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(a) Name the two sub-layers that make
up the data-link layer. [2]
(a) Media access control(MAC) sub-layer[1]
Logical-link control(LLC) sub-layer[1]
(b) Explain the difference between a
full-duplex and a half-duplex connection. [2]
(b) A full duplex connection is one where
2 connected stations can transmit and receive signals from each other
simultaneously.[1] whereas a half duplex connection is where the flow
is bi-directional but can only flow in only one direction at a time.[1]
.
(c) What is the major problem associated
with parallel transmission? Does
this problem also affect serial transmission? Explain your answer.
[3]
(c) Skew[1]. Each bit of a character is sent
over a separate channel and the
bits loose their timing relationship and arrive at different times
resulting
in errors .[1] NO it does not apply to serial transmission as there
all bits
are sent are sent over one path contiguously [1]
(d) Explain briefly the functional specifications
of the RS232C standard. [3]
(d) The functional specification describes
the function of each pin on the
connector.[1] It also states the circuits connected to the pins can
be
grouped into 4 groups namely data, ground, timing and control[2]
(e) Explain with the aid of a diagram
the concept of amplitude modulation. [2]
diagram[1]
The amplitude of the carrier signal changes according to the variation
of
the input signal[1]
(f) Despite the comparably high cost
factor, what is it that makes fiber optics a
desirable medium? [2]
(f) High Bandwidth[1]
Interference Immunity[1]
(g) Explain what is meant by noise in
the context of data communications.
What type of noise is unavoidable, and why? [3]
(g) Noise is unwanted energy from sources
other than the transmitter. Noise is a problem because it may mask
the original data wipe out bits or create spurious messages[1].
thermal noise - caused by random movement of electrons [1] and is
a
function of temperature [1] only materials at `absolute zero’
will have no
electron movement.
(h) What is the relationship between
encryption and compression? [2]
(h) When data is compressed the hacker will
not be able to receive the actual data unless the decompression technique
is known[1]. Therefore,
compression is a crude form of encryption[1]
(i) What is the unit of transfer of the
data-link layer? [1]
(i) the unit at the data-link layer is the
frame[1]
(j) Which layer of the OSI architecture
is responsible for preserving the meaning of
data that is transferred between 2 different machines? [1]
(j) The presentation layer[1]
(k) What is meant by CSMA/CA? How does
this media access control scheme
work? Give an example of an area where this technique is used. [3]
(k) Carrier-sense multiple access with collision
avoidance is a distributed
round robin technique for media access[1]. Each station listens to
the
carrier while a transmission is in progress. After it ends, each station
waits for a specific transmission period, based on its logical list
of
stations. After the time has elapsed the station will begin transmission.
[1]
e.g ISDN's basic rate Interface[1]
(l) Assuming one start and one stop
bit are used as delimiters in an asynchronous
transmission, what would be the effective utilization of this transmission?
Justify your answer. [3]
(l) 1 start and 1 stop it and assuming 1
byte represents a character the
effective channel utilization is [1]
8/10 X 100% = 80% [2]
(m) Is it accurate to say that a bridge
cannot be used to connect two networks with differing MAC layer protocols?
Explain your answer. [3]
(m) No[1] a bridge can be used to connect
networks with different MAC layer protocols provided they both adhere
to the IEEE 802.2 Logical link layer standard.[2]
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