Carrier-sense multiple access with collision avoidance: Difference between revisions

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{{Short description|Computer network multiple access method}}

In [[computer networking]], CSMA-CA belongs to a class of protocols called as CSMA-CA.

{{Use American English|date = March 2019}}

CSMA CA stands for: '''Carrier Sense Multiple Access With Collision Avoidance'''.

'''Carrier-sense multiple access with collision avoidance''' ('''CSMA/CA''') in [[computer network]]ing, is a [[link layer]] [[multiple access method]] in which [[Carrier signal|carrier]] sensing is used. Under CSMA/CA, [[Node (networking)|node]]s attempt to avoid collisions by beginning transmission only after the channel is sensed to have no traffic.<ref name="federalstandard">{{cite web|url=http://www.its.bldrdoc.gov/fs-1037/fs-1037c.htm |title=Federal Standard 1037C |publisher=Its.bldrdoc.gov |access-date=2012-09-09}}</ref><ref name="atis">{{cite web |url=http://www.atis.org/tg2k/ |title=American National Standard T1.523-2001, Telecom Glossary 2000 |publisher=Atis.org |access-date=2012-09-09 |url-status=dead |archive-url=https://web.archive.org/web/20080302071329/http://www.atis.org/tg2k/ |archive-date=2008-03-02 }}</ref> When they do transmit, nodes transmit frames in their entirety.

In CSMA-CA, a station wishing to transmit has to first listen to the channel for a predetermined amount of time so as to check for any activity on the channel. If the channel is sensed "idle" then the station is permitted to transmit. If the channel is sensed as "busy" the station has to defer its transmission. This is the essence of the "collision avoidance" part of the protocol.

This technique is primarily used in wireless networks, where the alternative with collision detection [[Carrier-sense multiple access with collision detection|CSMA/CD]] is not possible due to wireless transmitters de-sensing (turning off) their receivers during packet transmission.

==Detail==

CSMA/CA is a modification of pure [[Carrier Sense Multiple Access]] ('''CSMA'''). Collision avoidance is used to improve the performance of '''CSMA''' by attempting to be "less greedy" on the channel. If the channel is sensed busy before transmission then the transmision is deferred for a "random" interval. This reduces the probability of collisions on the channel.

CSMA/CA is unreliable due to the [[hidden node problem]].<ref>{{cite web |url=http://www.eunice-forum.org/eunice99/027.pdf |title=Study of different CSMA/CA IEEE 802.11-based implementations, Universitat Politècnica de Catalunya |access-date=2012-09-09 |archive-url=https://web.archive.org/web/20120306051958/http://www.eunice-forum.org/eunice99/027.pdf# |archive-date=2012-03-06 |url-status=dead }}</ref><ref name="eunice">{{cite arXiv|title=Comparative study of hidden node problem and solution using different techniques and protocols, Journal of Computing |date=2010 |author1=Viral V. Kapadia |author2=Sudarshan N. Patel |author3=Rutvij H. Jhaveri |class=cs.NI |eprint=1003.4070 }}</ref>

CSMA-CA is used where [[CSMA-CD]] cannot be implemented due to the nature of the channel. CSMA-CA is used in [[802.11]] based [[wireless LAN]]s. In a [[wireless LAN]], not all stations can "see" all other stations. Hence, collision detection is no longer an option.

[[Image:csma ca.svg|thumb|right|340px|Simplified algorithm of CSMA/CA]]

==Details==

Collision avoidance is used to improve the performance of the [[carrier-sense multiple access|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 [[802.11]] implementations 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 [[CSMA/CD]], it is very challenging for a wireless node to listen at the same time as it transmits (its transmission will dwarf any attempt to listen). Continuing the wireless example, the node awaits receipt of an acknowledgement packet from the Access Point to indicate the packet was received and checksummed correctly. If such acknowledgement does not arrive in a timely manner, it assumes the packet collided with some other transmission, causing the node to enter a period of [[binary exponential backoff]] prior to attempting to re-transmit.

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.<ref>{{cite web|title=How Effective is the IEEE 802.11 RTS/CTS Handshake in Ad Hoc Networks?|url=http://www.cs.purdue.edu/homes/park/cs536-wireless-3.pdf|publisher=UCLA|access-date=28 September 2012|author1=Kaixin Xu |author2=Mario Gerla |author3=Sang Bae }}</ref> 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 (RTS / CTS handshake, also [[Point coordination function]]), although they do not do so by default. By default they use a Carrier sensing mechanism called ''exponential backoff'' (or [[Distributed coordination function]]), that relies upon a station attempting to 'listen' for another station's broadcast before sending. CA, or PCF relies upon the AP (or the 'receiver' for Ad hoc networks) granting a station the exclusive right to transmit for a given period of time after requesting it (Request to Send / Clear to Send).<ref>{{cite web|last=Park|first=Kihong|title=Wireless Lecture Notes|url=http://www.cs.purdue.edu/homes/park/cs536-wireless-3.pdf|publisher=Purdue|access-date=28 September 2012}}</ref>

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&nbsp;dB above the [[noise floor]] (the thermal noise floor is around -101 dBm for 20&nbsp;MHz channels).<ref>{{Cite web|url=https://www.networkcomputing.com/wireless-infrastructure/channel-bonding-wifi-and-radio-frequency-physics|title=Channel Bonding in WiFi and Radio Frequency Physics &#124; Network Computing}}</ref> 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 P_th which, for non [[Wi-Fi 6]] systems, is between -76 and -80 dBm.<ref name="vilegas">Effect of adjacent-channel interference in IEEE 802.11 WLANs - Eduard Garcia Villegas; Elena Lopez-Aguilera; Rafael Vidal; Josep Paradells (2007)</ref>

==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.<ref name="rtscts">Comer, Douglas. (2009). Computer Networks and Internets. Upper Saddle River, NJ: Pearson Education Inc. {{ISBN|0-13-504583-5}}.</ref><ref name="MIT">{{cite web|url=http://ocw.mit.edu/courses/aeronautics-and-astronautics/16-36-communication-systems-engineering-spring-2009/lecture-notes/MIT16_36s09_lec21_22.pdf |archive-url=https://web.archive.org/web/20100613084531/http://ocw.mit.edu/courses/aeronautics-and-astronautics/16-36-communication-systems-engineering-spring-2009/lecture-notes/MIT16_36s09_lec21_22.pdf |archive-date=2010-06-13 |url-status=live |title=MIT Lecture - Communication Systems Engineering. Dr. Eytan Modiano |access-date=2012-09-09}}</ref>

==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), [[direct-sequence spread spectrum]] (DSSS) 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]] (FHSS) 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.<ref name=eunice/>

==Usage==

*[[ GNET]] – an early proprietary [[LAN]] protocol

* [[Apple Computer|Apple's]] [[LocalTalk]] implemented '''CSMA/CA''' on an electrical bus using a three-byte jamming signal.

* [[802.11 RTS/CTS]] implements '''virtual carrier sensing''' using short '''Requestrequest to Sendsend''' and '''Clearclear to Sendsend''' messages for [[Wireless LAN|WLAN]]s ([[802.11]] mainly 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 [[ITU-T]] [[G.hn]] standard, which provides a way to create a high-speed (up to 1 Gigabit/s) [[local area network]] using existing home wiring ([[Power line communication|power lines]], phone lines and [[Ethernet over coax|coaxial cables]]), uses CSMA/CA as a [[channel access method]] for flows that do not require guaranteed [[quality of service]], specifically the [[CSMA/CARP]] variant.

==See also==

* [[Carrier -sense multiple access with collision detection]] ('''CSMA/CD'''), (used for [[Ethernet]]) takes a different approach to the same problem.

* [[es:Carrier -sense multiple access with collision avoidancedetection]]

* [[IEEE 802.11 RTS/CTS]]

* [[Network allocation vector]]

==References==

{{reflist}}

* Computer Networks: a Systems Approach. Peterson & Davie. [[Morgan Kaufmann]], Burlington, MA, USA. {{ISBN|978-0-12-385138-3}}. pp128–139

==External links==

[[Category:{{Channel access methods]]}}

Source: derived in part from [[Federal Standard 1037C]]

CSMA CA stands for{{DEFAULTSORT: '''Carrier Sense Multiple Access With Collision Avoidance'''.}}

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