pstn

Public Switched Telephone Network (PSTN)Outline

Introduction

Elements of the PSTN

Circuit Switching

Local Loop Technologies Local Loop Technologies

Transmitted Signal in the PSTN

Analog-to-Digital Conversion

Transport Core and Signaling

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Introduction PSTN is a technology based on circuit switching by duplex

connections.

Temporary bidirectional connections are set.

Originally it was used for speech (voice) only at 300-3400 Hz. Earlier two subscribers were connected by purely physical

connection (physical switch contacts).

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connection (physical switch contacts). Nowadays different time slots are assigned for different

subscribers.

The digital PSTN system uses PCM for speech coding and TDM for multiplexing.

The PSTN is integrated with other telecommunication networks such as ISDN and PLMN.

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Elements of the PSTN

The main components of a PSTN include:

Customer premises equipment

Access system

Local loop and termination equipment at the end office switch

Transport core

Transport is the carriage of voice

Signaling

Signaling is the control of calling

Elements of the PSTN Contd.

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Customer premises equipment Customer premises equipment

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The main elements of the PSTN Contd.

The Access System consists ofthe access line to the customer

(called the local loop)and termination equipment at the end office

(nearest telephone office switch)

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End officeSwitch

Access Line(Local Loop)

Access Line(Local Loop)

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Transport Core

TrunkSwitch

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TrunkLine

The Transport Core connects end officeswitches (5 classes, with 1 being highest)

Trunk lines to connect switches

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Signaling System(SS7 in the U.S.)

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Signaling is the control of calling(setup, teardown, billing, etc.)

Transport is the actual transmission of voice

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Circuit Switching

A circuit is an end-to-end connection between two subscribers.

Capacity is reserved on all trunk lines and switches along the way.

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Circuit Switching Contd. Users are served by an access network that connects them to the

local central office (CO) switch. The CO switches themselves are interconnected with higher-speed

communication lines through tandem switches.

Tandem switches, in turn, connect to toll switches that are used to provide long-distance connections.

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provide long-distance connections.

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Development of circuit switching technology

April 2013

SPC: Stored program control

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Local Loop Technologies

Technology Use Status1-Pair Voice-GradeUTP

Residences Already installed

2-Pair Data-GradeUTP

Businesses for high-Speed access lines

Must be pulled to thecustomer premises

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UTP Speed access lines customer premises(this is expensive)

Optical Fiber Businesses for high-Speed access lines

Must be pulled to thecustomer premises(this is expensive)

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Transmitted signal in the PSTN

Local Local

Todays PSTN is predominantly digital with analog local loops

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Loop(Analog)

ResidentialTelephone(Analog)

Switch(Digital)

Switch(Digital)

Switch(Digital)

Loop(Digital)

PBX(Digital)

Trunk Line(Digital)

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Codec at the end office switch

Digital

End Office

Analog

The codec at the end office translates between analog customer signals and digital internal signals.

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Codec

DigitalInternalSignal

DigitalSwitch

Local Loop

AnalogSignal ADC

DACHome

Telephone

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Analog-to-Digital Conversion (ADC)

AnalogElectricSignal

AnalogVoiceSignal

Bandpass Filtering

Bandpass filtering to limit voice to 4 kHz is carried out at the end office switch.

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Filter at End Office Switch

SignalSignal

Subscriber

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Analog-to-Digital Conversion (ADC) Contd.

Signal

Energy Distribution forHuman Speech

Bandpass Filtering

The human voice can produce sounds up to 20 kHz, but most sound is between 300 Hz and 3.4 kHz.

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0 Hz 300 Hz 3,400 Hz 20 kHz

Bandwidth (3.1 kHz) The bandpass filter only passes this sound to reduce bandwidth.

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PCM

SignalDuration of Sample

(1/8000 sec.)AnalogSignal

In pulse code modulation (PCM), the bandwidth is assumed to be 4 kHz. This adds guard bands to the actual 300 Hz - 3.1 kHz signal.

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0

SignalAmplitude

TimeSample

(1/8000 sec.)Signal

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OneSample

One 8-bit

For signals going to the customer, sample bits are converted to amplitude levels for each sample.

8,000 samples per second, will sound smooth to the ear.

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GeneratedAnalog Signal DAC

00000100 00000011 00000111

One 8-bitSample

Arriving Digital Signal(8000 Samples/Second)

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TDM and ATM switch connections in the PSTN transport Core

Point-to-Point

Transport Core

Traditionally, the transport core used TDM trunk lines both point-to-point and ring trunk lines.

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SONET/SDHRing

Point-to-PointTDM

Trunk Line

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Speech Codecs

Codec Transmission RateG.711 64 kbpsG.721 32 kbps

Several different codecs can be used.

The codecs vary in compression and sound quality.

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G.722 48, 56, 64 kbpsG.722.1 24, 32 kbpsG.723 5.33, 6.4 kbpsG.723.1A 5.3, 6.3 kbpsG.726 16, 24, 32, 40 kbpsG.728 16 kbpsG.729AB 8 kbps

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Transport Core and Signaling

Transport Core

TDM: point-to-point and ring

SONET uses dual rings for reliability

If there is a break, the rings are wrapped

ATM uses packet switching ATM uses packet switching

More efficient than TDM, replacing TDM

Signaling

SS7 in the United States, C7 in Europe

Interoperable

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Switching systems at ETC

Analog Switches/Exchanges

Previously ARF-102 and AGF (of ERICSSON product), C400 (of NEC product) were analog automatic exchanges used for local services and their capacity extends from 500 to 10,000.

For small capacity of rural services there was an exchange called ARK (ERICSSON product).

ARM-201 and NC-82 exchange were used for national and international (one way) service.

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Switching systems at ETC Contd

Digital Switches/Exchanges:

Nowadays the switches that we have through out the country are all digital automatic switches

At the moment there are 4 main types of switches used through out the country. These are:

1. AXE-10.ERICSSON product (Sweden)1. AXE-10.ERICSSON product (Sweden)2. DMS-10..TELRAD (NORTEL) product (Israel)3. DMS-100TELRAD (NORTEL) product (Israel)4. C&C08HUAWEI product (China)

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Types of Switches/Exchanges The switches/exchanges at ETC can in general be classified as:

i. Combined national/international transit exchanges: AXE-10, C&C08

ii. National transit exchange: C&C08iii. Local Tandem exchange: AXE-10, C&C08, DMS-10, DMS-

100iv. Local (Parent, Stand alone, Remote subscriber stage)

exchange: AXE-10, C&C08, DMS-10 Previously AXE-10 at Nazret, D/Dawa, Jimma, Nekemt, B/Dar

and DMS-100 at Dessie, Mekele, Shashemene were used as local transit exchange. But nowadays they are replaced by C&C08 (HUAWEI) national transit exchange.

Nowadays, there are 2 combined national/international transit exchanges (1 AXE-10, 1 C&C08) and there are 8 national transit (C&C08) exchanges all over the country.

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Types of local exchanges

There are 3 types of local exchanges. These are:i. Parent Exchangeii. Stand aloneiii. Remote Subscriber Subsystems (RSS)

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Digital Switches/Exchanges Contd.. AXE-10 serves for medium, large and very large capacity

demand with a maximum capacity of around 50,000. All AXE-10 exchanges are installed in a building.

DMS-10 serves for small, medium and large capacity demand with a maximum capacity of around 10,000.

DMS-10 can be installed in a building as well as in a container. DMS-10 can be installed in a building as well as in a container.

DMS-100 is used for large capacity services and it serves as local and transit (combined) exchange when the need arises.

C&C08 extends from small capacity of 500 to large capacity of 10,000 subscriber lines.

C&C08 can be installed in buildings as well as a container and serves as local and national/international transit exchange. 26April 2013

Distribution of Switches/Exchanges AXE-10:

1 national/international switch 28 parent local switches 5 RSS

DMS-100: 3 parent local switches

DMS-10: 72 parent local switches 8 UNIREM

C&C08: 1 national/international switch

Totally 160 switches/exchanges are under installation from these 98 are operational

27April 2013

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