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Differences between Pure and Slotted Aloha

Last Updated : 02 Jun, 2023
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Pre-Requisite: Multiple Access Protocols

Aloha is a type of Random access protocol it was developed at the University of Hawaii in early 1970, it is a LAN-based protocol this type there are more chances of occurrence of collisions during the transmission of data from any source to the destination, Aloha has two types one Pure Aloha and another Slotted Aloha.

Pure Aloha

Pure Aloha can be termed as the main Aloha or the original Aloha. Whenever any frame is available, each station sends it, and due to the presence of only one channel for communication, it can lead to the chance of collision. 

In the case of the pure aloha, the user transmits the frame and waits till the receiver acknowledges it, if the receiver does not send the acknowledgment, the sender will assume that it has not been received and sender resends the acknowledgment.

Pure Aloha

Pure Aloha

For more, refer to Pure Aloha.

Slotted Aloha

Slotted Aloha is simply an advanced version of pure Aloha that helps in improving the communication network. A station is required to wait for the beginning of the next slot to transmit. The vulnerable period is halved as opposed to Pure Aloha.

Slotted Aloha helps in reducing the number of collisions by properly utilizing the channel and this basically results in the somehow delay of the users. In Slotted Aloha, the channel time is separated into particular time slots.

Slotted Aloha

Slotted Aloha

For more, refer to Slotted Aloha.

Differences Between Pure Aloha and Slotted Aloha

Pure Aloha Slotted Aloha
In this Aloha, any station can transmit the data at any time. In this, any station can transmit the data at the beginning of any time slot.
In this, The time is continuous and not globally synchronized. In this, The time is discrete and globally synchronized.
Vulnerable time for Pure Aloha = 2 x Tt Vulnerable time for Slotted Aloha = Tt

In Pure Aloha, the Probability of successful transmission of the data packet

= G x e-2G

In Slotted Aloha, the Probability of successful transmission of the data packet

= G x e-G

In Pure Aloha, Maximum efficiency

= 18.4%

In Slotted Aloha, Maximum efficiency

= 36.8%
Pure Aloha doesn’t reduce the number of collisions to half. Slotted Aloha reduces the number of collisions to half and doubles the efficiency of Pure Aloha.


1. What is the advantage of Pure Aloha over Slotted Aloha?


The advantage of pure aloha over slotted aloha is that there is no fixed size, and it has the ability to start transmission at any particular time and no need for synchronization.

2. What is the formula for Pure Aloha and Slotted Aloha?


Pure Aloha: S = G x e-2G

Slotted Aloha: S = G x e-G

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