A GENERAL TRUNKING

• A general trunking diagram for a telephone exchange contains Three kinds of switching networks:

ConcentratorsExpandersA central route switch

• A customer’s call is connected to a supervisory trunk by a concentrator.

• For a local call, the route switch connects the supervisory trunk to the particular expander on which is terminated the line of called customer.

• For a junction call, route switch connects the supervisory trunk to an ongoing junction on the route to the required distinction.

• A route switch also connects incoming junction calls to the expanders of called customers.

• The central processor interrogates all trunks which can be used for a required connection to determine which are free.

• The central processor selector a suitable set of these trunks and marks them to make the connection.

MANUAL SWITCHING SYSTEM

• In this, the concentrators consist of groups of answering jacks and plugs in front of the operators.

• The supervisory trunks are the cords circuits.• The operator performs the functions of a

register and the central control.• The expanders are provided by the jacks in the

multiple.

• This is unique because a single switch can provide access to all lines on even the largest exchange.

• Thus, there is no need for the route switch.

STROWGER STEP-BY-STEP SWITCHING

• Each concentrator is a group of customer’s uniselectors or line finders and each expander is a group of final selectors.

• The group selectors form the route switch.• Distribution control is used , there are no

register and no central control.• Supervisory function is normally at the

outgoing side of the route switch.

• For a local call, it is provided by relays in the final selector circuit.

• For a junction call, it is provided by an outgoing junction relay set.

CROSSBAR SWITCHING SYSTEM

• In this markers comprise the central control.• Each of the switching networks consists of a two

stage link network.• Since directory number(DN)to equipment

number(EN)translation is provided. The concentrator can also act as an expander.

• It serves two groups of trunks.• One group connects it to supervisory units and the

other is folded back to it from the outgoing side of the route switch.

SPC SWITCHING SYSTEM

• In this , all call processing functions may be performed by a central processor.

• Thus, there are no separate supervisory units and no registers between the trunks and the central control.

TANDEM EXCHANGE OR TRUNK EXCHANGE

• This exchange has no subscribers, so it contains no concentrator or expanders.

• Concentration is not required, because incoming junctions are more heavily loaded than customers lines.

• The originating local exchange has already concentrated the outgoing traffic from its customers on these junctions.

ELECTRONIC SWITCHING

• Electronic switching proved economic for common control.

• In Electro-mechanical exchanges, common controls mainly use switches & relays which were originally designed for use in switching networks.

• In common control systems, they were operated much more frequently & wear out.

• The life of electronic device is almost independent of its frequency of operation.

• Advances made in computer technology were incorporated and uses the stored program control(SPC).

SPC(STORED PROGRAM CONTROL)

• A digital computer is to be used as a central control and perform different functions with the same H/W by executing different programs.

• SPC exchanges can offer a wider range of facilities than earlier system.

• In SPC, the processor’s stored data can be changed electronically, some of these facilities can be controlled by customers.

FACILITIES

Call Barring:- Both outgoing and incoming calls can be barred by customers in SPC exchange. The customer can prevent un-authorised calls being made.

Repeat last call :- if a called subscriber line is engaged, the calling subscriber or caller can try again later without having to redial the full number.

Reminder calls:- the telephone exchange can be instructed to call the customer at a pre-arranged time.

Call diversion:- the telephone exchange can be instructed to connect calls to a different number when the customer goes away.

Three-way calls:- the customer can instruct the exchange to connect a third party to a call that is already in progress.

• Charge advice:- As a result of the caller sending the appropriate instruction when starting a call, the exchange calls back at the end of the call to indicate the call duration and charge.

DIODE CROSS-POINT SWITCH

• Cold cathode gas tubes and PNPN semiconductor devices have been used as electronic cross-point

• In fully electronic exchanges a multiplex system were used instead of multipled elements.

• FDM system could be used as a switch by bringing the two ends of its transmission path together.

• A multipled element switch with N incoming and N outgoing trunks needs N2 elements.

• A multiplex switch of the same size needs N devices on each side i.e. total of only 2N . Thus, if N is sufficiently large, the devices used can be much costly.

• A time division multiplex (TDM) switch can be used.

SWITCHING SYSTEM CLASSIFICATION

• Space –division (SD) system:-Each connection is made over a different path in space which exists for the duration of the connection.

• Time –division(TD) system:-Each connection is made over the same path in space, but at different instants in time.

DIGITAL SWITCHING SYSTEM

• When the I/P & O/P from the switching system can be directly support digital signal.

• Basic elements and its operation are very similar to stored program control (SPC)switching system.

• The cost of an analog switch is roughly proportional to the no. of cross points but the cost of digital switch is not proportionate.

FUNCTIONS OF DIGITAL SWITCHING SYSTEM

• The functions of the digital switching network is to connect pairs of a channels. So that information arriving at the switching centre in a particular channel on one PCM multiplex system can be passed to some other channel on an outgoing PCM multiplex system.

• There are two process for achieve this switchingTime division switchingSpace division switching

• If the time division transmission is used with space division tandem switching. It is necessary to provide de-mux equipment to demodulate every channel to audio before switching and multiplexing equipment to retransmit it after switching.

TRUNK OR TANDEM EXCHANGE

PCM TERMINAL

PCM TERMINAL

TDM SWITCH

Incoming Audio circuit

Incoming Digital circuit

OutgoingAudio circuit

Outgoing Digital circuit

LOCAL EXCHANGE WITH SPACE DEVISION CONCENTRATORS

PCM TERMINAL

PCM TERMINAL

TDM SWITCH

Digital Junction

Audio Junction

Subscriber line SPACE DIVISION

CONCENTRATOR

• A tandem exchange a mixture of PCM junction and analog audio junction , PCM terminal equipment is needed instead for analog junctions. A large cost saving can be made by using time division switching for trunk and tandem exchanges. Tandem exchanges have no customer’s lines.

LOCAL EXCHANGE WITH CODECS IN CUSTOMER’S LINE CIRCUIT

CODEC

CODEC

TDM SWITCH

Digital Junction

Audio Junction

Subscriber line

LOCAL EXCHANGE WITH DIGITAL CUSTOMER’S LINES CIRCUIT

CODECTDM

SWITCH

Digital Junction

Digital Subscriber line

BLOCK DIAGRAM OF SLIC FOR A DIGITAL EXCHANGE

TEST ACCESS

DECODER

RING TRIP

OFF-HOOK SUPERVISION

CODER

HYBRIDBATTERY FEED

OVERLOAD PROTECTIONRINGING

TEST EQUIPMENT

TWO WIRE LINE

RX

SEND

Digital switch

SUBSCRIBERS LINE INTERFACE CIRCUIT(SLIC)

• Functions of SLIC for digital exchange can be summarized by the acronym “BORSCHT”

‘B’ for Battery Feed ‘O’ for Overvoltage Protection ‘R’ for Ringing ‘S’ for Supervisory Signalling ‘C’ for Coding ‘H’ for Hybrid ‘T’ for Testing

Why ISDN?

ISDN - Integrated Services Digital Network Telephone services -> Telecommunication

services Used for voice, image and data.

WHAT IS ISDN?

INTEGRATED SERVICES DIGITAL NETWORKS (ISDN)

• In this network, the customer‘s terminal equipment & the local digital exchange can be used to provide many different services, all using 64kbps digit.

FUNDAMENTALS

• Types of channels– Bearer channel (B-channel=64 kb/s) clear pipe for

data– Delta channel (D-channel, 16 kb/s or 64 kb/s) call

signaling information:• who is calling• type of call• calling what number

ISDN BRI

• Access to an ISDN is provided in two forms: Basic Rate Access (BRA): the customer’s line

carries two 64kbps ‘B channels plus a 16kbps ‘D ’channels in each direction.

Basic Rate Interface (2 B channels + 1 D channel (16 kb/s))

Primary Rate Access (PRA): Two lines are provided to carry a complete PCM frame at 1.5 Mbps or 2 Mbps in each direction.

Primary Rate Interface (23 or 30 B channels + 1 D channel (64 kb/s))

ADVANTAGES OF ISDN

• Digital– reliable connection

• Speed– 128 kb/s (160 kb/s) for BRI– 1920 kb/s (2048 kb/s) for PRI

• Fast call setup– 2 seconds

• Bandwidth on Demand– adding new channels to the bundle of channels

• Multiple devices– phone, fax, PC, videoconferencing system, router,

terminal adapter,.. each with its own sub-address.