ALOHA is a simple communication protocol used in computer networks, specifically in wireless systems. Developed in the 1970s at the University of Hawaii, ALOHA was designed for data communication over radio waves.
There are two primary versions of ALOHA:
- Pure ALOHA
- Slotted ALOHA.
1. Pure ALOHA:
In Pure ALOHA, any device can send data whenever it has data to send. The downside of this approach is that data packets can collide if more than one device transmits simultaneously.
These collisions lead to data loss and require retransmissions.
To manage collisions, the sender waits a random amount of time before attempting to resend the data.
The maximum throughput of Pure ALOHA is 18.4%, meaning only 18.4% of the channel’s capacity is effectively utilized due to collisions.
Vulnerable Time of pure ALOHA
Vulnerable time is the time in which the collision of data packets occurs. If the first frame ‘B’ is sent at any particular time t , before the data is transmitted completely and the other frame ‘A’ starts before the completion of frame ‘B’ will lead to a collision.
Where T is the Transmission time of the frame Tt
The vulnerable time when the collision occurs (Vt) = [(t+Tt) -(t-Tt)]
= t+Tt - t+Tt
= 2 * Tt
Vulnerable time is also referred to as the propagation time or total time taken to transmit the complete data packet from the station.
Vulnerabletime=(Message Length) / (Transmission channel bandwidth)
Channel Throughput of Pure ALOHA
Here ‘G’ is the average number of frames transmitted through the channel during a period of one frame transmission. When the data packet is successfully or completely transmitted to the receiver end, the probability of the data packet is given below,
Maximum Efficiency
To find the maximum efficiency of the station during the data packet transmission
The Efficiency of Pure ALOHA in percentage is 18.4% and is very less due to the number of collisions.
2. Slotted ALOHA:
Slotted ALOHA improves the efficiency by introducing time slots. Devices can only send data at the beginning of each slot.
This reduces the likelihood of collisions because transmissions are aligned to discrete time intervals.
The throughput of Slotted ALOHA is higher than Pure ALOHA, reaching about 36.8%.
Vulnerable Time of slotted ALOHA
Vulnerable time is the time is the Transmission time of the frame Tt
The vulnerable time (Vt) = Tt
Channel Throughput of Slotted ALOHA
Here ‘G’ is the average number of frames transmitted through the channel during a period of one frame transmission. When the data packet is successfully or completely transmitted to the receiver end, the probability of the data packet is given below,
Maximum Efficiency
To find the maximum efficiency of the station during the data packet transmission
Difference between Pure ALOHA AND Slotted ALOHA
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