Page 127 - DCAP207_NETWORKS_DCAP406_COMPUTER_NETWORKS

Computer Networks/Networks

Notes S = G × e for Slotted Aloha
-G
Maximum throughput
-2G
-2G
dS/dG = e - 2G × e = 0 then
G = ½ and S = ½ e = 0.1839 for Aloha
-1
Max
-G
dS/dG = e - G × e = 0 then
-G
G = 1 and S = e = 0.3679 for Slotted Aloha
-1
Max
Managing Access to Networks
You may think if all the stations at a time begin to access network, there would be some sort of
chaos. Therefore, there are many methods of managing access to a network. If all network
stations tried to send data at once, the messages would become unintelligible, and no
communication could occur. It necessitates to device mechanism to avoid such deadlocks. Some
of important methods are listed below and discussed elsewhere in this study material:
Carrier-Sense Multiple Access with Collision Detection (CSMA/CD)

Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA)
Token Passing
Polling
Carrier Sense Multiple Access: In this case when a user wishes to transmit, he first listens to the
medium to ensure whether another transmission is in progress or not. It is known as carrier
sense. If the channel is in use, he must wait. If the medium is idle, he may transmit. If channel is
busy, he has to wait for a random period of time before trying to listen. This has been explained
in detail in other part of the tutorial.
CSMA with Collision Detection: If a user desires to transmit, he first listens to ensure whether
the channel is free or not. If the channel is idle, he transmits. If the channel is busy, he keeps on
listening until the channel is free, then transmits immediately. During the transmission, he
continues listening to detect collision. If a collision is detected, he stops transmitting immediately,
and waits a random period of time before goes back to step transmit again. Basically CSMA/CD
has three states. These are transmission period, contention period and idle period. This also
explained in other parts of this tutorial in detail.

Demand Priority: It uses services of intelligent hubs for controlling data transmission. A demand
signal is issued to the hub indicating that it wants to transmit. Depending on circumstances, the
hub responds with an acknowledgement that will allow the node to transmit. Likewise other
nodes are allowed to transmit in turn.
Token Passing: As name signifies, it uses a token or series of bits to allow a node to transmit. The
device after capturing token can transmit data into the network. When that particular node
completes sending its data, the node passes the token along to the next node in the topology.
Protocol specifications signifies how long a device may keep the token, how long it can transmit
for and how to generate a new token if there is not one circulating.

Polling: This method uses a central controller to ensure the demands of nodes in polling order.
Of so the central controller will allow them to transmit for a limited time, then the next device
is polled.

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