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Access control deals with the management
of the communications medium. Control can either be centralized or
distributed.
(a) What are the advantages and disadvantages
of having centralized controls? [4]
(a) Advantages - Fewer problems in coordinating
the activities of
multiple devices.[1]
Greater access control in terms of priorities, overrides
and guaranteed bandwidth to each station.[1]
Disadvantages - higher risk of single point of failure affecting the
whole
network[1]
Reduced efficiency if the control point a bottleneck[1]
(b) What does CSMA/CD stand for? What
kind of media access control technique
does this type of network employ? Explain how this type of network
operates. [4]
(b) Carrier sense multiple access with collision
detection, contention
technique [1] Award the mark only if both points are made.
All stations are attached to a shared medium and constantly listening
to
the medium[1]. If a station wants to send data it has to wait for
the media
to be idle. When it detects an idle media it will then send its data[1].
In
the instance that 2 stations send their data at the same time a collision
will occur, resulting in data from both station being destroyed[1].
A jam
signal is then sent out informing all stations that a collision has
occurred.
All stations will then wait for a random period of time before attempting
to
re-send their data again.[1]
up to a max of 3 marks (other relevant points may also earn credit)
(c) What is the IEEE standard for CSMA/CD?
Provide the standard frame format
for this standard. [5]
(c) IEEE 802.3 [1]
total
4 marks
any missing fields deduct 1 mark
missing size of fields deduct 1 mark
(d) Besides token-passing ring networks,
what is the other type of network that uses the token passing scheme
for media access? What is the IEEE standard for this type of network?
[2]
(d) IEEE 802.4 [1]
Token-bus networks[1]
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