Page 126 - DCAP406_DCAP_207_Computer Networks

Unit 8: Data Link Protocols

Soon there would be hundreds of such frequencies, and radios capable of listening to this Notes
number of frequencies at the same time are very expensive.
Time Division Multiplexing: In this case, each node is given time slots for sending message.
Unlike frequency multiplexing each node continue to have a single radio frequency. The
disadvantage of this method lies in wastage of time when a node has nothing to send in its
particular slot. This makes a node to wait long for sending data when there are several nodes.
Carrier Sense Multiple Access: In order to overcome the problem of empty time slots, Aloha
devised carrier sense multiple access techniques which has now become the de facto standard.
As you have already studies that this system has no fixed multiplexing at all. Instead each node
listens to see if anyone else is using the channel, and if they don’t hear anyone, they start talking.
However, this techniques saved the time but was not foolproof as if first node started using the
radio, then it might have it as long as it wanted and therefore left other nodes to wait indefinitely.
This gave the birth of breaking down of the messages into small packets, and interlaced them in
between the time slots of one node to other. This allowed other nodes to send out their packets
in between, so everyone could share the medium at the same time. In order to avoid the collision
problem when two nodes attempted to start their broadcast at the same time, Aloha protocol
devised the technique of acknowledgement when sender could always find out if its frame was
destroyed by listening to channel. For a LAN, feedback is immediate; while for a satellite there
is a long delay of 270 ms before sender knows. In this case, after sending any packet the nodes
listened to see if their own message was sent back to them by a central hub. If they got their
message back, they could move on to their next packet. If not, it meant that some collision with
another node’s packet had prevented it to reach to the intended destination. This prompted them
to send again after waiting for a random time. This collision avoidance system allowed any
node to use the entire network’s capability if no one else is using it.

Slotted Aloha

It uses small clock tick packets to allow intended nodes to send their packets immediately after
receiving a clock tick. This protocol does improve the overall channel utilisation, by reducing
the probability of collisions by a half. But this was not sufficient advantage and therefore further
work on the same for wired networks by Bob Metcalfe improved collision avoidance on busy
networks and established standards for Ethernet which is popularly known as CSMA/CD,
carrier sense, multiple access, collision detection. In the previous unit this has been explained in
quite detail.
Performance of Aloha or Slotted Aloha is determined with the help of throughput and average
delay. Throughput is average number of frames successfully transmitted per unit time. A high
value indicates a good performance. Poisson distribution can model frame transmission rate
with mean arrival rate l frames/s. We can assume average frame length tf second then Normalized
channel traffic or average number of old and new frames submitted per frame time is
G= λτ Erlang
f
The throughput is then given by S = Gλτ (no collision)
f
Probability that a frame does not have a collision is given by
P =e for Aloha
-2G
0
P =e for Slotted Aloha
-G
0
The throughput/frame time is then given by
S = G × e for Aloha
-2G

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