Rate Anomaly in CSMA/CA

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is a network protocol for carrier transmission that operates in the Medium Access Control (MAC) layer. CSMA/CA prevents collisions prior to their occurrence and is ideally used in wireless communications.

Rate anomaly occurs when the performance of a high speed station is impaired due to a low speed station, thus reducing the average throughput of the entire wireless network.

Explanation

The CSMA/CA allows each station to send a single frame at a time. Before sending a frame, the station waits for a certain inter-frame spacing (IFS). Following the IFS, it sends the request to send (RTS), clear to send (CTS), data and acknowledgement (ACK) as the situation may be. This sending of one frame at a time works fine if all the stations have equal or close transmission rates.

However, when the network allows a wide range of transmission rates of the stations as in IEEE 802.11 a/g, the disparity reduces the overall performance. This is called rate anomaly. This can be explained with the aid of the following example −

Suppose that there are two stations in the wireless network, X having rate of 6 Mbps and Y have a rate of 48 Mbps. Since, both of them are allowed to send one frame at a time, station X takes eight times more time than station Y. This reduces the speed of station Y to roughly that of station X. When the stations are transmitting individually, station X transmits at 6 Mbps and station Y transmits at 48 Mbps. However, the moment they are competing with each other, the average is reduced to nearly 6 Mbps.