Aloha protocol
|
|
The Aloha Protocol is a layer 2 (Layer 2 is the Data Link layer of the OSI model) protocol for LAN networks with broadcast topology. It was used for the first time in the Packet Radio System of the University of Hawaii in 1970. It is a predecessor to the Ethernet. There are two versions of Aloha, Pure Aloha and Slotted Aloha.
Pure Aloha
In Pure Aloha, any station can begin to send at any time, without listening to the medium first. Thus many collisions occur, and throughput decreases when too many stations are attached to the network.
If the packet transmission time is T, then vulnerable period of pure aloha is 2T.
Maximum throughput is 18% of physical channel capacity
Slotted Aloha
An improvement to the original Aloha protocol was Slotted Aloha, which introduced discrete timeslots. A station can not send anytime, but just at the beginning of a timeslot, and thus collisions are reduced.
If the packet transmission time is T, then vulnerable period of slotted aloha is T.
Because Listen before send, as used in the Ethernet, works a lot better than Aloha, Slotted Aloha is now only used on low band width tactical SATCOM networks by the US Military.