In the previous post, we made ourselves thorough with TDM, polling, CSMA/CD and token passing. We will further get acquainted with yet other method of access control called Aloha. In this tutorial, we will study about the rules of aloha and specifically Pure Aloha protocol and its functioning.

What is ALOHA?

ALOHA is a simple random access protocol that was developed at the University of Hawaii in the early 1970. ALOHA’s main purpose is to determine which competing station must get the next chance of accessing the multi-access channel at MAC Layer. We start this concept by understanding the rules of aloha.

Aloha Rules

• Any station could transmit data anytime.
• There is no carrier sensing.
• Collisions are possible.
• Acknowledgements exist in aloha. Hence, there is no need of collision detection.
• Retransmission occurs after some random amount of time. Thus, probability of collision is less since packet is transmitted after random amount of time. This is called back-off time.

The ALOHA initially developed by the University of Hawaii was called Pure ALOHA.

Working of Pure Aloha Protocol

Below picture depicts how pure aloha works –

Pure Aloha Example

Where,
– Blue Block represents data being transferred.
– White Block represents data being transferred at the same time or in the same block.

If a station starts transmitting data at any point of time, It is transmitted for TT time.
During this time, if any other station starts transmitting data, then, there can be a collision. Thus, TT is vulnerable time.

Note – TT stands for transmission time.

If collision occurs in the frame, then, the stations wait for a random amount of time. Then, they retransmit the frame until all the data are successfully transmitted. The waiting time of each station must be random and not same in order to avoid the collision of frames again and again. Thus, the throughput of Pure ALOHA is maximized when the frames are of uniform length.

Notes:

• Vulnerable time in pure aloha = 2 * TT
• Efficiency in pure aloha = $ {G} * {e}^{-2G} $
  where, G = no of stations who wants to transmit in TT slot
• On calculation, it was found that efficiency of pure aloha = 18.4 %

With this we conclude our studies of pure ALOHA protocol. Now, we will move ahead to an advanced version of pure ALOHA called slotted ALOHA in the next tutorial.

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