Types of Communication LinksTypes of Communication Links

• Modem dial-up

• Leased Line

• ISDN

• Broadband– ADSL– cable broadband– Ethernet broadband

• Modem Dial up– run-on telephone system– no connection until dial-up OK– a circuit-switched facility

• not packet-switched technology• = connection between 2 end-points in the network

is occupied once it is connected• when the connection is over, the line is freed up for

the other call

• Modem Dial-up (Cont’d)– a normal computer cannot perform connection

to and signal transmission through telephone system

• Computer uses digital signal– with only 0 / 1 …….. Either OFF or ON– no in-between states

• Telephone line can transmit only analogue pulses– signal constantly varies in amplitude and frequency

Digital Signal

Analogue Signal

• Modem dial-up (cont’d)– In order for the successful transmission of

signal• MODEM is required• MODEM = modulator demodulator• Convert Digital Signal to analogue signal and vice

versa

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• Modem dial-up (cont’d)– dial up connection– dedicated to the duration of the

communication session– no other dial up connection is allowed until

that dial up connection is finished– Circuit-switched facility / Circuit-

switching• connection between two end points is occupied

once connected

• Leased Lines– a telephone line that has been leased for privat

e use– also circuit-switched facility– adv :

• has guaranteed bandwidth• simplify planning

– Disadv : • billing is fixed regardless how much bandwidth is us

ed• no capacity to growth

• ISDN (Integrated Services Digital Network)– integrate both analogue data and digital data

over the same network (upgrade switches of ISP)

– can use the telephone system during data transmission

– Two types of services (by ISP)• BRI (Basic Rate Interface)

– for home use and small enterprise

• PRI (Primary Rate Interface)– for large corporation

• ISDN (Cont’d)– BRI

• consists of • 2 x 64 Kbps B-channels

– 1 channel for 1 job

• 1 x 16 Kbps D-channel– carrying control and signalling information

• Total bandwidth (2B + D) =– 2 x 64 + 1 x 16 = 144 Kbps

• ISDN (Cont’d)– PRI (in USA)

• consists of • 23 x 64 Kbps B-channels

– 1 channel for 1 job

• 1 x 64 Kbps D-channel– carrying control and signalling information

• Total bandwidth (23B + D) =– 23 x 64 + 1 x 64 + ??? = 1.544 Mbps

• ISDN (Cont’d)– PRI (in Europe)

• consists of • 30 x 64 Kbps B-channels

– 1 channel for 1 job

• 1 x 64 Kbps D-channel– carrying control and signalling information

• Total bandwidth (30B + D) =– 30 x 64 + 1 x 64 + ??? = 2.048 Mbps