ALOHA, the earliest random access protocol and was developed at the University of Hawi in early 1970. which is designed for a wireless LAN but it can be used in any shared medium. There is a potential collision because of the shared medium.
There are two types of ALOHA’s
- Pure ALOHA.
- Slotted ALOHA.
- Applicable to any shared transmission medium.
- When a station has a frame it sends at any time
- The station then listens for an amount of Time equal to the Maximum possible round-trip propagation delay on the network plus small fixed time in requirement.
- It receives ACK.
- If not re-transmit it.
- If no ACK after repeated transmission gives up.
- Slotted ALOHA was invented to improve the efficiency of pure aloha.
- Divide the time into slots of the frame time & force the station to send only at the beginning of the time slot.
Pure ALOHA vs. slotted ALOHA
- Pure ALOHA does not require time synchronization but slotted aloha does.
- Maximum channel utilization in pure aloha is 18% whereas 37% in case of slotted aloha.