The Telephone Network

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PSTN

public switched telephone network (PSTN) is the largest and most

widespread communications network in the world For computer communications used to

link remote sites of organisation

access LAN whilst away from office

access internet via internet service provider (ISP)

standards set by International Telecommunications Union —

Telecommunications Sector (ITU-T)

was Comité Consultatif International de Télégraphie et de Téléphonie (CCITT)

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Origins of the PSTN

S₁ S₂ S₃ E1 Exchange Local Loop Subscriber/Telephone started C.1880

telephones connected to exchange via local loop — UTP cable

telephones have stayed simple, exchanges have become very complex very little change in local loop

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Trunk Lines

S₁ S₂ S₃

E1

S₄ S₅ S₆

E2

networks were soon formed by linking exchanges link between exchanges is a trunk line

early trunks were made out of bunches of UTP cables then coaxial or microwaves used with FDM

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Structure of the Network

subscriber primary exchange secondary exchange tertiary exchange main trunk auxiliary trunk local loop S S S E S S S E S S E S S S S E S S S E S S E E E E E

basically a tree structure overall a partial mesh

extra trunks added to build in redundancy

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Numbering the Network

ITU

North American PTTs UK National Authority Norwegian PTT

subscriber area code IDD code Manhatten 212 Boston 617 London 20 Cambridge 1223 N America 1 UK 44 Norway 47

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Digital Trunk Lines

The basic telephone channel

One telephone channel stored using PCM in 8 bit samples at 8000Hz If used for data, give 64,000bs−1=64kbs−1

Trunk lines combine the telephone channels using TDM

The Bell System T1 carrier operates at 1.544Mbs−1

provides 24 64kbs−1 channels widely used in the US,

Australia and Japan

The CCITT E1 carrier

operates at 2.048Mbs−1

provides 32 64kbs−1 channel 30 user data

2 control

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The E1 Family of Trunk Lines

2.048Mbs−1 8.448Mbs−1 34.368Mbs−1

E1 E2 E3

Name No. E1 Mbs−1 _{No. User Channels}

E1 – 2.048 30

E2 4 8.448 120

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Dialup or Leased Lines

there are two basic modes of communication, both CO

dial-up each call

lease a permanent connection Use leased line when

an organisation is split between several sites

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Example: Comparing Costs

Company split between two sites

leased line costs £1,000 a year dial-up costs 10p per 3 minutes

how many hours use per working day makes leased line cheaper?

Cost of dial-up line = 0.10 × 603 = 2£h −1

Hours of dial-up use to reach cost of leased line = 1000₂ = 500_h

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Worksheet: Leased Lines

Two sites of a company have to exchange data at an average rate of 1.8Mbs−1. 1. Calculate the minimum number of dialup telephone lines that would be

needed to carry this load, if each line can carry 30kbs−1. (Ignore the overheads of framing data and the effects of errors in you working). 2. How many T1 leased lines would be needed to carry the same load? 3. How many E1 leased lines would be needed to carry the same load? 4. If the E1 leased line costs £240 a day, and a dialup line costs 10p for 5

minutes, calculate how many hours the leased line must be used each day in order to be cheaper than using dialup lines?

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Connecting to the POTS

General model

The Network DTE Computer RS-232-C - DCE Modem local loop

-Data Terminal Equipment

(DTE)

Data Circuit Terminating Equipment (DCE)

ITU-T V series of standards

Application of general model

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ISDN Channels

integrated services digital network (ISDN)

Channel Description

A a conventional 4kHz analogue telephone B 64kbs−1 of PCM voice or computer data C 8 or 16kbs−1 data

D 16 or 64kbs−1 control data

H data in some multiple of 64kbs−1 HO 384kbs−1

H11 1536kbs−1 H12 1920kbs−1

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ISDN Packages Available

basic rate interface (BRI)

2B+D service on a twisted pair line (144kbs−1) designed to replace standard telephone services

primary rate interface (PRI)

access to a trunk connection

capacity varies according to where you are (H11 or H12).

hybrid interface

gives a 1A+1C service

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Example: Choosing an ISDN rate

ISDN multirate allows you to bind B channels together to get any data rate which

is a multiple of 64kbs−1

small company with four PCs

if each PC generates 26 packets s−1 of length 64 bytes, select the suitable ISDN speed

Since total data arrival rate is 104 × 64 × 8 = 53kbs−1, we can try using just one B-channel (=64kbs−1): λ = 4 × 26 = 104 µ = 64000 64 × 8 = 125ps −1 ρ = λ µ = 104 125 = 0.83 d = 1 µ − λ = 1 125 − 104 = 0.048s l = ρ 1 − ρ = 4.9

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Asymmetric Digital Subscriber Line (ADSL)

phone ADSL modem MF .. . phone ADSL modem MF subscriber line subscriber line DSLAM to internet → .. . POTS exchange

G.DMT uses OFDM, to allow one analogue channel, 1Mbps upstream, 8Mbps downstream