Overview of the

Telecommunications Network

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Telecommunications Network

Telecommunication Networks

Basic purpose of a telecommunications network: transmit user information

in any form to another user of the network.

Many forms of networks, such as voice or data; subscribers may use different

access network technologies to access the network, for example, fixed or cellular

telephones.

Different services, such as data, fixed, or cellular telephony service.

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Different services, such as data, fixed, or cellular telephony service.

(1) Transmission

(2) Switching

(3) Signaling.

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Three technologies needed for communication through the telephone

network (POTS, plain old telephone service):

Transmission

Transmission is the process of transporting information between end points

of a system or a network.

Transmission systems use four basic media for information transfer from one point to

another:

1. Wire-pair (copper) cables, such as those used in telephone subscriber lines (access

network)

2. Optical fiber cables, such as those used in high-data-rate transmission in

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2. Optical fiber cables, such as those used in high-data-rate transmission in

telecommunications networks (especially in transport networks)

3. Radio waves, such as microwave radio links, cellular telephones and satellite

transmission

4. Free-space optics, and infrared communications (limited application, e.g. PC-to-PC

short range links).

17Fiber optics Twisted pairs Radio links

Manual Switching4

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A manual telephone exchange (1896)

An old rotary dial telephone

Automatic Switching

Switching: only a few cable connections are

needed between exchanges because the

number of simultaneously ongoing calls is

much smaller than the number of telephones.

A basic telecommunications network

Almon B. Strowger in

1887 developed the first

automatic switch (i.e.

First switches were not automatic so switching

was done manually using a switchboard.

Conceptual operation of a

step-by-step (SXS) switch

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automatic switch (i.e.

electro-mechanical).

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step-by-step (SXS) switch

If a subscriber dials a “3”, three

pulses from her telephone are

transmitted to the switch. The

switch then steps to level 3 in

the 1st relay bank. The 2nd relay

bank is now connected waiting

for the second dialed digit. It

accepts the second digit, a “7”,

and steps to appropriate position

and connects to the 3rd relay

bank, and so on.Called subscriber is “375”

Signaling

Signaling is the mechanism that allows network entities (customer premises

or network switches) to establish, maintain, and terminate sessions in a network.

Signaling examples on subscriber lines (i.e. between telephone and its central office):

• Off-hook condition

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• Dial

• On-hook condition

Signaling needed between exchanges as well because most calls have to be

connected via more than just one exchange.

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Operation of Ordinary Telephone 1

Local exchanges have a large-capacity battery that keeps the exchange and subscriber

sets operational for a few hours if the supply of electricity is cut off.

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A subscriber line, also called “local loop”, which carries speech signals is a twisted pair.

Power supply comes from the exchange site: basic telephone service independent of

the local electric power network.

Local loop

Telephone

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In its simplest form a telephone consists of:

(a) one microphone and one earphone, combined into a single handheld unit

(b) means of signaling the exchange when a call is to be made

(c) means of sending to the exchange the ‘address’ information of the called subscriber

(d) an incoming call alarm

(e) means of signaling the exchange that the call has been answered.

Telephone

Operation of Ordinary Telephone 28

Subscriber A Subscriber B

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Simplified illustration of the telephone connection

Operation of Ordinary Telephone 39

(a) One microphone and one earphone combined into a single handheld unit

Microphone

When we raise the telephone hook, the on/off switch is

closed and current starts flowing on the subscriber loop

through the microphone able to convert acoustic energy

into electrical energy. Originally telephone microphones

were carbon microphones (i.e. with carbon grains)

operating as variable resistors supplied with battery

voltage from the exchange site. Sound waves press the

(*)

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Earphone

Alternating current, generated by the microphone, is

converted back into voice at the receive end. The

earphone has a diaphragm with a piece of magnet

inside a coil. The coil is supplied by the alternating

current; this current generates a variable magnetic

field that makes the diaphragm to vibrate. This, in

turn, produces sound waves.

voltage from the exchange site. Sound waves press the

carbon grains more tightly, so loop resistance decrease

and current slightly increases. The variable air pressure

generates an alternating current into the subscriber loop.(*) Located in the Central Office

Operation of Ordinary Telephone 410

(b) Means of signaling the exchange when a call is to be made

Signaling functions include the detection of on/off-hook condition and dialing.

Each telephone has a switch that indicates an on- or off-hook condition. When the hook is

raised, the switch is closed and current starts flowing.

This is detected by a relay giving information to the control unit in the exchange.

(c) Means of sending to the exchange the ‘address’ information of the called subscriber

The control unit activates signaling circuits, which then receive dialed digits from subscriber

who initiates a call (Subscriber A). The control unit in the telephone exchange controls the

switching matrix that connects the speech circuit through to the called subscriber

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switching matrix that connects the speech circuit through to the called subscriber

(Subscriber B). Connection is made according to the numbers dialed by Subscriber A.

(d) An incoming call alarm

When the call is being routed to Subscriber B, the telephone exchange supplies to the

subscriber loop a ringing voltage and the bell of Subscriber B’s telephone starts ringing.

(e) Means of signaling the exchange that the call has been answered

The ringing voltage is switched off immediately when an off-hook condition is detected

on the loop of subscriber B, and then an end-to-end speech circuit is connected and the

conversation may start.

Summary of Signaling ProcedureSwitch open in the on-hook condition and closed when the hook is off

• indicates to the telephone exchange when a call is to be initiated

• prepare to receive dialed digits

Local loop closed and opened according to the dialed digits, and the number of

current pulses is detected by the exchange

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Subscriber signaling25

Rotary Dialing

Dialing plate with finger holes is rotated clockwise to the end and released

While homing, the switch is breaking the line current periodically and the

number of these periods indicates the dialed digit (digit 1 has one period, 2

has two periods, and 0 has 10 periods)

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Principle of rotary (or pulse) dialing

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Tone Dialing 1Electronic circuits make possible the implementation of better means for signaling.

Modern telephones have 12 push buttons (keys A to D in Figure normally are not

included) for dialing, each generating a tone with two frequencies.

One of the frequencies is from the upper frequency band and the other from the

lower band. All frequencies are inside the voice frequency band (300–3,400 Hz) and

can thus be transmitted through the network from end to end, when the speech

connection is established.

This signaling principle is known as dual-tone multifrequency (DTMF) signaling.

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This signaling principle is known as dual-tone multifrequency (DTMF) signaling.

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Tone Dialing 2Advantages of tone dialing are as follows:

• It is quicker and dialing of all digits takes the same time.

• Fewer dialing errors result.

• End-to-end signaling is possible.

• Additional push buttons available (*, #)

to activate supplementary services.

Supplementary services enable subscribers to

influence the routing of their telephone calls.

Each digit represented by

two audio tones.

DTMF signalling allows 10

digits, plus * and # symbols

to be coded as shown in

Table below:

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These services (for example call transfer) are not

available with telephones that use pulse dialing. To

control these services we need control buttons * and

#, which are available only in push-button telephones

that use tone dialing.

We use tone dialing also to control value-added

services (VAS), i.e. services that we can use via the

telephone network but that are usually provided by

another service provider, not the telecommunications

network operator. 28

2 Wires and 4 Wires Circuits

The local loop, which connects a telephone to a local exchange is a two-wire (2W)

circuit that carries the signals in both transmission directions.

Subscriber loops are two-wire circuits, because they are one of the biggest

investments of the fixed telephone network.

Long distance connections attenuate the speech signal and amplifiers are needed

on the line: use two wires for each direction, or four-wire (4W) connections.

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2W/4W circuits 29

2W/4W Hybrid 1To connect a 2W local loop to a 4W network a circuit called a 2W/4W hybrid is

needed (forchetta telefonica).

The signal from the receive pair is not connected to the transmit pair, or at least it

is much attenuated.

Balance is not ideal and attenuated signal is connected back, which is heard as an

echo from the far end of the telephone circuit if two-way propagation delay of the

circuit is long enough:

• e.g., satellite connections, digital cellular networks: both have long delays

and need echo cancellers

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2W/4W hybrid in long distance connection

and need echo cancellers

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2W/4W Hybrid 2The 2W/4W hybrid performs the following operations:

• separates the transmitting and receiving signals

• matches the impedance of the 2W local loop to the network circuit

• provides a loss to signals arriving on the receiving path, preventing

them from entering the transmitting path, which would cause echo.

In every subscriber set quite the same principle as the 2W/4W hybrid is used to

attenuate the subscriber’s own voice from the microphone to the earphone.

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31Generation of echo signal in a 2W/4W connection