ALOHA in Random Access

ALOHAnet was one of the first computer networking systems and a predecessor to the modern Internet. It was developed in the 1970s by Norman Abramson and his colleagues at the University of Hawaii. ALOHAnet was a wireless network that used a random access protocol known as ALOHA to allow multiple computers to transmit data over a shared communication channel.

In ALOHA, when a device has data to transmit, it simply sends it over the communication channel without checking if the channel is already being used by another device. If a collision occurs, the devices involved in the collision wait for a random amount of time before attempting to retransmit the data. This process is known as pure ALOHA.

To improve the efficiency of the network, a modified version of ALOHA known as slotted ALOHA was developed. In slotted ALOHA, the communication channel is divided into time slots, and each device is only allowed to transmit data at the start of a time slot. This helps to reduce the probability of collisions and improves the overall efficiency of the network.

There are two types of ALOHA protocol:

• Pure ALOHA:

Pure ALOHA allows users to transmit data at any time without checking if the channel is busy or not. If two or more users transmit data simultaneously, a collision will occur and the data will be lost. After a random amount of time, each user will attempt to retransmit the data.

The algorithm for Pure ALOHA is as follows:

1. When a station has a frame to transmit, it waits for the beginning of the next time slot.
2. The station transmits the frame in its entirety during the time slot.
3. The station listens to the channel to see if the transmission was successful. If it is successful, the station is done. If it is not successful, the station waits a random amount of time and then repeats the transmission.

• Slotted ALOHA:

In slotted ALOHA, the time is divided into fixed-size slots, and the users are required to transmit data only at the beginning of each slot. If a user misses a slot, it has to wait for the next slot to transmit its data. This approach reduces the chances of collisions and increases the efficiency of the network.

The algorithm for Slotted ALOHA is as follows:

1. When a station has a frame to transmit, it waits for the beginning of the next time slot.
2. The station transmits the frame in its entirety during the time slot.
3. The station listens to the channel to see if the transmission was successful. If it is successful, the station is done. If it is not successful, the station waits until the beginning of the next time slot and then repeats the transmission.

Flowchart of ALOHA:

Difference between Pure ALOHA and Slotted ALOHA:

Pure ALOHA and Slotted ALOHA are two different versions of the ALOHA protocol, which is a random access protocol for transmitting data in a shared channel. Here are the key differences between the two:

1. Time slots: In Pure ALOHA, a station can start transmitting at any time, whereas in Slotted ALOHA, the transmission can only start at the beginning of a time slot.
2. Efficiency: Slotted ALOHA is more efficient than Pure ALOHA because it reduces the probability of collisions. In Pure ALOHA, if two or more stations transmit at the same time, a collision occurs, and both transmissions are lost. In Slotted ALOHA, since all transmissions start at the beginning of a time slot, the probability of collisions is reduced, resulting in higher efficiency.
3. Throughput: The maximum throughput of Slotted ALOHA is twice that of Pure ALOHA. In Slotted ALOHA, since the transmission can only start at the beginning of a time slot, the total number of time slots is twice the number of transmissions per second. Therefore, the maximum throughput is 1/2, whereas in Pure ALOHA, it is 1/2e. The throughput of Pure ALOHA is given by:S = Ge^(-2G)where S is the throughput and G is the offered load, which is the average number of transmissions attempted during a time slot.The throughput of Slotted ALOHA is given by:

   S = Ge^(-G)

   where S is the throughput and G is the offered load, which is the average number of transmissions attempted during a time slot.

   

   Comparing the two equations, we can see that Slotted ALOHA has a higher throughput than Pure ALOHA for the same offered load. Slotted ALOHA can achieve a maximum throughput of 0.368, while Pure ALOHA can achieve a maximum throughput of only 0.184.

4. Implementation complexity: Pure ALOHA is simpler to implement compared to Slotted ALOHA, which requires synchronization between the stations to start transmitting at the beginning of a time slot.

In summary, Slotted ALOHA is a more efficient and faster version of ALOHA, but it requires more implementation complexity compared to Pure ALOHA.