Carrier-sense multiple access with collision avoidance
Carrier-sense multiple access with collision avoidance (CSMA/CA) in
It is particularly important for wireless networks, where the alternative with collision detection CSMA/CD, is not possible due to wireless transmitters desensing (turning off) their receivers during packet transmission.
CSMA/CA is unreliable due to the hidden node problem.[3][4]
CSMA/CA is a protocol that operates in the data link layer.
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.
- Carrier Sense: prior to transmitting, a node first listens to the shared medium (such as listening for wireless signals in a wireless network) 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.
- Collision Avoidance: if another node was heard, we wait for a period of time (usually random) for the node to stop transmitting before listening again for a free communications channel.
- Request to Send/Clear to Send (RTS/CTS) may optionally be used at this point to mediate access to the shared medium. This goes some way to alleviating the problem of hidden nodes because, for instance, in a wireless network, the Access Point only issues a Clear to Send to one node at a time. However, wireless 802.11implementations do not typically implement RTS/CTS for all transmissions; they may turn it off completely, or at least not use it for small packets (the overhead of RTS, CTS and transmission is too great for small data transfers).
- Transmission: if the medium was identified as being clear or the node received a CTS to explicitly indicate it can send, it sends the frame in its entirety. Unlike binary exponential backoffprior to attempting to re-transmit.
- Request to Send/Clear to Send (RTS/CTS) may optionally be used at this point to mediate access to the shared medium. This goes some way to alleviating the problem of hidden nodes because, for instance, in a wireless network, the Access Point only issues a Clear to Send to one node at a time. However, wireless
Although CSMA/CA has been used in a variety of wired communication systems, it is particularly beneficial in a
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 (the thermal noise floor is around -101 dBm for 20 MHz channels).[8] 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.[9]
IEEE 802.11 RTS/CTS Exchange
CSMA/CA can optionally be supplemented by the exchange of a Request to Send (RTS) packet sent by the sender S, and a Clear to Send (CTS) packet sent by the intended receiver R. Thus alerting 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.[10][11]
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 (simulations),
Usage
- GNET – an early proprietary LANprotocol
- Apple's LocalTalkimplemented CSMA/CA on an electrical bus using a three-byte jamming signal.
- 802.11mainly relies on physical carrier sensing though).
- IEEE 802.15.4 (Wireless PAN) uses CSMA/CA
- NCR WaveLAN – an early proprietary wireless network protocol
- HomePNA
- The CSMA/CARPvariant.
See also
- Carrier-sense multiple access
- Carrier-sense multiple access with collision detection
- IEEE 802.11 RTS/CTS
- Network allocation vector
References
- ^ "Federal Standard 1037C". Its.bldrdoc.gov. Retrieved 2012-09-09.
- ^ "American National Standard T1.523-2001, Telecom Glossary 2000". Atis.org. Archived from the original on 2008-03-02. Retrieved 2012-09-09.
- ^ "Study of different CSMA/CA IEEE 802.11-based implementations, Universitat Politècnica de Catalunya" (PDF). Archived from the original (PDF) on 2012-03-06. Retrieved 2012-09-09.
- ^ ].
- ^ Kaixin Xu; Mario Gerla; Sang Bae. "How Effective is the IEEE 802.11 RTS/CTS Handshake in Ad Hoc Networks?" (PDF). UCLA. Retrieved 28 September 2012.
- ^ "How does Carrier Sensing and Interference impact Wi-Fi performance?" (PDF). Retrieved 15 March 2023.
- ^ Park, Kihong. "Wireless Lecture Notes" (PDF). Purdue. Retrieved 28 September 2012.
- ^ "Channel Bonding in WiFi and Radio Frequency Physics | Network Computing".
- ^ Effect of adjacent-channel interference in IEEE 802.11 WLANs - Eduard Garcia Villegas; Elena Lopez-Aguilera; Rafael Vidal; Josep Paradells (2007)
- ISBN 0-13-504583-5.
- ^ "MIT Lecture - Communication Systems Engineering. Dr. Eytan Modiano" (PDF). Archived (PDF) from the original on 2010-06-13. Retrieved 2012-09-09.
- Computer Networks: a Systems Approach. Peterson & Davie. ISBN 978-0-12-385138-3. pp128–139