Carrier-sense multiple access with collision avoidance


Carrier-sense multiple access with collision avoidance in computer networking, is a link layer multiple access method in which carrier sensing is used. Under CSMA/CA, nodes attempt to avoid collisions by beginning transmission only after the channel is sensed to have no traffic. When they do transmit, nodes transmit frames in their entirety.
This technique is primarily used in wireless networks, where the alternative with collision detection CSMA/CD is not possible due to wireless transmitters that cannot effectively detect other transmissions while they themselves are transmitting.
CSMA/CA is unreliable due to the node problem.

Details

Collision avoidance is used to improve the performance of the CSMA method by attempting to divide the channel somewhat equally among all transmitting nodes within the collision domain.
  1. Carrier Sense: prior to transmitting, a node first listens to the shared medium to determine whether another node is transmitting or not. Note that the hidden node problem means another node may be transmitting which goes undetected at this stage.
  2. Collision Avoidance: if another node was heard, we wait for a period of time for the node to stop transmitting before listening again for a free communications channel.
Although CSMA/CA has been used in a variety of wired communication systems, it is particularly beneficial in a wireless LAN due to a common problem of multiple stations being able to see the Access Point, but not each other. This is due to differences in transmit power, and receive sensitivity, as well as distance, and location with respect to the AP. This will cause a station to not be able to hear another station's broadcast. This is the so-called 'hidden node', or 'hidden station' problem. Devices utilizing 802.11 based standards can enjoy the benefits of collision avoidance, although they do not do so by default. By default, they use a Carrier sensing mechanism called exponential backoff, that relies upon a station attempting to 'listen' for another station's broadcast before sending. CA, or PCF relies upon the AP granting a station the exclusive right to transmit for a given period of time after requesting it.
CSMA-CA requires a determination of whether a channel is 'idle', even when incompatible standards and overlapping transmission frequencies are used. Per the standards, for 802.11/Wi-Fi transmitters on the same channel, transmitters must take turns to transmit if they can detect each other even 3 dB above the noise floor. On the other hand, transmitters will ignore transmitters with incompatible standards or on overlapping channels if the received signal strength from them is below a threshold Pth which, for non Wi-Fi 6 systems, is between -76 and -80 dBm.

IEEE 802.11 RTS/CTS Exchange

CSMA/CA can optionally be supplemented by the exchange of a Request to Send packet sent by the sender S, and a Clear to Send packet sent by the intended receiver R. This alerts all nodes within range of the sender, receiver or both, to not transmit for the duration of the main transmission. This is known as the IEEE 802.11 RTS/CTS exchange. Implementation of RTS/CTS helps to partially solve the hidden node problem that is often found in wireless networking.

Performance

CSMA/CA performance is based largely upon the modulation technique used to transmit the data between nodes. Studies show that under ideal propagation conditions, direct-sequence spread spectrum provides the highest throughput for all nodes on a network when used in conjunction with CSMA/CA and the IEEE 802.11 RTS/CTS exchange under light network load conditions. Frequency hopping spread spectrum follows distantly behind DSSS with regard to throughput, with a greater throughput once network load becomes substantially heavy. However, the throughput is generally the same under real-world conditions due to radio propagation factors.

Usage