Media Access Control Sub Layer
Media Access Control Sub Layer
Round Robin Techniques
In Round Robin techniques, each and
every node is given the chance to send or transmit by rotation. When a node
gets its turn to send, it may either decline to send, if it has no data or may
send if it has got data to send. After getting the opportunity to send, it must
relinquish its turn after some maximum period of time. The right to send then
passes to the next node based on a predetermined logical sequence. The right to
send may be controlled in a centralized or distributed manner. Polling is
an example of centralized control and token passing is an example of
distributed control as discussed below.
Polling
The mechanism of polling is similar to
the roll-call performed in a classroom. Just like the teacher, a controller
sends a message to each node in turn. The message contains the address of the
node being selected for granting access. Although all nodes receive the
message, only the addressed node responds and then it sends data, if any. If
there is no data, usually a “poll reject” message is sent back. In this
way, each node is interrogated in a round-robin fashion, one after the other, for
granting access to the medium. The first node is again polled when the
controller finishes with the remaining codes.
The polling scheme has the flexibility
of either giving equal access to all the nodes, or some nodes may be given
higher priority than others. In other words, priority of access can be easily
implemented.
Figure
Polling can be done using a central
controller, which may use a frequency band to send outbound messages as shown
in Fig. 5.2.2. Other stations share a different frequency to send inbound
messages. The technique is called frequency-division duplex approach (FDD).
Main drawbacks of the polling scheme are high overhead of the polling messages
and high dependence on the reliability of the controller.
Polling can also be accomplished without
a central controller. Here, all stations receive signals from other stations as
shown in Fig. 5.2.3. Stations develop a polling order list, using some
protocol.
Token Passing
In token passing scheme, all stations
are logically connected in the form of a ring and control of the access to the
medium is performed using a token. A token is a special bit
pattern or a small packet, usually several bits in length, which circulate from
node to node. Token passing can be used with both broadcast (token bus) and
sequentially connected (token ring) type of networks with some variation in the
details as considered in the next lesson.
In case of token ring, token is passed from a node to the
physically adjacent node. On the other hand, in the token bus, token is passed
with the help of the address of the nodes, which form a logical ring. In either
case a node currently holding the token has the ‘right to transmit’. When it
has got data to send, it removes the token and transmits the data and then
forwards the token to the next logical or physical node in the ring. If a node
currently holding the token has no data to send, it simply forwards the token
to the next node. The token passing scheme is efficient compared to the polling
technique, but it relies on the correct and reliable operation of all the
nodes. There exists a number of potential problems, such as lost token, duplicate
token, and insertion of a node, removal of a node, which must
be tackled for correct and reliable operation of this scheme.
A token ring
network
Token passing mechanism
Performance: Performance
of a token ring network can be represented by two parameters; throughput,
which is a measure of the successful traffic, and delay, which is a
measure of time between when a packet is ready and when it is delivered. A
station starts sending a packet at t = t0, completes transmission at t = t0 +
a, receives the tail at t0 + 1 + a. So, the average time (delay) required to
send a token to the next station = a/N. and throughput, S = 1/(1 + a/N) for
a<1 and S = 1/a(1 + 1/N) for a>1.
Token passing mechanism
Contention-based Approaches
Round-Robin techniques work efficiently
when majority of the stations have data to send most of the time. But, in
situations where only a few nodes have data to send for brief periods of time,
Round-Robin techniques are unsuitable. Contention techniques are suitable for burst
nature of traffic. In contention techniques, there is no centralized control
and when a node has data to send, it contends for gaining control of the
medium. The principle advantage of contention techniques is their simplicity.
They can be easily implemented in each node. The techniques work efficiently
under light to moderate load, but performance rapidly falls under heavy load.
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