Pure ALOHA and Slotted ALOHA

In a computer network, there is a need for access to the common shared medium.  Aloha protocol is for a medium access control mechanism, for shared channel or medium. For example, LAN connects multiple computers over a shared physical medium, which in LAN wires, hubs, switches, routers etc.  When a packet needs to send to another computer over the LAN, the sender broadcasts the packet.  If there are simultaneous broadcasts from other computers then there may be a collision and the packet will be garbled.  Note that the collision problem is in only broadcast networks.  Aloha protocol provides a mechanism for communication over a common shared medium to transmit a packet reliably.

In other networks like peer to peer, no need for Aloha protocol, as there is no sharing of a medium, so no collision. In the following section, we will describe each type of aloha protocol in detail.

What is pure aloha and slotted aloha?

The first version of the aloha protocol is named Pure Aloha. After that, a more efficient version was developed which named as slotted aloha. Both pure aloha and slotted aloha works for a shared broadcast network, as LAN or WiFi LAN.

What is Pure Aloha?

As the name suggests pure aloha is the original version of the aloha protocol.  Following is the procedure in pure aloha for communication.

• When a network station needs to send the frame, it sends immediately and waits for the acknowledgment.
• If the sender receives an acknowledgment, the sender may send the next frame.
• If no acknowledgment sender assumes the frame has been garbled and retransmit the same frame after a random time to avoid collision again.

What is slotted Aloha?

The next version of aloha, more efficient. The following describes the protocol procedure for slotted aloha protocol.

• In slotted aloha, time is divided into slots. A sender may send a frame at the start of the time slot.
• If the sender does not send at the start then it will wait for the next slot.
• If two sender sends the frames at the beginning of the time slot, there will be a collision and the frame will be garbled.
• If the acknowledgment is not received, the frame is retransmitted in a new slot.