Difference Between Pure ALOHA and Slotted ALOHA

Pure ALOHA and Slotted ALOHA both are the Random Access Protocols, that are implemented on the Medium Access Control (MAC) layer, a sublayer of Data Link Layer. The purpose of the ALOHA protocol is to determine that which competing station must get the next chance of accessing the multi-access channel at MAC layer.

The main difference between Pure ALOHA and Slotted ALOHA is that the time in Pure Aloha is continuous whereas, the time in Slotted ALOHA is discrete. Let us discuss the other differences between Pure ALOHA and Slotted ALOHA in the comparison chart.

Content: Pure ALOHA Vs Slotted ALOHA

1. Comparison Chart
2. Definition
3. Key Differences
4. Conclusion

Comparison Chart

Basis for Comparison	Pure ALOHA	Slotted ALOHA
Introduced	Introduced by Norman Abramson and his associates at the University of Hawaii in 1970.	Introduced by Roberts in 1972.
Frame Transmission	The user can transmit the data frame whenever the station has the data to be transmitted.	The user has to wait till the next time slot start, to transmit the data frame.
Time	In Pure ALOHA the time is continuous.	In Slotted ALOHA the time is discrete.
Successful Transmission	The probability of successful transmission of the data frame is: S= G* e^-2G	The probability of successful transmission of the data frame is: S= G*e^-G
Synchronization	The time is not globally synchronized.	The time here is globally synchronized.
Throughput	The maximum throughput occurs at G = 1/2 which is 18%.	The maximum throughput occurs at G = 1 which is 37%.

Definition Of Pure ALOHA

Pure ALOHA is introduced by Norman Abramson and his associates at the University of Hawaii in early 1970. The Pure ALOHA  just allows every station to transmit the data whenever they have the data to be sent. When every station transmits the data without checking whether the channel is free or not there is always the possibility of the collision of data frames. If the acknowledgment arrived for the received frame, then it is ok or else if the two frames collide (Overlap), they are damaged.

If a frame is damaged, then the stations wait for a random amount of type and retransmits the frame till it transmits successfully. The waiting time of the each station must be random and it must not be same just to avoid the collision of the frames again and again.

The throughput of the Pure ALOHA is maximized when the frames are of uniform length. The formula to calculate the throughput of the Pure ALOHA is S-=G*e^-2G, the throughput is maximum when G=1/2 which is 18% of the total transmitted data frames.

Definition Of Slotted ALOHA

After the pure ALOHA in 1970, Roberts introduced an another method to improve the capacity of the Pure ALOHA which is called Slotted ALOHA. He proposed to divide the time into discrete intervals called time slots. Each time slot corresponds to the length of the frame.

In contrast to the Pure ALOHA, Slotted ALOHA does not allow to transmit the data whenever the station has the data to be send. The Slotted ALOHA makes the station to wait till the next time slot begins and allow each data frame to be transmitted in the new time slot.

Synchronization can be achieved in Slotted ALOHA with the help of a special station that emits a pip at the beginning of every time slot as a clock does.  The formula to calculate the throughput of the Slotted ALOHA is S=G*e^-G,  the throughput is maximum when G=1 which is 37% of the total transmitted data frames. In Slotted ALOHA, 37% of the time slot is empty, 37% successes and 26% collision.

Conclusion

The Slotted ALOHA is somewhat better than the Pure ALOHA. As the probability of collision is less in Slotted ALOHA as compared to Pure ALOHA because the station waits for the next time slot to begin which let the frame in a previous time slot to pass and avoids the collision between the frames.