ALOHA

RANDOM-ACCESS METHODS

The random-access methods we study in this chapter have evolved from a very interesting protocol known as ALOHA, which used a very simple procedure called Multiple Access (MA). The method was improved with the addition of a procedure that forces the station to sense the medium before transmitting. This was called carrier sense multiple access (CSMA). This method later evolved into two parallel methods: carrier sense multiple access with collision detection (CSMA/CD), which tells the station what to do when a collision is detected, and carrier sense multiple access with collision avoidance (CSMA/CA), which tries to avoid the collision.

ALOHA

ALOHA, the earliest random access method, was developed at the University of Hawaii in early 1970. It was designed for a radio (wireless) LAN, but it can be used on any shared medium. It is obvious that there are potential collisions in this arrangement. The medium is shared between the stations. When a station sends data, another station may attempt to do so at the same time. The data from the two stations collide and become garbled.

Pure ALOHA

The original ALOHA protocol is called pure ALOHA. This is a simple but elegant protocol. The idea is that each station sends a frame whenever it has a frame to send (multiple access). However, since there is only one channel to share, there is the possibility of collision between frames from different stations. Below figure shows an example of frame collisions in pure ALOHA.

The pure ALOHA protocol relies on acknowledgments from the receiver. When a station sends a frame, it expects the receiver to send an acknowledgment. If the acknowledgment does not arrive after a time-out period, the station assumes that the frame (or the acknowledgment) has been destroyed and resends the frame.

Vulnerable time

Let us find the vulnerable time, the length of time in which there is a possibility of collision. We assume that the stations send fixed-length frames with each frame taking Tfr seconds to send. Below figure shows the vulnerable time for station B.

The vulnerable time during which a collision may occur in pure ALOHA is 2 times the frame transmission time.

Slotted ALOHA

Slotted ALOHA was invented to improve the efficiency of pure ALOHA. In slotted ALOHA we divide the time into slots of T_fr seconds and force the station to send only at the beginning of the time slot. Below figure shows an example of frame collisions in slotted ALOHA.

A station is allowed to send only at the beginning of the synchronized time slot, if a station misses this moment, it must wait until the beginning of the next time slot. This means that the station which started at the beginning of this slot has already finished sending its frame. Of course, there is still the possibility of collision if two stations try to send at the beginning of the same time slot.

Vulnerable Time for Slotted ALOHA

The vulnerable time is now reduced to one-half, equal to T_fr. Below figure shows the situation.