ALOHA in Computer Networks -

ALOHA in Computer Network

The Multiple-access protocol ALOHA allows data to be exchanged over a common network channel. It works on the medium access control sublayer of the open systems interconnection (OSI) architecture. (Mac sublayer). Using this protocol, multiple data streams from different places are delivered over a multi-point transmission path. Norman Abramson and colleagues founded it at the University of Hawaii in the 1970s. Each ALOHA node or station delivers a frame without first determining whether or not the broadcast route is operational. The frames will be appropriately transferred if the channel is empty. Two frames will clash and be erased if they try to fill the channel at the same time. These channels may retransmit the damaged frames until effective transfer takes place.

In the below PDF we discuss about ALOHA & Types of Aloha in detail in simple language, Hope this will help in better understanding.

Rules of ALOHA in computer networks:

Any station has the ability to send information to a channel at any time.
Collisions and data packet loss are possible while transferring data across multiple sites.
Carrier detection is not necessary.
Aloha lacks impact recognition due to the existence of frame acknowledgment.
It requires that data be resent after an arbitrary period of time.

Types of ALOHA in computer networks

Aloha has two protocols in computer networks:

1. Pure Aloha:
We employ Pure Aloha whenever data may be transferred over a channel at a station. In pure Aloha, a conflict may occur and the data frame may be lost if each station sends data to a channel without first determining if it is free or not.When a station sends a data frame to a channel, pure Aloha waits for the recipient to acknowledge it. If the receiver’s response does not arrive within the given time, the station waits for an arbitrary amount of time, known as the backoff time. (Tb). The station can also believe that the frame has been misplaced or destroyed. It then sends the frame again until all of the data has been successfully sent to the recipient.The idea of four channels competing for access to the same channel is irrational. The diagram depicts that on the common medium, each location sends two frames, for a total of eight frames. Some of these frames conflict due to multiple frames competing for the same common channel.Only two frames—frame 1.1 from station 1 and frame 3.2 from station 3—are left, as shown in the image above. Even if a bit from one frame and a bit from another share the channel, a collision occurs, and both are destroyed.

Vulnerable Time for Pure Aloha:
Because the stations broadcast fixed-length frames, it is believed that each frame takes Tt time (Transmission Time) to be sent fully. The figure below depicts the vulnerable period for station A. To avoid frame collisions, we need to ensure that no other station begins broadcasting between Tt time before and Tt time after the transmitting station. Therefore, the vulnerable time for every station must be twice as long as the transmission time.

2. Slotted Aloha:
The slotted Aloha is designed to outperform pure Aloha since pure Aloha has a very high chance of impacting the frame. Slotted Aloha separates the shared channel into distinct time intervals. As a result, if a station desires to transmit a frame to a shared channel, it can only do so at the start of the slot and with a single frame. If the station is unable to send data at the start of the slot, it must wait until the beginning of the slot for the next transmission. However, there is still a possibility of a collision when broadcasting a frame at the start of two or more station time periods.The picture depicts the channel’s time divide. Only at the start of the slot can a station begin its broadcast. The only scenario that could lead to a crash is when two or more sites begin transmitting at the same time. Frame 1.2 of Station 1 and Frame 4.1 of Station 4 both show this situation.