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(a)A string of 200 characters is to be transmitted over a serial
data transmission line. How many bits will be transmitted in each
of the following modes?(Show your working.)
(i)Synchronous data transmission with 4 SYN
characters;message begun with STX character and ended with EOT character.[3
marks ]
(ii)Asynchronous transmission with 1 start bit and 2 stop bits.[3
marks ]
Award similar method marks for different methods of working (if correct).
(iii)Six additional c aracters (1).Total:206 haracters (1).Total=206
*8 =1648 bits (1).
(iv)Three extra bits per character (1).Total:11 bits per character
(1).Total=
200 *11 =2200 bits (1).
(b)Describe the main characteristics of twisted air cable.Explain
why it is ‘twisted ’and give one typical use.[3 marks ]
Award a mark for one of these or another
relevant point:
Made of copper,covered with insulating material.Two such strands twisted
along entire length.(1) Inexpensive and easy to install (1).
One mark for:
Twisting helps minimise the effects of noise or electromagnetical
interference (1).
and one for an example such as:
used for low-cost LAN,at 20Mbps (1).
(c)There are two modern standards for high-speed communications between
desktop computers and peripherals —USB and SCSI.
USB uses a thin cable with just a few wires,and a very simple 4-in
connector.It
achieves a maximum data rate of 12 Mbps.SCSI uses a fat cable with
many wires,and a 50-pin connector.It can achieve data rates up to
40 Mbps.
Use your knowledge of serial and parallel connections to explain how
these standards differ in their basic design,and how this affects
their erformance.USB cables are often longer than SCSI cables.Explain
why this is.[6 marks ]
Award marks for these or other relevant points:
USB is a serial standard (hence the name(!))(1),SCSI uses parallel
transmission (1).T is is indicated by the width (in terms of number
of wires)of the respective cables (1).
Parallel transmission is in principle much faster than serial,because
it transmits a whole word at each clock tick,whereas serial transmits
only one bit per tick (1). However,the overhead in keeping the words
synchronised means that parallel will tend to use a slower clock (1).
The parallel problems (‘skew ’)increase quickly with cable
length (1),so parallel cables must be shorter,in general,than serial,where
there can be no problem with bits arriving out of sequence or overlapping
(1).
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