MAC Access Modes and Timing
Access to the wireless medium is controlled by coordination functions. Ethernet-like CSMA/CA access is provided by the distributed coordination function (DCF). If contention-free service is required, it can be provided by the point coordination function (PCF), which is built on top of the DCF. Between the free-for-all of the DCF and the precision of the PCF, networks can use the hybrid coordination function (HCF), a middle ground for quality of service between the two extremes. Contention-free services are provided only in infrastructure networks, but quality of service may be provided in any network that has HCF support in the stations. The coordination functions are described in the following list and illustrated in Figure 3-4:
DCF
The DCF is the basis of the standard CSMA/CA access mechanism. Like Ethernet, it first checks to see that the radio link is clear before transmitting. To avoid collisions, stations use a random backoff after each frame, with the first transmitter seizing the channel. In some circumstances, the DCF may use the CTS/RTS clearing technique to further reduce the possibility of collisions.
PCF
The point coordination function provides contention-free services. Special stations called point coordinators are used to ensure that the medium is provided without contention. Point coordinators reside in access points, so the PCF is restricted to infrastructure networks. To gain priority over standard contention-based services, the PCF allows stations to transmit frames after a shorter interval. The PCF is not widely implemented and is described in Chapter 9.
HCF
Some applications need to have service quality that is a step above best-effort delivery, but the rigorous timing of the PCF is not required. The HCF allows stations to maintain multiple service queues and balance access to the wireless medium in favor of applications that require better service quality. The HCF is not fully standardized yet, but is being produced as part of the eventual 802.11e specification.
Adding quality of service to the 802.11 MAC was a significant undertaking. Due to the added complexity in terms of framing, queue management, and signaling, the quality of service specifications were still subject to standards committee wrangling as this book was written, and discussion of them must be postponed to a future edition.
Carrier-Sensing Functions and the Network Allocation Vector
Carrier sensing is used to determine if the medium is available. Two types of carrier-sensing functions in 802.11 manage this process: the physical carrier-sensing and virtual carrier-sensing functions. If either carrier-sensing function indicates that the medium is busy, the MAC reports this to higher layers.
Figure 3-4. MAC coordination functions
Physical carrier-sensing functions are provided by the physical layer in question and depend on the medium and modulation used. It is difficult (or, more to the point, expensive) to build physical carrier-sensing hardware for RF-based media, because transceivers can transmit and receive simultaneously only if they incorporate expensive electronics. Furthermore, with hidden nodes potentially lurking everywhere, physical carrier-sensing cannot provide all the necessary information.
Virtual carrier-sensing is provided by the Network Allocation Vector (NAV). Most 802.11 frames carry a duration field, which can be used to reserve the medium for a fixed time period. The NAV is a timer that indicates the amount of time the medium will be reserved, in microseconds. Stations set the NAV to the time for which they expect to use the medium, including any frames necessary to complete the current operation. Other stations count down from the NAV to 0. When the NAV is nonzero, the virtual carrier-sensing function indicates that the medium is busy; when the NAV reaches 0, the virtual carrier-sensing function indicates that the medium is idle.
By using the NAV, stations can ensure that atomic operations are not interrupted. For example, the RTS/CTS sequence in Figure 3-3 is atomic. Figure 3-5 shows how the NAV protects the sequence from interruption. (This is a standard format for a number of diagrams in this book that illustrate the interaction of multiple stations with the corresponding timers.) Activity on the medium by stations is represented by the shaded bars, and each bar is labeled with the frame type. Interframe spacing is depicted by the lack of any activity. Finally, the NAV timer is represented by the bars on the NAV line at the bottom of the figure. The NAV is carried in the frame headers on the RTS and CTS frames; it is depicted on its own line to show how the NAV relates to actual transmissions in the air. When a NAV bar is present on the NAV line, stations should defer access to the medium because the virtual carrier-sensing mechanism will indicate a busy medium.
To ensure that the sequence is not interrupted, node 1 sets the NAV in its RTS to block access to the medium while the RTS is being transmitted. All stations that hear the RTS defer access to the medium until the NAV elapses.
Figure 3-5. Using the NAV for virtual carrier sensing
RTS frames are not necessarily heard by every station in the network. Therefore, the recipient of the intended transmission responds with a CTS that includes a shorter NAV. This NAV prevents other stations from accessing the medium until the transmission completes. After the sequence completes, the medium can be used by any station after distributed interframe space (DIFS), which is depicted by the contention window beginning at the right side of the figure.
RTS/CTS exchanges may be useful in crowded areas with multiple overlapping networks. Every station on the same physical channel receives the NAV and defers access appropriately, even if the stations are configured to be on different networks.
Interframe Spacing
As with traditional Ethernet, the interframe spacing plays a large role in coordinating access to the transmission medium. 802.11 uses four different interframe spaces. Three are used to determine medium access; the relationship between them is shown in Figure 3-6.
Figure 3-6. Interframe spacing relationships
We've already seen that as part of the collision avoidance built into the 802.11 MAC, stations delay transmission until the medium becomes idle. Varying interframe spacings create different priority levels for different types of traffic. The logic behind this is simple: high-priority traffic doesn't have to wait as long after the medium has become idle. Therefore, if there is any high-priority traffic waiting, it grabs the network before low-priority frames have a chance to try. To assist with interoperability between different data rates, the interframe space is a fixed amount of time, independent of the transmission speed. (This is only one of the many problems caused by having different physical layers use the same radio resources, which are different modulation techniques.) Different physical layers, however, can specify different interframe space times.
Short interframe space (SIFS)
The SIFS is used for the highest-priority transmissions, such as RTS/CTS frames and positive acknowledgments. High-priority transmissions can begin once the SIFS has elapsed. Once these high-priority transmissions begin, the medium becomes busy, so frames transmitted after the SIFS has elapsed have priority over frames that can be transmitted only after longer intervals.
PCF interframe space (PIFS)
The PIFS, sometimes erroneously called the priority interframe space, is used by the PCF during contention-free operation. Stations with data to transmit in the contention-free period can transmit after the PIFS has elapsed and preempt any contention-based traffic.
DCF interframe space (DIFS)
The DIFS is the minimum medium idle time for contention-based services. Stations may have immediate access to the medium if it has been free for a period longer than the DIFS.
Extended interframe space (EIFS)
The EIFS is not illustrated in Figure 3-6 because it is not a fixed interval. It is used only when there is an error in frame transmission.
Interframe spacing and priority
Atomic operations start like regular transmissions: they must wait for the the relevant interframe space, typically the DIFS, to end before they can begin. However, the second and any subsequent steps in an atomic operation take place after the SIFS, rather than after the DIFS. Because the SIFS is shorter than the other interframe spaces, the second (and subsequent) parts of an atomic operation will grab the medium before another type of frame can be transmitted. By using the SIFS and the NAV, stations can seize the medium for as long as necessary.
In Figure 3-5, for example, the short interframe space is used between the different units of the atomic exchange. After the sender gains access to the medium, the receiver replies with a CTS after the SIFS. Any stations that might attempt to access the medium at the conclusion of the RTS would wait for one DIFS interval. Partway through the DIFS interval, though, the SIFS interval elapses, and the CTS is transmitted.