|
Please do not award any half marks
(a)What is a common fault associated with
parallel transmission?Explain this fault and why does it not apply
to serial transmission [4 marks ]
In parallel transmission,separate cables
carry separate bits simultaneously;one cable carries a clock signal
Over large distances the bits can lose their timing relationship (1).
Thus bits from the same character may arrive at the receiver with
different clock
signals;this is skew (1);the result will be that rwong characters
are received (1).This is not a problem in serial transmission because
all bits are in the same cable and cannot overtake each other (1).
Award credit for other relevant points.
(b)Distinguish between encryption and
compression.How are they related?[4 marks ]
Compression is a process of compressing the
data to reduce the amount of data transmitted.(1) Encryption is a
process of converting plain text into ciphertext (1),where the ciphertext
is readable only by the holder of the decryption key (1).Compression
also serves as a crude form of encryption,because unauthorized user
is unaware of the compression scheme (1)
Award credit for other relevant points.
(c)Differentiate between error correction
and error detection protocols.[3 marks ]
Error detection tells you which bytes are
wrong (and retransmission must be requested)(1);error correction contain
enough redundant information that errors can be both detected and
then corrected (1) without retransmission (1)
Award credit for other relevant points.
(d)In a contention based system,collisions
may occur.Explain what is meant by a
collision.Is this a network fault?How do the two hosts recover from
this situation?
[4 marks ]
A collision is when two hosts send their
data at the same time.Both messages get corrupted and has to be re
sent again.(1) NO,this is not a fault.(1) The system has been designed
in this way.Both hosts will back off for a random interval and then
attempt to retransmit their messages again (2).
Award credit for other relevant points.
|