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Aloha protocol in Computer Networks

 

The ALOHA protocol was designed at the University of Hawaii and provides data transmission between the computers on several Hawaiian islands involving packet radio networks. It is a multiple access protocol at the data link layer and aims for how the multiple terminals access the medium without interference or collision. 

 

There are mainly two different types of ALOHA:

  1. Pure ALOHA
  2. Slotted ALOHA

 

Pure ALOHA 

Pure ALOHA is a decentralized and not slotted protocol. In this protocol, the stations transmit frames whenever they want to send data. In a situation where two or more stations simultaneously transmit frames, then a collision occurs and the frames are destroyed, as it does not check whether the channel is busy or not before transmitting. 

When the stations transmit frames, then they expect an acknowledgement from the receiver, if the acknowledgement is not received within a specific time period, then the station assumes that the frame is destroyed and waits for some time before sending the frames again. This helps in avoiding collisions. This protocol works well when in small networks where the load is smaller, but in heavily loaded networks, this scheme is bound to fail. To assure Pure ALOHA, the throughput, and rate of transmission of the frame must be predicted, and for that, a few assumptions need to be made such as:

  1. All the frames are of the same length.
  2. Stations are not trying to generate frames while transmitting. 
  3. The population of the stations according to Poison distribution. 

To fix the problem with Pure ALOHA, slotted ALOHA has been developed. 

 

Vulnerable Time = 2 * Tt

The efficiency of Pure ALOHA:

Spure= G * e^-2G 

where G is the number of stations that want to transmit in the Tt slot. 

Maximum Efficiency:

Maximum Efficiency will be obtained when G=1/2

S(pure)max = 1/2 * e^-1  = 0.184 

 

Slotted ALOHA

It is similar to pure ALOHA, while the only difference being the method of transmission. In this scheme, the time of the shared channel is divided into discrete intervals called slots. The stations can send frames only at the beginning of the next time slot. There are still possibilities of collisions in slotted ALOHA, but those possibilities are largely reduced, and the performance is much better than pure ALOHA. 

 

The efficiency of Slotted ALOHA:

S(slotted) = G * e^-G

 

Maximum Efficiency:

S(slotted)max = 1 * e^-1 = 1/e = 0.368 

 

Reference

Aloha protocol in Computer Networks