Pure ALOHA protocol

Pure ALOHA protocol is a random access protocol used for data transfer.

User accesses a channel as soon as a message is ready to be transmitted. After a transmission the user waits for an acknowledgement on either the same channel or separate feedback channel. In case of collision (i.e. when a NACK is received), the terminal waits for a random period of time and retransmits the message. As the number of users increase, a greater delay occurs because the probability of collision increases.

Slotted ALOHA protocol

In Slotted ALOHA protocol, time is divided into equal time slots of length greater than the packet duration (time taken to transmit a fixed length packet).

The subscribers (users), each have synchronized clocks and transmit a message only at the beginning of a new time slot, thus resulting in a discrete distribution of packets (each subscriber is assigned a time slot in which he can transmit.). (Note here: two users may get same time slot at peak times)

This prevents partial collisions (i.e. packets collide with portion of another). This implies that either a packet will collide completely or not at all.

As the number of users increase a greater delay will occur due to duplicate time slot assignments and hence greater number of complete collisions. Repeated transmission of lost packets would make things even slower.

But, the vulnerable period for slotted aloha is only one packet duration, since partial collisions are prevented through synchronization.

Overall, slotted ALOHA provides a maximum channel utilization of .368 Erlangs, double that of pure ALOHA.

Limitation of Aloha in general

ALOHA protocols do not listen to the channel before transmission, and therefore do not exploit information about other users. By listening to the channel before starting transmission, greater efficiencies may be achieved. This is where CSMA protocols come into the picture which I will cover in a separate post.