The Telephone Network

Transcription

1 The Telephone Network 1

2 2 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) PSTN run by PTT or telco

3 3 Origins of the PSTN E 1 Exchange Local Loop Subscriber/Telephone started C.1880 S 1 S 2 S 3 telephones connected to exchange via local loop UTP cable telephones have stayed simple, exchanges have become very complex very little change in local loop plain old telephone system (POTS)

4 4 Trunk Lines E 1 E 2 S 1 S 2 S 3 S 4 S 5 S 6 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 now fibre optics, microwaves, or satellite with PCM and TDM Exchanges also digital, applying PCM to signal from local loop

5 5 main trunk auxiliary trunk local loop Structure of the Network E E E E tertiary exchange secondary exchange E E E E E E primary exchange S S S S S S S S S S S S S S S S S subscriber basically a tree structure overall a partial mesh extra trunks added to build in redundancy serve busy routes directly

6 6 Numbering the Network ITU N America 1 UK 44 Norway 47 IDD code Manhatten 212 Boston 617 London 20 Cambridge 1223 area code North American PTTs UK National Authority Norwegian PTT subscriber ITU-T sets out IDD codes for each country country decides how to run local system

7 7 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 widely used in Europe and UK

8 8 The E1 Family of Trunk Lines E1 E2 E Mbs Mbs Mbs 1 Name No. E1 Mbs 1 No. User Channels E E E E E

9 9 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 making frequent use of some public data network e.g. the Internet

10 10 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 = = 2 h 1 Hours of dial-up use to reach cost of leased line = = 500h Estimate the working year to be around 50 weeks of five days. Number hours of traffic per working day = = 2h

11 11 Worksheet: Leased Lines Two sites of a company have to exchange data at an average rate of 1.8Mbs 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? 5. If the peak load is about 2.2Mbs 1, suggest how you might deal with this if you were using T1 leased lines, and if you were using E1 leased lines.

12 12 The Network Computer Modem DTE RS-232-C local loop DCE Computer Modem Codec RS-232-C DTE local loop DCE digital analogue Exch Connecting to the POTS General model Application of general model trunk digital Data Terminal Equipment (DTE) Data Circuit Terminating Equipment (DCE) ITU-T V series of standards Codec Modem Computer local loop Exch analogue DCE RS-232-C DTE digital

13 13 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 H kbs 1 H kbs 1 H11 fits in T1 trunk, H12 fits in E1 trunk Some multiple of H0 fits in both T1 and E1

14 14 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 additional digital service in parallel with an analogue telephone service

15 15 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 = 53kbs 1, we can try using just one B-channel (=64kbs 1 ): λ = 4 26 = 104 µ = = 125ps 1 ρ = λ µ = = 0.83 d = 1 µ λ = = 0.048s l = ρ 1 ρ = 4.9 Seems reasonable delay and queue length, so one ISDN B-channel will be sufficient.

16 16 Asymmetric Digital Subscriber Line (ADSL) ADSL modem phone. MF subscriber line DSLAM. POTS exchange ADSL modem phone MF subscriber line to internet G.DMT uses OFDM, to allow one analogue channel, 1Mbps upstream, 8Mbps downstream Just a link to a digital subscriber line access multiplexer (DSLAM)