Pure VS. Slotted Aloha: Know the Differences between Pure and Slotted Aloha
The term Aloha refers to a random access protocol. It has two broad categories- pure and slotted. In this article, we will discuss the various differences between pure aloha and slotted aloha.
We utilise pure aloha when we have data available to be sent over any channel at any station. However, using a pure aloha poses the risk of data frame hitting. The slotted aloha, on the other hand, exists to overcome this problem. Let us discuss more on these two types of aloha. But before we do that, we will know more about what is aloha.
What is Aloha?
Aloha has basically been designed for WLAN or wireless Local Area Network- but we can also utilise it in a shared medium for data transmission. Multiple stations can easily transmit data and info to any channel in an aloha at any given time. Thus, aloha will never require carrier sensing.
What is Pure Aloha?
We use pure aloha when we have data and info to be sent over a channel at a station. In the case of a pure aloha, every station performs data transmission to another channel without checking the availability status of the channel. It means that we don’t know if that particular channel is idle at the moment. Such a case increases the overall chances of collision during the transmission, and we may eventually lose the data frame at times.
When a station transmits the given data frame to any channel, it will expect some sort of acknowledgment from the receiver’s end. If it receives this acknowledgment within a specified time, then it’s fine. Else, the station would assume that the data frame has been destroyed during transmission.
Practically, the station would wait for some random time for the acknowledgment. After that, it will consider the data to be destroyed and then retransmit this frame in the very same channel. This will occur repeatedly until the receiver successfully receives the intended data and reverts back with an acknowledgment.
What is Slotted Aloha?
Since the pure aloha possesses a very high possibility of data frame hitting, the slotted aloha is used to overcome this issue. Unlike pure aloha, a slotted aloha would disallow data transmission when the station is willing to send it.
The shared channel in a slotted aloha gets divided into slots. These are basically fixed time intervals. Now, if a station is sending a frame to the destined channel, it can only send the data frame at the beginning of this slot. Added to this, the station can only send a single frame in one slot. Thus, if the station fails to transmit/ send the data frame and the channel on the receiver’s end doesn’t receive it, the sender station HAS TO wait for the next slot to begin.
This method reduces the chances of collision to a great extent, but collision may still occur if two stations are trying to send data frames at the beginning of the very same slot.
Differences between Pure and Slotted Aloha
Let us talk about the differences between Pure and Slotted Aloha. To make this topic more understandable and clear, we are comparing both of the terms based on their individual characteristics in a table.
| Parameters | Pure Aloha | Slotted Aloha |
| Basics | Data transmission can occur from any station to any channel at any random time. | A station can only transmit a data frame to another station at the beginning of the available time slot. |
| Synchronisation of Time | There is no time synchronisation in the case of pure aloha. The time is continuous in this case. | The slotted aloha consists of a globally synchronised time for sending data frames across all the available channels. The time is discrete in this case. |
| Introduced By | Pure aloha was introduced at the University of Hawaii in 1970 under Norman Abramson’s leadership. | Robert introduced the slotted aloha to improve the overall capacity of the pure aloha, way back in 1972. |
| Total Number of Collisions | It is very high in pure aloha due to the random time of data frame transmission across various channels. | The slotted aloha, on the other hand, enhances the overall eligibility of data transmission. It thus decreases the chances of collision to almost half of the pure aloha. |
| Vulnerable Time | The vulnerable time of pure aloha is = 2 x Tt | The vulnerable time of pure aloha is = Tt |
| Probability of Successful Transmission | S = G * e-2G | S = G * e-G |
| Maximum Throughput | Pure aloha has a maximum throughput of about 18%. | Slotted aloha has a maximum throughput of about 37%. |
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