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

Revision as of 15:05, 9 February 2024

Revision as of 01:49, 19 April 2025

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::* '''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.<ref>{{cite web|title=How does Carrier Sensing and Interference impact Wi-Fi performance?|url=https://tetcos.com/pdf/WiFi-802.11g-2AP-2STA_CS-blocking-and-interference_NetSim-white-paper.pdf|access-date=15 March 2023}}</ref> 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>