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Pure ALOHA in Computer Networks
Aloha is a packet switching system. The time interval required to transmit one packet is called a slot. Aloha is a random access technique.
There are two ALOHA protocols as follows
- Pure ALOHA
- Slotted ALOHA
Now let us see what Pure ALOHA is.
Pure ALOHA
The mode of random access in which users can transmit at any time is called pure Aloha. This technique is explained below in a stepwise manner.
Step 1 In pure ALOHA, the nodes transmit frames whenever there is data to send.
Step 2 When two or more nodes transmit data simultaneously, then there is a chance of collision and the frames are destroyed.
Step 3 In pure ALOHA, the sender will expect acknowledgement from the receiver.
Step 4 If acknowledgement is not received within specified time, the sender node assumes that the frame has been destroyed.
Step 5 If the frame is destroyed by collision the node waits for a random amount of time and sends it again. This waiting time may be random otherwise the same frames will collide multiple times.
Step 6 Therefore, pure ALOHA says that when the time-out period passes, each station must wait for a random amount of time before re-sending its frame. This randomness will help avoid more collisions.
In PURE ALOHA, the vulnerable period is two slot times.
Vulnerable period is the maximum interval over which two packets can overlap and destroy each other. This phenomenon is shown the below figure
The throughput of pure ALOHA is given by the following
Probability of success (PSuccess) is equal to probability of no other packet transmission occurring within the vulnerable period.
Therefore, Throughput(s) is defined as the successfully transmitted traffic load and G is the total offered channel traffic load.
Assume that traffic generated for transmission obeys a Poisson distribution,
P(no other packet transmission occurs) = exp(-TG)
Where T is vulnerable period,
Psuccess = exp(-TG) = S/G S= Ge-TG
If vulnerable time is 2
Smax = 1/2e = 0.184
It means, in Pure ALOHA the channel utilization is 18 percent.