Transcript
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TELEPHONY
Public telecommunication networks are originally
designed for telephony and are known as Public
Switched Telephone Network (PSTN). They aredigitized to provide a variety of teleservices and are
known as Integrated Services Digital Network (ISDN).
1 Network Structure
Since voice communications take minutes to finish one
call, star configuration is the best choice in term of cost.
This is applied to connection between telephone
terminals and local exchanges. The connections
between exchanges have a 'tree' modified hierarchical
structure. The exchange centers are classified into
different levels which depend on their sizes and
locations (Fig. 1.1, 1.2)
Primary Centers
Secondary Centers
Local Exchanges
Tertiary Centers
Fig. 1.1 Hierarchical Structure Conform to ITU-T
Recommendation
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Fig. 1.2 Example of Hierarchical Switching Networks
1.2 Standards
Standardization of international telecommunication is
carried by two sectors of International
Telecommunications Union (ITU), namely:
ITU Telecommunications Sector (ITU-T). Its duties
include the study of technical questions, operating
methods and tariffs for telephony, telegraphy and data
communications.
ITU Radiocommunication Sector (ITU-R). Its duties
include the study of technical questions, operating
methods and tariffs for radio communications.
A telecommunications network consists of the
following interacting subsystems:
(i) Transmission systems
(ii) Switching systems
(iii) Signalling systems
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2 Transmission Systems
Basic telephone service involves the transmission of
speech to a distance place. In analogue system, atelephone line has a 300 to 3.4 kHz bandwidth mainly
for speech communication. PCM method is used to
convert analog speech to digital voice. They are then
time-division-multiplex TDM to form high capacity
trunk for transmission and switching.
2.1 Pulse Code Modulation (PCM)
It is an analog-to-digital conversion process that
converts analog signal into a format compatible with
digital transmission and switching.
Anti-
filter
Analogsignalaliasing Compressor
Encoder
PCMsignalQuantizer
and A/DSampling
Line
Fig. 2.1 PCM Generation
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Sampling
It is a conversion of analog waveform into pulse
amplitude modulation (PAM) form where the amplitude
of the pulse denotes the analog amplitude at thesampling time.
t
t
t
Analog waveform
Sampling Pulse
Resulting PAMsignal
fs
1
Fig. 2.2 Sampling of Analog Signal
Fig. 2.3 shows the spectrum of the PAM. Fig. 2.4
shows that if the sampling rate is least than the double
of the signal bandwidth, distortion will result and this
distortion is called aliasing. Therefore, an anti-aliasing
filter is added before sampling to safe-guard the bandwidth. This minimum sampling rate is known as
the Nyquist sampling rate:
fs = 2W (2.1)
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where W= bandwidth of the signal and
fs = sampling rate
Fig. 2.3 Spectrum of (a) the Modulating Signal and
(b) the PAM Wave
Fig. 2.4 (a)fs > 2W(b)fs < 2W
Quantization
It is the process of rounding off the amplitudes of thesamples to certain predetermined levels available to the
A/D converter.
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In Fig. 2.5(a), the straight line shows the linear input
and output relationship, and the staircase function
shows the quantized relationship. Fig. 2.5(b) shows the
quantization error as a function of input voltage. Thequantization error appears as noise after recovery and is
referred to as quantization noise.
The quantization error can lie between V/2, and
assuming it has a uniform probability density
distribution, the rms quantization error (noise) is
Enq =12
Vsee appendix (2.2)
where V = step size (resolution)
More generally, the ratio between the peak and rmsvalues of the signal voltage will be some value k =
Erms/Emax. If distortion is to be avoided, the peak signal
level must not be allowed to exceed the peak input of
the quantizer.
Emax = 2
LV
where L = total number of steps
The signal-to-quantization noise ratio in this case is
(S/N)q = Erms2/Enq2 = 3k2L2 (2.3)
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Fig. 2.5(a) Linear Quantization; (b) Quantization Error
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Sign-Magnitude Code
The MSB of the 8-bit codeword is the sign bit and the
remaining bits are the magnitude bit. The example
below show a 3-bit PCM code:
Sign Magnitude
1 11
1 10
1 01
1 00
0 00
0 01
0 10
0 11
0.5V
1.5V
2.5V
3.5V = Emax
-0.5V
-1.5V
-2.5V
-3.5V
Transfer Function Recovered
Voltage
3 V
2 V
1 V
0 V
0 V
-1 V
-2 V
-3 V
V
Fig. 2.6 Staircase Transfer Curve
From Fig. 2.6, it clearly shows that the total number ofstep L:
L = 2n - 1 where n = no of bit per codeword (2.4)
Companding
With speech the peaks of the signal only infrequently
extend over the full range of the input. In effect, the
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signal does not have a uniform probability density
function, and the (S/N)q is lower than that given by Eq.
2.3. To compensate for this, compression is needed. It
has variable gain characteristics with a lower gain at
higher input.
If we keep the bits per sample fixed, then compression
will increase the S/Nq for low amplitude signals and
will decrease S/Nq for higher amplitude signals as
compared with the linear PCM. Ideal companded PCM
encodes signal levels with quantization error
proportional to signal level to keep the S/Nq constant atall amplitude levels.
In North America, -law compression characteristic is
used. In Europe and many other parts of the world, A-
law characteristic is used. They all use 8-bit
quantization.
Fig. 2.7 Compressor Characteristics of and A-law
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Fig. 2.8 Nonlinear Quantization
The reverse of compression is expansion. The
combined process is known as companding.
Some Important Data
With 3.4 kHz being the maximum voice frequency in
telephony, the standard sampling rate selected is
8,000 samples per second. With 8-bit quantizer, the
overall PCM data rate is 64 kbps.
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Line Encoding
It is used to convert standard logic levels to suitable
format for telephone line transmission. The followings
are some examples of line encoding method:
Line Encoding
Unipolar NRZ
Bipolar NRZ
Unipolar RZ
Bipolar RZ
AMI
0 0 1 1 0 1 0 1
0 V
+2 V
+1 V
-1 V
0 V
+2 V
0 V
-1 V
+1 V
0 V
-1 V
+1 V
NRZ = Non-Return-to-Zero
RZ = Return-to-Zero
AMI = Alternate Mark Inversion
Fig. 2.9 Line Encoding Methods
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Major factors in selecting a line encoding format:
Transmit Power
Unipolar uses more power than the bipolar methods.
DC Component
Unipolar methods have DC components whilst bipolar
methods have not. The presence of DC in the signal
does not favour the use of transformer for isolation.
Clock Recovery
Bipolar RZ has the highest voltage transitionswhich is the best for clock recovery.
Unipolar RZ and AMI lack of transitions for longstrings of zero.
NRZ lacks of transitions for long strings of zeroand one.
Bandwidth
The bandwidth of RZ doubles to that of NRZ and AMI.
Error Detection
AMI possesses error detection ability while all others
do not.
The conclusion is that AMI is the best line encodingmethod among all others in our example.
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2.2 Time-Division Multiplexing
In the European digital hierarchy, it uses the ITU-T 30
PCM voice channels to form the primary multiplex
carrier and has a data rate of 2.048Mbps.
30-channel Frame Format
Fig. 2.10 shows the ITU-T 2.048 Mbps
recommendation. Sixteen frames form a multi-frame
and in each frame, there are 32 eight-bit time slots, 30
for PCM voices, one for synchronization and one for
channel signalling.
1 2 3 4 13 14 15 16 17 18 19 28 29 30 310 0 1
1 3 5 72 4 6 8 8-bit PCM
One frame, 256 bits, 125usecs
ch 1 2 .... 15 16.... 17 18 .... 30....
Synchronizationchannel
Next frame
Signalling
channel
Fig. 2.10 30 Channels PCM Multiplexing
One frame contains one sample per user, therefore
Sampling rate = Frame rate = 8k per second
Frame duration = 1/8k = 125 s
Total number of bit per frame = 32 8 = 256 bits
Multiplex data rate = 256 /125 s = 2.048 Mbps
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Plesiochronous Digital Hierarchy (PDH)
Higher-order digital multiplex systems are first based
on PDH since all the primary inputs operate from
independent clock sources. It operates at 2.048M,8.448M, ... , multiplex hierarchy.
Synchronous Digital Hierarchy (SDH)
It is for fully digitized networks operating
synchronously using high-capacity optical-fiber
transmission systems and TDM switching. It operates
at basic rate of 155.52Mbps and multiplexes by factor
of 4. Any of the existing plesiochronous rates up to
140Mbps can be multiplexed into the SDH.
24-channel Frame Format
The other common multiplex format is the 24-channel
multiplexing used in USA.
2 3 4 5 10 11 12 13 14 15 16 21 22 23 241 1 2
1 3 5 72 4 6 8 8-bit PCM
One frame, 193 bits, 125usecs
Synchronization Bit
Next frame
Fig. 2.11 24 Channels PCM Multiplexing
Total number of bit per frame = 24 8 + 1 = 193 bits
Multiplex data rate = 193 /125 s = 1.544 Mbps
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3 Switching
3.1 Single-stage Space Switch
It is a simple full-matrix switch. However, a largeswitch system cannot be formed simply using full-
matrix switches because the number of crosspoints N2 is
huge when N is large.
1
2
N
1 2 N
N xN::
....
Fig. 3.1 (a) Matrix switch (b) Symbol
Concentrator and Expander
N xN N xN N xN. . . . .
1 2 M
N M x N
(a)
NM x N
Concentrator Expander (b)
Fig. 3.2 (a) Concentrator and Expander using SquareMatrix Switches; (b) Symbolic Representations
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3.2 A General Trunking
In order to utilize the switching network more
effectively, the low-usage subscriber lines are grouped
into high-usage trunk-groups before switching.
Switching
ConcentratorSubscriber
lines
Expander
IncomingJunctions
OutgoingJunctions
Central control
Fig. 3.3 General Trunking for a Switching System
3.3 Two-stage Space Switch Network
To reduce the total number of crosspoints, multi-stage
switch network is constructed using smaller matric
switches. The method is illustrated in Fig. 3.4 with a
10 10 matrix switch as the basic building block.
The basic design criterion is that the number of link
(intermediate connections) should be equal to the
number of trunk (incoming or outgoing connections).
This ensures the utilization of links will not be under or
over to that of the trunks. This leads to:
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The matrix switch should be square, i.e., N N The optimum N = Trunkof. No (3.1)
10x 10
10x 10
10x 10
10x 10
(10) (10)100 incoming 100 outgoing
trunks trunks
link
Fig. 3.4 Two-stage Switch Network
The blocking probability is high since there is only one
link between any two input and output building blocks.
3.4 Three-stage Space Switch Network
Three-stage network resolves or reduces network
blockage by the introduction of additional stage in the
middle. In Fig. 3.5, there are ten intermediate links
instead of one in the 2-stage network.
10x10
(10)100 incoming
10x10
trunks
10x 10
(10) 100 outgoing
10 x 10
trunks
10x10
(10)
10x10
Fig. 3.5 Example of 3-stage Switch Network
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3.5 Time Switching
If TDM transmission is used with space switching (Fig.
3.6), it is necessary to provide de-multiplexing and
multiplexing equipment before and after switching. Iftime switching is employed (Fig. 3.7), the multiplex
links can be switched directly.
Space-
division
switch
Incoming
PCM
trunks
PCM
muldexes
Outgoing
PCM
trunks
PCM
muldexes
Fig. 3.6 Space Switching with PCM
Time-
division
switch
Outgoing
PCM
trunks
Incoming
PCM
trunks
Fig. 3.7 Time Switching with PCM
The principle of a time switch is shown in Fig. 3.8.
Since any incoming channel can be connected to anyoutgoing channel, it is equivalent to an N N matrix
switch.
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0
1
2
3
X
Cyclic
write
From input
trunk
Decoder
Speech
store
Connection
store
S p e e c h
address of x
0
1
2
3
Y
Random
read
indirectly
To output
trunk
Fig. 3.8 Operation of a Time Switch
Time switching and space switching are used togetherin tandem switch and are known as Time-Space-Time
(TST) switching (Fig. 3.9).
m outgoingtrunksm incomingtrunks
Time
switch
nn
Time
switch
nn
Space
switch
mm
Time
switch
nn
Time
switch
nn
Fig. 3.9 TST Switch Networkm = no. of PCM trunks,
n = no. of timeslots.
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Time switch doesn't have the size problem when
compared with space switch. It can be expanded
rapidly by increasing the memory size and the operating
speed. However, time switch has the problem of
delay. Each time switch will introduce at most 125 sdelay time.
4 Signalling
To maintain smooth operations of a telephone network,
there are a lot of signalling information needed to be
sent through the telephone network. When the signals
are sent along with the same voice circuits, this is
known as channel-associated signalling. However, it
is more efficient for the central processor of one
exchange to send information to the other exchange
through separate high-speed data link, and this is known
as common-channel signalling. An example is thetimeslot-16 of European primary muliplex. It provides
a 64 kbps common-channel for signalling between
processors.
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Appendix
From Fig. 2.13(b), we find the relationship to describe
the error voltage:
y =2
V- x for 0 < x