a channel translating equipment ( C T E ) subdivides a group into

twelve individual channels

Not all the available bandwidth needs be broken down into individual 4kHz

channels If required, some of it can be used directly for large bandwidth applications

such as concert grade music or television transmission In Figure 3.9, two 48 kHz

circuits are derived from the supergroup, together with 36 individual telephone

12 individual

circuits

( & -wire 1

12 individual

circuits

,

El CTE

H

12 individual

circuits

2 X 1 8 k H z {

bandwith,

&-wire lines

ST E

One L - wire

( supergroup FDM line 1

+ Transmit

Receive