Carrier sense multiple access with collision avoidance
Carrier sense multiple access with collision avoidance (CSMA/CA) is the term for the several methods used in WLANs to avoid collisions, which uses distributed control function (DCF), the mandatory access method in IEEE 802.11 communications.
In wireless communication, medium contention is somewhat more difficult to achieve. Wireless is an unbounded medium; that is, it is not constrained to a physical conduit such as a fiber or a copper wire. This makes detecting a collision impossible. Several wireless stations can be transmitting simultaneously and the access point (AP) will see nothing but radio frequency (RF) noise in a broad spectrum. No effective communication will occur because no single station can successfully acquire the medium and associate with, let alone find, the AP. A method was needed to allow a mobile client, or any other station, to acquire exclusive rights to the medium in order to make wireless networking possible and cost-effective. This has developed into a complicated choreography that will be discussed below.
Carrier sense multiple access with collision avoidance (CSMA/CA) is the term for several methods used in WLANs to avoid collisions. CSMA/CA is not a perfect system, but it works in most cases. The fundamental realization of CSMA/CA is distributed control function (DCF), which is the mandatory access method in IEEE 802.11 communications. DCF utilizes four key features to ensure that any station can acquire the wireless medium and effectively communicate: physical carrier sense (also called clear channel assessment, or CCA), virtual carrier sense, random back-off timers, and interframe spacing (IFS). Two other access methods, point coordination function (PCF) and hybrid coordination function (HCF), are also used.
A wireless station wishing to transmit data will first monitor the channel to see if it is idle. All mobile stations can hear the traffic of all other mobile stations. This is a crucial feature that is exploited to allow effective communication. All stations associated with a given base station subsystem (BSS) will be monitoring the medium if they're not transmitting. If the medium is not idle, the station will not attempt to transmit; this is true for all other wireless stations on the network.
Stations detect the frames transmitted by the station that has successfully acquired the medium; within this frame is an ID/duration field containing the value representing how long the medium will be used by the station until another station can contend for the medium (ranging from 0-32,768 µs). This is called virtual carrier sense, and the value is read into all listening stations' network allocation vector (NAV), which waits for this time period to expire before attempting transmission.
The NAV notifies all listening stations of the amount of time the medium will be busy and includes any interframe spacing and message acknowledgment. Each unicast frame must be acknowledged in order to indicate it has been received intact. The obverse is also true-a wireless station will acknowledge each frame sent by the AP. If an acknowledgment is not received, the frame is assumed to be lost and is subsequently retransmitted. It should be noted that only unicast frames require acknowledgment; multicast and broadcast frames do not require acknowledgment. If the medium is idle at the end of the time period, the station attempts to transmit. If successful in acquiring the medium, the AP will begin transmitting data.
In addition to the CCA and NAV, another protocol is used to avoid transmissions. Once the channel becomes idle as indicated by the above methods, the station will attempt to transmit immediately; unfortunately, so will every other station with the same NAV value counted down to zero. One method is IFS, which is the spacing between frames, acknowledgments, and the differential times when the medium is open for contention. Essentially, it is the time between wireless frame transmissions. The type of IFS determines what type of IEEE 802.11 traffic follows it. For instance, only acknowledgment, data, and clear to send (CTS) frames are permitted to follow a short interframe spacing (SIFS). There are several types of IFS, here listed from the shortest to the longest:
- Reduced interframe spacing (RIFS), highest priority
- Short interframe spacing (SIFS)
- PCF interframe spacing (PIFS), not used
- DCF interframe spacing (DIFS), lowest priority
- Arbitration interframe spacing (AIS), which is used by quality of service (QoS) traffic
- Extended interframe spacing (EIFS), which is used with retransmissions.
All IFS durations are based on the SIFS duration. Depending on the type of WLAN being used, the SIFS will be either 10µs or 16 µs. For 802.11b/g/n in the 2.4Ghz band, the duration is 10µs. For 802.11a/n in the 5 Ghz band, the duration is 16 µs. The RIFS is used only with IEEE 802.11 multiple-input and multiple-output (MIMO) and is used to precede data frames; the RIFS duration is 2 µs. DIFS is used to train stations to wait for higher priority traffic, which is preceded by a SIFS, to be transmitted. The DIFS is relatively long, being the applicable SIFS plus two 9µs "slot times." Stations that do not conform to QoS standards requiring the use of enhanced distributed channel access (EDCA) or HCF controlled channel access (HCCA) use DIFS.
The EIFS is the longest duration IFS because it is designed to allow retransmission of a corrupted or otherwise lost frames. Each unicast frame must be acknowledged or it is presumed lost. However, the EIFS allows sufficient time to determine if the frame is lost and subsequent retransmission. When the AP and associated stations detect a corrupt frame, they will wait an EIFS to allow retransmission. An EIFS equals a SIFS plus the applicable DIFS plus the time for an acknowledgment frame to be transmitted. For 802.11b/g/n in the 2.4Ghz band, an EIFS is 364µs; for 802.11a/n in the 5Ghz band, it is 160µs.