1-persistent CSMA is an aggressive version of Carrier Sense Multiple Access (CMSA) protocol that operates in the Medium Access Control (MAC) layer. Using CMSA protocols, more than one users or nodes send and receive data through a shared medium that may be a single cable or optical fiber connecting multiple nodes, or a portion of the wireless spectrum.
In 1-persistent CSMA, when a transmitting station has a frame to send and it senses a busy channel, it waits for the end of the transmission, and transmits immediately. Since, it sends with a probability 1, the name 1 – persistent CSMA is given.
It is used in CSMA/CD (Carrier Sense Multiple Access with Collision Detection) systems including Ethernet.
The algorithm of 1-persistent CMSA is:
When a frame is ready, the transmitting station checks whether the channel is idle or busy.
If the channel is busy, the station waits and continually checks until the channel becomes idle.
If the channel is idle then it transmits the frame immediately, with a probability 1.
A collision may occur if two or more channels transmit simultaneously. If collision occurs, the station waits for a random period of time and restarts the algorithm all over again.
It has better throughput than ALOHA protocols.
There are chances of collisions in the following situations:
Situation 1: Suppose that a station A has transmitted a frame, which has not yet reached another station B due to propagation delay. Station B assumes that the channel is idle and transmits its frame. Thus a collision occurs.
Situation 2: Suppose that a station A is transmitting while stations B and C are waiting for the transmission to complete. At the instance station A completes transmission, both stations B and C start transmitting simultaneously at the same time. This results is collision.
The throughput of a network system is defined as the number of successful transmissions per frame time. The following graph compares the throughput of 1-persistent CMSA with that of other MAC protocols: