§ The Aloha protocol was designed as part of a project at the University of Hawaii.
§ It provided data transmission between computers on several of the Hawaiian Islands using radio transmissions.
§ Communications was typically between remote stations and a central sited named Menehune or vice versa.
§ All messages to the Menehune were sent using the same frequency.
§ When it received a message intact, the Menehune would broadcast an ack on a distinct outgoing frequency.
§ The outgoing frequency was also used for messages from the central site to remote computers.
§ All stations listened for message on this second frequency.
§ Pure Aloha is an unspotted, fully-decentralized protocol.
§ It is extremely simple and trivial to implement.
§ The ground rule is -“when you want to talk, just talk!”.
§ So, a node which wants to transmit will go ahead and send the packet on its broadcast channel, with no consideration whatsoever as to anybody else is transmitting or not.
§ One serious drawback here is that, you don’t know whether what you are sending has been received properly or not (so as to say, “whether you’ve been heard and understood?”).
§ To resolve this, in Pure Aloha, when one node finishes speaking, it expects an acknowledgement in a finite amount of time – otherwise it simply retransmits the data.
§ This scheme works well in small networks where the load is not high.
§ But in large, load intensive networks where many nodes may want to transmit at the same time, this scheme fails miserably. This led to the development of Slotted Aloha.
§ This is quite similar to Pure Aloha, differing only in the way transmissions take place.
§ Instead of transmitting right at demand time, the sender waits for some time.
§ This delay is specified as follows – the timeline is divided into equal slots and then it is required that transmission should take place only at slot boundaries.
§ To be more precise, the slotted-Aloha makes the following assumptions: All frames consist of exactly L bits.
§ Time is divided into slots of size L/R seconds (i.e., a slot equals the time to transmit one frame).
§ Nodes start to transmit frames only at the beginnings of slots.
§ The nodes are synchronized so that each node knows when the slots begin.
§ If two or more frames collide in a slot, then all the nodes detect the collision event before the slot ends.
§ In this way, the number of collisions that can possibly take place is reduced by a huge margin. And hence, the performance become much better compared to Pure Aloha.
Flow Chart of Aloha:
§ A station which has a frame ready will send it.
§ Then it waits for some time.
§ If it receives the acknowledgement then the transmission is successful.
§ Otherwise the station uses a back off strategy, and sends the packet again.
§ After many times if there is no acknowledgement then the station aborts the idea of transmission.