Chapter 4. IEEE 802.11 s MAC Sub-layer ProtocolFrames, etc.

Chapter 4. IEEE 802.11's MAC Sub-layer Protocol Frames, etc.

While the Physical layer options discussed in Chapter 3 are of fundamental importance (and are very tangible to the end user, because they are directly related to perceivable characteristics such as the performance of the WLAN), the unifying feature that all IEEE 802.11-based WLANs have in common is their MAC sub-layer protocol. In contrast to the PHY protocol(s), the MAC sub-layer protocol is virtually invisible to end users, except when they initially configure their WLAN adapter.

In this chapter, we describe the MAC sub-layer protocol that has been developed for IEEE 802.11-based WLANs. The discussion in this chapter is based on the MAC sub-layer protocol that was specified in the document IEEE 802.11-1999, which is the base standard for IEEE 802.11 WLANs.

The MAC sub-layer is formally the lower portion of the Data Link layer, but the upper sub-layer, known as the Logical Link Control (LLC) sub-layer, is comprised of up to two protocols. In contrast, there is exactly one MAC sub-layer protocol at the MAC sub-layer of the IEEE's subdivided OSI Data Link layer. Confusingly, one of the two protocols at the LLC sub-layer is named LLC, which might lead one to conclude that, as with the MAC sub-layer, there is only one LLC sub-layer protocol. The other LLC sub-layer protocol is known as the Sub-Network Access Protocol (SNAP), which if used must be layered over LLC. However, LLC can be used alone…SNAP is optional, but LLC is mandatory over all IEEE MAC sub-layer protocols. In an attempt to avoid confusion, the author will make every effort to distinguish between the LLC sub-layer and the LLC sub-layer protocol(s) whenever the context is not clear.

Besides examining the inner workings of the IEEE 802.11 MAC sub-layer protocol, in this chapter we will also examine the remainder of the Data Link layer in detail, at least as it pertains to IEEE 802.11-based WLANs. Besides learning facts such as the mandatory usage of the LLC sub-layer protocol over IEEE 802.11's MAC sub-layer protocol, which has already been alluded to, there are a number of fundamental (and mandatory) MAC sub-layer features, and there are some optional extensions, which are either defined in the original IEEE 802.11-1999 specification or in an extension document (e.g., IEEE 802.11e or IEEE 802.11i, the two most prominent examples of MAC sub-layer protocol enhancements under way in the IEEE 802.11 WG). One of the most important invariant features is the MAC sub-layer's frame formats, as well as the basic channel access method, known as the Distributed Coordination Function (DCF).

The MAC sub-layer protocol design for IEEE 802.11 is in the process of evolution, being marginally extended and modified by various TGs within the IEEE 802.11 WG. Because of the importance of the MAC sub-layer protocol, there is reluctance to modify it very much. To a greater or lesser extent, TGs "e," "h," "i," and "j" are modifying part of the behavior of the MAC sub-layer protocol, because of necessary functionality that the IEEE 802.11 WG perceived to be missing in the IEEE 802.11-1999 specification.

Keep in mind that anything that the author states regarding the output of the existing TGs of IEEE 802.11 is based on the specifications as they existed when this book was finished. As such, it is possible that the details may have been changed between when this book was written and whenever the TG ultimately completed work on its standard. At the time of this writing, the most recent TG is "n," and it is not expected to have its first official meeting until September, 2003. In order to minimize the divergence, the author is trying to only refer to the parts of each draft that seem to be the most "solid," but there is no guarantee that the details of even these portions of the draft(s) will not change. Please look at the completed drafts for the final information. The IEEE gives away its standards for free after they have been finished for 6 months; search the web for the "Get IEEE 802" program.

Task Group "e" is adding facilities to enable "Quality of Service" within WLANs. This affects the basic WLAN frame header (in QoS-enhanced data frames, there is an additional two-octet "QoS Control" field that is appended at the very end of the frame's header that allows the frame to be marked according to the desired QoS parameters). TGe is also defining some procedures to enable two STAs to establish a "Direct Link" with each other, so that they can exchange traffic without sending it through the AP. This feature, if approved in TGe's final draft, will optimize certain traffic since that traffic will only need to cross the wireless medium once (instead of needing to cross it twice, once to the AP, and once from the AP).

Task Group "f " has defined an independent IEEE 802.11F-2003 standard (known as a "recommended practice") for inter-AP procedures. TGf's standard was approved in June of 2003 and was published in July 2003.

Task Group "i " is defining new procedures that will enable STAs to join and leave a wireless LAN securely. As the reader may be aware, the security that was defined in IEEE 802.11-1999 (known as "Wired-Equivalent Privacy," or WEP) has been found to be so weak as to be virtually useless. TGi will be defining techniques that enable a STA to securely associate with an AP, and also to send and receive encrypted data. Finally, TGi will likely also define procedures to enable "fast roaming" to happen while maintaining the strong security protection that has already been negotiated for an association between a STA and an AP. By "fast" we mean that the secure roaming from one AP to another can happen rapidly enough that a user who is engaged in, for example, a Voice-over-IP-over-WLAN phone conversation, would not notice any degradation in the quality of the sound conveyed through the phone, such as an audible dropout or click. In order to protect a higher-layer data packet as it crosses the wireless medium, TGi has defined several new "encapsulation" headers that carry the necessary information to help a receiver (in possession of the negotiated keys) decode the frame. Security will be discussed in considerably more detail in a later chapter.

Task Group "g" has made some minor adjustments to the usage of one of the basic MAC-layer Control frames. TGg's standard was published in June of 2003 as IEEE 802.11g-2003.

Task Group "h" was discussed in the previous chapter, and it provides control mechanisms for a STA to discover the regulatory domain in which it is located, and to make necessary accommodations to the other non-WLAN users that might be sharing the channel, such as being able to detect if a periodic radar system is in use. TGh also provides mechanisms to control the output power of the WLAN device, to permit it to share the medium with other non-WLAN users. TGh's work is nearly complete, and seems likely to be published in 2003. It is actually debatable whether or not TGh is modifying the MAC or PHY layer it has aspects of both.

Task Group "j" is a new PHY similar to IEEE 802.11a that supports operation in Japan's 4.9 and 5.0 GHz bands. Due to the fact that this portion of the radio frequency spectrum is not dedicated to WLANs, STAs that support the TGj PHY are not permitted to send any traffic until they have heard from an AP first, so only "passive scanning" is allowed…"active scanning" is prohibited. IEEE 802.11's MAC sub-layer protocol, as amended by TGj, relies on the presence of a properly configured AP. In particular, this prohibits TGj devices from operating in "ad-hoc" mode. TGj may complete its work in 2003, but publication in early 2004 is more likely.