Computer Networking

Transport Layer

Carrier Sense Multiple Access (CSMA)

Basic principle of CSMA,

“Sense the medium before to transmit” or “Listen before talk”. Through this Principle, the chance of collision reduced and performance becomes high.

Important: Sensing the medium does not mean to sense entire medium. It simply means to sense the medium at just transmission point.

When multiple channels are connected through the common medium and want to communicate with others then the chances of collisions are very high. To reduce this collision CSMA comes into picture.

According to CSMA, every channel senses the medium (busy or idle) before transmission. If the medium is idle then station can send data to the medium. Otherwise, it must wait until the medium becomes idle. So, it reduces the chances of a collision.

Collision in CSMA

The possibility of collision still exists because of propagation delay. The case when a station X has transmitted a frame, which has not yet reached another station Y due to propagation delay. Then Station Y assumes that the channel is idle and transmits its frame. Thus a collision occurs.

When two frames moves on the same channel but opposite direction then they may collide. The following figure shows all.

After collision, CSMA use Acknowledgement signal for retransmission of collided frame.

Note: Higher the propagation delays lowest the performance.

CSMA Modes

There are four major modes of CSMA as

1. 1-Persistent

After sensing the medium in 1-persistent, if the channel is idle then station starts transmission immediately. Otherwise, it senses the transmission medium continuously until it becomes idle.

Wait for transmission is minimum because of continuously sensing the medium. So, probability of frame transmissions is always 1.

2. Non-Persistent

After sensing the medium in non-persistent, if the channel is idle then station starts transmission immediately. Otherwise, it does not senses medium continuously. It senses the medium for random period of time.

Transmission of data in non-persistent, having longer delay than 1-persistan.

3. P-Persistent

 P-persistent is applied in slotted channels. It is the combination of 1-Persistent and Non-persistent modes. It transmission probability (p) is in-between 1 and 0.

In P-persistent transmission only occur when it satisfy the following conditions

  • Medium is idle
  • And beginning of slot is also available for sensing station.

If above both conditions satisfy then probability of frame transmission is P. Otherwise it waits for a (q = 1-p probability) random time for its retransmission in next slot.

Note: Generally Wi-Fi use P-persistent

Descriptive Diagram of 1-persistent, Non-persistent and P-persistent are given below

4. Zero- Persistent

  • Each station in 0-persistent is assigned a transmission order (superiority).
  • When the medium is idle then stations transmit data according to their order numbers.

Suppose Station A, B and C are assigned number 1, 2 and 3 respectively. The time when the transmission medium becomes idle then station A starts transmission First. The station C has to wait for 2-time slots after the medium becomes idle.

Comparison of Throughputs

The throughput is the number of successful transmissions per frame time. The following diagram compares the throughput of various medium access protocols



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