Computer Network Unit 1 q what are the topologies in computer n/w ?

Q 5. Explain puere ALOHA and Slotted ALOHA.
Ans:
ALOHA is a medium access protocol that was originally designed for ground based radio broadcasting however it is applicable to any system in which uncoordinated users are competing for the use of a shared channel. Pure ALOHA and slotted ALOHA are the two versions of ALOHA.

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.

Advantages:_·_Superior_to_fixed_assignment_when_there_is_a_large_number_of_bursty_stations._·_Adapts_to_varying_number_of_stations._Disadvantages'>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


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.

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.