1
The Telephone Network
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
Origins of the PSTN
E1
Exchange
Local Loop
S1
S2
Subscriber/Telephone
S3
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
plain old telephone system (POTS)
4
Trunk Lines
E1
S1
S2
E2
S3
S4
S5
S6
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
Structure of the Network
main trunk
auxiliary trunk
local loop
tertiary
exchange
E
E
E
E
E
S S S
S S S
E
S
basically a tree structure
overall a partial mesh
S
secondary
exchange
E
E
E
S S S S
S S S
E
S
primary
exchange
S subscriber
extra trunks added to
build in redundancy
serve busy routes directly
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Numbering the Network
ITU
Manhatten
212
Boston
617
North American PTTs
Norway
47
UK
44
N America
1
London
20
area
code
Cambridge
1223
UK National Authority
ITU-T sets out IDD codes for each country
country decides how to run local system
IDD
code
Norwegian PTT
subscriber
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
The CCITT E1 carrier
operates at 1.544Mbs−1
operates at 2.048Mbs−1
provides 24 64kbs−1 channels
provides 32 64kbs−1 channel
30 user data
2 control
widely used in the US,
Australia and Japan
widely used in Europe and UK
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The E1 Family of Trunk Lines
E1
E2
E3
2.048Mbs−1
8.448Mbs−1
34.368Mbs−1
Name No. E1
Mbs−1 No. User Channels
E1
–
2.048
30
E2
4
8.448
120
E3
16
34.368
480
E4
64 139.264
1920
E5
256 565.148
7680
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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
making frequent use of some public data network — e.g. the Internet
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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 ×
60
3
= 2£h−1
Hours of dial-up use to reach cost of leased line =
1000
2
= 500h
Estimate the working year to be around 50 weeks of five days.
Number hours of traffic per working day =
500
50×5
= 2h
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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?
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.
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Connecting to the POTS
General model
The Network
Computer
Modem
local loop
RS-232-CDTE
DCE
Data Terminal Equipment
(DTE)
Data Circuit Terminating
Equipment (DCE)
ITU-T V series of standards
Application of general model
Computer
Computer
Modem
Modem
Codec
Codec
RS-232-C
- DCE local loop
- Exch trunk- Exch local loop
- DCE RS-232-C
- DTE
DTE
analogue
analogue
digital
digital
digital
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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
H11 fits in T1 trunk, H12 fits in E1 trunk
Some multiple of H0 fits in both T1 and E1
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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
additional digital service in parallel with an analogue telephone 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
d=
µ=
64000
= 125ps−1
64 × 8
1
1
=
= 0.048s
µ−λ
125 − 104
ρ=
l=
λ
104
=
= 0.83
µ
125
ρ
= 4.9
1−ρ
Seems reasonable delay and queue length, so one ISDN B-channel will be
sufficient.
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Asymmetric Digital Subscriber Line (ADSL)
ADSL
modem
MF
subscriber line
..
.
phone
..
.
ADSL
modem
phone
POTS
exchange
DSLAM
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)