The most important difference between Pure ALOHA and Slotted ALOHA is the time in Pure Aloha is constant whereas, the time in Slotted ALOHA is different.
Pure ALOHA and Slotted ALOHA are the Random Access Protocols, that have implemented on the MAC (Medium Access Control) layer, a sublayer of Data Link Layer. The objective of the ALOHA protocol is to determine what competing channel must find the next chance of accessing the multi-access station at the MAC layer.
Let’s talk about the other differences between Pure ALOHA and Slotted ALOHA in the comparison chart.
Comparison Chart
| Basis | PURE ALOHA | SLOTTED ALOHA |
| Introduced | Introduced by Norman Abramson at the University of Hawaii in 1970. | Introduced by Roberts in 1972. |
| Frame Transmission | The user can transmit data frame whenever the channel has the information to be transmitted. | The user needs to wait until the next time slot begins, to transmit the data frame. |
| Time | In Pure ALOHA the time is constant. | In Slotted ALOHA the time is different. |
| Probability of Successful Transmission | S= G* e^-2G | S= G*e^-G |
| Throughput | Throughput is maximum at G = 1/2 that is 18%. | The maximum throughput occurs at G = 1 that is 37%. |
| Globally Synchronization | Not | Yes |
What is Pure ALOHA?
Pure ALOHA is firstly introduced in 1970 by Norman Abramson and his partners at the University of Hawaii. Pure ALOHA allows each station to transmit the data each time they have the information to be sent. When every channel transmits the data without assessing whether the channel is free or not there is always the chance of the crash of data frames. If the acknowledgement came for the received frame, then it’s ok, or if the two frames collide (Overlap), they are ruined.
If a frame is damaged, then the channels waiting for a random quantity of type and retransmit the frame until it transmits successfully. The waiting period of each channel has to be random and it shouldn’t be same just to prevent the crash of the frames over and over. The throughput of the Pure ALOHA gets maximized when the frames are of uniform length. The formula to compute the throughput of the Pure ALOHA is S-=G*e^-2G, the throughput is maximum when G=1/2 that is 18 per cent of the total transmitted data frames.
What is Slotted ALOHA?
Following the pure ALOHA in 1970, Roberts introduced another system to enhance the capacity of the Pure ALOHA that’s called Slotted ALOHA. He suggested dividing the time into discrete intervals called slots. Every time slot corresponds to the length of the framework. Compared to the Pure ALOHA, Slotted ALOHA doesn’t allow to transmit the information whenever the channel has the information to be sent. The Slotted ALOHA makes the channel to wait until the next time slot starts and let every data frame to be transmitted in the new time slot.
Synchronization could be accomplished in Slotted ALOHA with the support of a unique station that emits a pip at the start of every time slot for a clock does. The formula to compute the throughput of the Slotted ALOHA is S=G*e^-G, the throughput is maximum when G=1 that is 37 per cent of the total transmitted data frames. In Slotted ALOHA, 37 per cent of the time slot is vacant, 37% successes and 26% crash.
Key Differences between Pure ALOHA and Slotted ALOHA
- Pure ALOHA introduced by Norman and his partners at the University of Hawaii in 1970. wheres, Slotted ALOHA was introduced by Roberts in 1972.
- In pure ALOHA, every time a station has data to send it transports it without waiting , in slotted ALOHA an individual wait until the next time slot beings to transmit the information.
- In pure ALOHA the time is constant whereas, in Slotted ALOHA the time is discrete and divided into slots.
- In pure ALOHA probability of successful transmission is S=G*e^-2G. while in slotted ALOHA the probability of successful transmission is S=G*e^-G.
- The period of receiver and sender in pure ALOHA isn’t globally synchronized whereas, the period of receiver and sender in slotted ALOHA is globally synchronized.
- The maximum throughput occur at G=1/2 that is 18% whereas, the maximum throughput occurs at G=1 that is 37%.
Conclusion
The Slotted ALOHA is somewhere better than the Pure ALOHA. Since the probability of collision is less in Slotted ALOHA when compared with Pure ALOHA because the channel waits for the next time slot to start which let the framework in an earlier time slot to pass and avoids the collision between the frames.