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Home > Protocol >  Aloha Protocol > Pure Aloha Protocol
 

  Aloha Protocol

 
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Pure Aloha Protocol
 
Pure Aloha Protocol

With Pure Aloha, stations are allowed access to the channel whenever they have data to transmit. Because the threat of data collision exists, each station must either monitor its transmission on the rebroadcast or await an acknowledgment from the destination station. By comparing the transmitted packet with the received packet or by the lack of an acknowledgement, the transmitting station can determine the success of the transmitted packet. If the transmission was unsuccessful it is resent after a random amount of time to reduce the probability of re-collision.

Figure Pure Aloha Protocol

Advantages:

· Superior to fixed assignment when there is a large number of bursty stations.

· Adapts to varying number of stations.

Disadvantages:

· Theoretically proven throughput maximum of 18.4%.

· Requires queueing buffers for retransmission of packets.

Comparison

Slotted Aloha

The first of the contention based protocols we evaluate is the Slotted Aloha protocol. The channel bandwidth is a continuous stream of slots whose length is the time necessary to transmit one packet. A station with a packet to send will transmit on the next available slot boundary. In the event of a collision, each station involved in the collision retransmits at some random time in order to reduce the possibility of recollision. Obviously the limits imposed which govern the random retransmission of the packet will have an effect on the delay associated with successful packet delivery. If the limit is too short, the probability of recollision is high. If the limit is too long the probability of recollision lessens but there is unnecessary delay in the retransmission. For the Mars regional network studied here, the resending of the packet will occur at some random time not greater than the burst factor times the propagation delay.

Another important simulation characteristic of the Slotted Aloha protocol is the action which takes place on transmission of the packet. Methods include blocking (i.e. prohibiting packet generation) until verification of successful transmission occurs. This is known as "stop-and-wait". Another method known as "go-back-n" allows continual transmission of queued packets, but on the detection of a collision, will retransmit all packets from the point of the collision. This is done to preserve the order of the packets. In this simulation model queued packets are continually sent and only the packets involved in a collision are retransmitted. This is called "selective-repeat" and allows out of order transmission of packets.

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Slotted Aloha Protocol

By making a small restriction in the transmission freedom of the individual stations, the throughput of the Aloha protocol can be doubled. Assuming constant length packets, transmission time is broken into slots equivalent to the transmission time of a single packet. Stations are only allowed to transmit at slot boundaries. When packets collide they will overlap completely instead of partially. This has the effect of doubling the efficiency of the Aloha protocol and has come to be known as Slotted Aloha.

Figure 11: Slotted Aloha Protocol

Advantages:

· Doubles the efficiency of Aloha.
· Adaptable to a changing station population.

Disadvantages:

· Theoretically proven throughput maximum of 36.8%.
· Requires queueing buffers for retransmission of packets.

Synchronization required.

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