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Public Switched Telephone Network (PSTN)
Chapter 11/12 Text book Hioki
Subscriber Loop:
Fig below (fig 11-17 P275 Text book) shows a simplified schematic of a conventional
Telephone handset and how it is connected to the Public Switched Telephone Network.

The connection between your telephone and the world wide interconnecting
network is a two-wire circuit that the phone company calls Subscriber Loop.

The signals that your telephone company must send and receive are listed on page
275 of your Text book Hioki and are repeated below

Ringing Signal
20 Hz, 90 v R.M.S
ON Hook/OFF Hook Signal
A current of about 20 ma OFF hook, 0 ma ON Hook
Dial Tone
350Hz + 440 Hz
Busy Tone
480Hz + 620 Hz
Ring Back
440 Hz + 480 Hz
Congestion
480 Hz + 620 Hz
Receiver Off hook
1400 + 2060 + 2450 + 2600
RINGER: First device we encounter as we proceed from the Central Office (CO) or
Local Office (LO) or Local Exchange (LE) into the telephone is the RINGER, which
responds only to an A.C ringing signal and produces an audible ring.

SWITCH HOOK: After that, in parallel with the Ringer is the Switch Hook, which
has two contacts that are in the open position when the telephone is ON HOOK. The
DC current in the loop is 0 ma when the telephone is ON HOOK because of the open
circuit. The – 48 V battery at the LO is disconnected from the loop when the
telephone is ON HOOK. When the handset is lifted OFF the HOOK, the Switch
Hook contact closes, the battery is connected to the loop causing a 20 ma of DC
current to flow through the loop into the telephone circuit. This is how the
Telephone company knows that you have lifted the telephone Off the hook and are
about to Dial a telephone number
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
As soon as a subscriber lifts the handset off the hook and current begins to flow a
special device in the LO is attached to the subscriber loop which produces a dial
tone and also stores the dialed digits.

As mentioned earlier, the RINGER responds to a.c signal only and does not
interfere with the DC loop current
Dialing:

Next event is dialing. There are two kinds of dialing. PULSE and TONE

Pulse dialing is disappearing so I will not cover it. However I would still like you
to be familiar with the relevant voltages and currents and waveform. Please
study the handout. In pulse dialing, the switch hook opens and closes as many
times as the dialed digit, thereby interrupting the circuit as many times as the
number being dialed. So if the number 5 is dialed it will generate 5 pulses. For
“0” 10 pulses are generated. Pulse dialing is slow and time consuming. So they
came up with Tone Dialing.
Touch Tone Dialing

Modern method of dialing

Fig shows the keypad

This is called DTMF (Dual Tone Multi Frequency) dialing

It uses a unique set of two tones to represent each dialed digit. These tones are
actually sine waves that are sent to the LO as each digit is pressed

Consider the keypad diagram shown on p265 of your textbook.

When a key is pressed, the tones corresponding to its ROW and COLUMN are
transmitted.

Fastest rate is 0.1s, typically about 0.25s/key pressed. And 0.25s between keys

Frequencies are so chosen that (i) None of the tone is a harmonic or a close
harmonic of any other frequency, e.g 2nd harmonic of 1209 is 2418 which doesn’t
exist on the keypad. (ii) The amplitude modulation of one tone by another does not
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generate the sum and difference frequencies- 1447 + 697=2144 which is not found on
the keypad.

Note also that in DTMF, the sum of the amplitude of the two frequencies is
transmitted not the sum of the frequencies.
Ringer:

Ringer responds to the ac signal sent from the LO

Fig shows a schematic of the Bell 500 telephone (fig 11.5 p262 Hioki)

You can see that the ringer is really a coil ( inductor ) in series with a capacitor.

The capacitor blocks dc and passes only ac ringing signal.

Thus the ringer does not interfere with the dc loop current and dc operation of
the telephone circuit

When the conversation begins, the signal voltage during talking is only a few
volts at about 3ma.

The voice frequencies transmitted are between 300 HZ to 3300 HZ, since most of
the voice energy lies in this range. This is called Telephone Bandwidth ( BW ).
Thus the Telephone BW is 3 KHz. Telephone lines are designed to carry
frequencies in this frequency range.
Telephone Switching System: Local Exchange and Hierarchy of Exchanges.

Now that we understand how a telephone is hooked to the nearest Local
Exchange or Local office (LE or LO), how a subscriber loop works as well as
associated voice and control signals, we turn our attention to how a call is routed
across the telephone network

The first 3 digits of a telephone number represent the city code such as 416 for
Toronto, 905 for Thornhil, Markham and Brampton etc.

The next 3 digits specify Local Office address such as 675 for Rexdale, 453 in
Brampton, 276 Mississauga etc.

The last 4 digits indicate the customer address

Thus 416 – 675 - 6111
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City LO
subscriber

LO usually serves an area within a 10KM radius

Thus a city can have a maximum of 000 to 999 (1000) Local Offices and each
office can serve a maximum of 0000 to 9999 (10,000) customers.

That’s why Toronto was split into 416 and 905 regions, because the number of
exchanges grew more than 1000 due to population explosion.

Fig below shows how a call is connected to another customer.
6610
1234
INTER OFFICE TRUNKS
1
BRAMPTON
1234
MISSISAUGA
LO A
LO B
4422
2
455
276
5678
5000
1
1
2
5678
REXDALE
LO C
675
1100
9110
2457

Fig above shows 3 neighboring towns served by 3 Local offices.
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
The lines connecting Local Offices are called Inter Office trunks or simply
TRUNKS

There may be more than one trunk installed between a pair of offices (depending
upon the level of traffic)

Consider the case when 455-5000 calls 455-4422

A Switch in the Local Office A makes a connection between 5000 and 4422. In good
old days a human operator did it manually. Now a day it is done electronically
under the control of a microprocessor or a computer.

If 4422 is talking to some one, a busy signal is returned, which is a combination of
audio tones 480 Hz and 620 Hz pulsing at a rate of one per second.

If 4422 is not busy, a ringing signal is sent to 4422 and when the called party picks
up the phone, a connection is made. LO also sends a ring back to the calling party.

These control or call progress signals are manufactured electronically inside the
Local office


When the called party picks up the phone, the LO
(i)
Detects the off-hook by sensing the current
(ii)
Ringing and ring back ceases
(iii)
Conversation begins
When the call is finished, both parties put the phone back on-hook. DC current
stops. Connection is broken

This was an example of a call within the confines of LO
Interoffice Calls:

Now suppose LO A 455-5000 calls LO C 675-1100

The prefix 675 will indicate to LO A that the call is for another LO and therefore
routs the call to LO C via inter Office trunk 1

This is called Routing. The last 4 digits of the called party are transmitted down the
trunk to the LO C. this process is called Inter Office Signaling. The Inter office
Signaling techniques are a sophisticated area of growth and development in
communications technology. It concerns with how to transmit control information
between offices and allow inter office communication in the most efficient way.
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
There are several schemes used by Network operators and Carriers such as Telus,
Bell mobility, microcell, FIDO, Rogers Cantel etc. They fall in two major categories

The Common Channel Interface Signaling (CCIS) and SS#7. The Signaling System
#7 is the most commonly used mechanism in Telephone system as well as Wireless
system. We shall learn more about these systems later in the program

At this point, you should understand that the primary purpose of the signaling
protocols is to transfer control information between parts of the Network for the
control of connection establishment, management of the call in progress and
disconnection

When the LO C receives the customer number 1100, then the same process is
repeated as described earlier for the call between 5000 and 4422 in the 455 Office.

Now suppose, before this call is broken, LO A 453-5678 calls LO C 675-2457.

Since another trunk is available, fortunately the call goes through.

Suppose now, while both trunks are occupied453 4422calls 675 9110.

This call cannot go through, even if the called party is not currently using the phone.

This is called BLOCKING or CONGESTION caused by the insufficient resources
either inside the LO or inadequate number of trunks

For this kind of blocking, the caller will hear a busy tone, but it will pulse at twice
the rate of a normal busy signal.

This system of switching calls is called Circuit Switching, because the resources of
the Network are dedicated to caller for the duration of the call and then switched to
somebody else.

Blocking is unavoidable in Circuit Switching because, the telephone company
cannot provide a dedicated line for each customer. The cost becomes extremely
prohibitive, while most of the circuits are not busy most of the time.

The number of lines needed to provide a dedicated service to n customer = n( n-1)/2.
Try calculating for 1000 customers????
North American Telephone Hierarchy:

In order to provide efficient, cost effective service across the Nation, the Telephone
Companies use a telephone Switching hierarchy as shown in Fig 12.1 12.2 P290/292
in you text book Hioki
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
As shown, the customers are connected to the LO via subscriber’s loop.

The Los are interconnected directly by Trunk lines or indirectly through a facility
called TANDEM CENTER or TOLL CENTER

4 Classes of these Tandem centers exist in North America
Toll Centers (Class 4)
Primary Centers (Class 3)
Secondary Centers (Class 2)
Regional Centers (Class 1)

When a direct trunk doesn’t exist between the called and the calling parties, the call
is routed through one of these alternate paths. The sophisticated Switch software
decides which the most efficient path based on the traffic condition at that time.
Terms Used in Telecommunication Systems:

Simplex Communication: Transmission in one direction only for example radio or
TV broadcast.
T

R
Half Duplex: Transmission in both directions but only in one direction at a time.
This is called Time Division Switching. e.g human conversation, Intercome system,
walkie-talkie etc
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T
R
R
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
Full Duplex: Transmission in both directions simultaneously. It uses either two
separate lines or two different modulation frequencies for forward and reverse
channels e.g Most modems use two sets of modulation frequencies, also most
modern protocols are full duplex such as HDLC,LAP B etc
R
T
T
R

4 WIRE to 2 WIRE Hybrid: Show the picture of hybrid and discuss its function and
implementation in the Telephone hand set and earlier subscriber loops. Also discuss
the Modern technique of using CODEC for 2 Wire to 4 Wire conversions.

Switched and Leased lines:
Switched Line: Regular non-dedicated line. Time-shared with other customers
Leased Lines: Also Called Private lines. Refers to renting a private dedicated line
-
Customer pays a fixed price per month.
-
Connection always exist, no need to dial the number
-
Low cost for high volume
-
Usually used by corporations to connect their branch offices
-
No blocking
.
Echo Suppressors: Voice operated Relays used to suppress echoes.
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Special Telecom Services:
Call Forwarding: Transfers the call automatically to where you can be found.
Call waiting: A short beep alerts you to an incoming call.
3-way call: Allows you to have a 3-way conversation.
Return Call: Lets you know who was the last caller
Caller ID or Call Display: Allows you to see the name and Telephone number of the
person calling you.
Caller ID Block : Blocks your number and name from appearing on Call ID screen.
You can do some research and find some more special services that the Telephone
companies are providing for an extra fee.
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DECIBELL (dB):
-
Indicates power level.
-
Ratio of output power to input power
-
DB = 10 log Po/Pi
-
If Po >Pi, it is called Gain
-
If Po<Pi, it is called Loss or Attenuation (a negative number)
-
In terms of voltage ratio, dB=20 log Vo/Vi (provided Ro=Ri). Can you prove
it??
-
Also dBm is defined as dBm=10 log Po/1mw. Thus dBm refers to output
power with 1mw of reference input power.dBm therefore indicates absolute
power in mw.
-
Consider the table shown
DB
Po/Pi
dB
Po/Pi
0
1
0
1
3
2
-3
0.5
6
4
-6
0.25
9
8
-9
0.125
10
10
-10
0.1
13
20
-13
0.05
20
100
-20
0.01
On the other hand consider the table below for dbm
Dbm
Po
dbm
Po
0
1mw
0
1mw
3
2mw
-3
0.5mw
6
4mw
-6
0.25mw
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9
8mw
-9
0.125mw
10
10mw
-10
0.1mw
13
20mw
-13
0.05mw
20
100mw
-20
0.01
The main advantage of dB (or dBm) is that while calculating power gain or
loss of interconnected networks, the individual gains or losses can be simply
added or subtracted.
Example:
I/P=0dbm
O/P=?
-30dbm
35dbm
-22dbm
20dbm
-2dbm
O/P = 0 + (-30) + (+35) + (-22) + (+20) + (-2) = +1dbm
Example: For the telephone transmission system shown below, calculate the gain
needed for the output amplifier to produce an output voltage of 6V. Also calculate
the output power level in dbm.
Cable section
S.G
Cable loss
26 db loss
Out put Amp = 6V Vo
29 db loss
10
dbm
X
A
Line Amp
13db gain
B
Boaster Amp
20 db gain
C
600
Ro
GND
Solution:
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(a)
First calculate the output power level using the Equation Po = Vo2/Ro
= 62 / 600 = 36 /600 = 60mw.
Now compute the output power level in dbm = 10 log Po/1mw
=10log60mw/1mw = 17.78 dbm
(b)
To calculate the gain of the output amplifier C, first calculate the total loss or
gain up to point X.
10 +13 –26 +20 – 29 = -12 dbm
There for the output amplifier gain required = 17.78 dbm – (-12 dbm) = 29.78 dbm.
Assignment:
For the Telephone transmission system shown, calculate the output voltage and the
output power in dbm.
GAIN 16 dbm
Cable loss = 6dbm
S.G V
1.1 V
O/P
Vo = ?
600
GND
600
GND
GND
ANS:
Output power level = 13dbm
Output voltage Vo = 3.48V
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