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Switching Systems for Public Networks

Amila PereraSri Lanka Telecom Training Centre

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Description of Unit

The aim of this unit is to provide an understanding of the architecture, operation and major design issues relating to switching and transmission systems.

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Description of Unit

This unit has been designed to enable the learner to understand the importance of switching and transmission systems within public and private networks.

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Description of Unit

Learners will consider the need for seamless and transparent global networks and the requirement for unified networks incorporating differing technologies for multi-media applications.

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Summary of learning outcomes Investigate switching systems for public

networks Investigate switching systems for private and

integrated networks Investigate the properties of the

Plesiochronous Digital Hierarchy (PDH) Investigate the properties of the Synchronous

Digital Hierarchy (SDH)

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Summary of learning outcomes Investigate switching systems for public

networks Investigate switching systems for private and

integrated networks Investigate the properties of the

Plesiochronous Digital Hierarchy (PDH) Investigate the properties of the Synchronous

Digital Hierarchy (SDH)

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Links

The unit is designed to stand alone but it has links with:

Unit 8: Data Communications and Networks Unit 17: Digital Networks.

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Switching systems for public networks Call set up process Call routing Signalling techniques

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Call Set up Process

Number allocation/recognition Use of tones Network hierarchy (eg: local exchange,

tandem/transit, international gateway)

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Call Routing

Subscriber trunk dialling (STD) Access networks Transmission equipment Concentrators Remote concentrator units (RCU) Digital cell centre exchange (DCCE) Digital main switching units (DMSU) Time and space switching Stored programme control (SPC)

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Signaling Techniques

DC Tones in-band Tones out-of-band Supervisory Channel associated signalling (CAS Common channel signalling (CCS) Signalling system no. 7 (SS7) Service logic (eg: service switching point (SSP),

service control point (SCP) call processing)

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Switching systems for private and integrated networks PBX architecture PBX features Voice over internet protocol

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PBX Architecture

Hardware (eg: leased lines, leased carriers, leased trunks, multi-line key handsets); Centrex

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PBX features Range and modularity of services

time-of-day routing toll-free calling, call line identification local number portability

Enterprise switching hunt groups key systems

Connectivity options PBX-PBX, PBX-PSTN ISDN

Unified network concept Call/contact centres

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Voice over internet protocol internet telephony major system components

gateways, gatekeepers, IP telephones PC based software phones

development issues Voice quality bandwidth constraint transparency to the user TCP/UDP issue Security accounting/billing

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Switching systems for public networks describe the call set up process explain how calls are routed over the PSTN evaluate the range of signalling techniques

used in public networks

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Switching systems for private and integrated networks describe the architecture of a private branch

exchange (PBX) describe the features of a PBX explain the principles of voice

communication by means of the internet protocol (voice over IP)

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Textbooks

Ronayne J – Introduction to Digital Communications Switching (UCL Press, 1991) ISBN 0273034529

Russell T – Signalling System #7 (McGraw-Hill, 2000) ISBN 0071361197

Sexton M and Reid A – Broadband Networking: ATM, SDH and SONET (Artech House, 1997) ISBN 0890065780

Shepard S – SONET/SDH Demystified (McGraw-Hill, 2001) ISBN 0071376186

Thompson R – Telephone Switching Systems (Artech House, 2000) ISBN 1580530885

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Switching systems for public networks describe the call set up process explain how calls are routed over the PSTN evaluate the range of signalling techniques

used in public networks

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The Phone

transmitter

receiver

Hybrid network

(inside telephone)

Line cordTip

Ring

RJ11

RJ11

RJ11

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Speak

transmitter

receiver

Sound waves

Hybrid network

(inside telephone)

Line cordTip

Ring

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Listen

transmitter

receiver

Sound waves

Hybrid network

(inside telephone)

Line cordTip

Ring

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Tip & Ring

two wires ordinary telephone line Tip is the ground side (+), (also called A –

earth) Ring is the working voltage (-), (also called B

– battery)

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Origin of the Term

tipring

sleeve

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Early Phones

Early phones were self-powered Used batteries for powering the transmitter Ringer on the phone B was activated by the

electrical energy generated by the hand crank on A.

When the receiver was lifted, the hook switch would connect the transmitter and the battery

When in idle they were kept disconnected to save battery.

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Ringing Tone

To ring the phone to alert to an incoming call, about 90 volts of 20 Hz AC current is superimposed over the DC voltage already present on the idle line. (America)

In Europe it is around 60-90 volts AC at a frequency of 25 Hz

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Manual Central Exchange

The first telephone exchange installed in 1878

First phones were self-powered Ringing by means of using a hand crank Ringing signal would operate a relay and a

lamp, at the central office. The operator would answer the call To connect A and B, the operator had patch

cords.

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Manual Central Exchange

Main disadvantage?

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Manual Central Exchange

Main disadvantage: There was no indication when the call was

completed.

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Operator switchboard

To overcome the above disadvantage the operator switchboard was invented

It provided the operator with a monitor switch The monitor switch had 3 positions 1 – allow A and B talk 2 – talk to A in private 3 – talk to B in private

31Bell Telephone Company Central Exchange in New York (in 1888)

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Operator did not listen to the ongoing call But would interrupt from time to time (in a few

minutes) either A or B. One operator could handle about 10 calls at a

time.

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Numbering

In those early times the operator could remember the people (subscribers) in a small town by name.

In large cities this was not practical Telephone numbering started

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North American Telephone Numbering Plan In about 1957, 3-digit area code 3-digit central exchange code 4-digit line number

7 –digit ‘subscriber number’

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Centralized Power

When the concept of central exchanges were introduced, ringing the phone became a task of the switchboard operator.

The use of hand crank as well the battery in the phone were no longer necessary

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Line Relay

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Later, a third lead, called sleeve, was introduced to the plug of the patch cord.

It was wired to contact in the line relay, which operated a ‘monitor’ or ‘supervisory’ lamp for the duration of the call.

Operators no longer needed to interrupt into the call, since they could watch the lamps.

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Automation of Switching Process As the number of subscribers grew, Look for more efficient ways for switching Automation of switching function became

necessary

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If Not Automated…

Whole population would be required to work as switchboard operators!

Or, limit the number of users to 10% of present users!

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1892 Strowger Switch

Handling of switching instructions that were previously done by the operator, was now assigned to the user from the telephone unit itself (telephone dial).

This switch had stepping relays that worked according to the dial pulses.

Upon completing one digit, the first stepping relay would activate another stepping relay.

Complete connection was achieved by serial activation of several stepping relays.

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Strowger switch was also called the step-by-step switch.

They came into wide use only by 1920’s. Still in rural areas, manual switchboards were

in operation until 1950’s.

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SPC Switching Systems

With the invention of the transistor and computers, Strowger switches have been replaced SPC electronic switches.

They take much smaller space, and economical in power consumption.

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