Background: ALOHA Protocol

The story of our first MAC starts out in Hawaii in the early $1970$s$.$ Some areas did not have

working telephone system. This did not make life more pleasant for researchers and engineers

in University of Hawaii who were trying to connect users on remote islands to the main

computer in Honolulu, see Figure $1.$ Stringing their own cables under the Pacific Ocean was

not in the cards, so they looked for a different solution and decided to use wireless

communication. And thus, ALOHA protocol was born. Remember from the lecture that there

are two types of ALOHA: pure and slotted. They differ with respect to whether time is

continuous, as in the pure version. Or divided into discrete slots into which all frames must fit,

as in slotted ALOHA.

The basic idea of pure ALOHA system is simple: let users transmit whenever they have data

to be sent. There will be collisions, of course, and the colliding frames will be damaged.

Senders need some way to find out if this is the case. In the ALOHA system, after each station

has sent its frame to the central computer, this computer rebroadcasts the frame to all of the

stations. A sending station can thus listen for the broadcast from the hub to see if its frame has

gotten through. If the frame was destroyed, the sender just waits a random amount of time and

sends it again.