Iv The Telephone And Multiplex Systems
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Transcript of Iv The Telephone And Multiplex Systems
The Telephone System and Multiplex Systems 1
THE TELEPHONE SYTEM AND
MULTIPLEX SYSTEMS ANALOG TELEPHONE SYSTEMS F When two computers owned by the same company or
organization and located close to each other need to communicate, it is often easiest to just run a cable between them. This is how local area networks work.
F However, when the distances are large, or there are
many computers, or the cables would have to pass through a public road or other public right of way, the costs of running private cables are usually prohibitive.
F Consequently, the network designers must rely upon
the existing telecommunication facilities such as the Public Switched Telephone Network (PSTN).
F The PSTN was designed many years ago with a
completely different goal in mind: transmitting human voice in a more or less recognizable form. Their suitability for computer-to-computer communication is often marginal at best.
The Telephone System and Multiplex Systems 2
F The typical error of using telephone lines for computer
communication is about one error per 100,000 bits sent. The error rate for a direct cable connection (LAN) is about one error per 10,000,000,000,000 bits sent.
F However, the situation is changing rapidly with the
introduction of fiber optics and digital technology. F Analog telephone systems provide either two or four
wires that connect the telephone handset and a local telephone company central office (also called switching office or end office).
CENTRALOFFICE
TELEPHONE TELEPHONE
TELEPHONE
TELEPHONE
The Telephone System and Multiplex Systems 3
The Telephone System and Multiplex Systems 4
F The lines connecting the subscriber’s telephone and
the central office are know as local loop lines. Local loops consist of twisted pairs nowadays, although in the early days of telephony, uninsulated wires spaced 25 cm apart on telephone poles were common.
F Central offices are connected to each other via toll
offices through high-bandwidth lines called trunk lines. Trunk connections are often implemented using coax cables, fiber optics, or microwave transmission.
CENTRALOFFICE
TELEPHONE
TELEPHONE
TELEPHONE
CENTRALOFFICE
TELEPHONE
TELEPHONE
TELEPHONE
TRUNK LINES
TOLLOFFICE
The Telephone System and Multiplex Systems 5
F With the advent of digital technology, long-distance
trunk lines within the telephone system are rapidly being converted to digital. The old system used analog transmission over copper wires; the new one uses digital transmission over optical fibers.
F Telephone lines that go through a central office switch
can make two types of connections with other telephones:
1. Connecting two telephones within the same
central office. In this scenario, the switching mechanism within the central office sets up a direct connection between two local loops. This connection remains intact for the duration of the call.
2. Connecting two telephones belonging to
different central offices.
In either case, the call requests the closure of electrical switches to make the connection (circuit-switched connection).
F Telephone calls between central offices are more
complex than local-loop calls. F The calls go through trunk lines and these lines
aggregate several telephone calls through wire or fiber cable.
The Telephone System and Multiplex Systems 6
MULTIPLEXING F Telephone companies have developed elaborate
schemes in combining several conversations over a single physical trunk line.
F Multiplexing is the process of combining several
signals and transmitting them through the same channel simultaneously.
F Types of Multiplexing Techniques 1. Frequency Division Multiplexing (FDM) 2. Time Division Multiplexing (TDM) F Frequency division multiplexing relies on the general
rule that signals with different frequencies, if transmitted simultaneously, can be easily separated at the receiver.
If signals have the same frequency, FDM translates
each signal to a new frequency range.
The Telephone System and Multiplex Systems 7
0 4 KHz
0 4 KHz
0 4 KHz
60 KHz 64 KHz
65 KHz 69 KHz
70 KHz 74 KHz
60 K
Hz
64 K
Hz
65 K
Hz
69 K
Hz
70 K
Hz
74 K
Hz
AFTER MULTIPLEXING:
The Telephone System and Multiplex Systems 8
The FDM technique of multiplexing requires guard
bands (spaces between adjacent signals) to keep signals from contaminating each other.
A radio-frequency modem (or RF modem) is just one
of the several devices that can do frequency translation.
FDM and adequate guard bands allow several
telephone connections to take place through the same trunk.
The trunk should have a high
bandwidth. The multiplexed signals should be demultiplexed to
bring the signals back to their original frequencies.
The Telephone System and Multiplex Systems 9
DIGITAL TELEPHONE SYSTEMS F While long-distance trunk lines are now largely digital
in the more advanced countries, the local loops are still analog and are likely to remain so for at least a decade or two, due to the enormous cost of converting them.
F To switch from analog-based equipment to digital-
based communications networks requires an analog-to-digital conversion.
F A codec (coder/decoder) is a device that translates
analog voice signals into digital signals.
Telephone
CentralOffice
Analog Local Loop Digital Trunk
The Telephone System and Multiplex Systems 10
F Steps in analog-to-digital conversion:
1. Pulse-Amplitude Modulation (PAM). The first step in analog-to-digital conversion is to convert the analog signal into discrete signals that have amplitudes that simulate the original signal (pulses). This technique is commonly called as signal sampling.
2. Pulse Code Modulation (PCM). PCM converts
the stream of continuously varying PAM signals into a stream of binary digital signals
PCM requires two steps:
A. Quantization. This reduces the PAM signal to a limited number of discrete amplitudes.
B. Coding. This converts each PAM pulse
into a binary word.
The Telephone System and Multiplex Systems 11
F Pulse-Amplitude Modulation A PAM signal consists of a sequence of pulses in
which the amplitude of each pulse is proportional to the amplitude of the analog information signal at the corresponding point where the sample was taken.
analoginformation
signalx(t)
samplingpulsetrainp(t)
sampled-datasignalxs(t)
T s
Pfs
0
The Telephone System and Multiplex Systems 12
F A sampling pulse train, p(t), guides the sampling
process. F In order for the sampling to take place, x(t) should be
observed during short intervals of time of width τs seconds (which is the aperture time), which corresponds to a pulse. So at the presence of a pulse, the system takes a sample.
F The system takes a sample after every Ts seconds,
where Ts is the sampling period (the time between the beginning of one sample to the beginning of the next sample). From this, the sampling rate fs = 1 / Ts samples per second.
F Therefore, the amplitudes of the pulses in the pulse
train are modulated by the information signal. In other words, this train of pulses acts as the carrier instead of the usual analog sine wave.
F The amplitude of the pulses of the sampled-data signal
xs(t) corresponds to the amplitude of the modulating signal x(t) at the point where it was sampled
F The frequency of the sampling pulse train (the number
of samples per second) should be at least 2 x the highest frequency of the analog signal. This is known as Shannon’s Sampling Theorem.
The Telephone System and Multiplex Systems 13
F Pulse-Code Modulation For the discussions, assume that the number of bits to
be used is n = 4 bits. This choice results in m = 24 = 16 digital or PCM words.
Natural Binary Number
Decimal Value
Unipolar Normalized
Decimal Value
Bipolar Normalized
Decimal Value 0000 0 0/16 = 0.0000 -8/8 = -1.000 0001 1 1/16 = 0.0625 -7/8 = -0.875 0010 2 2/16 = 0.1250 -6/8 = -0.750 0011 3 3/16 = 0.1875 -5/8 = -0.625 0100 4 4/16 = 0.2500 -4/8 = -0.500 0101 5 5/16 = 0.3125 -3/8 = -0.375 0110 6 6/16 = 0.3750 -2/8 = -0.250 0111 7 7/16 = 0.4375 -1/8 = -0.125 1000 8 8/16 = 0.5000 0/8 = 0.000 1001 9 9/16 = 0.5625 1/8 = 0.125 1010 10 10/16 = 0.6250 2/8 = 0.250 1011 11 11/16 = 0.6875 3/8 = 0.375 1100 12 12/16 = 0.7500 4/8 = 0.500 1101 13 13/16 = 0.8125 5/8 = 0.625 1110 14 14/16 = 0.8750 6/8 = 0.750 1111 15 15/16 = 0.9375 7/8 = 0.875
The Telephone System and Multiplex Systems 14
Because of the different possible voltage levels and
the widely different decimal values of the binary number system as the number of bits is changed, it is frequently desirable to normalize the levels of both the analog signal and the digital words so that the maximum magnitudes of both forms have (or at least) approached unity.
Normalized Input = Actual Input Analog Voltage Analog Voltage FSV of A/D Converter where FSV = Full-Scale Voltage Actual Output = Normalized Value of x FSV of Analog Voltage Digital Word DAC Two Most Common Forms Employed in A/D
Conversion 1. Unipolar 2. Bipolar
The Telephone System and Multiplex Systems 15
Unipolar Encoding The unipolar representation is most appropriate
when the analog signal x(t) is always of one polarity (including zero). If the signal is negative, it can be inverted before sampling.
The normalized range, x, is therefore:
0 ≤ x < 1 Let xu be the unipolar quantized decimal representation of x following the A/D conversion. The maximum value of xu is therefore:
xu(max) = 1 - 2-n Let ∆xu be the normalized step size, which represents in a decimal value the difference between successive levels.
∆xu = 2-n
The Telephone System and Multiplex Systems 16
Bipolar Encoding The bipolar representation is most appropriate
when the analog signal x(t) has both polarities. The normalized range, x, is therefore:
-1 ≤ x < 1
Let xb be the bipolar quantized decimal representation of x following the A/D conversion. The maximum value of xb is therefore:
xb(max) = 1 - 2-n + 1 Let ∆xb be the normalized step size, which
represents in a decimal value the difference between successive levels.
∆xb = 2-n + 1
The Telephone System and Multiplex Systems 17
Quantization may be done in two ways:
1. Rounding. The sampled value of the analog
signal is assigned to the nearest quantized level.
2. Truncation. The sampled value is adjusted
to the next lowest quantized level.
Example:
A certain 5-bit A/D converter with a FSV of 8V is
to be employed in a binary PCM system. The input analog signal is adjusted to cover the range from zero to slightly under 8V, and the converter is connected for unipolar encoding.
1. What is the normalized step size? 2. What is the actual step size in volts?
3. What is the normalized maximum quantized analog level?
4. What digital word would the value 0.51 V
correspond to? 5. What voltage would the digital word 00101
correspond to?
The Telephone System and Multiplex Systems 18
Solution:
1. Normalized Step Size
∆xu = 2-n = 2-5 = 0.03125
2. Actual Step Size Actual Step Size = ∆xu x FSV = 0.03125 x 8 = 0.25 v
3. Normalized Maximum Quantized Analog
Level
xu(max) = 1 - 2-n = 1 - 2-5 = 0.96875
The Telephone System and Multiplex Systems 19
Natural Binary Number
Decimal Value
Unipolar Normalized Decimal Value
Actual Value in volts
00000 0 0/32 = 0.0000 0 v 00001 1 1/32 = 0.03125 0.25 v 00010 2 2/32 = 0.0625 0.5 v 00011 3 3/32 = 0.09375 0.75 v 00100 4 4/32 = 0.125 1.0 v 00101 5 5/32 = 0.15625 1.25 v 00110 6 6/32 = 0.1875 1.5 v 00111 7 7/32 = 0.21875 1.75 v
.
.
.
.
.
.
.
.
.
.
.
. 11011 27 27/32 = 0.84375 6.75 v 11100 28 28/32 = 0.875 7.0 v 11101 29 29/32 = 0.90625 7.25 v 11110 30 30/32 = 0.9375 7.50 v 11111 31 31/32 = 0.96875 7.75 v
4. What digital word would the value 0.51 V
correspond to? 00010
5. What voltage would the digital word 00110
correspond to? 1.5 v
The Telephone System and Multiplex Systems 20
Rounding or truncation always results in quantization
error since the rounded or truncated value can never be recovered at the receiver side. For unipolar representation: Normalized Resolution = ± ½ ∆xu = ± 2-(n+1) Actual Resolution = ± 2-(n+1) x FSV % Resolution = ± 2-(n+1) x 100% For bipolar representation: Normalized Resolution = ± ½ ∆xb = ± 2-n Actual Resolution = ± 2-n x FSV % Resolution = ± 2-n x 100%
To minimize quantization errors, the system should use more bits.
The Telephone System and Multiplex Systems 21
Example: A certain 5-bit A/D converter with a FSV of 8V is
to be employed in a binary PCM system. The input analog signal is adjusted to cover the range from zero to slightly under 8V, and the converter is connected for unipolar encoding.
1. What is the normalized resolution? 2. What is the actual resolution in volts? Solution: Normalized Resolution = ± ½ ∆xu = ± 2-(n+1) = ± 2-(5+1) = ± 0.015625 Actual Resolution = ± 2-(n+1) x FSV = ± 0.015625 x 8 = ± 0.125 v
The Telephone System and Multiplex Systems 22
F Time division multiplexing is used to combine several
digital voice signals into one channel. This technique interleaves more than one individual digital signal into another channel by giving each original signal time slots in the multiplexed channel.
TIM
E D
IVIS
ION
MU
LT
IPL
EX
ER T
IME
DIV
ISION
MU
LT
IPL
EX
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ADC
Telephone
DAC
Telephone
ADC
Telephone
DAC
Telephone
ADC
Telephone
DAC
Telephone
1
2
n
1
2
n
byte 1from
channel1
byte 1from
channel2
. . .byte 1from
channeln
byte 2from
channel1
byte 2from
channel2
. . .byte 2from
channeln
. . .
MULTIPLEXED DATA:
The Telephone System and Multiplex Systems 23
F Case Study: T1 Channel Banks AT&T combines 24 digitized voice signals into one
high-capacity channel called the T1 Carrier.
data rate per phone = 8000 samples/sec x 8
bits/sample (output of each codec) = 64,000 bps total data rate of T1 = 64,000 bps x 24 = 1.536
Mbps
TIM
E D
IVIS
ION
MU
LT
IPL
EX
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CO
DE
C
Telephone
Telephone
Telephone
1
2
24
Central Office
T1 Channel
1.544 Mbps
The Telephone System and Multiplex Systems 24
channel