What are the various Channel Control Protocols ? What is Aloha and Slotted Aloha ? What is CSMA/CD ?
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The Aloha protocol(developed in Hawaii) is an OSI layer 2 protocol for LAN networks with broadcast topology.
The first version of the protocol was basic:
* If you have data to send, send the data
* If the message collides with another transmission, try resending later
Pure Aloha had a maximum throughput of about 18.4%. This means that about 81.6% of the total available bandwidth was essentially wasted due to losses from packet collisions.
An improvement to the original Aloha protocol was Slotted Aloha, which introduced discrete timeslots and increased the maximum throughput to 36.8%. A station can send only at the beginning of a timeslot, and thus collisions are reduced.
CSMA(Carrier sense multiple access) - In pure CSMA, only the carrier sense is used to avoid collisions. If two nodes try to send a frame at nearly the same time, neither detects a carrier so both begin transmitting. The transmitters do not detect collisions, so transmit the entire frame (thus wasting the bandwidth used).
In Carrier Sense Multiple Access With Collision Avoidance (CSMA/CA) - each node must inform other nodes of an intent to transmit. When the other nodes have been notified, the information is transmitted. This arrangement prevents collision because all nodes are aware of a transmission before it occurs. However, collisions are still possible, and are not detected so have the same consequences as in pure CSMA.
In Carrier Sense Multiple Access With Collision Detection (CSMA/CD), sending nodes are able to detect when a collision occurs and stop transmitting immediately, backing off for a random amount of time before trying again. This results in much more efficient use of the media since the bandwidth of transmitting the entire frame is not wasted. It is used in Ethernet as it’s the most efficient among all.
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