Telecommunication
Systems Solution
Manual
ICE-3205
MUHAMMAD MAHBUB SARWAR SHAFI
7/25/22
TS Solution Manual
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Contributed By:
Mahan Choudhury (2054901002)
Jannatul Ferdous Mirza (2054901042)
Farah Ireen (2054901045)
Samrin Shanjana Flavia (2054901046)
Muhammad Mahbub Sarwar Shafi (2054901049)
Solution Manual of:
Fundamentals of Telecommunications, Second Edition, Roger L. Freeman
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Contents
Chapter-01 .............................................................................................................................................. 3
Chapter-02 .............................................................................................................................................. 8
Chapter-03 ............................................................................................................................................ 13
Chapter-04 ............................................................................................................................................ 17
Chapter-05 ............................................................................................................................................ 23
Chapter-06 ............................................................................................................................................ 26
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Chapter-01
1. Define telecommunications.
Answer: The term "telecommunications" describes the transmission of information over
a long distance using electronic and electrical devices. A comprehensive
telecommunication system consists of two or more stations that are each outfitted with a
transmitter and a receiver.
2. Identify end-users.
Answer: The word "end user" refers to the individual for whom a hardware or software
product is created, as opposed to the product's creators, installers, and service providers.
3. What is/are the function(s) of a node?
Answer: A node is a point or junction in a transmission system where lines and trunks
meet. A node usually carries out a switching function.
4. Define a connectivity.
Answer: A connectivity links an end-user to a node, and from there possibly through other
nodes to some final end-user destination with which the initiating end-user wants to
communicate.
5. What are the three phases of a telephone call?
Answer: There are three sequential stages to a telephone call.
a. Call setup
b. Information exchange
c. Call takedown
6. Describe on-hook and off-hook.
Answer: On-hook is a state where a landline phone is currently "hung up." It cannot dial
out to another number. When in this position, the line becomes available, and no calls can
be made from the phone.
Off-hook describes a state that a phone currently is in when the phone is taken off the
hook or placed in the off-hook position. Off-hook causes the phone line to be busy and
allows the user to place phone calls or answer incoming calls.
7. What is the function of the subscriber loop?
Answer: This is just a pair of copper wires connecting the battery and switch out to the
subscriber premises and then to the subscriber instrument. The action of current flow
alerts the serving exchange that subscriber requests service. When the current starts to
flow, the exchange returns a dial tone, which is audible in the headset
8. What is the function of the battery?
Answer: The current source is a “battery” that resides at the local serving switch. It is
connected by the subscriber loop.
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9. Describe address signaling and its purpose.
Answer:
If the called subscriber and the calling subscriber are in the same local area, only seven
digits need be dialed. These seven digits represent the telephone number of the called
subscriber (user). This type of signaling, the dialing of the digits, is called address signaling.
The digits actuate control circuits in the local switch, allowing a connectivity to be set up.
If the calling and called subscribers reside in the serving area of that local switch, no further
action need be taken.
10. Differentiate trunks from subscriber loops (subscriber lines).
Answer:
Subscriber loops
1. A subscriber loop is a pair of copper wires
that connect the battery and switch out to the
subscriber premises and then to the subscriber
instrument.
2. Subscriber loops connect end-users or
subscribers to a local serving switch.
Trunks
1. Trunk is a transmission path
between exchanges or central offices.
2. Trunk’s interconnect exchanges or
switches.
11. What is the theoretical capacity of a four-digit telephone number? Of a three-digit
exchange number?
Answer:
The telephone number, as we used it above, is seven digits long. For example:
234–5678
The last four digits identify the subscriber line; the first three digits (i.e., 234) identify the
serving switch (or exchange). For a moment, let’s consider theoretical numbering capacity.
The subscriber number, consisting of the last four digits, has a theoretical numbering
capacity of 10,000. The first telephone number issued could be 0000; the second number,
if it were assigned in sequence, would be 0001, the third would be 0002, and so on. At the
point where the numbers ran out, the last number issued would be 9999. The first three
digits of the example above contain the exchange code (or central office code). These three
digits identify the exchange or switch. The theoretical maximum capacity is 1000. If again
we assign numbers in sequence, the first exchange would have 001, the next 002, then 003,
and finally 999.
12. What is the common colloquial name for an NPA code?
Answer: When long-distance service becomes involved, we must turn to using still an
additional three digits. Colloquially we call these area codes. In the official North American
terminology used in the NANP is “NPA” for numbering plan area, and we call these area
codes NPA codes.
13. What is the rationale for having a tandem switch?
Answer: A tandem switch is used to interconnect other switches via trunks. Thus, tandem
switches are always part of a series of switches and lines that connect telephone callers to
each other.
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14. Define grade of service. What value would we have for an objective grade of service?
Answer: Networks are sized/dimensioned for a traffic load expected during the busy hour.
The sizing is based on probability, usually expressed as a decimal or percentage. That
probability percentage or decimal is called the grade of service. The grade of service can
be defined as “the proportion of total calls, usually during the busy hour, that cannot be
completed immediately or served within a prescribed time.” Grade of service and blocking
probability are synonymous. Blocking probability objectives are usually stated as B =0.01%
or 1%. This means that during the busy hour, 1 in 100 calls can be expected to meet
blockage
15. How can we improve grade of service? Give the downside of this.
Answer: BH traffic intensities are used to dimension the number of trunks required on a
connectivity as well as the size of switches involved. Now a PSTN company
(administration) can improve its revenue versus expenditures by cutting back on the
number of trunks required and making switches “smaller.” Of course, network users will
do a lot of complaining about poor service. Let’s just suppose the PSTN does just that,
cuts back on the number of circuits. Now, during the BH period, a user may dial a number
and receive either a voice announcement or a rapid-cadence tone telling the user that all
trunks are busy (ATB) and to try again later. From a technical standpoint, the user has
encountered blockage. This would be due to one of two reasons or may be due to both
causes. These are: insufficient switch capacity and not enough trunks to assign during the
BH.
16. Give the basic definition of the busy hour.
Answer: Busy hour is defined as the uninterrupted period of 60 minutes during the day
when the traffic offered is maximum.
17. . Differentiate simplex, half-duplex, and full duplex.
Answer: Simplex is one way operation; there is no reply channel provided. Radio and
television broadcasting are simplex. Certain types of data circuits might be based on
simplex operation.
Half-duplex is a two-way service. It is defined as transmission over a circuit capable of
transmitting in either direction, but only in one direction at a time.
Full duplex or just duplex defines simultaneous two-way independent transmission on a
circuit in both directions. All PSTN-type circuits discussed in this text are considered using
full-duplex operation unless otherwise specified.
18. . What is double seizure?
Answer: In two-way trunk operation, any trunk can be selected for operation in either
direction. The incisive reader will observe that there is some fair probability that the same
trunk can be selected from either side of the circuit. This is called double seizure
19. On what kind of trunk would double seizure occur?
Answer: In two-way trunk operation double seizure occur.
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20. What is a full-mesh network? What is a major attribute of a mesh network?
Answer: If every switch in a network is connected to all other switches (or nodes) in the
network, we call this “pattern” a full-mesh network. A mesh topology has multiple
connections, making it the most fault tolerant topology available. Every component of the
network is connected directly to every other component.
21. What are two major attributes of a star network?
Answer: Two major attributes are:
• Most economic nodal patterns both to install and to administer.
• Free to modify by adding direct routes
22. Define a traffic relation.
Answer: Traffic relation simply means the traffic intensity (usually the BH traffic intensity)
we can expect between two known points.
23. On a hierarchical network, what is final route?
Answer: If we remove the high-usage routes, the backbone structure remains. In the
terminology of hierarchical networks, the backbone represents the final route from which
no overflow is permitted.
24. Give at least three reasons for the trend away from hierarchical routing.
Answer:
The change is brought about due to two factors: transmission and switching. Since 1965,
transmission techniques have taken leaps forward. Satellite communications allowed direct
routes some one-third the way around the world. This was followed by the introduction
of fiber-optics transmission providing nearly infinite bandwidth, low loss, and excellent
performance properties. These transmission techniques are discussed in Chapter 9. In the
switching domain, the stored program control (SPC)11 switch had the computer brains to
make nearly real-time decisions for routing. This brought about dynamic routing such as
AT&T DNHR (dynamic nonhierarchical routing). The advent of CCITT Signaling System
No. 7 (Chapter 7) working with high-speed computers made it possible for optimum
routing based on real-time information on the availability of route capacity and shortest
routes. Thus, the complex network hierarchy started to become obsolete.
25. . List at least six QoS items.
Answer: Elements to be listed under QoS are:
• Can connectivity be achieved?
• Delay before receiving dial tone (dial tone delay).
• Post dial(ing) delay (time from the completion of dialing the last digit of a number
to the first ring-back of the called telephone). This is the primary measure of
signaling quality.
• Availability of service tones [e.g., busy tone, telephone out of order, time out, and
all trunks busy (ATB)].
• Correctness of billing.
• Reasonable cost of service to the customer.
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26. List at least one international standardization body, one regional standardization
group, and three U.S. standardization organizations.
Answer:
ISO (International Organization for Standardization) is a standardization body.
ETSI, (European Telecommunication Standardization Institute) is a regional
standardization group.
Three U.S Standardization Organizations are:
• Electronics Industries Association (EIA)
• Telecommunication Industry Association (TIA)
• American National Standards Institute (ANSI)
27. Define a POP and POT
Answer: A Point of Presence is a location where a long-distance carrier could terminate
services and provide connections into a local telephone network LATA.
A Point of terminations (POT) the last point of service rendered by a commercial carrier
under applicable tarif or networks.
The POT generally is a distribution frame or other item of equipment at which the LEC’s
access facilities terminate and where cross-connection, testing, and service verification.
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Chapter-02
1. Name at least four different ways of communicating at a distance prior to the
advent of electrical communication.
Answer: Here are the four different ways of communicating at a distance prior to the
advent of electrical communication:
a. People use the bell in the church tower to call people for the religious service. Or
they can use the bell to announce a death
b. For the fall of Troy, the Greeks used a relay of signal fires. The also used the
signal fire in the distance to announce the victory of Troy
c. They are trained to use the semaphore. The semaphore is slow, but it can be used
in some miles of distance using the manual version
d. The American Indian used the smoke at day and the fire at night. Or the African
can use the drum for distance communication
2. What kind of energy is stored in a battery?
Answer: A battery is made up several cells. Commonly, a battery is made up by four
cells, a dry cell stores chemical energy. Its positive electrode is connected through some
resistive device to the negative electrode.
3. How did the old electric telegraph communicate intelligence?
Answer: We connect the battery with a length of copper wire looping. It is back to the
other electrode. A buzzer or other sound generating device is inserted into that loop at the
farthest end of the wire before looping back. Now, we will have the essentials of a telegraph
circuit and let it communicate intelligence.
4. What limited the distance we could transmit with electrical telegraph before using
a repeater? Give at least two ways we could extend the distance.
Answer: We see that the loop has a certain resistance. This is the function of its length
and the diameter of the wire. The resistance will increase when we try to make the loop
longer. As the length increases, the current in the loop decreases.
Therefore, there will be some point where the current is so low, and the buzzer will not
work. We can maximum the loop length by using the wire with a greater diameter. Or we
can use the relay technique. In this technique, at the far end of the loop, a human operator
copies the message and retransmits it down the next leg of the circuit.
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5. How could that old-time electrical telegraph operate with just one wire?
Answer: We use one metallic wire connecting the west station to the east station. The
second wire is replaced with ground. The earth is a good conductor, and so we use earth,
called ground, as the second conductor (or wire).
6. Name at least four ways we might characterize a “sine wave” either partially or
wholly.
Answer: There are four ways that we can characterize a “sine wave” as:
• The frequency is the number of times per second that a wave cycle repeats at given
amplitude
• If we see the y = sin x, “a sine wave” is developed where x is expressed in radians.
• The lower the frequency and the amplitude is down at any single point, and the
maximum excursion is in negative direction
• The up the frequency and the amplitude is up at any single point, and the maximum
excursion is in positive direction
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7. What is the equivalent wavelength (λ) of 850 MHz; of 7 GHz?
Answer:
Radio waves travel at 3 × 108 m/sec.
850MHz in Hz is 850,000,000 (in Hz).
We know that Fλ=3×108
So now,
850,000,000 λ = 3 × 108 m/s
850×106 λ = 3 × 108 m/s
850×λ = 3 × 102 m/s
850×λ = 300
300
λ = 850
λ = 0.375 m
Again,
7,000,000,000 λ = 3 × 108 m/s
7×109 λ = 3 × 108 m/s
70×λ = 3 m/s
3
λ = 70
λ = 0.043 m
8. What angle (in degrees) is equivalent to 3π/2? to π/4?
Answer: We know that,
2πrad=360 degree
So,
3𝜋
2
360×
𝑟𝑎𝑑=
3𝜋
2
=270 degree
2𝜋
Same goes for,
2πrad=360 degree
𝜋
360×
So, 4 𝑟𝑎𝑑=
2𝜋
𝜋
4
=45 degree
9. Give two examples of baseband transmission.
Answer: Two examples of baseband transmission as:
• The 1s and 0s transmitted electrically from a PC
• The alternating current derived from the mouthpiece of a telephone handset.
10. Define modulation.
Answer: Modulation is the process where some characteristic of a high-frequency carrier
signal (bandpass), is changed according to the instantaneous amplitude of the information
(baseband) signal.
11. What are the three generic forms of modulation? What popular device we find in
the home utilizes all three types of modulation simultaneously. The answer needs
a modifier in front of the word.
Answer: There are three generic forms of modulation:
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a. Amplitude modulation (AM): The amplitude of carrier signal is varied according
to the instantaneous amplitude of the modulating message signal m(t).
b. Frequency modulation (FM): In FM the carrier amplitude remains constant, the
carrier frequency varies with the amplitude of modulating signal.
c. Phase modulation (PM): The process by which changing the phase of carrier signal
in accordance with the instantaneous of message signal.
The popular device that we find in the home utilizes all three types of modulation
simultaneously is the television. The video in the TV uses amplitude modulation, the sound
subcarrier uses frequency modulation, and the color subcarrier employs phase modulation.
All of them are in the analog formats.
12. Differentiate an analog signal from a digital signal.
Answer:
Analog Signal
Digital Signal
1) Continuous Signal
1) Discrete Signals
2) Represented by sine waves
2) Represented by square waves
3) Continuous range of values
3) Discontinuous values
4) Records sound waves as they are
4) Waves converts into a binary
waveform.
5) Human Voice, Analog Electronic
5) Computer, optical drives
Devices
13. Give at least four applications of a 1-bit code. We suggest using imagination.
Answer: We can see four applications of 1 bit code as:
• A car engine is running or not running
• A telephone line is active or busy
• A light is on or off
• A door is open or closed
14. What is the total capacity of a 9-bit binary code? The Hollerith code was a 12-bit
code. What was its total capacity?
Answer: We know that an 8-bit binary code has 256 distinct 8-bit sequences (i.e., 28 =
256).
So now for 9-bit binary code, 29=512 capacity and for the
12-bit Hollerith code, 212=4096 capacity.
15. Name four different transmission media.
Answer: There are four types of transmission media:
a. Wire pair: consists of two parallel conductors or wires. The wires commonly use
a copper conductor, although aluminum conductors have been employed.
b. Coaxial cable: A coaxial cable is an electrical cable with a copper conductor and
an insulator shielding around it and a braided metal mesh that prevents signal
interference and cross talk.
c. Fiber-optic cable: Optical fiber cable constitutes very thin glass fibers clustered
in a single cable used for long-distance transmission and high-performance data
networking.
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d. Radio: It is the sending of something (usually information) with the use of radio
waves.
16. What is the opposite of loss? What is the most common unit of measurement to
express the amount of loss?
Answer: The opposite of loss is gain. An attenuator is a device placed in a circuit to
purposely cause loss. An amplifier does just the reverse; that is, it gives a signal gain. An
amplifier increases a signal’s intensity.
Figure 1: Attenuator
Figure 2: Symbol of an amplifier
Loss or attenuation is usually expressed in decibels (dB).
17. What is the reason of twists in twisted pair?
Answer: The reason for twists in twisted pair is that the wire pair transmission suffers
impairments besides loss. One of the impairments is crosstalk. Most of us have experience
with crosstalk in our telephone line. It seems that we hear other voices in our telephone
conversation.
18. What is the principal cause of data rate limitation on wire pair.
Answer: The usual twisted copper pair of wires has limited bandwidth because the signal
frequencies attenuate the longer the wire is due to electromagnetic radiation and due to
conversion of signal energy to heat.
19. What is the principal drawback of using coaxial cable for long-distance
transmission?
Answer: The principal drawback of using coaxial cable for long-distance transmission is
its frequency response was exponential. In other words, its loss increased drastically as
frequency was increased. Thus, equalization was required which tends to level out the
frequency response.
20. What is the principal, unbeatable advantage of fiber-optic cable?
Answer: The principal, unbeatable advantage of fiber optic cable is that the fiber optic
cable has a much greater bandwidth and comparatively flat frequency response. Also, it
can transmit signal in longer distance and have high level security.
21. Regarding limitation of bit rate and length, a fiber-optic cable may be either ____
or ______?
Answer: Regarding limitation of bit rate and length, a fiber optic cable may be either loss
limited, or dispersion limited. If a fiber-optic link is limited by loss, it means that as the
link is extended in distance, the signal has dissipated so much that it becomes unusable.
Dispersion-limited means that a link’s length is limited by signal corruption.
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22. Explain dispersion (with fiber-optic cable).
Answer: Dispersion-limited means that a link’s length is limited by signal corruption. As
a link is lengthened, there may be some point where the bit error rate (BER) becomes
unacceptable. This is caused by signal energy of a particular pulse that arrives later than
other signal energy of the same pulse.
23. What are some typical services of LMDS?
Answer: LMDS stands for Local Multipoint Distribution System in 28/38-HGz bands,
1.2Ghz bandwidth is for some typical services they are CATV, Internet, data, and
telephony services.
Chapter-03
1. Define signal-to-noise ratio.
Answer: Signal-to-noise ratio (S/N or SNR) is the most widely used parameter for
measurement of signal quality in the field of transmission where it compares the level of a
desired signal to the level of background noise. Signal-to-noise ratio expresses in decibels
the amount that signal level exceeds the noise level in a specified bandwidth.
𝑆𝑖𝑔𝑛𝑎𝑙 𝑃𝑜𝑤𝑒𝑟
SNR=
𝑁𝑜𝑖𝑠𝑒 𝑃𝑜𝑤𝑒𝑟
2. Give signal-to-noise ratio guidelines at a receiving device for the following three
media: voice, video-TV, and data. Base the answer on where a typical customer
says the signal is very good or excellent.
Answer: The following are S/N guidelines at the corresponding receiving devices:
• Voice: 40 dB
• Video (TV): 45 dB
• Data: ∼15 dB, based upon the modulation type and specified error performance
3. Why do we use a sinusoidal test tone when we measure S/N on a speech channel
rather than just the speech signal itself?
Answer: Signal-to-noise ratio really has limited use in the PSTN for characterizing speech
transmission because of the “spurtiness” of the human voice. We can appreciate that
individual talker signal power can fluctuate widely so that S/N is far from constant during
a telephone call and from one telephone call to the next. In lieu of actual voice, we use a
test tone to measure level and S/N
4. The noise level of a certain voice channel is measured at −39 dBm, and the testtone signal level is measured at +3 dBm. What is the channel S/N?
Answer: We know that,
(S/N) dB=level (signal in dBm) – level (noise in dBm)
=+3dBm – (-39dBm)
=42dB (Excellent)
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As we know the minimum SNR for voice is 40dB and here the SNR is 42dB. So, this SNR
satisfy the user or to make a receiving instrument function within certain specified criteria
5. If we know the loudness rating of a telephone subset earpiece and of the subset
mouthpiece, what additional data do we need to determine the overall loudness
rating (OLR) of a telephone connection?
Answer: The overall loudness rating (OLR) is defined as the loudness loss between the
speaking subscriber’s mouth and the listening subscriber’s ear via a telephone connection.
• The send loudness rating (SLR) is defined as the loudness loss between the
speaking subscriber’s mouth and an electrical interface in the network.
• The receive loudness rating (RLR) is the loudness loss between an electrical
interface in the network and the listening subscriber’s ear.
• The circuit loudness rating (CLR) is the loudness loss between two electrical
interfaces in a connection or circuit, with each interface terminated by its nominal
impedance.
OLR=SLR+CLR+RLR
6. The BER of an underlying digital circuit is 1 × 10−8, for data riding on this circuit.
What is the best BER we can expect on the data?
Answer:
7. What are the three basic impairments on a telecommunication transmission
channel?
Answer:
There are three basic impairments found in all telecommunication transmission systems.
These are:
• Amplitude (or attenuation) distortion: Change in attenuation (amplitude) at any
frequency with respect to that of a referenced frequency
• Phase distortion: Nonlinear phase shift relative to the frequency of the signal.
(Impacts data transmission VERY much)
• Noise: Noise is an unwanted signal which interferes with the original message
signal and corrupts the parameters of the message signal.
8. Of the three impairments, which one affects data error rate the most and thus limits
bit rate?
Answer: Phase distortion has little effect on speech communications over the
telecommunications network. However, regarding data transmission, phase distortion is
the greatest bottleneck for data rate (i.e., the number of bits per second that a channel can
support). It has probably more effect on limiting data rate than any other parameter.
9. Explain the cause of phase distortion.
Answer: A modulated signal gets distorted on passing through the channel when phase
shift doesn’t change uniformly with frequency, whereas if the phase shift is nonlinear with
respect to frequency, the output signal is distorted with respect to frequency. If the phase–
frequency relationship over a passband is not linear, phase distortion will occur in the
transmitted signal.
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10. Name the four types of noise we are likely to encounter in a telecommunication
system.
Answer: There are four types of noise we will encounter in telecommunication. They are:
• Thermal noise: Thermal noise is a noise based on thermal agitations of electrons.
• Intermodulation noise: caused by nonlinearity in devices; similar to white noise
• Impulse noise: Impulse noise is noncontinuous, consisting of irregular pulses or
noise spikes of short duration and of relatively high amplitude.
• Crosstalk: Crosstalk is the unwanted coupling between signal paths
11. What will be the thermal noise level of a receiver with a noise figure of 3 dB and a
bandwidth of 1 MHz?
Answer: We know,
𝑃𝑛 = −204𝑑𝐵𝑊/𝐻𝑧 + 𝑁𝐹𝑑𝐵 + 10𝑙𝑜𝑔(𝐵)
Here,
Pn=-204dbW/Hz is bandwidth for perfect receiver (at room temperature 290K or
17 degree C)
NFdB (Noise Figure in decibel) =3dB
B (Bandwidth in Hz) = 1 MHz = 1 × 106
𝑃𝑛 = −204𝑑𝐵𝑊/𝐻𝑧 + 3𝑑𝐵 + 10𝑙𝑜𝑔(1 × 106 )
𝑃𝑛 = −204𝑑𝐵𝑊/𝐻𝑧 + 3𝑑𝐵 + 60 𝑑𝐵
𝑃𝑛 = −141 𝑑𝐵𝑊
12. Define third-order products based on the mixing of two frequencies F1 and F2.
Answer:
13. Give four causes of impulse noise.
Answer: The causes of impulse noise are
• lightning,
• car ignitions,
• mechanical switches (even light switches),
• fluorescent lights.
14. Relate VU to dBm for a simple sinusoidal signal. Relate VU to dBm for a complex
signal such as human voice.
Answer: VU, standing for volume unit. When a VU meter is used to measure the level of
a voice signal, it is difficult to exactly equate VU and dBm. One of the problems, of course,
is that speech transmission is characterized by spurts of signal. However, a good
approximation relating VU to dBm is the following formula:
Average power of a telephone talker ≈ VU − 1.4 (dBm).
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15. Echo as an annoyance to a telephone listener varies with two typical causes. What
are they? What is the most important (most annoying)?
Answer: Echo as an annoyance to a telephone listener varies with two typical causes,
where annoyance is a function of the delay time and the intensity (level). Delay time is the
most annoying.
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Chapter-04
1. Define switching in light of transmission.
Answer: Transmission provides the transport of a signal from an end-user source to the
destination such that the signal quality at the destination meets certain performance criteria.
Switching selects the route to the desired destination that the transmitted signal travels by
the closing of switches in either the space domain or the time domain or some
combination(s) of the two.
2. Define signaling.
Answer: Signaling develops and carries the control information for switches. If a
transmission path becomes impaired, signaling becomes ineffectual and the distant-end
switch either will not operate or will not function correctly, misrouting the connectivity.
3. Define calling rate and holding time.
Answer:
a. Calling rate, or the number of times a route or traffic path is used per unit time
period; or more properly defined, “the call intensity per traffic path during the busy
hour (BH)”;
b. Holding time, or “the average duration of occupancy of one or more paths by
calls.”
4. Define lost calls using the terms offered traffic and carried traffic.
Answer: Carried traffic is the volume of traffic actually carried by a switch, and offered
traffic is the volume of traffic offered to a switch. Offered traffic minus carried traffic
equals lost calls. A lost call is one that does not make it through a switch. A call is “lost”
usually because it meets congestion or blockage at that switch.
5. Why are call attempts so important in the design of modern SPC switches?
Answer:
6. Suppose the average holding time of a call is 3.1 minutes and the calling rate in the
busy hour is 465 on a particular work day. What is the traffic flow?
Answer: The traffic flow (A) would then be 465 × 3.1, or 1441.5 call-minutes (Cm) or
1,441.5 /60, or about 24.025 call-hours (Ch).
7. Under normal operating conditions, when can we expect blockage (of calls)?
Answer: The probability of encountering blockage is an important parameter in traffic
engineering of telecommunication systems. If congestion conditions are to be met in a
telephone system, we can expect that those conditions will usually be encountered during
the BH.
8. The statistics during the BH for a particular exchange is 5 lost calls in 841 offered
calls. What is the grade of service?
Answer:
Grade of service = Number of lost calls/Number of offered calls
= 5/(841 + 5)
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= 6/846
p ≈ 0.007
9. When dealing with traffic formulas and resulting traffic tables, we have to know
how lost calls are “handled.” What are the three ways that lost calls may be
handled?
Answer:
In conventional telephone traffic theory, three methods are considered for the handling or
dispensing of lost calls:
1. Lost calls held (LCH): the telephone user will immediately reattempt the call on
receipt of a congestion signal and will continue to redial.
2. Lost calls cleared (LCC): the user will hang up and wait some time interval before
reattempting if the user hears the congestion signal on the first attempt
3. Lost calls delayed (LCD): the user is automatically put in queue (a waiting line or
pool)
10. On a probability-distribution curve, define the mean.
Answer: The mean is a point on the probability distribution curve where an equal number
of events occur to the right of the point as to the left of the point. Mean is synonymous
with average.
11. What percentage of events are encompassed in one standard deviation?
Answer: one standard deviation either side of the mean in above figure will contain about
68% of the population or measurements.
12. What traffic formula (and resulting traffic tables) would be used for modern digital
exchanges (we assume such exchanges are SPC based)?
13. Consider one traffic relation that has been designed to meet grade of service
objectives. We vary one characteristic—the number of traffic channels. Argue the
case for efficiency for 10 circuits versus 49 circuits during the BH. Assume grade of
service as 0.01 and $1.00 per erlang.
14. Define serving time when dealing with waiting systems (Erlang C).
Answer: The term serving time is the time a call takes to be served from the moment of
arrival in the queue to the moment of being served by the switch.
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15. Leaving aside issues of survivability, describe how alternative routing can improve
grade of service or allow us to reduce the number of trunks on a traffic relation and
still meet grade of service objectives.
16. Why is data traffic so different from telephone traffic?
Answer: Data traffic usually consists of short, bursty transactions from a few milliseconds
duration and on the other hand telephone traffic as the aggregate of telephone calls over a
group of circuits or trunks with regard to the duration of calls as well as their number to
several seconds, depending on the data transmission rate
17. Distinguish between a tandem/transit switch and a local serving switch.
Answer: A transit switch is just a tandem switch that operates in the long distance or “toll”
service. A local switch has an area of responsibility. We call this its serving area. All
subscriber loops in a serving area connect to that switch responsible for the area.
18. Why use a tandem switch in the first place?
Answer:
• Because of protocols
• The tandem is eliminated for that traffic relation.
• Tandem switch that operates in the long distance or “toll” service.
19. What are the three basic requirements of a telephone switch?
Answer:
a. Establishing Connection: An exchange (a switch) must be able to connect any
incoming call to one of a multitude of outgoing circuits.
b. Terminating Connection: It has the ability not only to establish and maintain (or
hold) a physical connection between a caller and the called party for the duration
of the call, but also to be able to disconnect (i.e., “clear”) it after call termination.
c. It also has the ability to prevent new calls from intruding into circuits that are
already in use. To avoid this, a new call must be diverted to another circuit that is
free or it must be temporarily denied access where the caller will hear a “busy back”
(i.e., a tone cadence indicating that the line is busy) or an “all trunks busy” tone
cadence signal or voice announcement (i.e., indicating congestion or blockage).
20. For a local serving switch with 10,000-line capacity, exemplify concentration ratios
(lines/trunks) for a residential area, for an industrial/office area.
21. List six of the eight basic functions of a local switch.
Answer:
There are eight basic functions that must be carried out by a conventional switch or
exchange:
a. Interconnection
b. Control
c. Alerting
d. Attending
e. Information receiving
f. Information transmitting
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g. Busy testing
h. Supervisory
22. Give the three basic functions of a local serving switch.
Answer:
• The supervision function
• the control function
• attending–alerting function.
23. In a very small town, its local serving exchange has only three-digit subscriber
numbers. Theoretically, what is the maximum number of subscribers it can serve?
Answer: Theoretically, the maximum number of subscribers the local serving switch can
serve is 999.
24. How many switch banks will a step-by-step (SXS) switch have if it is to serve up to
10,000 subscribers?
25. What happens to a line that is “busied out?”
Answer:
Busied Out means that a line or connection is taken out of the pool because it is busy, it is
being used, and is not available for others to utilize.
26. Give at least three advantages of an SPC exchange when compared with a registermarker crossbar exchange.
27. What are the three basic functional blocks of an SPC exchange (SPC portion only)?
28. Differentiate remote concentrators from remote switches. Give one big advantage
each provides.
Answer:
• Concentrators or line concentrators consolidate subscriber loops, are remotely
operated, and are a part of the concentration and expansion portion of a
switchplaced at a remote location.
• A remote switch, sometimes called a satellite, or satellite exchange, originates and
terminates calls from the parent exchange. It differs from a concentrator in that
local calls are served by the remote switch
29. Describe two-wire operation and four-wire operation.
Answer:
• Two-wire operation:
A telephone conversation inherently requires transmission in both directions.
When both directions are carried on the same pair of wires, it is called two-wire
transmission.
• Four-wire operation:
Carrier and radio systems require that oppositely directed portions of a single
conversation occur over separate transmission channels or paths. Thus we have
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two wires for the transmit path and two wires for the receive path, or a total of
four wires, for a full-duplex (two-way) telephone conversation.
30. What is the function of a hybrid (transformer)?
Answer: A hybrid, in terms of telephony (at voice frequency), is a transformer with four
separate windings. Based on a simplified description, a hybrid may be viewed as a power
splitter with four sets of wire-pair connections.
The functions of a hybrid transformer are controlling the grid voltage, current, active and
reactive power flow, balancing three-phase loads and can actively filter harmonics as well.
31. Describe the function of the balancing network as used with a hybrid.
Answer: Balancing network, which electrically balances the hybrid with the two-wire
connection to the subscriber subset over the frequency range of the balancing network.
Balancing, in this context, means matching impedances—that is, the impedance of the
two-wire side to the hybrid two-wire port.
32. There are two new telephone network impairments we usually can blame on the
hybrid. What are they?
Answer: Echo and Singing. The cause of echo is defined as any impedance mismatch in
the circuit. It is most commonly caused by this mismatch that occurs at the hybrid. Echo
that is excessive becomes singing. Singing is caused by high positive feedback on the
intervening amplifiers. Singing on the analog network could take the network down by
overloading multiplex equipment.
33. What is the balance return loss on a particular hybrid connectivity when the
balancing network is set for a 900-loop, and this particular loop has only a 300 Ohm
impedance?
34. What are the two generic methods of multiplexing?
Answer: There are essentially two generic methods of multiplexing information channels:
a. In the frequency domain, we call this frequency division multiplex (FDM).
b. In the time domain, which we call time division multiplex (TDM)
35. A mixer used in an FDM configuration has a local oscillator frequency of 64 kHz
and is based on the CCITT modulation plan. After mixing (and filtering), what is
the resulting extension of the desired frequency band (actual frequency) limits at 3
dB points)?
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36. The standard CCITT supergroup consists of groups and occupies the frequency
band kHz to kHz? How many standard voice channels does it contain?
Answer: A supergroup contains five standard CCITT groups, equivalent to 60 voice
channels. The standard supergroup, before further translation, occupies the frequency
band of 312 kHz to 552 kHz.
37. What are pilot tones and what are their purpose in an FDM link?
Answer: Pilot tones are used to control level in FDM systems. They may also be used to
actuate maintenance alarms.
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Chapter-05
1. Define the voice channel using the band of frequencies it occupies from the CCITT
perspective. What is its bandwidth?
Answer: A voice-band channel as “a channel that is suitable for transmission of speech or
analog data and has the maximum usable frequency range of 300 to 3400 Hz.”
CCITT/ITU-T also defines it in the range of 300–3400 Hz. Its Bandwidth is 3100Hz.
2. Where does the sensitivity of a telephone set peak (i.e., at about what frequency)
from a North American perspective, from a CCITT perspective?
Answer: The frequency response of the carbon transmitter peaks between 800 and 1000
Hz.
3. A local serving switch provides a battery emf source for the subscriber loop. What
is the nominal voltage of this battery? Name at least three functions that this emf
source provides.
Answer: Battery voltages have been standardized at −48 V dc. The three functions that
this emf source provides:
a. Talk battery.
b. An ac ringing voltage for the bell or other alerting device on the telephone
instrument supplied from a special ringing voltage source.
c. The telephone dial, when operated, makes, and breaks the dc current on the
closed loop, which indicates to the switching equipment the telephone number of
the distant telephone with which communication is desired.
4. A subscriber loop is designed based on two limiting conditions (impairments).
What are they?
Answer: The two basic criteria, which limit loop length, must be considered when
designing a subscriber loop:
a. Attenuation (loss) limits: Attenuation (loss) must be limited to keep within
loudness rating requirements.
b. Resistance limits
5. What is the North American maximum loss objective for the subscriber loop?
Answer: In North America the maximum loss objective is 8 dB for a subscriber loop. In
some other countries, that value is 7 dB. It takes two subscriber loops to make a
connection: the subscriber loop of the calling subscriber and the loop of the called
subscriber.
6. What happens when we exceed the “signaling” limit on a subscriber loop?
Answer: Once we exceed the signaling limit when the telephone goes off-hook, no dial
tone is returned. This just means that there is insufficient loop current to actuate the switch,
telling the switch we wish to make a call.
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7. What are the two effects that a load coil has on a subscriber loop or a metallic VF
trunk?
Answer: Loading a particular subscriber loop consists of inserting loading coils into the
loop at fixed distance intervals. Adding load coils tends to:
a. Decrease the velocity of propagation
b. Increase the impedance
8. Using resistance design or revised resistance design, our only concern is resistance.
What about loss?
Answer: If all of the RD requirements are followed, 8 Db standard for loss is achievable.
In addition, certain other strategies are employed to ensure this is the case. (Add inductive
load to longer loops etc.)
9. What is the switch design limit of RD, of RRD?
Answer: RRD covers subscriber loop as long as 24 kft. Loop length is broken down into
two ranges: From 0 to 18,000 ft the maximum loop resistance is 1300 Ω, and from 18,000
to 24,000 ft the maximum loop resistance is 1500 Ω.
10. Where will we apply long route design (LRD)?
Answer: The subscribers which are outside of the boundary of the resistance design
boundary that is served by the switch, there Long Route Design (LRD) is applied. Also,
Scarcely Populated and in remote areas LRD is used.
11. Define a range extender.
Answer: A range extender increases the battery voltage to either −84 or −96 V dc. In
some texts the term loop extender is used rather than range extender.
12. With RRD, on loops greater than 1500 Ω, what expedients do we have using
standardized design rules?
Answer: With RRD, on loops greater than 1500, what expedients do we have using
standardized design rules?  The range beyond 1500 ohm is served by MLRD (modified
long route design), CREG (concentration range extender with gain) or DLC (digital loop
carrier). Loop resistances up to 1500 ohm are served by RRD procedures.
13. What kind of inductive loading is used with RRD?
Answer: H88 inductive loading is used with the RRD on loops longer that 18,000ft. Twogauge combinations may be employed selected from the following three wire gauges: 22,
24, 26 gauge.
14. Beyond what limit on a subscriber loop is it advisable to employ DLC?
Answer: Of the three long route design techniques, digital loop carrier is the most
attractive, especially for facilities greater than 28,000 ft.
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15. For VF trunks, if D is the distance between load coils, why is the first load coil
placed at D/2 from the exchange?
Answer:
The distance (D) between load coils is all important. The spacing (D) should not vary more
than ±2 from the specified spacing. The first load coil is spaced D/2 from an exchange
main frame, where D is the specified distance between load coils.
16. Define a main frame.
Answer: This is a facility, often a frame at a switching center, where all circuits terminate
and where they may be cross connected. This is a location where we can get physical or
virtual access to a circuit and where we may reconfigure assets.
17. What does line build-out do?
Answer: Line build-out networks are used to increase the electrical length of a wire-pair
cable section. These networks range in complexity from a simple capacitor that simulates
the capacitance of the missing cable length to artificial cable sections.
18. What are the two types of VF repeaters that may be used on VF trunks or subscriber
loops? What are practical gain values used on these amplifiers?
Answer: Trunks using VF repeaters gains may be as high as 6–8 dB. Another repeater
commonly used on two-wire trunks is the negative-impedance repeater. This repeater can
provide a gain as high as 12 dB, but 7 or 8 dB is more common in practice.
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Chapter-06
1. What is the major overriding advantage of binary digital transmission? Give at least
two secondary advantages.
2. Name the three steps to develop a PCM signal from an analog signal.
Answer: There are three steps in the development of a PCM signal from that analog
model:
a. Sampling
b. Quantization
c. Coding
3. Based on the Nyquist sampling theorem, what is the sampling rate for a nominal
4-kHz voice channel? for a 56-kHz radar product signal?
Answer: The nominal 4-kHz voice channel: sampling rate is 8000 times per second (i.e., 2
× 4000). An analog radar product channel 56 kHz wide: Sampling rate is 112,000 times
per second (i.e., 2 × 56,000).
4. If I’m transmitting 8000 frames per second, what is the duration of one frame?
5. Give a simple definition of quantization distortion.
Answer: The distortion introduced in the process of quantization. The distorted signal can
be considered as the original signal with quantization noise added.
6. Our system uses linear quantization. How can we reduce quantization distortion
(noise)?
7. What is the negative downside to increasing quantization steps?
8. Companding used in PCM systems follows one of two laws. Identify each law—its
name and where is it applied.
9. How many bits are there in a PCM word? How many different binary possibilities
can be derived from a PCM word or codeword?
10. If a PCM code received starts with a 0, what do we know about the derived voltage
sample?
11. Bits 2, 3, and 4 of the PCM codeword identify the segment. How many total
segments are there?
12. In DS1, of what use is the framing bit?
13. How is the identification of frame beginning carried out in E1?
14. How is signaling carried out in DS1? in E1?
15. How do we arrive at 1.544 Mbps for DS1?
16. With the AMI line code, how is the zero coded?
17. If, with bipolar transmission, under “normal” circumstances, the first 1 is a −1.0 V,
what would the second “1” be?
18. Where does a wire-pair cable or light-wave regenerative repeater derive its timing
from?
19. What is the purpose of stuffing on a higher-order PCM multiplex?
20. In the North American digital hierarchy, just from its nomenclature, what does an
M34 multiplex do?
21. On a digital link, how would excessive loss affect signal quality?
22. Define jitter from the perspective of a PCM link.
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23. The principal cause of systematic phase jitter is a function of.
24. What value BER can we expect as a mean for the North American digital network?
25. What is the threshold BER for the digital network? Why is it set at that point?
26. Give at least three “economic” advantages of digital switching.
27. Define the function of a time-slot interchanger.
28. If we have a switch that is only a single time-slot interchanger for an E1 system,
how many different subscribers could I be connected to?
29. What are the three functional blocks of a conventional time-slot interchanger (i.e.,
a time switch)?
30. How can the capacity of a time-slot interchanger be increased? Give two methods.
31. How can a switch manufacturer sell just one switch to cover both E1 and DS1
regimes?
32. What is the function of a junctor in a digital switch?
33. What are the two primary factors that can change a national digital network
structure?
34. What causes a slip on a digital network?
35. What is the difference between controlled slips and uncontrolled slips?
36. Argue the efficacy of using an external timing source to synchronize network
elements.
37. What is a disciplined oscillator?
38. What is holdover stability?
39. According to CCITT, if we can maintain long-term frequency departure to better
than
1 × 10−11, what kind of slip performance can we expect?
40. Define a burst errored second.
41. What kind of slip rate could we expect if all network timing references were lost?
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