ALOHA AND SLOTTED ALOHA

Multiple access protocols can be subdivided further as –"

1. Random Access Protocol:

In this, all stations have same superiority that is no station has more priority than another station. Any station can send data depending on medium’s state( idle or busy). It has two features:

1. There is no fixed time for sending data
2. There is no fixed sequence of stations sending data

The Random access protocols are further subdivided as:
(a) ALOHA – It was designed for wireless LAN but is also applicable for shared medium. In this, multiple stations can transmit data at the same time and can hence lead to collision and data being garbled.

• Pure Aloha:
  When a station sends data it waits for an acknowledgement. If the acknowledgement doesn’t come within the allotted time then the station waits for a random amount of time called back-off time (Tb) and re-sends the data. Since different stations wait for different amount of time, the probability of further collision decreases.
• Slotted Aloha:
  It is similar to pure aloha, except that we divide time into slots and sending of data is allowed only at the beginning of these slots. If a station misses out the allowed time, it must wait for the next slot. This reduces the probability of collision.