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
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