t 12a - irot

T 12A

TELEPHONE EXCHANGE PRINCILPES &

TELEPHONE INSTRUMENTS

Issued in October 2008

INDIAN RAILWAYS INSTITUTE OF SIGNAL ENGINEERING & TELECOMMUNICATIONS

SECUNDERABAD - 500 017

T 12A

TELEPHONE EXCHANGE

PRINCILPES & TELEPHONE INSTRUMENTS

CONTENTS

S.No Chapter Page No

1 Constituents of Telephony 1

2 Switching & Signalling functions of Telephone Exchange 5

3 MDF & Protective devices 24

4 Telephone Exchanges in Indian Railways 34

5 Telephone instruments 58

© IRISET

http://www.iriset.ac.in

Prepared by P.L. Narasimha Rao, ILP - 1

Checked by P.V. Sreekanth, Professor - Tele

Approved by V.K.Goyal, Sr. Professor - Tele

DTP and Drawings K.Srinivas, JE II(D)

Date of Issue October 2008

Edition No 01

No. of Pages 101

No.of Sheets 52

“This is the Intellectual property for exclusive use of Indian Railways. No part of this publication may be stored in a retrieval system, transmitted or reproduced in any way, including but not limited to photo copy, photograph, magnetic, optical or other record without the prior agreement and written permission of IRISET, Secunderabad, India”

IRISET 1 T12A - TELEPHONE EXCHG PRINCIPLES & INSTRUM ENTS

CHAPTER 1

CONSTITUENTS OF TELEPHONY

1.1 Principles of Telephony: Telephony provides a means of sending information through

human speeches when required between two persons situated at a distance apart. In line

telephony the information is sent through the medium of line conductors between them.

a) Telephones: The apparatus that are used for transmitting and receiving speech signals are

called ”Telephones” and the persons who use them for sending information between them are

called “Subscribers”.

The telephone transmitter and telephone receiver must be such that the conversion from

speech into electrical currents and vice-versa be perfect i.e. free from frequency distortion,

amplitude distortion.

b) Telephone exchange: It is a place where switching between two subscribers is done

through either manually or electronically. In addition to switching, signalling and controlling are

also done at “Exchange”.

c) Human speech and its transmission: Human speech consists of a large number of

frequency components of different values between 0.3 to 3.4 KHz having different amplitudes

and different phase relations between them.

Steps in Telephone transmission:

i) Conversion of speech into the electrical voice frequency currents at transmitting end.

ii) Transmission of speech currents through a media lines to the distant end.

iii) Conversion of voice frequency currents into speech sounds at the receiving end.

FIG.1.

As shown in the Fig.1 when any subscriber speaks into the “Transmitter” of his telephone set,

his speech is converted into oscillatory electrical currents and then it is sent through lines /

Transmission lines and then exchange to the other subscriber’s telephone set where it is



converted into speech sounds by “Receiver”. One pair of conductors are required between two

subscribers. All the line conductors from different subscribers premises are concentrated at a

place known as the telephone exchange.

d) Types of currents:

There are two types of currents to be generated in an exchange.

i) Speech currents- carry information between subscribers

ii) Signalling currents – carry the signalling information.

1.2 Cables used in Telephony:

In line telephony speech currents are carried by lines first into the exchange and there to the

receiver of the called subscriber through separate pair of lines. The type of transmission lines

used are generally insulated copper conductors, which are packed into a bunch of 10, 20, 50 or

100 pairs called as “Telephone cables”.

Copper wires are used for Telephony transmission due to, less attenuation and less distortion

provided that the insulation resistance of conductor is within the given values.

a) Characteristics of Telecom cables:

Sl.No Lb/Mile Kg/Km Copper conductor

Diameter in mm

Loop resista nce

per Km in Ω

dB loss per

KM

1 6.5 1.85 O.51 182 1.379

2 10 2.84 0.63 114 O.91

3 20 5.68 0.90 56 0.75

4 40 11.36 1.24 27.4 0.43

1.3 Basics of Telephone Exchange: Telephone Exchange a place where switching between two subscribers is done through either

manually or electronically. In addition to switching, signalling and controlling are also done at

exchange.

It consists of the following functional blocks:

a) Main Distribution Frame with protective devices.



b) Card Frame.

c) Mother board.

d) Power supply panel with protective devices.

a) Main Distribution Frame (MDF):

In a Telephone exchange different subscribers from different places are terminated on a frame

called “ Main Distribution Frame” (MDF) in the exchange and from there they are extended to

subscribers line cards/Trunk cards kept in the exchange rack. Protective devices are located in

the MDF.

Purpose of MDF:

There are three purposes of MDF,

1) It is the place where both outdoor and indoor cables are terminated

The cross connection between the two cables conductors is done on the MDF and this is

done by means of jumper wires (Red & White).

2) It carries all the protective devices used in the exchange. They are Fuses, Heat coils &

Lightning protectors.

3) The MDF is the most suitable place for testing purposes.

b) Card Frame:

It contains different slots in which the nominated cards are to be inserted. It is different in

different types of exchanges.

c) Mother board:

It provides connectivity between different cards. It is a multilayer PCB.

d) Power supply panel:

It provides power supply to different cards in the exchange at different low D.C. voltages. It also

includes protective devices like fuses etc.

1.4 Main functional areas in Telephone Exchange:

a) Switching Function: The switching functions are carried out through the switching network,

which provides a temporary path for simultaneous, bi-directional speech between the following:-



a) Two subscribers connected to the same exchange. This is called as “Local switching”

ii) Two subscribers connected to different exchanges. This is known as “Trunk switching”.

iii) Pairs of trunks towards different exchanges. This is known as “Transit switching”

b) Signalling function: The signaling function enables the various equipment in a network to

communicate with each other in order to establish and supervise the calls. It is of two types,

i) subscriber line signalling: It enables the exchange to identify calling subscribers line,

extend dial tone, receive the dialed digits, extend the ringing voltage to the called

subscriber, extend the ring back tone to the calling subscriber to indicate the called

subscriber is being connected. In the event the called subscriber is busy, engage

tone is sent to the calling subscriber.

ii) Inter exchange signalling: It enables a call to be set up, supervised and cleared

between exchanges.

c) Controlling function: The controlling function performs the task of processing the signalling

information and controlling the operation of the switching network.

The control functions may be,

i) Wired logic control: In this pre wiring is done between different speech path devices

and common control. If any changes are required in facilities of subscribers or

introduction of new services require wiring changes.

ii) Stored Programme Control (SPC): After introduction of microprocessor, stored

programme control system came into use. In this system the establishment and

supervision of the connections in the exchange is under the control of

“Microprocessor”, which is suitably programmed.

Exercise Objectives:

1. Switching: Signalling and Controlling are the main functions of Exchange.

2. Two Subscribers of two different Exchanges are connected through Trunk Switching.

3. Loop Signalling is extended from Subscriber to Exchange.

4. Stored Program Control has over all control on the Exchange.

5. MDF is meant for connecting indoor and outdoor cable pairs.

6. Protective devices are used on MDF for protecting the Exchange SLC/TRK cards from

damage.

7. Copper wires are used for transmission due to less attenuation & less distortion.

8. A Trunk line uses 3 pairs of copper wire for its termination.

9. Mother Board is a multilayered PCB for placing cards.

10. Card Frame is used to suitably connect the card in slots.


IRISET 4(i) T12A - TELEPHONE EXCHG PRINCIPLES & INSTR UMENTS

Objective: 1. The main function of SPC exchange are___________ and ______________.

2. Two subscribers of two different exchanges are connected through _________________.

3. Loop signal is extended from _____________ to ______________.

4. Over all control of the exchange is done by __________________.

5. Indoor and outdoor cables are terminated on _______________.

6. Protective devices are used on ____________ to protect ___________, _____________ in exchange.

7. To terminate a 2w trunk line number of pairs required are ______________.

8. Transmission through copper wire offers less _________ and less _____________.

Answer the following in short: 1. What is telephony?

2. What is MDF (Main Distribution Frame)? What are the requirements of MDF?

3. What are the functional areas of telephone exchange?

4. What are the requirements to install a new telephone exchange?

SWITCHING & SIGG. FUNCTIONS OF TELEPHONE EXCHANGE


CHAPTER-2

SWITCHING AND SIGNALLING FUNCTIONS OF

TELEPHONE EXCHANGE

2.1 Switching:

a) Introduction : Switching and associated signalling systems is essential for the operation of

telecommunication networks. Depending on the design of the switching system, customers are

provided additional facilities and to facilitate operation and maintenance. There are some basic

functions, which must be performed by all switching systems.

b) Functions of switching system:

i) Attending: The system should be continuously monitoring all the lines to detect call requests.

ii) Information receiving: In addition to receiving call and clear signals the system must

receive information from the caller as to the called line (or other services)required. This is called

as the “Address signal”.

iii) Information processing: The system must process the information received, in order to

determine the actions to be performed and to control the actions. Since both originating and

terminating calls are handled differently for different customers, class of service information

must be processed in addition to the address information.

iv) Busy testing: Having processed the received information to determine the required

outgoing circuit the system must make a busy test to determine whether it is free or already

engaged on another call. If a call is to a customer with a group of lines to a PBX or to an

outgoing junction route, each line in the group is tested until a free one is found. In an automatic

system, busy testing is also required for both the switches of exchanges.

v) Interconnection: Interconnection between the two subscribers is to be made in the following

sequence:

- A connection to calling subscriber

- A connection to called subscriber

- A connection between the two subscribers



vi) Alerting: Having made the connection, the system sends a signal to alert the called

subscriber e.g. by sending ringing current to a subscriber telephone.

vii) Supervision: After called terminal has answered, the system continues to monitor the

connection in order to able to clear it down when the call has ended. In stored - programme –

controlled system the data for call charging can be generated by a central processor as it sets

up and clears down connection.

viii) Information sending: If the called subscriber’s line is located on another Exchange the

additional function of information sending is required.

The originating exchange must signal the required address to the terminating exchange (and

possibly to intermediate exchanges if the call is to be routed through them)

c) Types of switching: The following are the different types of switching used in different

areas,

i) Message switching: It is the type switching used to send the message from town’ A’ to ‘B’

via circuits A and C and one between C and B. The operator at ‘A’ sent the message to C,

where it was written down by the receiving operator. This operator recognized the address of

the message as being at B and then retransmitted the message over the circuit to B. This

manual process is shown in the Fig. 2.1 .

Fig.2.1



Evolution of message switching. (a) Manual transfer of hand copy (b) Manual transfer of paper

tape (c) Manual transfer of paper tape with automatic route selection (torn-tape relay system).

(d) Automatic message switching system. T= teleprinter, R/P= reperforaor, A/T=automatic

transfer, S= store

A modern message switching center is thus a special purpose electronic computer. In fact,

message switching was the first area in telecommunications to adopt stored-programmed-

control (SPC). In a message switching center an incoming message is not lost when the

required outgoing route is busy. It is stored in a queue with any other messages for the same

route and transmitted when the required circuit becomes free.

ii) Circuit switching: In circuit switching the circuit of calling subscriber’s telephone to that of

called subscriber’s telephone on demand and to maintain this connection for the duration of the

call. The disadvantage of circuit switching is, if the required outgoing circuit from a switch is

already engaged on another call, the new call offered to it cannot be connected. The call cannot

stored, as in message switching.

iii) Manual switching : In manual switching the interconnection between calling and called

subscribers is done by man (operator) through multiple jack field. Any operator can make

connections easily to all customer lines, a jack for every line appears in the field of rows and

columns and this is repeated at intervals around the switchboard. The operator in a manual

exchange provides many different services, using the basic switchboard apparatus, by

performing different sequence of actions which are stored in his or her memory.

iv) Electronic switching: In this, electronic devices are used as switching devices in switching

network. In SPC systems digital computer to be used as a central control and perform different

functions with same hardware by executing different programmes.

Electronic switching systems may be classified as,

- Space – division(SD) systems: Each connection is made over a different path in space

which exists for the duration of the connection.

- Time-division(TD) systems: Each connection is made over the same path in space, but at

different of time.



The distinction between SD and TD in between the control arrangements of an exchange as

well as to its switching network. For example, if an exchange uses individual registers, this is

space division. In time a central processor, handling each call in turn, performs division of the

register function.

v) Digital switching: When signals are transmitted through the switch in digital form, then it is

called ”Digital switching”.

These signals may represent speech samples or data. The digital signals of several speech

samples are time multiplexed on a common media, while transmitted through the switching

system. However, it has become synonymous with digital switching.

(vi) Time and Space switching:

Generally a digital switching system serves several time division multiplexed (PCM) signals.

These PCM signals are conveyed on PCM highways (the common path over which many

channels can pass with the separation achieved by time division). Switching of calls, in this

environment, placing digital signals from one time-slot of one PCM multiplex in the same or

different time-slot of another PCM multiplex. Fig.2.2

Fig.2.2 For example, PCM samples appearing in TS6 of I/C PCM HWY1 are transferred to TS 18 of

O/G PCM HWY2, via the digital switch as shown in Fig.2.2.

The interconnection of time slots, i.e., switching of digital signals, can be achieved using two

different modes of operation. These modes are

- Space switching

- Time switching



Usually, a combination of both the modes is used.

In space switching, corresponding timeslots of I/C and O/G PCM Highways are

interconnected. A sample in a given time slot, TS1 of I/C HWY, say HWY1, is switched to

the same time slot, TS1 of an O/G HWY, say HWY2. Obviously there is no delay in

switching of the sample from one highway to another highway, since the sample transfer

takes place in same time-slot of the PCM frame.

Time switching on the other hand, involves the interconnection of different time-slots on the

incoming and outgoing highways by re-assigning the channel sequence. For example, a

time-slot TSx of an I/C highway can be connected to a different time-slot, TSy, of the O/G

highway. In other words, a time switch is basically, a time-slot changer.

vii) Packet switching: For transmitting the data by a data network, packet switching is

used. Long messages are split into a number of short ones, called packets, which are

transmitted separately, as shown in the Fig.2.3 Thus the single packet from the VDU

operator is sent between packets of the large computer file, instead of waiting until its

transmission is complete and the delay is minimized.

Fig.2.3. Pocket switching

The format of a typical packet is shown in Fig 2.3. Since each packet is handled as a

complete message, its data must be preceded by a header, which contains the destination

address of the message. It is possible for pockets sometimes to arrive at their destination in


IRISET 10 T12A - TELEPHONE EXCHG PRINCIPLES & INSTRU MENTS

a different order from that in which they are sent. Each header is therefore contains a

sequence number, to enable packets to be reassembled in the correct order at the receiving

terminal. Other bits are added to the header for controlling purposes, e.g. to indicate

whether a packet contains a message or is being sent to control the network. The packet

ends with bits added for error detection and correction.

The data network and its terminals handle the packets by procedures known as “Protocols”.

Packet switching was first developed for the use in private data network. It is widely used in

local-area networks (LAN’s), in wide-area networks (WAN’s).

2.2 Signalling:

a) Introduction:

A telecommunication network establishes and releases temporary connections, in

accordance with the instructions and information received from subscriber lines and inter-

exchange trunks, in the form of various signals. Therefore it is necessary to interchange the

information between an exchange and subscriber lines and exchange, and between

different exchanges.

A signaling system uses a language which enables two switching equipments to converse

for the purpose of setting up calls.

b) Evolution of Signalling in Telephony:

i) Magneto phone - Ringing current generated by magneto

ii) Manual exchange - Call request / Call release

iii) Signalling in Analog exchanges & SPC exchanges – Subscriber line signaling & Inter-

exchange signalling

iv) Signalling in PCM & IDN (CAS) - Subscriber to NW signalling (Between sub. line &

network) & Internal Network signalling (Signalling within network)



v) Signalling in ISDN (CCS#7 - also called SS7) - Subscriber to NW signaling & Internal

Network signalling

2.2.1 Types of Signalling:

- Signalling in Telephony can be classified in to:

1. Subscriber line Signalling: Signalling concerned with Calling subscriber line / Called

subscriber line, basically subscriber loop signaling comes under this category. Different signals

are explained below.

(a) Call report signal:

i) When the subscriber line is idle, the line impedance is high.

ii) The impedance falls as soon as the subscriber lifts the handset, resulting in increase

of line current

iii) This increase is detected as a new call signal by the exchange

iv) The exchange connects appropriate equipment / module to the line to receive the

address information (subscriber is going to dial)

v) The exchange notifies its readiness to receive the ‘ to be dialed number information’,

by extending the dial tone to the subscriber

vi) Address signal:

vii) After receipt of dial tone, subscriber dials the number (decadic/tone)

viii) This causes sending of address digits (called subscriber’s number ) in sequence to

the exchange

(b) End of selection signal:

(i) Establishment of connection by the exchange as per dialed number can be successful /

Un-successful .The exchange sends one of the following signals to calling subscriber

– Ring-back tone if the called line is free

– Busy tone if called line is engaged or otherwise inaccessible

– Recorded announcement to give reasons for the call failure other than ’line-busy’

(c) Answer-back signal:

i) On hearing the ringing, the subscriber lifts the handset.

ii) A battery reversal signal is transmitted on the line of the calling subscriber.



iii) This signal is used to operate special equipment attached to calling subscriber (e.g.

short circuiting the transmitter of CCB till the coin is inserted)

(d) Release signal:

i) When the calling subscriber goes on hook, i.e. releases the call, the line impedance

goes high.

ii) This is detected by the exchange which releases all equipment / modules involved in

the call.

(e) Permanent line or permanent glow (PG):

i) A PG signal is sent to the calling subscriber if the subscriber fails to release call even

after the called subscriber has gone on-hook and the call is released after a time

delay.

ii) The PG signal is also sent if the subscriber takes too long to dial.

(f) Ring signal:

i) On receipt of call to a line which is free, the exchange sends ‘Ring signal’.

ii) Ring signals is 25 Hz or 50Hz signal with suitable interruptions

iii) Ring-back tone is fed to calling subscriber line.

(g) Answer signal:

i) When the called subscriber lifts the handset on receipt of ring, the line impedance

goes low.

ii) This is detected by exchange, which cuts off ringing tone on called subscriber and

ring-back tone on calling subscriber.

(h) Release signal:

i) After completion of conversation if the called subscriber goes on-hook (before the

calling subscriber), the high line impedance of called subscriber line is detected by

the exchange.



ii) The exchange sends PG to the calling line and releases the call after a time delay if

the calling subscriber is still holding

(i) Register re-call signal:

i) Using DTMF dialing, another number can be dialled while holding on to the call in

progress in order to set up a call to a third subscriber.

ii) The signal to recall the dialing phase during the talking phase is called ‘Register

recall signal’.

iii) It is achieved through interruptions on the calling subscriber‘s loop for a duration less

than the release signal.

2.2.2 Inter Exchange signalling: It is the signalling between interconnected exchanges.

Signalling as per trunk circuit configuration viz., loop, E&M, tone.

Classifications based on duration of signaling, type of transmission etc. will be discussed.

It is concerned with management of subscriber calls as well as network itself.

It has complex set of commands, responses & set of parameters.

a) Classification based on type of transmission:

i) Pulsed Signalling - Signal information transmitted as pulses.

ii) Continuous Signalling - Presence of a signal is characterized by transition from one

condition to another

iii) Compelled Signalling - Similar to pulsed signalling, but transmission of signalling

information continues till acknowledgement is received from other end.

b) Classification based on Trunk Circuit Configurat ion:

i) Loop Signalling:

– Trunk connection between exchanges comprises of dc loop similar to exchange-

subscriber loop

– DC related condition like high/low ohmic loop, normal/reverse polarity on a lead,

earth/battery on a lead are detected and interpreted by the system. Hence, also

known as dc signalling



– Signalling and speech are carried on the same pair of limbs which form the loop.

Signalling states in loop signalling

State Out-going end In-coming end

Idle High resistance loop -ve potential on A-limb

+ve potential on B-limb

Forward

Seizure Low resistance loop -ve potential on A-limb

+ve potential on B-limb

Release Disconnection followed by

high resistance loop

On recognizing forward disconnection

signal-

open circuit/reversed potential, followed by

normal potential when incoming eqpt. has

been released

Backward

Answer Low resistance loop +ve potential on A-limb

-ve potential on B-limb

Clear back High resistance Change to normal potential

Blocking High resistance loop Disconnection of potential

(ii) E&M Signalling:

– In E&M Signalling, an additional pair of line is utilized per trunk

– One wire is dedicated to the forward signals (M-wire for transmit or mouth) which

corresponds to receive or R-lead of the destination exchange.

– The other wire is dedicated to the backward signals (E-wire for receive or ear)

which corresponds to the transmit or send wire or S-lead of the destination

exchange.



Signalling states in E&M Signalling

State Out-going from exchange In-coming to exchange

E-lead M-Lead E-lead M-Lead

Idle Earth Open Earth Open

Forward

Seizure Battery Open Earth Earth

Release Earth Earth/Open Battery/Earth Open

Backward

Answer Battery Earth Battery Earth

Clear back Battery Open Earth Earth

Blocking Earth Earth Battery Open

iii) Single Frequency Tone Signalling:

– Exchanges far apart cannot be connected by loop or E&M leads. Hence, Single

Frequency Tone is used to transmit signalling states.

– Tone can be In-band (2400 Hz or 2600 Hz) or

Out-of band (3825 Hz)

– In case of in-band signalling, it shall be ensured that simulated tone of 2400 Hz

or 2600 Hz does not switch-off the circuit.

Signalling states in Single Frequency Tone Signalli ng:

State Out-going

from exchange

In-coming

to exchange

Idle ON ON

Forward

Seizure OFF OFF

Release ON OFF/ON

Backward

Answer OFF OFF

Clear back OFF ON

Blocking ON OFF



c) Classification based on duration of Signalling O peration:

i) Line Signalling:

- Signalling is operative through-out the duration of call.

Examples:

Loop Signalling

E&M Signalling

Single Frequency Tone Signalling

ii) Register Signalling:

– Signalling is operative only during relatively short phase of setting up of call –

essentially for transmitting the address information.

– Trunk register holds the address information

– Register information is interchanged between exchanges during the phase between

the receipt of trunk seizure signal and switching of exchange to speech phase.

– Register signals can be transmitted ‘In-band’ or ‘Out-of-band’

– In ‘out-of-band’ method, only Single Frequency Tone is used and fewer combinations

of signals are possible

– In ‘In-band’ method, which is easy as no interference can take place in ‘Register

Signalling’, 2 out of 6 frequencies are used . For reliability, ‘Compelled Sequence’ is

used. This is known as ‘CSMF’ or ‘R2’ Signalling.

d) Classification based on Setting up signalling ov er multiple hops:

i) End-to-end Signalling:

Signalling is between ends of a connection as the call progresses . On A-B-C Exchange

connection, it first between A-B , then B-C and finally A-C.

ii) Link-by-link Signalling:

Signalling is confined to individual links i.e. A-B and B-C and not A-C.



2.2.3 How switches carried signalling

A. Channel Associated Signalling (CAS)

In this method, signalling as well as information (voice/data) are carried in the sme circuit or

channel through Compelled Sequence Multi Frequency (CSMF) Signalling , also known as R2

Signalling in CCITT terminology. In this method :

- 2 out of 6 Frequencies used for forward Signals (1380,1500,1620,1740,1860,1980 Hz)

- 2 out of 6 Frequencies used for backward Signals (1140,1020,900,780,660,540 Hz)

- 6C2 i.e.15 Combinations are possible either-way

- Hence, 15 types of signals can be sent either way

Modified R2 Signalling:

In India, DOT (BSNL) prescribed use of 5 frequencies either way (1980 Hz not used in forward

group & 540 Hz in backward group). Thus, 5C2 i.e.10 Combinations are possible either way.

Hence, 10 types of signals can be sent either way known as ‘Modified R2 Signalling

Since mentioning of frequency each time is cumbersome, index or weight code

is used. As we send 2 frequencies for each signal, we denote them by weight

codes (0+1 for f0 + f1 and so on).

Signal Frequency (Hz)

Forward 1380 1500 1620 1740 1860

Backward 1140 1020 900 780 660

Index fo f1 f2 f3 f4

Weight code 0 1 2 3 4

B. Common Channel Signalling (CCS)

In a network of SPC exchange, a connection that is made through two exchanges require call

processing by the central processor in each exchange. If channel associated signaling is used

for calls from the exchange A to exchange B, as shown in the figure 2.4 , it necessary for the



central processor of exchange A to send its outgoing forward signals to the individual speech

circuit for transmission to exchange B. At this exchange, the signals must be detected on the

speech circuit and passed to the central processor. Similarly backward signals from processor B

must be sent over the speech circuit, detected at exchange A and extended to its processor.

This is an inefficient arrangement for signallig between processors. If a high-speed data is

employed between the processors, as shown in fig , it can be provided a channel for all signals

between exchanges A and B. This is known as “Common Channel Signalling” (CCS).

Fig. 2.4. Principles of common-channel signaling (C CS). (a) Channel associated

signaling between central processors (b) Common-cha nnel signaling between central

processors

Switching network

Processor

Switching network

Processor

Switching network

Switching network

Processor Processor

Exchange A Exchange A Junction Exchange B

Exchange A (a)

Exchange B

(b)



Advantages of CCS are:

- Information can be exchanged between the processors much more rapidly when channel-

associated signaling is used.

- As a result, a much wider sets of signals can be used and this enables more services to

be provided to customers.

- Signals can be added or changed by software modification to provide new services.

- There is no longer any need for line signaling equipment on every junction, which results

in a considerable cost saving.

- Since there is no line signaling, the junctions can be used for calls from B to A in addition

to calls from A to B. Both-way working requires fewer circuits to carry the traffic than if

separate groups of junctions are provided from A to B and from B to A.

- Signals relating to a call can be sent while the call is in progress. For example, a customer

can transfer a call elsewhere, or request a third party to be connected into an existing

connection.

- Signals can be exchanged between processors for functions other than call processing,

for example for maintenance or network-management purpose.

- The use of CCS for inter-exchange signaling has been followed by its application to

customer’ lines in Integrated-Services digital Network (ISDN). Basic-rate of ISDN access

provides two 64 K bit/s channels and a 16 K bits/s common signaling channel in each

direction. Primary-rate access provides a 64Kbits/s common channeling channel for either

23 channels (in 1.5 M bits/s-based network) or 30 channels (in 2 M bits/s-based

networks).

Practical CCS systems:

Currently a signaling system termed as CCITT No.6 recommended by CCITT is in use

Another system termed as CCITT No.7 is being experimented for compatibility with ISDN.

The signaling data is interchanged in digital systems between the two processors via a

special dedicated signaling interface.



Frame structure for high-level data-link control (HDLC) protocol

2.2.4 Digital subscriber line signaling:

Digital transmission is used on customer lines to provide access to an ISDN. Each line may

give access to several terminals on the customer’s premises and common channel signaling

is used to serve them.

The CCITT has defined the Digital Subscriber Signaling system No.1 (DSS1) for signaling

over D channel. The transfer of information in each direction between the customer’s

premises and the exchange is by messages, called frames, which are similar to the CCITT

no.7 signal units The form of HDLC protocol used is known as LAP-D (link access protocol

for D channel).

The format of frame is shown in Fig 2.5 shown below.

Flag Address Control Information Check Flag

1 octet 1 or 2 octets

1 octet Variable 2 octets 1 octet

Fig.2.5 HDLC Frame format

Each frame begins and ends with 8-bit flag and there is a 16-bit cyclic redundancy check

field. The address field is not used for routing in the network ; it selects ports at either end of

the line. Its first octet contains the terminal-end- point identifier, which indicates the

exchange terminal to be used (e.g. for a circuit switched call or a pocket-switched call). The

second octet contains the terminal-end-point identifier, which identifies the equipment on the

customer’s premises to which the message refers. Each octet begins with an extension bit.

If this is ‘0’, it indicates that another octet is to follow; if it is’1’, it indicates that the octet is the

last. Normally, the extension bit of the first octet is ‘0’ and that of the second octet is ’1’.

However, the latter can be changed to ‘0’ to add a third octet, or more, if required. The

second bit in the address field is the command/response bit (C/R). If the frame is command,

it indicates the receiver (i.e. either the exchange or the customer). If the frame is response, it

indicates the sender.

The control field (1 or2 octets) indicates the type of frame being transmitted. There are three

types of frames,

First bit



(i) I format for information transfer

(ii) S format for supervisory functions

(iii) U format for un-numbered transfers

The I frame and the S frame contain sequence numbers to ensure that frames are not lost.

The U frame does not contain sequence number.

The information field contains the layer-3 information to be transferred between customer

and exchange.

Primary rate access with common-channel signaling is used between digital PBXs and

public exchanges. Common-channel signaling based on similar principles can also be used

between PBXs in a digital private network.

2.3 Local call connection sequence:

A typical call connection sequence is explained below to understand the process of a call within

the exchange.

a) When the calling subscriber lifts his handset to make a call, a call request is originated.

b) The exchange sends dial-tone to the calling subscriber to indicate him to start dialing.

c) The called number is transmitted to the exchange, when calling subscriber dials the

number.

d) If the called number is free, the exchange sends ringing current to him.

e) Feed back is provided to the calling subscriber by the exchange by sending,Ring back

tone if the called subscriber is free. Busy tone, if the called subscriber is busy.

f) Recorded message, if provision exists, for non-completion of call due to some

g) other constraint.

h) The called subscriber indicates acceptance, of the incoming call by lifting the handset.

i) The exchange, recognizing the acceptance, terminates the ringing current and the ring-

back tone, and establishes a connection between the calling and called subscribers.

j) The connection is released when either subscriber replaces handset.

k) When the called subscriber is in a different exchange, the follow inter-exchange trunk

signal functions are also involved, before the call can be set up.

l) The originating exchange seizes an idle inter-exchange trunk, connected to a digit

register at the terminating exchange.



m) The originating exchange sends the digits. The steps iv to viii are, then, performed to set

up the call.

2.4. Inter exchange call connection sequence:

a) The line which connects two subscribers of different locations is known as” Tie line” or “Trunk

line” or “Junction line”.

i) The tie line is the line connecting two similar type of Electronic exchanges located with in local

area.

ii) The junction line is the line connecting one main exchange to intercom exchange located with

in local area.

iii) The trunk line is the line connecting two main exchanges located at distant places located at

different towns or cities. These lines may be MW channels or OFC channels.

b) Different interface circuits(Cards) are used to work with different signalling system eg. Loop-

no loop, E & M, DTMF, R2MFC etc. In case of C-DOT, RAX exchange, the following cards

are used to provide communication between the subscribers of two exchanges.

i) 2 way trunk card (TWT):- It interconnects C-DOT exchanges with 2W trunk, working

on loop basis and can be directly connected to TBE card in OMNI S3E exchange or

via BWTDE card in OKI exchange. There are 8 circuits per card, each one of which

can be programmed as either incoming (I/C), or outgoing (O/G) or both way (B/W)

circuit.

ii) 4W E&M trunk card: - This is used to interface the RAX to another exchange through

carrier equipment. For this card also having 8 circuits can be programmed for

I/C,O/G or BW. It can be connected to OKI exchange via. PFWTA card.

c) In case of OKI exchange the following cards are used for connecting to other exchanges.

To connect to main exchange on junction lines,

i) BWTLE card is located in the LTE rack, which can be programmed for incoming,

outgoing or both way on loop signaling.

ii) ODBWTDE card is located in the LTE rack, which works on E&M signaling via trunk

lines on MW or OFC.

iii) BWTLE card is located in the LTE rack, which uses ringing voltage for incoming, and

loop signaling for outgoing calls.



Objective: 1. Enabling switching between two subscribers a card called _____________ is required.

2. To check the idle or busy status of the subscriber, exchange performs ____________.

3. To alert the called subscriber _____________ is sent on line.

4. If the subscriber is busy the messages are stored in _____________ switch.

5. When the signals are transmitted from the switch in digital form then it is known as___________.

6. Signaling in ISDN network is _____________ or __________.

7. E&M signaling is used to connect _____________.

8. 4wire E&M trunk will require ______ pairs of wires.

9. IDC means insulation displacement connector (true/false).

Subjective Answer the following in short: 1. Explain in short various types of switching taking place in electronic exchange?

2. Why signaling is essential in telecom circuits?

3. What are different types of signaling used in telecom?

4. Explain packet switching.

5. Explain time and space switching.

MDF & PROTECTIVE DEVICES


CHAPTER 3

MDF AND PROTECTIVE DEVICES

3.1 Main Distribution Frame (MDF) – Introduction:

MDF means “Main Distribution Frame” which is the first distribution point (DP) from the

Telephone Exchange towards subscriber. All the incoming cables from subscriber telephones

and trunk lines from the other Telephone exchanges are terminated on one side of MDF known

as “Cable side” . The cables coming from the exchange equipment room are terminated on the

other side known as ”System side” . These are known as KRONE TERMINAL BLOCKS .

MDF use IDC (Insulation Displacement Connector) terminal strips on which pairs of wires can

be terminated. IDC terminal strips contains two parallel rows of terminals in the same molded

plastic block. The terminals in one row are internally connected to the corresponding terminals

in the second row. However the connection is made through contacts that may be broken by the

insertion of a monitoring plug or a special disconnection plug known as” Wedge Connector”.

IDC insertion tool: It is used to make connections to the IDC terminals of sockets and

connecting cables, and is frequently referred to as a ’Wire Inserter 2A’ using the BT

identification code number. The manufacturer of the tool is Krone (UK) Technique Ltd., and their

part number is 6417/1/810/02.

The two features of the tool are,

a) It includes a scissor-action cutter that is designed to cut off surplus wire after the termination

has been made. This occurs automatically as part of the punch-down action.

b) It is provided with a folded-out metal hook for removing the wires from the terminals. It is

shown in the figure.3.1.

3.2 Main Distribution Frame (MDF) – Functions:

a) All incoming lines from subscribers and outgoing lines towards exchange are terminated

at MDF.

b) It is a place suitable for testing the line side and equipment side with line test in case.

c) It carries protective devices like fuses & Integrated Protection Module (IPM).



3.3 Terminations: The incoming and outgoing lines are terminated at MDF on Krone Tag Blocks. These are

available in capacities of 50 pair &100 pair. Tag Block of 10 pair is shown in the below figure.

The specifications are also given below.

Fig.3.1 Professional IDC insertion tool



Krone Tag Blocks 100 pair: » MDF TAG BLOCK 100 PAIR

MDF Tag Block 2/10 for 100 pair with 10 LSA Plus Disconnection Module for use on

“Line Side” (subscriber) of MDFs.

Fig.3.2 » Technical Specifications

Conductor dia 0.40….0.80 mm

Outside dia 0.70…..1.60 mm

Conductor Insulation Plastic material PE or PVC

No of wires/contact 1 (2 wires only with conductor dia 0.4 to 0.65 mm)

Temperature Operation -20…+80 deg C

Relative Humidity 93% at +40 deg C

Contact type IDC Silver Plated

Connection Dry gas plated

No of reterminations > 200

Wire Clamping & Guide Double sided

Vibration IEC 50A (CO) 145, (IEC 68-2-6 Test Fc), DIN 57804

Contact resistance <2.5 m ohm

Insulation resistance > 50,000 mega ohm

Dielectric strength 2 KV RMS

Impulse voltage resistance > 3.6 KV

Current impulse strength > 5 KA

3.4 The Main Distribution Frame for RAX 128 port & 512 port:

The main Distribution Frame for RAX 128p is of wall mountable type having two parts, VIZ. Wall

mount frame and hinged panel. The Frame is made up of anodized Aluminum material for better

finish and insulation. The hinged angel is an open able one, on which the fuse mount assembly,

connector modules and label holders are assembled.



IDC Terminal strips Aluminium

Frame

The Main Distribution Frame is equipped with LSA plus 2/10 disconnection modules capable of

taking insulation displacement connections for the conductor diameter of 0.4 to 0.8mm, with an

overall diameter including the insulation up to 1.5mm. Provisions are made for jumpering

between line side and exchange side by providing jumper rings.

Protective elements like anti-surge fuses of 200mA and 3 terminal gas discharge tubes are

used in the circuit. Add-on arrestor magazines are used for easy and quick replacement of G.D.

Tubes.

The Main Distribution Frames are supplied with insertion tool for making IDC connections, 2/4 -

4 Pole Test Cord for testing with the line side or exchange side isolation, patch cords and hand

tools etc. Main Distribution Frame for 512P Max. is floor mountable

3.4.1 Main Distribution Frame (MDF) 128Port:

Fig.3.3

a) Features of 128P MDF:

i) Equipped for 128P but expandable for another 128P on the same frame.

ii) LSA-PLUS system components providing facilities for add-on Single Pair Integrated

Protection Module, quick connect IDC type avoiding wire stripping, soldering or

screwing for wire termination.

iii) Number of accessories such as for making and identification purpose and as well as

for measuring and testing facilities.

iv) Frame design lends for quick and easy installation.

v) Dust covers provided for avoiding entry of dust on the contact point on the tag block.



IDC Terminal strips

Aluminium Frame

3.4.2 Main Distribution Frame (MDF) 512Port:

Fig.3.4

a) Features of 512P MDF:

i) Stand on wall mountable type of 2 meter high.

ii) Expandable by ganging similar units.

iii) LSA-PLUS system components providing facilities for add-on Single Pair Integrated

Protection Module, quick connect IDC type avoiding wire stripping, soldering or

screwing for wire termination.

iv) Accessories for monitoring, testing, labelling, disconnection of lines either in single or

10 pairs and test adopter makes the installation quicker and efficient system

functioning.



3.5 Protective devices used in MDF:

The Exchange equipment to be protected against A.C. power across, power induction, and

lightning faults on Telecommunication lines. Otherwise such hazards can potentially travel into

the exchange modules and severely damage sensitive switching and transmission equipment.

To minimize the effects of such occurrences, poly switch resettable devices can be used as

over current protection as “Primary protection” at MDF in modular form.

In India many standards use the ITU-TK.20 specification as a guideline. Most specifications

include lightning power cross surges intended to mimic the worst case electrical faults that can

be expected. Typical power cross surges are from 100V to 300V with current levels from 0.250

A to 3A. Lightning surges with open-circuit voltages of 1000V to 2500V and short circuit currents

of 10 to 100A peak are common.

In MDF primary protection is provided. Primary protectors are Gas Discharge Tube (GDT) OR

Carbon blocks or Integrated Protection Modules(IPM). The primary protectors limits the very

high energy transients.

The secondary protectors are incorporated on Telecom line card itself, which limit the voltage

and current to acceptable levels.

Here are other types of over voltage protectors such as “Metal Oxide Varistors (MOV) and

Thyristor Surge protection devices (TSPD) preferred choice for Telecom protection applications.

A typical protection design shown below.

a) Gas Discharge Tube- Basic principle of operation :

Gas Discharge Tube (GDT), operates on the principle of the arc discharge phenomenon.

Electrically, GDT act as voltage-dependent switches. As soon as the voltage applied to the GDT

exceeds the spark over voltage (70 volts to a few KV, depending on the type), an arc is formed

in the hermetically sealed discharge region within nanoseconds. When the discharge has died

down, the GDT extinguishes and the internal resistance immediately returns to values of several

hundred mega ohms.



Under normal operating conditions the high insulation resistance and the low self-capacitance

contribute to the fact that a gas-filled surge arrestor has virtually no effect on the system to be

protected.

i) Applications:

Typical applications are,

- Telephone exchange substations

- Cable distribution system

- Telephone MDF

- Subscriber’s terminals

- Satellite reception system

ii) Definitions of terminology:

- Spark over/breakdown voltage : An electrical breakdown of a discharge gap of a gas

discharge tube.

- Impulse discharge current: The peak value of the impulse current passing through the

gas discharge tube.

- Nominal alternating discharge current: For currents with a frequency of 15 Hz to 62m

Hz , the alternating discharge current which the gas discharge tube is designed to carry

for a defined time.

- Arc voltage: The voltage appearing across the terminals of the gas discharge tube during

the passage of the arc current.

- Transverse voltage : For a gas discharge tube with several gaps, the difference of the

discharge voltages of the gaps assigned to the two conductors of a telecommunication

circuit during the passage of discharge current.

- Holdover voltage: The maximum DC voltage across the terminals of a gas discharge

tube which it may be expected to clear and to return to the high impedance state after the

passage of surge, under specified circuit conditions.

iii) Principle characteristics:

Electrical: The electrical parameters of GDT with typical values are given below:

- DC park-over voltage

(breakdown voltage) - 70 to 1400V

- Impulse discharge current - up to 40KA

- Nominal alternating



discharge current - 20 A

- Arc voltage - <25 V

- Insulation resistance - ≥ 10 GΩ

- Self capacitance - 1 to 5 Pf

- Transverse voltage - <35V

- Hold-over voltage - > 100 V type

- Operating temperature range - -40º to 100º C

b) Metal Oxide Varistors (MOV) :

Metal oxide varistors are voltage dependent, non- linear devices which are manufactures

using a semiconducting metal-oxide, i.e., ZnO (zinc oxide), and has the typical I-V

characteristics.

i) Applications:

- Consumer electronics (video recorders, colour TV, amplifiers, and car radios)

- Domestic applications ( heating controls, washing mechiines, and microwave ovens).

- Telecommunications (telephone handsets, telephone exchanges, fax, telex, and modems).

- General industrial applications (machine control, airconditioning, medical analysis,

transformers etc).

- Lighting (electric ballasts)

- Power supplies ( washing machines, power switches etc).

- Automotive electronics (protection of an ignition circuit output transistor, protection of the

solenoid driver circuit).



ii) Features:

The min features of the varistors are:

- Excellent voltage clamping.

- A fast response time to suppress high voltage transients and surges.

- Excellent temperature characteristics and a wide operating temperature range.

- Large surge current withstanding capability and a very low leakage current.

- Low capacitance value which makes it suitable for protecting switching circuits.

- Completely non-flammable in accordance with IEC standards, even under severe loading

conditions.

(iii) Typical Protection design: Typical Protection design is shown in the figure shown below. It has ‘Primary protection’ in

MDF by GD tubes and ‘Secondary protection in the Line Card by providing over current and

over voltage devices like Resistor, PTC, Fuse & Thyristors.


IRISET 33(i) T12A - TELEPHONE EXCHG PRINCIPLES & INS TRUMENTS

Objective: 1. Normally an IDC can accommodate _______ pairs of copper cable on it.

2. MDF’s are to provide with _____________ devices no it.

3. Gas discharge tubes works on the principle of __________________ phenomenon.

4. MOV are _______ linear devices.

5. IDC are more reliable terminations than other terminations.

Subjective: 1. What care should be taken while installing the MDF?

2. What is surge protection? Why it is provided and what are different types of protection?

TELEPHONE EXCHANGES IN INDIAN RAILWAYS


CHAPTER 4


4.1 Classification of Exchange on the basis of mode of operation: Telephone exchanges Manual type Automatic type “Operator Electronic exchanges El ectro-mechanical type Dependent” 1) C-DOT excha nge 2) OKI exchang e 3) ISDN exchan ge 4.1.1 Manuel exchanges: - Connection between two subscribers is established by a person

known as ”Operator” sitting at Manual Trunk exchange.

4.1.2 Automatic exchange: - Connection between two subscribers of same exchange is

established automatically by a system called “Switching system”.

In case of “Strowger exchanges” the switching is done by “Electro-mechanical switches”.

Now a days in digital exchanges the switching is done by “Digital switches” which is known

as “Digital switching”.

To overcome the manual switching, automatic exchanges, having electromechanical

components were developed.

a) Evolution of exchanges:

First automatic exchanges having direct control feature appeared in 1892, in Laporte

(Indiana).

First cross-bar exchange having indirect control feature appeared in 1928 in

Sundavell, Sweden.



First electronic exchange employing SDM (Space Division Multiplexing) was

commissioned in 1965 at Succasunna, New jersey.

First digital exchange with TDM (Time Division switching-Digital switching) was

commissioned in 1970 in BRITTANY, FRANCE.

b) Advantages of Electronic exchanges over Electrom echanical exchanges.

S.No. Electro -mechanical exchanges Electronic exchanges

1. Category analysis, Routing,

translation etc. done by relays.

Translation, speech path, sub’s

Facilities etc. managed by MAP

(Maintenance administration &

Programming) and other DATA.

2. Any changes in facilities required

require addition of hardware and/

large amount of wiring change.

Flexibility limited.

Changes can be carried out by

simple commends.

3. Testing is done manually externally

and is time consuming. No

logic analysis carried out.

Testing carried out periodically,

automatically and analysis printed

out.

4. Partial fault availability hence

blocking.

Full availability, hence no blocking

5. Limited facilities to the subscribers. A large number of different types of

services possible.

6. Slow in speed. Dialing speed is

maximum 11IPS and switching speed

is in milli-seconds.

Very fast dialing speed up to 11

digits/sec possible. Switching is

achieved in a few microseconds.

7. Switching noise is large. Switching noise is almost Nil.

8. Occupies large space. Occupies lesser space.

9. Installation and testing time is long. Installation and testing time is

Small/short.

10. Maintenance is more & Preventive

maintenance is required.

Remidical maintenance is very easy

due to plug-in type PCB. No

preventive maintenance.

c) Facilities provided by Electronic exchanges:

Facilities are THREE types-



i) Facilities to subscribers

ii) Facilities to administration

iii) Facilities to the maintenance personal

i) Some of the Subscriber’s facilities are,

- MFB push button dialing (Multi Frequency Buttons)

- Priority subscriber lines

- Toll (outgoing calls) restriction

- Service Interception

- Abbreviated dialing

- Call Forwarding

- Do not Disturb

- Conference calls

- Camp on busy

- Call waiting

- Malicious call identification

- Call charge print out/immediate billing

- Interception or announcement

- Hot line

- Automatic wake up

- Denied Incoming calls

- Instrument locking / Dynamic locking

- Free of charge calls / tool free

ii) Facilities to administration:

- Reduce switch room accommodation

- Faster installation and easy extension

- Versatility

- Economic consideration

- Automatic test of subscriber line

iii) Maintenance Facilities:

- Fault processing is automatic.



- Diagnostics: Location of fault by maintenance staff on demand can be done by

programme.

- Statistical programs – traffic condition, trunk occupancy rate etc.

- Blanking – Stopping of calls from subscribers.

iv) Points to be remembered during installation:

- Traffic handling capacity.

- Power supply.

- Total protection from dust.

- Temperature and Humidity control –Air-conditioning system.

- Static electricity and electromagnetic interference.

- PCB repair.

4.2 C-DOT Exchange:

4.2.1 Introduction:

C-DOT means “Center for Development of Telematics”

This is a central government organization of India set up to develop the necessary

equipments (infrastructure) suitable for Indian climatic and environmental conditions.

4.2.2 Special features of this exchange :-

a) Single frame terminal unit capable of 128 ports

b) Standard programme control

c) It is modular in design and hence expandable

d) Man machine communication

e) Low power consumption

f) No air condition is requires

g) No single LCC fault effects more than 8 terminations

h) It is reliable

i) The digital switching system of C-DOT technology will offer 100% non-blocking voice

and data network.

j) Easy system start up.

k) Faults are indicated through maintenance panel in the form of audio and visual alarms.

l) Calling party release.

m) Called party release (effected after 60 seconds).



n) Automatic system alarms in case of duplicate unit failure, battery low and power supply

unit failure.

o) Remote testing facility to check system status.

4.2.3 Working Voltages:

a) Input voltage to PSU - -48V ± 4V D.C.

b) Out put voltages of PSU are,

i) +5V.D.C –8A – For IC’s

ii) –9V.D.C - 0.5A – for Codec

iii) + 12V.D.C – 1A - for relays

iv) –5V.D.C -0.1 A – for other digital components

v) –48V.D.C -Speech voltage.

4.3 Types of C-DOT exchanges:

There are two types of C-dot exchanges, used in Indian Railways.

4.3.1 C-DOT 128 port RAX (Rural Automatic Exchange) :

It is designed to meet the telecommunication needs of small sized rural areas. These

exchanges are also suitable for Indian Railway applications, where the Telephone line

capacity is less. It can be expandable up to 400 lines.



(a) C-DOT RAX 128 port Exchange INCOMING LINES

Fig.4.1 Block diagram of C-DOT RAX 128 port Exchange

(b) Physical description:

It is a cabinet type exchange. Its front side is provided with hinged doors.

The cabinet contains equipment frame, in which there will be 26 card slots guides filled with

26 cards.

Each card is provided with two numbers of “FEMALE EURO CONNECTORS”. These get

connected with “MALE EURO CONNECTORS” on mother board when the card is inserted

into the respective slot guide.

The Mother Board is the Multilayer PCB which connects all the cards. The cards in C-DOT

exchange are classified as,

(i) Peripheral cards or Terminal cards:

These include subscriber’s cards (LCC), Trunk and Tone cards. These cards are not

duplicated.

(ii) Control Cards:

This card controls the working of the exchange. These cards are duplicated.

(iii) Power Supply Unit (PSU) CARDS:

This generates necessary D.C voltages required for the exchange. This is also duplicated.

MDF

TERMINAL GROUPS

SWITCHING NETWORK

PSU CONTROL CARDS



(c) Maintenance :

The maintenance and system administration is done by a ”Maintaince panel” which is

connected to RAX control processor card, through RS 232C. Microprocessor 65C02 is

provided for performing the above functions.

(d) Accessories for maintenance:

(i) LINE TEST UNIT: Different tests on subscriber’s lines or on subscriber’s telephone can

be conducted with the help of this unit.

(ii) TOTAL FAILURE ALARM UNIT: It gives an audible and visible alarm to maintenance

staff in case of ” Total Failure of Exchange”.

(e) Technical Specifications:

(i) Switching: Digital PCM’A’ law CCITT standards non-blocking.

(ii) Control: Microprocessor based Stored Programme Control (SPC).

(iii) Capacity: Up to 96 subscribers with 8 Trunks.

(iv) Line loop resistance: Max 1000Ω.

(v) Ringing: 75 V RMS.

(vi) Tones used in C-DOT exchange:

Sl. No Type of Tone Significance of Tone Frequency

1 Dial Tone Gives an indication that the equipment is ready to receive dial pulses.

400 Hz. Continuous

2 Ring back Tone Gives an indication that the called subscribers bell is ringing.

400 Hz, 0.4 Sec ON, 0.2 Sec OFF and so on..

3 Busy tone Indicates that the called subscribers telephone is busy

400 Hz, 0.75 Sec ON,0.75 Sec OFF.

4 Number un -obtainable Tone

Indicates that the dialed number is not pertaining to the exchange.

400 Hz, 0.28 Sec’s ON, 0.2 Sec OFF

(vii) Primary power: - 48V ± 4V, D.C. 6Amps.

(viii) Working Temperature: 0ºC to 45ºC.

(ix) Relative Humidity: 5% to 95%.



4.3.2 C-DOT 128 Port PBX:

PBX stands for ”Private Branch Exchange”. This is normally used within the office to

connect a number of internal users to a smaller number of outside lines on the public

network.

The extensions may be served by a conventional analog telephone network.

The block diagram is same as RAX.

(a) Physical description:

The cabinet contains equipment frame, in which there will be 26 slots guides. In this only

23 cards can be housed. Each card is provided with two numbers of “EURO

CONNECTORS”. The motherboard is Multilayer PCB, which connects all the cards,

external voice, and data connections are made from Main distribution frame.

(b) Slot allocation of C-DOT 128 port Exchange:

The equipment frame contains the following c ards:

Cards Slots

Tone Generation and Diagnostic(TGD) Card 2 No’s 2 Slots

Terminal Cards 14 No’s 14 Slots

Terminal Interface Controller/Switching network (TIC/SN) Car 2 No’s 2 Slots

Signal processor Controller (SPC) 2 No’s 2 Slots

Power Supply Unit(PSU) Card 2 No’s 4 Slots

Data Card 1 No 1 Slot

Spare Slot - 1 Slot

Total 23 Cards 26 Slots



(c) Port allocation of C-DOT 128 port exchange:-

Slot No Card Name Ports

1 & 2 PSU -0 -

3 TGD 00-07

4 JUN 08-15

5 LCC 16-23 -port. No. 16 is for console 1.

6 LCC 24-31 -port. No.17 is optional for paging.

7 LCC 32-39

8 LCC 40-47

9 LCC 48-55

10 LCC 56-63

11 TIC-0 - Copy `0`

12 SPC-0 - Copy `0`

13 DTA -

14 Spare -

15 TIC-1 - Copy `1`

16 SPC-1 - Copy `1`

17 LCC 64-71

18 LCC 72-79

19 LCC 80-87

20 LCC 88-95

21 LCC 96-103 port. No-103 is for console `2`

22 LCC/CONF 104-111

23 JUN 112-119

24 TGD 120-127

25 & 26 PSU-1 -

Maximum number of Extensions can be provided are –94

Minimum number of Extensions can be provided are- 86

Maximum number of Junctions can be provided are – 16

Maximum number of Data terminations can be provided are –32



(d) Features:- Some provisions are made in C –DOT exchange to use the system effectively ,

These are,

i) System Features

ii) Extension Features

iii) Operator Features

(i) SYSTEM FEATURES:

These features are provided along with the system. These are

- Class of service : The user is allowed or disallowed the facilities like, ‘0’ dialing, STD

facility, conference facility, paging access facility, tie line accesses facility etc.

- D.C. Battery back up : -48 V DC battery backup is provided to work during power Ac

power failure.

- Extensive Automatic Diagnostics : The diagnostic information regarding the status of

cards, junctions, positions, links are displayed by LED’s, audio beeps are given, making of

extension numbers OUT OF SERVCE, indicating the battery LOW condition when voltage

falls below 44 DC.

- Metering: Counting the number of calls made by each extension, junction and Operator is

called as ‘Metering’.

- Power Failure cut through: - In case of power failure, the PBX will not work, During this

period 2 pre determined extensions are immediately connected to the main exchange

through the junction lines.

So they can pass the information to the main exchange.

- Redundancy Scheme: - Providing the duplicate cards in the exchange is called as

“Redundancy scheme“.

- Tones: - These are necessary in the exchange to progress a call between two extensions

and to know the condition of the exchange. The TGD card generates different tones,.



They are,

Sl.

No.

Type of Tone

Significance

Frequency

Cadence in ms

1 Dial Tone indicates the system is

ready

400 Hz Continuous

2

Ring back Tone Indicates the called

Extension is ringing.

400 Hz

modulated

by 25 Hz

400 ms ON

200 ms OFF

400 ms ON

2000 ms OFF

3

Busy Tone Indicates the called

Party is busy

400 Hz 750 ms ON

750 ms OFF

4

N U Tone indicates that invalid or

Unlisted number is

dialed

400 Hz 2800 ms ON

200 ms OFF

5

Wait/Operator

Intervention Tone

Indicates that the

operator going to

enter

the conversation

400 Hz

Modulated

by 25 Hz

250 ms ON

250 ms OFF

6

Feature Tone Indicates that the

feature can be

activated

400 Hz 100 ms ON

100 ms OFF

7

Assurance Tone Feature sequence is

valid

400 Hz 200 ms ON

200 ms OFF

8

Silent Tone Indicates extension is

OOS

-

4.4 OKI Electronic Exchange:

4.4.1 Introduction: It comes under NE 1000 series. It in fully Non-blocking Digital Electronic

Exchange PBX type. It works under SPC (Stored Programme control). The technology

involved is PCM. Voice and Data Communication can be simultaneously transmitted,

unidirectional as well as bi-directionally. It can be maintained remotely. Capacity is 512

to 6144 extensions.



4.4.2 Services:-

a) Multimedia Communication

b) Connection with various Business systems

c) Hybrid switching

d) Intelligent Electronic mail service

e) One-System Design concept

f) Building Block (multiunit) Structure

g) Simultaneous and Bi-directional Communication of voice and data

h) Connection of a Varity of terminals

i) Compact, light weight, and low power consumption

j) Remote maintenance

4.4.3 System Description:

This Exchange uses three levels of Microprocessor in its architecture.

Each exchange has a Central Processor 68000 in CPU, which further controls the Z80

processors located in each LTE. SDLC is used between LTE and CPU.

Each Z 80 processor controls 20 Microcontrollers in polling mode. Each user card has

microcontroller IC 8049 for analogue circuit and IC 8031 for digital circuit. The hardware

architecture of NE 1200 exchange is shown below.

Fig.4.2 Architecture of NE 1200



It is having 3 types of Racks.

a) Central module Equipment (CME):- It houses all control cards, CPU, memory devices

and I/P & O/P devices.

b) Miscellaneous Equipment (MSE)-It houses the main SMPS, Disc drive, Drive controller

card, and Fuse panel.

c) Line Terminal Equipment (LTE):- It contains all Line and Trunk cards.

The cabinet (CAB) is a modular type. Number of lines per cabinet and number of LTE’ s are

listed below.

In 1100 model there is no duplicate CPU hence it is not recommended for Railway use. Model

NE 1200 has got duplicated CPU, so it is recommended by RDSO. The duplicated CPU works

on hot strand by mode. It has Single unit configuration with 8 LTE’ s.

Number of lines per cabinet and number of LTE’ s ar e listed below:-

Number of

Lines

Equipment

Type

Number of

CAB’s

Number of

LTE’s

128 NE 1100 1 1

256 NE 1100 1 2

384 NE 1100 1 3

512 NE 1100 1 4

1024 NE 1200 2 8

2048 NE 1200 4 16

3072 NE 1300 7 24

6144 NE 1300 13 48

a) Central module Equipment (CME):

In CME (Control Module Equipment) Rack, cards cannot be interchanged from one position

to another. It is restricted to its own position, otherwise it damages the cards. It is provided

with 32 slots. Each slot is allotted for a particular type of card only as shown in the block

diagram.



It consists of the following cards:

(i) DATA TRANSFER UNIT CARD (DTC):

This card is placed in CME rack slot number 15. It helps intercommunication between two

CPU’s.

(ii) SYSTEM STATUS CHECK CIRCUIT (SSC) CARD:

This Card is mounted in CME rack slot 15 and it reports system status eg. Major, Minor

faults, system CPM or SWM failures etc. The card itself displays different status information

by several LED’s.

(iii) TONE SENDER (TSNE) CARD:

This card is placed in CME rack in slot number 08 and 25. It generates 30 audible tones.

Fig.4.3 Block Diagram of CME Rack



b) Miscellaneous Equipment (MSE):

It houses the main Switch Mode Power Supply (SMPS), Disk Drive, Drive Controller and

Fuse panel. These are show in

the housing diagram.

Fig.4.4 Block diagram of MSE rack

c) Line Terminal Equipment (LTE):

It contains all Line and Trunk cards. The cards in LTE rack are,

(i) SLICAE Card (Subscriber’s line interface card ) for analog Extension.

This card has 8 subscriber circuits. This will perform BORSCHT function.

This card is placed in LTE slots 2 to 21.

It is provided with INTEL Microcontroller 80C49.



Its function is to convert the analog voice signals to digital information.

There are 6 types of SLICA Cards.

SLICAE 02 – Have line testing and reversal.

SLICAE 06 – Has line testing and message function.

It is provided with a busy LED (RED). It glows only during port is busy.

Maximum loop resistance permitted for telephone is- 1200 Ohms.

It extends –48v DC line voltage to lines via. MDF through the normal contacts of Ring Relay.

Different voltages required for the smooth working for this card are as follows:-

-48V DC – Current supply for calling (Line voltage).

-5V DC & +5 V DC

- Power supply to logical and analog circuits.

(ii) BWTLDE Card (Both Way/Trunk Loop Detection Equ ipment)

- This trunk card is mounted in LTE rack and contains 6 circuits for sending and receiving the

loop signals from Trunk lines.

- It works with a line resistance up to 4000 Ohms and offers a DC loop resistance of 640Ω to

1040Ω.

- A RED led indicates the busy condition of the junction.

- We can make a particular trunk busy by operating a switch on the card.

- It has one front connector CND for line terminals.

ITS FUNCTIONS: -

- It sends and receives various signals to and from other exchanges by sending and

decting loop during transmission and reception.

- It converts voice signals to PCM signals.

- It offers low DC loop and high AC resistance to central office (Main Exchange).

PRECAUTIONS:-

- Before installing the card see that the lines do not have any fault.

- Before removing the card set the switches (operate) and see that LED’s are off.

- Set all switches before inserting the card.

- The card be handled with care.



(iii) BWTLE Card ( Both Way Trunk Line Equipment):

- This card is mounted on LTE rack and uses for 8 circuits for connecting to the Central

Office .

- When Port is busy LED will glow.

- The Port can be made busy by operating the “Busy Switch”.

- The circuit works up to a loop resistance of 1500 Ohms.

- Dc loop resistance permitted is 380 Ohms.

(iv) POTE Card (Positional Trunk Equipment):

- This card is used to connect attendant consoles Max. 2 No’s.

- It is kept at the 21 slot of LTE.

- This is having 60 pin front connectors CNA & CNB for ATT 0 & ATT 1.

- The distance permitted is,

For ATT console – 500 Mtrs.

ATT console 7 touch Call Box – 440 Mtrs,

ATT console & EBY – 140 Mtrs.

ATT console, OTC & EBY – 120 Mtrs.

(v) HIGH WAY INTERFACE CARD(HWIFC):

- It is kept in the slot number 22 of LTE rack. It controls 20 line/trunk cards of LTE.

- This card is having an Z-80 processor.

(e) Attendent Console:

- It is a peripheral device and it is connected by a 64 pin connector through POTE card

which is kept in LTE.

- It is used to connect trunk circuits, Tie line circuits & Stations.

- It has the following keys,

Mode changing over keys – 4 No’s (R’0’ to R’3’)

Function Keys – 10 No’s (C’0’ to C’9’)

End key – 1 no.



- It has 33 squires under which functional lamps are provided which glows during different

operations.

- It is provided with a hand set for speech.

- There is a volume control knob to control the audibility of the tone for alerting the operator.

- A liquid crystal display is provided on the top.

- It has many features like, station calling, Call hold, direct outward dialing Camp-On –

Ringing, Camp –On –Busy, One touch calling Automatic recall etc.

4.5. I S D N Exchange:

4.5.1 Introduction:

ISDN means Integrated Services Digital Network. It is a set of digital transmission

protocols defined by the international standards body for telecommunications, the ITU-T.

These protocols are accepted as standards virtually by every telecommunications carrier

all over the world.

ISDN is compatible with the traditional telephone system so that a single pair of telephone

wires is capable of carrying voice and data simultaneously. It is a fully digital network

where all devices and applications present themselves in a digital form.

The essential difference between ISDN and the conventional telephone system is that it is

digital not analog. Information travels as bits rather than as waves. In addition, it also

allows multiple streams of these bits to occupy the same connection providing the user

with greater versatility of services.

4.5.2. OBJECTIVE:

The objective of the ISDN is to provide the user with easy access to a multiplicity of

services over a single connection and digitalization up to sub’s premises.

ISDN has spread as a powerful tool through out the world in providing different services

mainly voice, data and Image transmission. High speed data transfer and video

conferencing are among the main facilities of the ISDN. With the introduction of video

conferencing the dream to see each other face to face during conversation has come true.



4.5.3 Advantages:

a) High speed and high quality communication.

b) Reliability and security.

c) Better use of existing facility.

d) International standardization.

e) Simplified wiring.

f) Efficiency of network usage.

g) Standard data transport rate.

4.5.4 SRVICES OFFERED BY ISDN:

a. Telecommuting,

b. Simultaneous transmission of voice, Fax, Data, and e-mail.

c. Video Conferencing,

d. Remote broadcasting,

e. High quality audio transmission,

f. Supplementary services:

(i) Calling line identification presentation (CLIP),

(ii) Calling line identification restriction (CLIR),

(iii) Advice of charge (AOC),

(Iv) Multiple subscriber number (MSN),

(v) Call forwarding service (CF),

(vi) Call forwarding busy (CFB),

(vii) Call forwarding no answer (CFNR)

(viii) Call forwarding unconditional (CFU),

(ix) Terminal portability (TP),

(x) Call hold (CH),

(xi) Closed user group (CUG),

4.5.5. Type of information handled by ISDN:

ISDN handles all types of information – voice, data, studio-quality sound, static and

moving images. They are all digitized, and transmitted at high speed. It can handle many

devices and many telephone numbers on the same line. Up to 8 separate telephones, fax

machines or computers can be linked to a single BRI ISDN connection.



A single BRI connection can have different phone numbers assigned to each service.

A basic rate ISDN line can support up to two calls at the same time. Any combination of

voice, fax, or PC connections can take place at the same time, through the same ISDN

line.

From a digital ISDN telephone you can place a call to an analogue telephone on the

PSTN (Public Switched Telephone Network) and vice-versa. Both networks are

interconnected by the network carrier in a way similar to the connection between the

mobile phone network and the analogue phone network. For the user, it is completely

transparent whether he is calling a GSM telephone, a conventional telephone or an ISDN

digital telephone.

a) Basic Rate:

i) It is provided through a Basic Rate Interface. This kind of interface is also called as S’0’

(zero) interface.

ii) There are two channels of 64 kbps each.

iii) The total data rate across this interface 192Kbps. (64 kbps + 64 kbps + 16 kbps) (2B +

1D) + 48 kbps over Heads.

iv) This bit rate was chosen because the wiring already installed by the telephone

companies under the streets can carry base band (digital) transmission at this speed.

Normally, BRI would be for domestic use, or smaller remote offices.

b) Primary Rate:

i) It is provided through a Primary Rate Interface.

ii) This kind of interface is also called an S2 Interface.

There are either 30 channels or 23 channels (North America, Japan) that you can use.

i) The total data rate across this type of interface is 2.048 MBPS or 1.5 Mbps. (30B + 1D) +

64 kbps over Heads

ii) This kind of access requires the installation of a high speed line to the customer

premises.

Primary Rate would typically be used for large remote access servers, fax servers or

PBX s in medium size or large offices.

4.5.6 Services provided by ISDN:

a) Network services:



- Network services carry the interactions between the user and the network

For example: Setting up calls and disconnecting them. Network services define how the

user and the network interact with each other in order to manage calls.

- The user can use Network services to quest the network to perform functions such as

making and clearing calls, transferring calls to another user, and so on. This activity is

known as signaling.

b) Bearer services:

- Bearer services carry data between two users

For example: voice or fax information encoded as a bit stream.

- Bearer services carry the call activity that the user is performing at any given moment.

- This includes voice calls, fax and modem calls, and connections to the internet.

- Broadly speaking, there are two forms of bearer services.

c) Structured Data: The information passing over the bearer service is in a format that is

understood by the network.

Voice is an example of structured data. Because the network knows that the connection

carrying voice, it can convert the data into an analogue signal in the event that the call is

connected to an ordinary analogue phone.

d) Unstructured Data: The format of the information is not understood by the network, but

is understood by the two users at either end of the service.

4.6. POWER SUPPLY ARRANGEMENTS IN TRELEPHON EXCHAN GES:

a) Types of batteries used for Telephone Exchanges:

The following types of Lead Acid batteries are used for telephone exchanges,

(i) Conventional Lead Acid HDP Tubular type.

(ii) Sealed Maintenance Free type.(VRLA)

(iii) Low Maintenance type.



The comparison between the three is given under:

Conventional Lead

Acid type

SMF / VRLA type Low Maintenance type

1. Application Ideal for

charge/discharge

cycling duties

For short time

back-up with low

depth of

discharge

Ideal for float application

in power

plants/substations .

2. Reliability Quite reliable Reliability depends

on ambient

temperature,

charger voltage

setting and depth

of discharge.

Most reliable in Float

operation

3. Monitoring state

of charge

Measuring specific

gravity

Sp. Gravity cannot

be monitored

Visually through the cell

container and measuring

the specific gravity of

cells.

4. Susceptibility of

high temperature

Satisfactory

operation up to

48/50ºC

Prone to damage if

operated above

35ºC continuously.

Up to 48/50ºC electrolyte

temperature.

5. Maintenance

requirement

(a)Topping up once

in a month.

(b) State of charge

to be monitored by

using Hydrometer.

(a) No topping up

is required

(b)State of charge

cannot be known

exactly.

(a)Topping up once in 8-

12 months.

(b) State of charge to be

monitored by using

Hydrometer

6. Recharge Quick chare by

Boost chare

Float charge at

2.23v/cell.

Occasional quick

charge may be

given to 2.4v/cell

Capable of quicker

recharge by boost

charge than conventional

type.

7. Ageing Capacity degrades

continuously during

service life.

Capacity loss is

negligible if

operated under idel

conditions.

Capacity does not drop

below rated initial

capacity during service

life.

9. Life expectancy 10-12 years 10 years 18-20 years



b) POWER SUPPLY ARRANGEMENT AT ELECTRONIC EXCHANGE:

The power supply arrangement at Electronic exchange is shown in Figures below.

Battery C-DOT

POWER SUPPLY ARRANGEMENT AT ELECTRONIC EXCHANGE

A C

170-270 V,

FCBCCharger

A T

SUPPLY

Exchange128 Points

Stabiliser

50Hz

Stabiliser230 V

_

+ +

_

48 V80 AH

48 V20 Amps

Ferroresistant

50Hz160-270 V,

A C 230 V A C

ExchangeElectronic

OKIAutomaticBattery

48 V, 100 A

_

+48 V400 AH

_

+

StabiliserCharger

~

C - DOT

128 Port Exchnage : 5 Amps.

Normal Battery Current : 8 Amps. 13 Amps.

OKI/512 PORT

Switch

70 Amps.Battery Current : 40 Amps.

so 100 Amps. is taken

Exchnage Current : 30 Amps.

Total Current :

Total Current :

Voltage

Fig.4.5 Power supply arrangement for OKI Exchange

The electronic exchange requires - 48V - 64V DC. This can be provided with a battery of

- 48V DC with 24 cells connected in series. The capacity of the battery can be calculated

taking the current drawn by the equipment.

A 80 AH battery is shown in the figure 4.5.

A battery charger of suitable type with required current rating which can charge the

battery and supply current to the load should be used.



Example:

Current drawn by the 128 port electronic exchange - 3 Amps.

Capacity of the battery required is – 3A X 10 H = 30 AH.

But as for the RDSO the battery should not be discharged to 50% of its Capacity to get

maximum life from it.

So the Capacity of the battery required is double to that of the calculated value. Which

becomes 2 X30 = 60 AH.

So we can go for a 80 AH battery.

Calculation of charger current:

The charger current can be calculated from the battery capacity.

If the battery capacity is 80 AH then the current capacity = 80/10 X 2.5 =20 Amps.

This can supply 3 Amps load current and also 8 Amps for charging battery if fully

discharged.



Objective: 1. Automatic connection between two subscribers is done by ___________ system.

2. Electronic exchanges are easy to ___________.

3. SPC stands for ______________________________.

4. Digital switch provides 100% _______________ voice and data network.

5. RAX stands for _______________________________________.

6. User cards and trunk cards are located in ___________________.

7. All control cards are duplicated in RAX. (True/False)

8. TGD card generates different types of ____________.

9. Total number of slots in RAX exchange is _______.

10. OKI NE 1200 exchange have ______ LTE’s.

11. SMPS is located in ________ rack of OKI NE1200.

12. In CME rack interchange of slots is not allowed. (T/F)

13. ISDN network is a _____ layer network.

14. BRI stands for Basic Rate Interface. (T/F)

15. BRI transmits _____________ Basic channels and one digital channel.

16. PRI contains ______________ Basic channels and one digital channel.

Subjective: 1. Draw the block diagram of CDOT 128 port and explain each functional block.

2. What are the advantages of electronic exchange?

3. Draw a simple block diagram of OKI exchange and explain.

4. Write some of the features of OKI exchange.

5. Write short note on BRI and PRI.

6. Draw a simple block diagram showing one BRI connection made at subscriber’s premises.

7. Write some of the services provided by ISDN exchange.



CHAPTER 5


5.1 Types of telephone instruments used in railways :

a) Push button Telephones:

(i) Single line (ii) Double line

b) Main & Extension (1+1) type Telephones.

c) Hand free Telephone with caller ID (CLIP phone)

d) Integraded telephones (Panasonic make)

(i) Single line with Data connection

M.No: KX-T 2375 MXW, speaker phone facility.

(ii) Two line with speaker phone facility

M.No: KX-T 2378 JXW.

(iii) Caller ID compatible

M.No: KX-T SC11 MXW

(iv) Two line CLIP with speaker phone facility

M.No: KX-TS3282BXW

(v) Cord less phone

M.No: KX-T C1085 BXB

5.2 PUSH BUTTON TELEPHONE:

The phone in which the dial is a ‘Push button key board’ type is called as “Push button

telephone”. In this the Ringing circuit, Speech transmitting circuit & receiving circuits

incorporates solid state devices and IC’s.

A simple push button telephone circuit consists of the following stages:



a) Ringer stage – produces audible ringing sound.

b) Key board matrix stage – to dial the required number or to perform the function of dialing

the digits.

c) Dialer stage – to generate the tone or pulse corresponding to the key dialed.

d) Sound amplifier stage – to produce the speech sounds during transmitting and receiving.

e) Voltage dropper stage – to drop the from 48V DC to 5V to 12V when the hand set is

lifted.

f) Rectifier and protection stage – to give voltage of proper polarity, to limit any high

voltage peaks due to induction on lines.

All the above stages are inter connected as shown in the given block diagram (Fig.1.)

L 1 L 2

Fig.5.1 BLOCK DIAGRAM OF PUSH BUTTON TELEPHONE

KEY BOARD MATRIX STAGE

DIALLER STAGE

RECTIFIER PROTECTOR STAGE

SOUND/SPEECH AMPLIFIER STAGE

VOLTAGE DROPPER STAGE

RINGER STAGE



a) Ringer Stage:

It consists of a Ringer IC, which operates an “Piezo electric buzzer’ or ‘Speaker’. The IC

operates on AC ringing signal of 20Hz frequency with its associated circuit.

It consists of,

i) Ringer IC: It consists of 8 terminal IC. It includes bridge rectifier circuit, power supply

control circuit, output amplifier, low and high frequency oscillator circuit. The generally

used ringer IC’s are LS1240, LS1240A, LS1240C, TCM 1536P etc.

ii) Piezo electric buzzer – It gives audiable sound. It is operated by the tone produced by the

output of the oscillator. It is connected between the output terminal of output stage of IC,

and ground (+ve) through an volume control which regulates the sound volume.

iii) Cradle hook switch – When the hand set is on the cradle the entire telephone circuit is

disconnected from the lines and only ringer section is kept connected to the lines L1 and

L2.

When the hand set is OFF the cradle hooks, the ringer section is disconnected by the

cradle switch contacts and the bridge rectfier is connected to the voltage dropper stage,

and further to dialer, sound amplifier stage.

iv) Other components: The other components of ringer section are,

- Capacitor(C1) of value 0.1uF. It is to change the frequency of oscillator.

- Resistance(R5) of value about 212Ω. It decides the frequency of oscillations. To change

the frequency of oscillator, the resistance value to be changed.

- Capacitor (C4) of value 10Uf 63V, is used to filter the rectifier output.

Description of ringer stage circuit used in GCEL 501 telephone is given below (ref.

Fig.5.2): The ringer section consists of LS 1240 (8 pin) IC, which consumes less current.

This IC has an internal oscillator to generate two tone frequencies and connects them

across the output amplifier.

The tone and oscillator frequencies can be adjusted externally with the help of resistor R3

and capacitor C2. This stage consists of a ringer switch having S1,S2, and S3 switches.

(v) Working: When the hand set is on the cradle hooks, switch S1 and S2 are OFF and S3

contact on ‘B’ side. So the ringing current of 20 Hz is applied to the Bridge rectifier inside the

IC as input. This AC ringing current is rectified by the bridge rectifier and the DC output is

used for the working of oscillator IC. The oscillations produced by the oscillator are



connected between pin.No.5 and 2 via output stage in the IC. These are connected to the

piezo electric buzzer via volume control. The volume of the buzzer can be adjusted by the

volume control. IC pin.No.3 and 4 are connected with a capacitor of 0.1uF and a resistance

of 212Ω which decides the frequency of oscillations(tone). The 10 uF,63V capacitor is a filter

capacitor which filters the output of the bridge rectifier.

As soon as the hand set is lifted, the ringer switch operates and S3 changes over its position

to ‘A’ side and the input to IC pin.No1 is disconnected and ring stops.

Fig. 5.2 Ringer circuit (vi) Fault finding: - No ring: Check: 1) Check ringer IC pin.no 1 & 8 for 24V,AC supply while ringing.

- If 24V,AC supply is ok, then check,

• Ringer IC

• Buzzer/Speaker faulty

• Volume control faulty

• Check the resistance, on pin.4 of ringer IC(may be open)

- If the 24V,AC on pin.no 1&8 not getting, then check ,



• Resistance on pin.no1 of ringer IC

• Resistance on pin.no8 of ringer IC

• Check the line capacitor (1uF or 1.5uF /250V)

• Check the ringer switch (cradle hook switch) contacts if any carbon formation is

there.

- Continuous ringing like sound:

• Capacitor on pin.no.3 of Ringer IC may be faulty (open)

• Change Ringer IC.

- Intermittent ringing:

• Clean the ringer switch (cradle hook switch) contacts and remove dust & carbon.

- Ringing low:

• Check telephone line supply

• Check the buzzer

• Volume control faulty

• Ringer IC faulty

• Ringer IC pin.no.7 capacitor faulty.

• Line capacitor C1 faulty

b) Key board matrix stage:

It is provided with a “Key pad” to enter the subscriber’s telephone numbers.

It has 4 Rows R1, R2, R3 & R4 and 3C columns C1, C2 & C3. The key pad is

connected to dialer IC. Minimum 12 numbers of keys are provided on key pad.

Number buttons -10 No’s (0-9 numbers)

Special keys – 2 to 6 No’s

Function of some special buttons are:

i) Redialing ® - To repeat the last dialed number.

ii) Tone-Pulse dialing



iii) Star key(*) – or Ashtreik key (#) – To get the Tone mode dialing.

iv) Mute (M) – To prevent outgoing speech.

v) Pause(P) To get multiples of 2.2 seconds delay between two digits of a number.

A row and the column are shorted when a number on the key pad is pressed. This

generates equal number of pulses or DTMF tone pair which will be available at dialer

IC.

Fault tracing:

i) No number gets dialed,

(a) Clean well the key matrix PCB

(b) Clean the key matrix push button carbon points

ii) Wrong number gets dialed,

This is due to wrong connection of key matrix connector.

Refer Fig.5.3 For key board matrix connection diagram.

Fig.5.3 Key-board matrix connection diagram

c) Dialer Stage:

This section is connected to the key board matrix stage on one side, and other side it is

connected to rectifier and protection stage, and sound amplifier.



When any key on the key board matrix is pressed the corresponding pins of dialer IC are

stored and pulses are produced. These are connected with the lines via rectifier protection

stage.

It sends the dial pulses at speed of 10 IPS. It can store the dial pulses, when the keys are

operated fast to an extent of 1720 digits.

The dialer section has the following parts:

(i) Dialer IC :- It is 16 pin IC. It consists of,

- Key board interface circuit

- Oscillatory circuit

- Transmit Amplifier circuit

- Mute circuit

- Memory circuit etc.

The following are the some dialer IC’s,

- IC M2560G –18 pin IC

- UM 1032 CP – 18 pin IC

- UM 91214 B or C –18 pin IC

- HM 91510 B – 22 pin IC

- HT 9202 G – 16 pin IC

(ii) Pulse / Tone switch: This switch is having two positions ie. Pulse dialing mode, or Tone

dialing mode in which DTMF signals are generated. The dialer IC should be pulse/tone

switchable IC.

Dialler section of GCEL telephone is shown in Fig.5.4. In this stage IC2560G is used. The

connections to different pins are shown in the circuit.

Pin. No’s 1,2,3,4,16,17,18 – Connected to the key board matrix to generate the required

number of pulses by pressing the keys.

Pin. No 5 – It is dialing pulse output pin which sends the dialing pulses to the exchange.

These pulses are connected to the telephone lines via Q10 and Ring switch contacts.

Pin. No 6,7,8 – Internal oscillator pins. This oscillator gives output only when keys are

pressed. Pins 6&8are connected with two resistances and pin 7 is connected with capacitor.

Pin. No 9 – Detects the hook switch contacts.



Pin. No 10 – Negative supply pin which is earthed

Pin. No 11 – Gives out Mute pulse. The pulse during dialing mute the speech amplifier by

this pin.

Pin. No 12,14,15 – Grounded

Pin. No 13 - +ve supply from bridge rectifier and sound Amplifier is given to this pin for

working of IC . It works on 1.5 to 3.5 V.D.C.

Working of stage: When ay key of the keyboard is pressed corresponding pins are shorted.

Thus the corresponding pulses are generated by UIC via keyboard interface, control logic

these are conned to pin.5 of IC. Then these are connected to telephone lines, through Q10

Transistor and ringer switch contacts. These generated pulses are stored in 20 bit memory if

the dialing is fast. If any key is pressed in ON-HOOK position, it is not considered and the

oscillator will not function.

Fault tracing:

I) No number gets dialed:

(a) Check the +ve voltage on dialer IC pin. No 13. It should be 1.5 to 3.5 Volts. DC.

(b) Check the resistance on pin.No.9 (270KΩ) for open.

(c) Check the components on pin. No’s 6,7,8.

(d) Check the dialer IC pin.No.11 resistance.

II) Continuous dial tone but no dialing:

(a) Check the dialer IC pin 6,7 and 8 components

(b) Check dialer IC pin 5 resistance (560KΩ), Transistor Q10 , Capacitor C2 4,7uF

and C1 100uF / 16 V.

(c) Change the dialer IC.

III) Wrong number gets dialed,

Dialler IC may be faulty.



FIG.5.4 Dialler stage



d) Sound Amplifier Stage:

This is also called as “Speech stage”. It consists of a speech IC having 18 or 20 pins in

special cases. This IC performs al interfacing functions between the Microphone, the ear

phone, the dialer IC and the Telephone lines.

This stage performs 3 functions:

i) It receives the incoming speech signals, amplifies and are connected to the loud speaker,

which reconverts them to sound signals by sound output circuit in the IC.

ii) The outgoing speech sounds are converted to corresponding speech currents by

microphone, amplified by microphone amplifier and connected to telephone lines.

iii) Side tone suppression is done by “Side Tone’ circuit in the IC.

iv) The gain of the sound amplifier is controlled by “Automatic gain control(AGC) circuit”

provided in the IC.

v) Impedance matching between the Telephone lines, IC circuit, Mike, and Loud speaker is

done by “Equavalisation Amplifier” provided in the IC.

Some of the sound Amplifier IC’ are TEA1060,TEA1061

These IC’s are bipolar integrated circuits performing all speech and line interface functions

required in fully electronic circuits.

The circuits internally perform electronic switching between dialing and speech.

Sound Amplifier section in GCEL-510 telephone:

It uses digital IC TEA1060 with 18 pins. Please refer fig.5.5 Function of each pin are given

under:

Pin. No 1 – Positive power supply of +5V DC is given to this pin for the function of IC from

voltage dropper stage.

Pin.No 2&3 – The resistance R10(30KΩ) connected between these pins to adjust the gain.

Pin. No 4&5 – The sound output is available between these pins which is connected to

loudspeaker which reproduces the incoming sound.

Pin. No6 – The gain of the receive amplifier is adjusted by the R13 (260 KΩ) connected

between pins 5&6.

Pin. No 7&8 – The mike output is connected to the microphone input amplifier between these

pins. The R17,R18 & R12 acts as LPF.



Pin. No9 – It provides internal current stabilization.

Pin.No 10 – This is negative supply pin, at ground potential.

Pin. No 11 – The incoming signal received on this pin 1 is internally applied between pins 11

and 18.

Fig.5. 5 Sound amplifier stage Pin No 12 – The tone signals from dialer section are connected to this pin.

Pin. No 13 – The DTMF tone output from dialer IC is fed to this pin for internal signal processing

and feeding on line.



Pin. No 14 – This pin receives mute input from the dialer IC. It disables the receiver amplifier

during pulse dialing to prevent D.C clicks.

Pin No 15 – It provides stabilized +ve supply for the working of IC. The current signals from

microphone are received on this pin and sent to Telephone Exchange. Incoming speech signals

are received and given to Loud speaker.

Pin. No 16 – Filter capacitor C17(4.7 uF) filters the regulator output.

Pin. No 17 – Automatic gain control is provided for short and long lines by the resistance R23

(212 Ω) connected on this pin.

Pin. No 18 – Completes the IC internal circuit.

Working of Sound Amplifier:

Speech Transmission: The microphone output is connected to input of microphone amplifier.

The output of the amplifier is connected to telephone lines from Pin 15 via pin1.

Speech Reception: The incoming signal received on Pin.No1 is internally applied to pin11 and

pin18. The output of the speech out put amplifier is connected to loudspeaker.

Fault tracing:

i) Sound Amplifier not working/ No incoming and outgoing speech:

Check for,

(a) Any diode of bridge rectifier faulty.

(b) Transistor Q7.Q8,Q11 and Q12 any one faulty.

(c) Transistor Q12 emitter Zener diode D9 (12PH) faulty.

(d) Sound Amplifier IC may be faulty.

(e) Pin 15 not getting signals

(f) Check supply on Pin1

ii) No dialing but when pushbutton are pressed, illusion of dial tone comes.

Check for,

(a) Sound amplifier IC TEA 1060 pin12 resistance R24 (100KΩ) open circuited.

(b) Pin 11 capacitor C14(100KPF) open circuited.

(c) Diode on Pin.No12 D9 (IN 4148) opened.

(d) Pin 11 resistance R21 (3.9KΩ) and capacitor C15(330KPF,100V) may be faulty.

iii) Instead of dial tone high frequency oscillations comes

(a) IC TEA1060 Pin 14 capacitor C16(10KPF) open



(b) IC TEA 1060 Pin 18 resistance R24(390Ω) OR capacitor C18 (10KPF) open.

(c) IC TEA 1060 faulty

iv) Faults:

(a) Speaker sound generally very low

(b) Microphone self sound is heard on speaker

(c) Incoming speech not able to here

Check for,

Change Sound Amplifier IC

e) Voltage Dropper Section:

Please refer fig. 5.6 .Telephone instrument gets 40 to 50V DC supply from the telephone

exchange through telephone lines. Till the ring comes this high voltage is maintained. But as

soon as the receiver is lifted, with two way switch, this high voltage goes to voltage dropper

section, where it is converted into +9V or +12V as per the telephone needs. Then dial tone

signals are given and are heard after amplification.

Fig.5.6 Voltage dropper stage



f) Rectifier / Protection Section (fig.5.7):

It is also called as “Polarity guard”

Functions of this section are,

1. To supply voltage of the proper polarity to the input of the telephone circuit for proper

functioning and avoids damage to electronic circuit. This is done by bridge rectifier.

2. A Metal Oxide Vristor (MOV) or Voltage Dependent Resistor(VDR) of 95V rating is

connected across the input of the bridge circuit to limit any high peaks of voltages due to

induction on telephone lines.

3. The output of the bridge rectifier is connected to the dialer section, voltage dropper section

and Ringer section. IN4004 diodes are used in bridge rectifier.

Fig.5.7 Rectifier protection stage

DTMF Phone

DTMF means “Duel Tone Multi Frequency” signaling is used in telephony transmission

system due to the following advantages over pulse signaling.

1) Dialing speed is fast – each digit takes just a few ms to transmit. However, pulse digits

will take between 1 and 2 seconds each.

2) It is more reliable because the decoding and switching operations are accomplished

electronically, so sensitive electromechanical devices can be eliminated.

3) The tones may be used for signaling purposes after connection has been made.



This method of signaling system uses 16 distinct voice band frequency signals, each consisting

of sinusoidal signals one from a ‘low group” and other from a “high group”

Higher group Tones: H1 H2 H3 H4 1209 HZ 1336 HZ 1477 HZ 1633 HZ Lower group tones L1= 697 HZ L2 = 770 HZ L3 = 852 HZ

L4 = 941 HZ

Fig.5.8. DTMF Key-pad & frequency selection

The A, B, C and D buttons are used in special applications and are not part of the common

telephone key board.

Column H4 is normally available on a telephone keypad and reversed for special signaling.

DTMF Decoder

DTMF decoder is necessary in any system using DTMF signaling. The purpose of decoder is to

decode a valid pair of signaling tone and provide output data corresponding to DTMF signaling

received.

The DTMF decoder must have the following characteristics: -

1) The decoder should decode DTMF signals within ±1.5% of their frequencies and should not

decode signals with either frequency deviation more than 3.5% from the standard.

1 2

4 5

3

6

8 7

4

9 C

B

0 * # D

A



2) The decoder should decode DTMF tone bursts on short as 40 mS and recognize inter digital

intervals as short as 40ms. It should not recognize tone burst or inter digital intervals shorter

than 20 ms.

3) The decoder should decode DTMF signals in the presence of a dial tone that has each of its

frequencies at a level of –16 dB to +3dB.

4) The decoder should decode DTMF signals that have a power per frequency of 25 to 0 dBm,

with the high frequency tone –4 to –8 dB relative to the low frequency tone.

5) The decoder, in the presence of message circuit noise, should miss decoding less than 1 in

10,000 valid DTMF tone bursts.

5.4 Main and Extension (1+1) Telephones:

It is also called as Twin set. It is used between Manager and his secretary. The Instrument

kept at Manager is known as “Extension” and that kept at secretary is known as “Main”.

a) Installation and Connection (fig.5.9):

The sets can be installed as shown in the connection and wiring diagram. The lines L1&L2

coming from the exchange are connected to the jack of the main telephone instrument . The

same lines are extended to the extension telephone through a SB cable. Power supply of

about 12V DC is extended to the two telephones by an adaptor. “Control and “Buzzer” lines

are also extended between the two telephones. A total 3 pairs are required between the two

telephones. Both the telephones are identical and can be used in place of each other. In case

of no power both the instruments will become parallel on exchange lines. The inter

connection wiring diagram is shown below (Fig.5.9)

b) # HOW TO USE:

This is a tone / Pulse Switchable system which can be used to access both Pulse dialing (Loop

signalling) and Tone Dialing (DTMF Signalling). The instruments are also having the facility of

dynamic change over to Tone mode by pressing ‘*’ key while in Pulse mode.

c) Main & Extension interconnection

Please refer fig. 5.9 showing wiring for main and extension interconnection.



Fig.5.9(a) Main & Extension interconnection

Fig.5.9(b) PB Phone Front View



d) # OUTGOING CALLS:

i) PULSE DIALING( Tone/Pulse switch on Pulse)

Go off hook.

Press ’LINE’.

Dial the desired number.

After conversation, replace the handset.REDAIL (In case the line is not connected to desired

number after dialing)

Go off hook.

Press ‘LINE’.

Press ‘REDIAL’.

After conversation, replace the handset.

ii) DTMF DIALING (Tone/Pulse switch is on Pulse)

Go off hook.

Dial’*’ and then the desired number.


RDIAL

Go off hook.

Press ‘LINE’.

Press ‘REDIAL’.

After conversation, replace handset.

iv) DTMF DIALING (Tone/Pulse switch is on Tone)

Go off hook.

Press ‘LINE’.

Dial the desired number.


REDIAL

Go off hook

Press ‘LINE’.

Press ‘REDIL’.




e) # INCOMING CALLS:

Whenever there is any incoming call, there comes ring on both the instruments. The volume

of the ring can be adjusted as per your requirement. Press ‘LINE’ to receive incoming calls

from any of the instruments.

f) # INTERCOME FACILITY:

If you want to consult your companion on other side, press ‘INTERCOME’ to call from any of

the extension to other extension.

g) # CALL TRANSFER:

To transfer the call, press ‘INTERCOM’ from the extension which is on line. The ‘BEEP’ will

indicate the transfer. The outer party will get the music. Press ‘LINE’ from the other extension

to complete the transfer.

h) # HOLDING THE LINE:

To hold the line, press ‘INTERCOME’. The line will be put to hold. You can speak through the

intercom without the caller over-hearing your conversation. Press ‘LINE’ to get back the line.

i) # RINGER VOLUME CONTROL:

The extensions are provided with a3- position ringer volume control switch on the right side.

The volume can be adjusted at OFF, MEDIUM & HIGH as per requirement. The OFF option

is primarily given for the situations in which the Manager/Boss does not want to be disturbed

on incoming calls.

j) #PAUSE:

When the Pause button is pressed, it introduces a delay of about 2.2 Sec. between two digits

dialed before and after the PAUSE.

k) # MUTE:

As long as the MUTE button is pressed, no sound will be transmitted to other party.

l) # FLASH:

The FLASH button can be used for call waiting. It may be used to access facilities offered by

some EPABXs or Network services provided by the telephone company.



5.5 Hand free telephone with caller ID compatible:

This type of telephone is compatible with a caller ID service offered by your telephone

company. The calling part’s information will be displayed after the first ring.

The unit can record information of up to 50 different callers for both lines combined,

including the time and date received, the called line, and the number of times called, in the

caller list.

5.5.1 Integrated telephones (Panasonic make)

(a) Single line with Data connection:

M.No: KX-T 2375 MXW, speaker phone facility.

It is a programmable telephone. It is provided with a LCD. For the function of LCD and

speaker phone 3 AA type Alkaline or Manganese cells are to be installed into the space

provided on the back side of the instrument. The location of controls are shown tin the figure

5.10.

It is provided with TWO line jacks, one is to connect with single-line Telephone Jack and the

other is Data jack. Data jack can be connected to,

• Computer

• Modem

• FAX

• Answering machine

It is provided with a PROGRAM button. We can program different functions. For each setting

the PROGRAM button to be pressed at the starting and ending of each setting.

(i) Different types of programs are :

- Selecting the Dialing mode either TONE or PULSE.

- Time adjustment

- Setting the LCD Contrast.

- Storing Phone Numbers in Memory:

• You can store 10 phone numbers in memory 0 to 9 number buttons function as memory

stations.

• You can store up to 20 phone numbers in the one-touch auto dial buttons (10 number in

UPPER memory locations, 10 numbers in LOWER memory locations.



• Other facilities are :

- Erasing the stored numbers :

- Turning Music Played during the Hold ON/OFF.

- Selecting the flash time.

- Setting the PIN code to prevent unauthorized persons from using your unit.

- Dial lock

- Call Restriction.

Fig.5.10 Location of controls on Panasonic phone



(ii) Special Features:

- Call waiting service

- Temporary Tone dialing

- Using PAUSE BUTTON

- Muting the conversation.

(b) Two line with speaker phone facility

M.No: KX-T 2378 JXW.

It is provided with 2 line jacks. One is used to connect telephone line and the other to

connect the another telephone line or Data line.

It is not provided with caller ID. Location of controls are shown in the Fig 5.11.

(c) Caller ID compatible ( Fig.5.12): M.No: KX-T SC 11 MXW

It is a telephone with Caller ID , Call waiting service and Voice Mail Service.

The different controls are shown in the figure. It is having a LCD display. It requires 3 ‘AA’

size Alkaline cells (LR6) or Manganese cells (R6,UM-3) for the LCD working.

(i) Settings:

- Time and Date setting

- Dialing mode setting

- LCD contrast setting

- Ringer volume setting

(ii) Making calls:

- Outgoing calls

- Answering incoming calls



Fig.5.11 Location of controls on caller ID phone

(iii) Features:

- Caller Identification feature.

- Viewing the caller list.

- Calling back from the caller list

- Editing the Caller’s Phone Number.

- Storing Caller List Information in the Phone Book List.

- Erasing Caller List Information.

- Storing Names and Phone Numbers in Phone book list.

- Dialing from the Phone book.

- Phone book Editing.

- Phone book Erasing.



(iv) Special Features:

- Voice mail service- Listening to voice mail services.

- Temporary tone dialing

- Using the PAUSE button if pause is required.

- Answering call waiting service.

- Setting the password.

- Dial Lock.

Fig.5.12 Caller-ID phone with navigator key



(v) Battery replacement procedure:

If “ “flashes. The battery power is low. Install new batteries as soon as

possible.

- Disconnect the telephone line cord from the unit.

- Press down in the direction of the arrow and remove the battery cover.

- Replace the batteries with new ones using correct polarity (+,-)

- Connect the telephone line cord to the unit.

• After battery replacement, the information stored in the Redial List will be cleared. Store

the desired item in the Phone Book List and Caller List.

• The time will be shown as “12:00AM 31/12 or “0:00 31/12” after replacing the batteries,

readjust the time and date.

(d) Two line CLIP with speaker phone facility.

M.No: KX-TS3282BXW.

It also 2 line telephone like Model:KX-T2378JXW, in addition it is provided with CLIP (Caller

Line Identification Programme).

Location of different controls are shown in the Figure 5.13a & 5.13b.

(i) Battery Installation:

Install the three “AA” size Alkaline(LR6) or Manganese (R6,UM-3) batteries in battery

compartment. They work as emergency power during a power failure.

During power failure the battery will retain the clock memory and redial memory. If you do not

install the battery, the data in the memory will be lost during a power failure.

If “ “flashes. The battery power is low. Replace all the cells with new

ones.

Disconnect the telephone line cords before opening the battery cover.

- You do not need to disconnect the AC adaptor, otherwise the clock memory and the redial

memory will be lost. If ” Ο “ flashes on the display, adjust the clock.



(ii) Programmable settings:

- Time and Date adjustment.

- Assigning the extension number.

- Dialing Mode (PULSE/TONE).

- LCD Contrast.

- Ringer volume adjustment.

- Ringer pattern selection.

Fig.5.13a Two line CLIP with speaker phone



Fig.5.13 b Two line CLIP with speaker phone (iii) Making outgoing calls:

- Using the speakerphone

- Adjust the speaker volume

- To redial the last number dialed

- To redial using the redial list (Memory redial)

- To release the hold

(iv) Answering Incoming calls:

- Using the speakerphone.

- Using the Caller List



- Calling Back from the Caller List

- Editing the Caller’s Phone Number

- Storing the Caller List Information in the Directory or in One-Touch Dialer Memory

- Erasing Caller List Information

(v) Directory facility:

- Storing Names and Numbers

- Finding stored items

- Editing

- Erasing

(vi) One-Touch Dialer:

- Storing Names and Numbers

- Dialing a stored number

(vii) Intercom facility;

- Paging a Designated Extension

- Paging all extensions

- Transferring an External Call to another Extension

- Room Monitor Feature

- Making/Answering another call during a conversation

- Conference facility

(ix) Special Features:

- How to use the PAUSE button

- Muting your conversation

- For call waiting service users

- Temporary Tone dialing

- Incoming call tone

- Line selection

- Setting a password

- Dial lock

- Call restriction

- Call Privacy Features

- Connecting on optional Handset to the Unit.

- Wall mounting facility



(e) Card less phone (Fig.5.14)

M.No: KX-T C1085 BXB

(i) Introduction:

It is a TWO line cordless Telephone having a base unit & a hand set. Calls are transmitted

between the two using wireless radio waves.

For maximum distance and noise-free operation , the recommended base unit location is:

- Away from electrical appliances such as TV, radio or personal computer.

- In HIGH and CENTRAL location with no obstructions such as walls.

It works on a rechargeable Ni-Cd battery power for handset. Charge the battery for about 10

hours before initial use.

- The IN USE/CHARGE indicator lights.

An AC adaptor is required for working between the base unit and handset.

- USE ONLY WITH Penosonic AC ADAPTOR PQLV14BX.

- The AC adaptor must remain connected at all times. (It is normal for the adaptor to feel

warm during use)

- When more than one unit is used, the units may interfere with each other. To prevent

or reduce interference, please leave ample space between the base units.

Connecting the Telephone Line Cord:

- One 4-wire Telephone Line Cord can be connected to a two-line telephone jack.

- One number 4-wire telephone line cord & One number of 2- wire telephone line cord can

be connected to the respective line jacks.

• To use as single line telephone One 2-wire telephone line cord can be connected to

single line telephone jack.

The location of controls are shown in the below figure 5.14a&b.

(ii) Settings:

You can programme the following:

- Selecting the dial mode i.e., TONE or PULSE.

- Selecting the line mode: Line 1 and Line 2 can be set to select “A” or select “B”.

- Ringer volume can be selected to HIGH or LOW



- Outgoing calls can be made either from base unit or hand set.

- 10 Phone numbers can be stored in the handset.

- 10 Phone numbers can be stored in memory of base unit.

- The stored numbers can be dialed from either Hand set or Base unit,

- The handset and base unit can be used at the same time on separate telephone lines.

Conference Call :

While having a conversation on one line, you can make or answer a second call on the other

line and then combine the calls to make a conference call.

A two way intercom is possible between the handset and the base unit.

The intercom can be used during a call. This feature enables you to transfer a call between

the handset and the base unit.

(iii) SPECIAL FEATUERES:

- Call waiting service.

- Temporary Tone Dialing.

- Automatic security code setting.

- Using the PAUSE button.

- Using the FLASH button to use special features.

- Selecting the line automatically.

- Indicating another incoming call by TONE.

- Wall mounting facility.

(iv) Recharging the Battery:

When the RECHARG indicator flashes, or the unit beeps intermittently, recharge the

battery for about 10 hours.

If you do not recharge the handset battery for more than 30 minutes, the RECHARGE

indicator will continue to flash.



Fig.5.14 a 2-Line Cordless phone base station



Fig.5.14 b 2-Line Cordless phone handset



(v) Battery information:

After Panasonic battery is fully charged:

Operation Operating time

While in use (TALK) Up to about 6 hours

While not in use (Stand-by) Up to about 30 hours

• The battery operating time may vary depending on usage conditions and ambient

temperature.

• Clean the hand set and the base unit charge contact s with a soft, dry cloth once in

a month. Clean more often if the unit is subject to grease, dust or high humidity.

Otherwise the battery may not charge properly.

• If the battery is fully charged, you do not have to place the handset on the base unit until

the RECHARGE indicator flashes. This will maximize the battery life.

• The battery cannot be overcharged.

5.6 ISDN TELEPHONE: (a) Introduction:

An ISDN telephone is an all-digital telephone designed to take full advantage of the many features of ISDN

(b) Features of ISDN Telephone:

Features of ISDN Phone are listed below:

i) PABX functionality

ii) password protection

iii) Calling / Connected Line Identification (CLIP, CLIR, COLP, COLR)

iv) Call Hold, Toggling (HOLD)

v) Terminal Portability (TP)

vi) Call Waiting (CW)

vii) Call Forwarding (CFU, CFB, CFNR, CD)

viii) Callback (CCBS, CCNR)

ix) Three-Party Conference (3PTY)

x) Explicit Call Transfer(ECT)



xi) Charge Information (AOC-S, AOC-D, AOC-E) (xxxii) display of charges and charge

accounts

xii) DTMF and keypad dialling possible

xiii) ISDN S0 basic access (point to point or point to multipoint)

xvi) operation also in restricted powering mode.

xvii) Display of list of Missed calls, receive calls, redial numbers, and Hotkey numbers.

(c ) Frequently asked questions about ISDN Telephon es (i) Can I use my existing telephone with an ISDN li ne? Yes. Your existing telephone can be connected to an analog port on the TA. (ii) Can I use a dial pulse (DP) type telephone?

That depends on your TA. For example, only push-button (PB) telephones can be used with

the NEC A term IT55DSU. The OKI PCLINK TA322DSU, however, supports both PB and

DP type telephones.

(iii) Can telephones connected to the TA be used in ternally?

You mean like an intercom? Yes, although it depends on your particular TA. Both the OKI

PC Link TA212 and NEC A term IT55DSU support this feature.

(iv) Will I be able to use my telephone in the even t of a power failure?

Unlike analog telephone lines, which continue working in the event of a power failure, ISDN lines require power from the customer site. So if the power fails, your telephone lines fail as well. Of course, if your telephone requires a power source, usually in the form of a plug-in adaptor, to drive its convenience features, it wouldn't work in the event of power failure, anyway. The same applies to most cordless phones and fax machines. (The telephone on some fax machine may still work even with the power turned off.) Some TAs, such as the NEC A term IT55DSU, have a battery backup feature, whereby the ISDN line can still be used in the event of a power failure. To use this feature you'll need a plain, unpowered push-button telephone.

(v) If I connect both my fax machine and telephone to the TA, will they both ring when a call comes in, and is it possible for the TA to det ect the type of call and route it to the appropriate device?

If the caller is also using INS64, your TA should be able to tell whether the incoming call is voice or fax and route it through to the appropriate device. This information is transmitted on the D channel before communications are established on a B channel. Of course, this may depend on the capabilities of your particular TA. If the call originates from a regular telephone, i.e., from a non-INS64 subscriber, it is not possible to detect the type of call and both devices will ring. With the NEC Aterm IT55DSU it is possible to switch off call type detection and ringing for ports individually.



(vi) So what happens if I pick up the telephone and hear beeping fax tones?

With the NEC A term IT55DSU you simply transfer the call to your fax machine.

(vii) What about additional telephone numbers?

The call type detection problem can be remedied using additional telephone numbers, and that's what NTT's Dial-In service is all about. Dial-In provides additional numbers for each device, so fax and telephone can be called independently.

In the illustration below, the caller dials 474-XX01, which is the subscription right number. At the destination site, the telephones and fax machine all ring at the same time.

In the next illustration, the subscription right number has been assigned to the main telephone and Dial-In numbers assigned to the other telephone and fax machine. This time the caller dials 474-XX02 and only the other telephone rings.

(viii) How much does Dial-In cost?

Dial-In costs ¥1,000 to install and ¥900/month for each number.

(ix) How many additional numbers can I have?

Up to 100 per INS64 line.

(x) What if I want all devices to ring when the sub scription right number is called?

For this situation NTT offer the Global service. With Global, all devices ring when the caller dials the subscription right number, as shown below,



but when a Dial-In number is called only the device assigned that number rings, as shown below.

Your TA should allow you to select which analog ports work with Global. For example, you may want to enable it for telephones only, as shown in the following example.

You have to apply to NTT to use the Global service, and there's an option on the INS64 application form. Global is available only if you purchase additional Dial-In numbers.

(xi) What's sub-addressing?

By entering a sub-address number after the main telephone number callers can direct calls to specific devices on the ISDN line. Up to 19 sub-addresses can be used per line.



(xii) Can anyone use sub-addresses?

No. The caller must be using an ISDN telephone. Analog telephones connected to a TA, however, can be called using sub-addresses, because the TA handles the settings.

(xiii) Is it possible to check call charges without an ISDN telephone?

Possibly. The Raku-Raku utility bundled with the NEC A term IT55DSU can display call charges for each port.

(xiv) Can we do call-redirect to any number?

Calls can be redirected to analog telephones, ISDN telephones, or cellular telephones, but not to PHS phones. Calls originating from analog telephones, ISDN telephones, and public telephones can be redirected to another destination. Calls from portable and PHS phones cannot.

(xv) Can you illustrate call-redirect, call-transfe r, call-waiting and 3-party conference?

Call re-direction redirects incoming calls to a specified telephone.

Call Transfer allows you to transfer a call in progress to another telephone.



Call Waiting allows you to put a call on temporary hold and answer an incoming call.

Three-party Service allows you to hold a three-way telephone conversation.

The Three-party service has two modes of operation: Mixing and Switching. In Mixing mode, all three callers can talk simultaneously. In Switching mode, only two callers can talk simultaneously. For example, A can talk to either B or C, but B cannot talk to C and versa.



Objective: 1. Ringer stage produces audible ringing sound. (T/F)

2. Lifting the handset results in voltage drop from -48volt to +5v or +12v. (T/F)

3. One row and one column frequency is selected for pressing one digit on key pad.(T/F)

4. # and * key are special function key. (T/F)

5. DTMF stands for Dual Tone Multi Frequency. (T/F)

6. The purpose of decoder is to decode a valid pair of signaling tone. (T/F)

Subjective: 1. What are different types of telephone instruments used in Indian Railways?

2. What is DTMF? How they are produced?

3. Write short note on Cordless Telephone instrument.

4. What are the main features of ISDN telephone?

t 12a - irot

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