Xem mẫu
- 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)
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 haulwe 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 thischapter we discusspredominantlyhowtheselineissues affect analoguetransmission
systems and how they may be countered. The effects on digital transmission are discussed in
later chapters.
3.1 ATTENUATION AND REPEATERS
Sound waves diminish thefurther they travel and electrical signals become weaker as they
pass along electromagnetic transmission lines. With electrical signals the attenuation
(as this type loss in signal strength called) is caused by the various electrical properties
of is
of the itself. These properties are known asresistance, the capacitance, the leakance
line the
and the inductance. The attenuation becomes more severe as theline gets longer. On very
long haul links thereceived signals become weak as to be
so 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 signalto its original wave shape and strength.
29
- 30 LONG-HAUL
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
attenuationconstantforanyparticular signal frequency onan electrical wire line
transmission system is given by the following formula:
cli + + +
= J ( i { J [ ( R 2 47r2f2L2)(G2 47r2f2C2)] (RG - 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 impeding the onwardpassage of the
by
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.
- Transmitted signol omplitude Ta
1 c =
Amplltude at any pomt = Ta X e-&
U
3
Dosronced
Figure 3.1 The effect of distance on attenuation. cr = attenuation constant
- LINE LOADING 31
3.2 LINE LOADING
As the inductance and capacitance workagainstone another, asimplemeans of
minimizing the effect of unwanted attenuation distortion
and is to increasethe
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, find that attenuation
we
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 (ordiameter) wire andgoodquality
insulating sheath.
A second conclusion from the formulafor the attenuation constant that its value is
is
minimized when the inductance has a value given by the expression
L = CR/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 someothermagneticmaterial
directly around the conductor, but it is cheaper and easier to provide a lumped loading
coil at intervals (say 1-2 km) along theline. The attenuationcharacteristics of unloaded
and loaded lines are shown in Figure 3.2.
Unloaded line /
-
.
a Lump loaded line
Continuously
loaded line
I
-
I
Speech
band
I
Signalfrequency
Figure 3.2 Attenuation characteristics of loaded line
- 32 LONG-HAUL
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, tomakesurethatthewanted 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
intheconversationwouldbedisproportionatelyattenuated,resultingin heavy and
unacceptable distortion of the signal at the receiving end.
3.3 AMPLIFICATION
Although lineloading 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.
Normallya repeater comprisestwo amplifiers, oneforeachdirection of signal
transmission. A splitting device is also required to separate transmit and receive signals.
This is so that each signal can befed to a relevant transmit or receive amplifier, as
Figure 3.3 illustrates.The splitting device is called a hybrid or hybridtransformer.
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
r--- - _ _ _ _ - - -
Repeater
- - -l
I Amplifier 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 telephonerepeater in the line. Eachrepeaterconsists of twohybrid
transformers and two amplifiers (one for each signal direction).
The amplijication introduced at eachrepeaterhas to be carefully controlledto
overcomethe 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,
causingfeedback and even moreamplification.Thisin 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 attenuation by boosting the signal by 13 dB, so that at
this
+ 1 dB
3
Line loss Arnplifler
+ l 3 dB
Figure 3.4 An unstablecircuit
- 34 LONG-HAUL
its output the signal is actually 4dB louder than it was at the outset. However, by the
time the second hybrid loss (in H2)and Q’s access lineattenuation 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 thehybridtransformerscanactuallycarryouttheirline-splittingfunctionto
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 retransmittedsignal (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 andamplified 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 sectionof the circuit, is 2 dBless than that of the original
signal emanating from telephone P. However, on its first ‘lap’ it had a strength 4dB
lower than the original. In other words, the re-circulated signal is actually louder than it
was on thefirst lap!What is more, if it goesaround again it will gain 2dB 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 fromthenon-idealperformance of thehybrid.Inpracticeit 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
probablymore expensive) ones. solution
This requirescarefulbalancing 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 instabilityproblem is simply to
reduce the amplificationinthefeedbackloop.This is done simply by adding an
attenuating device.Indeedmostvariableamplifiers are in fact fixed gainamplifiers
(around 30 dB) followed immediately be variable attenuators or pads. For example, in
the case illustrated in Figure 3.4 a reduction in the gainboth amplifiers A1 and A2 to
of
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 signalheardintelephone Q will be 6dB quieterthanthattransmitted by
telephone P, but this is unlikely to trouble the listener.
Crosstalk is the name given toanoverheard signal onanadjacentcircuit.It is
broughtabout by electromagneticinduction of an over-amplifiedsignalfrom one
circuit onto its neighbour, and Figure3.5 gives a simplified diagram of how it happens.
Becausethe transit amplifier on the circuitfromtelephone Ato 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
- CIRCUITS
TWO- AND FOUR-WIRE 35
Repeaters
Figure 3.5 Crosstalk. Q hears A
the conversation from P. It is resolvedeither 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.
3.4 TWO- AND FOUR-WIRE CIRCUITS
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 inmaintainingcircuitstabilityandacceptable
received signal volume simultaneously; this difficulty arises from the interaction of the
variousrepeaters.Aneconomicconsiderationisthehighcost of themanyhybrid
transformersthat need to beprovided. All butthefirstandlasthybrids 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 repeaters are needed, and
of
Repeater 1 Repeater 2 Repeater 3
2 -wire 2 -wire
I ine I ine
Figure 3.6 A repeatered 2-wirecircuit
- 36 LONG-HAUL
Repeater 1 Repeater 2 Repeater 3
L-wire L- wire
line Line
Figure 3.7 A repeatered4-wirecircuit
it eases the maintenance burden. This is why most amplified longhaul (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.
3.5 EQUALIZATION
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 entirehigh or low-frequency range couldbe 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 -
listortedsignal
I /
I
p- Speechband -
4
I I
D Signal frequency
Figure 3.8 Amplitude spectrum of distorted and undistorted signals (attenuation distortion)
- FREQUENCY DIVISION MULTIPLEXING (FDM) 37
In Figure3.8 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.
To counteract this effect, equalizers are used, which are circuits designed to amplify
orattenuate differentfrequencies by different amounts.Theaim 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 normallyincluded in repeaters, so that the effects of distortion can be
corrected all the way along the in the same way that amplifiers counteract the effect
line,
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 alargenumberofindividualphysical wire
circuits, one for each channel, can a very expensive business. For this reason, whatis
be
known as multiplexingwasdeveloped as a way of making better use of lineplant.
Multiplexing allowsmany transmission channels share the same
to 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 savingin cost because the
number of wire pairs required between the end points can be reduced.
Table 3.1 Frequency division multiplex (FDM) hierarchy
Consists Bandwidth of
of name
Channel 24 telegraph 4 kHz 0-4 kHz 1
(1 telephone channel) subchannels
120 Hz spacing
Group 12 channels 48 kHz 60-108 kHz 12
Supergroup 5 groups 240 kHz 3 12-552 kHz 60
Basic hypergroup 15 supergroups 3.7 MHz (3.6 MHz used) 312-4082 kHz 900
(also called a ‘super (3 mastergroups) (240 kHz per supergroup (4 MHz line)
mastergroup’) with 8 kHz spacing
normally between each)
Basic hypergroup 16 supergroups 4 MHz 60-4028 kHz 960
(alternative)
Mastergroup 5 supergroups 1.2 MHz 3 12-1 548 kHz 300
Hypergroup 9 mastergroups 12 MHz 312-12336kHz 2 700
(12 MHz)
Hypergroup 36 mastergroups 60 MHz 4404-59 580 kHz 10 800
(60 MHz)
- 38 LONG-HAUL
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 capable of supporting avery large bandwidth. Some
be
FDM cables have a bandwidth as high as 12 MHz (million cycles per second), or even
60 MHz. This compareswith the modest 3. l kHz (thousandcycles per second) required
for asingle telephone channel. The large bandwidththe key to the technique, as it sub-
is
divides readily into a much larger number of individual small bandwidth channels.
The lowest constituent bandwidth that makes up an FDMsystem is a single channel
bandwidth of 4 kHz. This comprises the 3. l kHz needed for a normal speech channel,
togetherwithsomesparebandwidth to createseparationbetweenchannelsonthe
system as a whole. Various other standard bandwidths are then integral multiples of
asinglechannel.Table3.1illustratesthis and gives thenames of these standard
bandwidths.
The overall bandwidth of the FDM transmission line is equalto 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 bandwidth is called
of translating equipment.
Thus supergrouptranslatingequipment ( S T E ) subdividesasupergroup 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 broken
be downinto individual 4kHz
channels. If required, some of it can be used directly for large bandwidth applications
suchasconcertgrademusicortelevisiontransmission.InFigure 3.9, two 48 kHz
circuits are derived the
from supergroup, togetherwith 36 individual telephone
. , -
One. L wire
( supergroupFDM line 1
12 individual
circuits
( & -wire 1
+Transmit
12 individual
circuits
El CTE
H ST E
12 individual
circuits
2 X 18 k H z
bandwith,
{ Receive
&-wirelines
Figure 3.9 Breaking-up bandwidth in FDM
- DIVISION FREQUENCY (FDM) 39
channels. The same principle can be applied at other levels in the hierarchy. Thus, for
example, all the supergroups of a hypergroup could be broken down into their compo-
nent groups using HTE and STE.Alternatively, some of the supergroups could be used
directly for bandwidth applications of 240 kHz.
Before multiplexing,the audio signals which areto bemultiplexed-up are first
converted to four-wire transmission, if they are not so already. The signals on each
transmit pair are then accurately filtered so that stray signals outside the allocated
bandwidth are suppressed. In fact, a telephone channel is filtered to be only 3.1 kHz in
bandwidth (of the available 4 kHz). The remaining 0.9 kHz separation prevents speech
interference between adjacent channels. Groups are filtered to 48 kHz, supergroups to
240 kHz, etc. Each filtered signal is then modulated by a carrier frequency, which has
the effect offrequency shifting the original signal into another part of the frequency
spectrum. For example,atelephonechannelstarting out in thebandwidthrange
300-3400 Hz might end up in the range 4600-7700 Hz. Another could be shifted to the
range 8300-11 400Hz, and so on.
Each component channel of an FDM group is frequency shifted by the CTE to a
different bandwidth slot within the 48 kHz available;so that in total 12 individual
channels may be carried. Likewise, in a supergroup, five already made-up groups of
48 kHz bandwidth are slotted in to the 240 kHz bandwidth by the STE.
The frequency shift is achieved by modulation of the component bandwidths with
different carrier signal frequencies. The frequency of the carrier signal which is used to
modulate the original signal (or baseband) will be equal to the value of the frequency
shift required. Each carrier signal must be produced by the translating equipment.
Themodulation of a signal inthefrequency band 300-3400Hz using acarrier
frequency of 8000Hz produces a signal of bandwidth from 4600Hz (8000-3400) to
1 1 Hz (8000 + 3400). The original frequency spectrum of
400 300-3400 Hz is reproduced
in two mirror image forms,called sidebands. One sideband is in the range 4600-7700 Hz
and the other is in the range 8300-1 1 400 Hz. Both sidebands are shown in Figure3.10.
As all the information is duplicated in both sidebands, only one of the sidebands
needs to be transmitted. For economy in the electrical power needed to be transmitted
to line, it is normalfor FDM systems tooperate ina singlesideband ( S S B ) and
Amditude Carrier
frequency
t I
Original
signal
spectrum [ baseband)
300 Hz 3600 HZ 6600 Hz 7700 Hz 8300 Hz llLO0 Hz
Signal
Lower Baseband
sideband sideband
Figure 3.10 Frequency shifting by carrier modulation
- 40 LONG-HAUL
suppressed carrier mode. The original signal is reconstructed at the receiving end by
modulating (mixing) a locally generated frequency. (Note: single sideband operation
may also be used in radio systems, but the carrier not suppressed in this case because
is
it is often inconvenient to make a carrier signal generator available at the receiver.
When the carrier is not suppressed a much simpler and cheaper detector can be used.)
Each baseband signal to be included in an FDM system is modulated with a different
carrier frequency, the lower sideband is extracted for conveyance. It may seem ineffic-
ient not to double up the use of carrier frequencies, adopting alternating upper and
lower sidebands of different channels, but by always using the lower sidebandwe obtain
a better overall structure, allowing easier extraction of singlechannelsfromhigher
order FDM systems. The carrier frequencies needed to produce a standard group are
thus 64 kHz, 68 kHz, 72 kHz, . . . ,108 kHz and the overall structure is as shown in
Figure 3.1 1.
...
1L
1 2 1 1 1 0 9 8 7 6 5 0 3 2 7
Audio
Channels
!
I I
Baseband 0 - h kHZ
Channel
modulating
frequency 6 6 , 6 8 , 7 2 , 7 6 , 8 0 , 8 h , 8 8 , 9 2 , 9 6 , 1 0 0 , 1 0 h , 108 kHz
( lower
sideband used 1
1211 1 0 9 8 7 6 5 h 3 2 1
Basicgroup
structure
kHz 60 108 kHz
(when
frequency 120 k H z
1 2 3 h 5 6 7 8 9 1 0 1 1 1 2
12 kHz kHz 60
Figure 3.11 The structure of an FDM group
- CROSSTALK AND ATTENUATION ON FDM SYSTEMS 41
Supergroups hypergroups
and may be modulated in a similar
fashion,
using
appropriate carrier frequencies and single sideband operation.
3.7 CROSSTALK AND ATTENUATION ON FDMSYSTEMS
Therecan be as muchcrosstalkandattenuation in FDM systems as in the single
channel or audio circuits which we discussed in the first part of the chapter. They
require just as muchif not more planning, as FDM systems are generally more sensitive
and complex.
nguon tai.lieu . vn
