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Please do not award any half marks
(a)Explain the term modulation.Briefly
explain,with the aid of a diagram,one
modulation technique.[3 marks ]
Modulation is a signal process which shifts
the signal from one frequency range to another so that the signal
can be transmitted properly (1).
Explanation of Frequency modulation (1) OR Amplitude modulation (1)
OR Phase modulation (1).
Appropriate diagram (1).
(b)Name and explain briefly describe
any four data communications services provided by a typical telecommunications
service providers.[8 marks ]
Public switched telephone network (PSTN)(1).Designed
for voice transmission.A modem is required for data transmission as
signaling if analog (1) OR A Dial-up to some remote server is required
henever data transfer is required (1)
Analog leased lines (1) similar to PSTN but connection is permanent
Modem is still needed for data transmission (1)
Packet switched network (1) Access is either via a leased line or
dial-up.Typically rates of up to 64kbps are available (1)
Digital leased line (1) designed for private data networks.Speeds
of up to 140 Mbps are available (1)
Other valid points may be given marks. Answers involving PSPDN, PLN,
ISDN may score marks.
(c)Video on Demand is a service where
customers can select a film from an on-line catalogue and it will
be transmitted directly to their TV sets.
A 2-hour digitized movie ccupies 3GB 0f storage.What is the minimum
data-rate that the network must achieve?What sort of medium would
make a suitable econo mical choice?[4 marks ]
3 GB 3000000000 bytes =24000000000 bits.(1)
2 hours =7200 seconds
24000000000/7200 =3333333 bits per second (1)
data-rate needed approximately 3.5 Mbps (1)
co-axial is suitable (1) [although .ber optics ould be a valid choice,it
could not be
economical hen compared to coaxial cables because of its high cost
].
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