lecture 4

© 2013 Information & Communication Technology Institute

PSTN (Public Switched Telephone Network)

04, May 2013Hamidullah

Mohammad RIAZ


Outline

• Introduction• PSTN Network Elements• Pulse dialing and DTMF dialing• Local loop• Switching Concepts• Trunking Systems• Signaling Inband signaling, CAS and CCS

• Summary• References


Introduction

• Ever since Alexander Graham Bell made the first telephone call, the telephone has expanded by leaps and bounds. The technical term for the telephone network is PSTN.

• Switched Network series of inter-linked nodes (switches)


PSTN Network Elements

The Elements of PSTN are: • CPE - Subscriber terminals• Local loops• Switches• Trunks


Subscriber terminal

• CPE (Customer Premise Equipment): Term used to identify any piece of equipment supplied by the customer to interface with the PSTN.

• In its simplest form, the subscriber terminal is the ordinary telephone with a keypad to dial the numbers.

There two types of dialing:• Pulse dialing• DTMF (Dual Tone Multi Frequency)


Local loop

• The local loop is a dedicated link between a subscriber terminal and the switch.

• In remote and rural areas, where laying the cable is costly or infeasible, radio is used.


Switch

• Hardware and/or software devices• Capable of creating temporary connections between two or

more devices linked to the switch• In earlier days, mechanical and electromechanical switches

were used extensively. Present switches use digital technology. These digital switches have the capacity to support several thousand to a few million telephones.

• To cater to large areas, the switching system is organized as a hierarchy. At the lowest level of the hierarchy, the switches are called end offices or local exchanges. Above that, there will be toll exchanges (class 4 switches), primary (class 3) switches, secondary (class 2) switches, and regional (class 1) switches.


Con…


Trunk

• Trunks interconnect the switches.• Nowadays, trunks are mostly digital: speech is converted

to PCM formats, multiplexed, and transmitted through the trunks.


Local loop

• The local loop which is a dedicated connection between the terminal equipment and the switch, is the costliest element in the telephone network. Generally, from the switch, a cable is laid up to distribution box (Distribution Point) from which individual cable pairs are taken to the individual telephone instruments.

• To reduce the cable laying work, particularly to provide telephone connections to dense areas such as high-rise residential complexes, DLC (Digital Loop Carrier) systems are being introduced. The telephone cables are distributed from the DLC system, and the DLC is connected to the digital switch using a single high-bandwidth cable.


Con…


Switching Concepts

• The PSTN operates on the circuit switching principle. When a subscriber calls another subscriber, a circuit is established that is a concatenation of various channels on the trunks between the switch connected to the calling subscriber and the switch connected to the called subscriber.

Circuit switching operation involves the following steps:• Call establishment • Data transfer (conversation)• Call disconnection


Con…


Call Processing Software

Call processing software resides in the switch. The functions of the call processing software are:• To keep track of the subscriber terminals and to feed the dial tone when

the subscriber goes off-hook.• To collect the digits dialed by the subscriber. Note that the subscriber

may dial a few digits and then pause – the software should be capable of handling such cases as well.

• Analyze the digits and switch the call to the right destination by seizing the trunk.

• Feed various tones to the subscriber terminal such as busy, call hold and etc.

• When the subscriber goes on-hook, free the trunk.• Keep track of the call records known as CDRs (Call Detail Records) that

contain call information such as date and time when the call is made, the called party number, whether the call is local/ long distance, and duration of the call.

• Based on CDRs, do an offline analysis to generate billing information.


Trunking Systems

Two switches are connected together through trunks. The trunks are of different types:1. Two-wire analog trunks, which are used to interconnect

small switches.2. Four-wire trunks, which are also used to interconnect

small switches.3. T1 carriers which are digital trunks. Each T1 carrier

carries 24 voice channels. In Europe, the equivalent standard is referred to as E1 trunk. Each E1 trunk supports 30 voice channels.


Architecture of External Plant


Cable Chamber


External Plant Cable Network


Pillar Architecture


MDF

• MDF is Main Distribution frame ,which interconnects theexternal line plants and exchange equipment.• Arrangement is made by the use of iron frames. These

iron frames are called main distribution frame (MDF)• A means for permanently terminating the external

cables.• A means for terminating the Exchange cables.• Mean for mounting the protective devices connected to

the incoming cables.


T1 Carrier

In T1 carrier, a frame consists of 193 bits – 192 bits corresponding to 24 voice channels’ data and one additional bit for framing. The frame duration is 125 microseconds. Hence, the gross data rate of T1 carrier is 1.544 Mbps.


T2, T3 and T4 carriers

In major towns and cities, because of high traffic, T1 carriers will not suffice. In such a case, T2, T3, and T4 carriers are used. Higher capacity trunks are obtained by multiplexing T1 carriers.


Signaling

In a telephone network, before the conversation takes place, a circuit has to be established. Lot of information is to be exchanged between the subscriber terminal and the switch, and between the switches for the call to materialize and later get disconnected. This exchange of information is known as signaling.


Con…

Signaling is used to indicate/ exchange the following information:• Calling and called party numbers.• Availability/ non-availability of network resources such as

trunks.• Availability of the called party.• Billing information.• Network information such as busy trunks, faulty

telephones and etc.• Routing information as regards how the call has to be

routed.• To provide special services.


Types of Signaling

In the telephone network, there are three types of signaling:1. In-band Signaling2. CAS (Channel Associated Signaling)3. CCS (Common Channel Signaling)


Summary


References

• Principles of Digital Communication Systems and Computer Networks == Dr. K.V.K.K Prasad


Any

lecture 4

Documents

digital technology

digital switches

electromechanical switches

present switches

telephone connections

ordinary telephone

telephone cables

exchanges class