Virtual Carrier Sense, RTS/CTS, and the Network Allocation Vector (NAV)
In the 802.11 MAC protocol, a virtual carrier sense mechanism operates by observing the Duration field present in each MAC frame. This is accomplished by a station listening to traffic not destined for it. The Duration field is present in both RTS and CTS frames optionally exchanged prior to transmission, as well as conventional data frames, and provides an estimate of how long the medium will be busy carrying the frame.
Physical Carrier Sense (CCA)
Each 802.11 PHY specification (e.g., for different frequencies and radio technology) is required to provide a function for assessing whether the channel is clear based upon energy and waveform recognition (usually recognition of a well-formed PLCP). This function is called clear channel assessment (CCA) and its implementation is PHY-dependent. The CCA capability represents the physical carrier sense capability for the 802.11 MAC to understand whether the medium is currently busy. It is used in conjunction with the NAV to determine when a station must defer (wait) prior to transmission.
DCF Collision Avoidance/Backoff Procedure
Upon determining that the channel is likely to be free (i.e., because the NAV duration has been met and CCA does not indicate a busy channel), a station defers access prior to transmission. Because many stations may have been waiting for the channel to become free, each station computes and waits for a backoff time prior to sending. The backoff time is equal to the product of a random number and the slot time (unless the station attempting to transmit already has a nonzero backoff time, in which case it is not recomputed). The slot time is PHY-dependent but is generally a few tens of microseconds. The random number is drawn from a uniform distribution over the interval [0, CW], where the contention window (CW) is an integer containing a number of time slots to wait, with limits aCWmin ≤ CW ≤ aCWmax defined by the PHY. The set of CW values increases by powers of 2 (minus 1) beginning with the PHY-specific constant aCWmin value and continuing up to and including the constant aCWmax value for each successive transmission attempt. This is similar in effect to Ethernet’s backoff procedure initiated during a collision detection event.
HCF and 802.11e/n QoS
Clauses 5, 6, 7, and 9 of the 802.11 standard [802.11-2007] are based in part on the work of the 802.11e group within IEEE, and the terms 802.11e, Wi-Fi QoS, and WMM (for Wi-Fi Multimedia) are often used. They cover the QoS facility—changes to the 802.11 MAC-layer and system interfaces in support of multimedia applications such as voice over IP (VoIP) and streaming video. Whether the QoS facility is really necessary or not often depends on the congestion level of the network and the types of applications to be supported. If utilization of the network tends to be low, the QoS MAC support may be unnecessary, although some of the other 802.11e capabilities may still be useful (e.g., block ACKs and APSD). In situations where utilization and congestion are high and there is a need to support a lowjitter delivery capability for services such as VoIP, QoS support may be desirable. These specifications are relatively new, so QoS-capable Wi-Fi equipment is likely to be more expensive and complex than non-QoS equipment.
Physical-Layer Details: Rates, Channels, and Frequencies
The [802.11-2007] standard now includes the following earlier amendments: 802.11a, 802.11b, 802.11d, 802.11g, 802.11h, 802.11i, 802.11j, and 802.11e. The 802.11n standard was adopted as an amendment to 802.11 in 2009 [802.11n-2009]. Most of these amendments provide additional modulation, coding, and operating frequencies for 802.11 networks, but 802.11n also adds multiple data streams and a method for aggregating multiple frames (see Section 3.5.1.3). We will avoid detailed discussion of the physical layer, but to appreciate the breadth of options, Table 3-2 includes those parts of the 802.11 standard that describe this layer in particular.
Channels and Frequencies
Regulatory bodies (e.g., the Federal Communications Commission in the United States) divide the electromagnetic spectrum into frequency ranges allocated for various uses across the world. For each range and use, a license may or may not be required, depending on local policy. In 802.11, there are sets of channels that may be used in various ways at various power levels depending on the regulatory domain or country. Wi-Fi channels are numbered in 5MHz units starting at some base center frequency. For example, channel 36 with a base center frequency of 5.00GHz gives the frequency 5000 + 36 * 5 = 5180MHz, the center frequency of channel 36. Although channel center frequencies are 5MHz apart from each other, channels may be wider than 5MHz (up to 40MHz for 802.11n). Consequently, some channels within channel sets of the same band usually overlap. Practically speaking, this means that transmissions on one channel might interfere with transmissions on nearby channels.