• It was designed for a radio (wireless) LAN, but it can be used on any shared medium.

• It is obvious that there are potential collisions in this arrangement. The medium I shared between the stations. When a station sends data, another station may attempt to do so at the same time. The data from the two stations collide and become garbled.

Pure ALOHA

Figure shows an example of frame collisions in pure ALOHA.

• There are four stations (unrealistic assumption) that contend with one another for access to the shared channel.

• The figure shows that each station sends two frames; there are a total of eight frames on the shared medium. Some of these frames collide because multiple frames are in contention for the shared channel.

• Figure shows that only two frames survive: frame 1.1 from station 1 and frame 3.2 from station 3. We need to mention that even if one bit of a frame coexists on the channel with one bit from another frame, there is a collision and both will be destroyed.

• It is obvious that we need to resend the frames that have been destroyed during transmission. The pure ALOHA protocol relies on acknowledgments from the receiver.

• When a station sends a frame, it expects the receiver to send an acknowledgment. If the acknowledgment does not arrive after a time-out period, the station assumes that the frame (or the acknowledgment) has been destroyed and resends the frame.

• A collision involves two or more stations. If all these stations try to resend their frames after the time-out, the frames will collide again. Pure ALOHA dictates that

• when the time-out period passes, each station waits a random amount of time before resending its frame.

• The randomness will help avoid more collisions. We call this time the back-off time TB.

• Pure ALOHA has a second method to prevent congesting the channel with retransmitted frames. After a maximum number of retransmission attempts Kmax' a station must give up and try later

ALOHA

It is an earliest random access method.

It is the multiple access protocol developed in 1970.

It was developed for wireless LAN and can be used for any shared media.

Following image shows the flow chart of ALOHA

There are 2 types of ALOHA

1. Pure Aloha

2. Slotted Aloha

Pure ALOHA

Developed in 1970.

Pure aloha allows the multiple stations to send the signal at any time.

Whenever data is available the station can transmits the signals.

In this frame sent by each station is of fixed length.

In pure aloha the station sends frames the it expects the acknowledgement from the receiver.

If acknowledgement doesnot receive the station assumes that the frames are destroyed.

If In pure Aloha multipe station sends signals(frames) simultaneously then collision occurs then frames are destroyed. If frames are destroyed due to collisions then the stations has to wait for random amount of time and after this they can resend the frames.

In pure aloha the chances of collisions are very high.

Slotted Aloha

Developed in 1972.

It was invented to improve efficiency of pure aloha that is to reduce collisions.

In this the time of shared station is divided in discreet time intervals called slots.

Each station is allowed to send frames within the specified slot only.

Each station has to send the frames at the beginning of the slots and only one frame is placed in one slot.

If the station is failed to place the frame at the beginning then station has to wait for the beginning of next slot.

Two station can collides if they try to place the frames at the beginning of the same slot then collisions are occurred.

The slotted aloha reduces the collisions to one-half of pure aloha.