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Differences between Pure Aloha and Slotted Aloha
The Aloha Protocol allows several stations to send data frames over the same communication channel at the same time. This protocol is a straightforward communication method in which each network station is given equal priority and works independently.
Aloha is a medium access control (MAC) protocol for transmission of data via a shared network channel. Using this protocol, several data streams originating from multiple nodes are transferred through a multi-point transmission channel.
There are two types of Aloha protocols − Pure Aloha and Slotted Aloha.
In Pure Aloha, the time of transmission is continuous. Whenever a station has an available frame, it sends the frame. If there is collision and the frame is destroyed, the sender waits for a random amount of time before retransmitting it.
In Slotted Aloha, time is divided into discrete intervals called slots, corresponding to a frame.
In this article, we will highlight the major differences between Pure Aloha and Slotted Aloha.
What is Pure Aloha?
Pure Aloha is the basic form of Aloha contention mechanism, in which demand-driven data frames from numerous VSATs are sent to the satellite through a shared channel. It was first used at the University of Hawaii in 1970, under the direction of Norman Abramson.
In Pure Aloha, the time of transmission is continuous. Whenever a station has an available frame, it sends the frame.
A collision occurs if more than one frame tries to occupy the channel at the same time. If there is collision and the frame is destroyed, the sender waits for a random amount of time before retransmitting it.
After transmitting a frame, a station waits for a finite period of time to receive an acknowledgement. If the acknowledgement is not received within this time, the station assumes that the frame has been destroyed due to collision and resends the frame.
Due to the bursty nature of traffic inside a network, the possibilities of data frames colliding are quite high when using the Pure Aloha protocol.
None of the stations are concerned whether or not another station is transmitting at the time. As a result, when several data packets are broadcast over the same channel, they collide.
What is Slotted Aloha?
Slotted Aloha was introduced in 1972 by Robert as an improvement over Pure Aloha.
In slotted aloha, successful data transmission occurs only when each slot sends just one data frame at a time. The chance of a collision is considerably reduced by doing so.
Here, time is divided into discrete intervals called slots, corresponding to a frame. The communicating stations agree upon the slot boundaries.
Any station can send only one frame in each slot. Also, the stations cannot transmit at any time whenever a frame is available. They should wait for the beginning of the next slot.
It will stay idle if no data packets are sent in any of the slots. It should be noted that if a packet does not get acknowledgment after a collision, it is deemed lost and is retransmitted in a different slot after back-off time is taken into account.
However, there still can be collisions. If more than one frame transmits at the beginning of a slot, collisions occur.
Difference between Pure Aloha and Slotted Aloha
The following table highlights the important differences between Pure Aloha and Slotted Aloha.
| Key | Pure Aloha | Slotted Aloha |
|---|---|---|
| Time Slot | In Pure Aloha, any station can transmit data at any time. | In Slotted Aloha, any station can transmit data only at the beginning of a time slot. |
| Time | In Pure Aloha, time is continuous and is not globally synchronized. | In Slotted Aloha, time is discrete and is globally synchronized. |
| Vulnerable time | The vulnerable time or susceptible time in Pure Aloha is equal to (2×Tt). | In Slotted Aloha, the vulnerable time is equal to (Tt). |
| Probability | The probability of successful transmission of a data packet $S\:=\:G\:\times\:{e^{-2G}}$𝐺 | The probability of successful transmission of data packet $S\:=\:G\:\times\:{e^{-G}}$ |
| Maximum efficiency | Maximum efficiency = 18.4%. | Maximum efficiency = 36.8%. |
| Number of collisions | Does not reduce the number of collisions. | Slotted Aloha reduces the number of collisions to half, thus doubles the efficiency. |
Conclusion
Both Pure Aloha and Slotted Aloha are Random Access Protocols. Slotted Aloha is an improvement over Pure Aloha, as it reduces the number of collisions and doubles the capacity of Pure Aloha.
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