11.3 IEEE 802.11 Protocol Model
| The IEEE 802.11 standards were developed with the primary goal of being very similar to the makeup of the 802 LAN family that we introduced earlier. This means that all the applications, protocols, and management mechanisms need to execute seamlessly in the IEEE 802.11 environment as well. Users accustomed to the mode of operation in a LAN environment (for example, the IEEE 802.3 LAN), should not notice any significant difference while operating in a IEEE 802.11 environment. One of the key protocol layers that was discussed in the IEEE 802 LAN family is the logical link control (LLC) layer. This layer creates the transparency between the network layer protocols like IP and the data link and physical layer mechanisms. The IEEE 802.11 protocol is designed to operate as a peer protocol to the IEEE 802.3 LAN, as another data link and physical layer mechanism. This means that the LLC layer is the same across multiple LAN technologies, including the IEEE 802.11 family, as shown in Figure 11-2. Figure 11-2. IEEE 802.11 family. The LLC layer in the IEEE 802.11 protocol model isolates the various LAN and WLAN protocols from the network layers like IP. This allows seamless execution of applications, upper layer protocols, and management mechanisms over IEEE 802.11. The 802.2 LLC layer makes the IEEE 802.11 WLAN protocol indistinguishable from the other IEEE 802 protocols. The IEEE 802.11 WLAN protocol model includes two layers below the LLC layer, the MAC layer and the PHY layer. The PHY layer is the layer responsible for transmission over the air. There are many mechanisms used to transmit over the air, and they all work with the same MAC layer above. The MAC layer in 802.3 LANs provides a reliable communication mechanism that allows multiple users fair and consistent access to the shared medium. These same functions are needed in IEEE 802.11 LANs as well, along with additional functions needed due to the wireless nature. There are differences brought about due to the wireless nature that create new functions of the MAC and PHY layer. 802.3 LAN users are used to the privacy afforded by the use of wires and the security of all information transmitted over wires. Going wireless with IEEE 802.11 WLANs raises the level of awareness regarding security and privacy. These issues need to be addressed when creating the standards for the IEEE 802.11 family. Another implication of going wireless is that the medium is no longer as predictable as when using cables in a LAN environment. Wireless interface creates additional issues such as reliability of the wireless medium and power used to transmit. These issues need to be addressed in IEEE 802.11 to create a viable WLAN mechanism. Mobility is one of the fundamental results of going wireless. Wireless communication allows users to access the network in any location, if appropriately configured. Mobility allows users to roam around campus with no need to look for ports of access and may allow large-scale deployments in public areas as well. Going wireless allows a new set of applications that were not possible with wired 802.3 LANs. However, the fundamental premise of 802.3 LANs, such as secure, reliable, high-speed communication that allows multiple users fair and consistent access to the medium, needs to be realized in the 802.11 WLAN family as well. 11.3.1 IEEE 802.11 and Its FeaturesThe initial IEEE 802.11 protocol introduced the IEEE's standard to a wide audience and was standardized in 1997. It introduced the use of new MAC and PHY layers. It also introduced a logical architecture that allowed devices with IEEE 802.11 capability to communicate with each other. It introduced the notion of devices communicating with an access point, which in turn connects to the wired LAN network. The data rates of the initial IEEE 802.11 went from 1 Mbps to a peak rate of 2 Mbps. The data rate of 1 Mbps is mandatory, and the 2 Mbps is an optional data rate depending on the type of physical layer mechanism used. There are three different types of PHY layer transmission technologies that have been standardized in IEEE 802.11. Two of these methods are based on radio frequency (RF) technologies, and the third is an infrared mechanism. One of the key requirements of IEEE 802.11 Project Authorization Request (PAR) is the need for global availability. This prompted the use of the unlicensed 2.4-GHz spectrum that is available in most parts of the world known as the industrial, scientific, and medical (ISM) band . The amount of bandwidth available in the 2.4-GHz range depends on the country. In the United States, 79 MHz of spectrum is available in the 2.4-GHz range. Countries also regulate the different types of technologies and the amount of power used, as there are other technologies that operate in this band, including Bluetooth, HomeRF, and even garage door openers and baby monitors . By using the ISM band consistent with the appropriate amount of power allowed in each country, IEEE 802.11 can be deployed throughout the world. This ISM band is used for the two modes of the PHY layer that operate in the RF layer. The infrared option operates in the nearly visible 850 to 900 nanometers range at a maximum transmit power level of 2 watts peak optical power. The data rates of 1 and 2 Mbps, although not very fast when compared with today's data rates available in IEEE 802.3 LANs, provides enough data rates to allow a vast array of applications. The IEEE 802.11 WLAN also provides a very viable alternative to mobile users and compares favorably with other 2.5G and 3G cellular technologies, which also provide wireless Internet access, albeit at much lower data rates. Realistically, the actual transmission rate will be lower than the peak of 2 Mbps, given the wireless media and the need for reliability, which requires retransmissions. It should still allow for devices to use IEEE 802.11 in residential, manufacturing, retail, warehousing, remote monitoring, hospitals , and other areas where high data rates are not a requirement. An example of this is bar code applications, which should work well with the 2 Mbps data rates offered by IEEE 802.11. 11.3.2 IEEE 802.11b and Its FeaturesOne of the concerns with the initial IEEE 802.11-1997 standard was the data rate that can be achieved. The upper limit of 2 Mbps was slow for situations where large data blocks needed to be transmitted (increasing transmission delay in the network) and multimedia applications, resulting in the arrival of IEEE 802.11b. The introduction of newer standards was also facilitated by advancements in digital signal processors and chipsets. In October 1997, the IEEE 802 executive committee allowed the creation of two new projects to increase data rates beyond the initial IEEE 802.11 standard. This led to the development of IEEE 802.11a and an IEEE 802.11b standard, which only modifies the PHY layer with the same MAC layer defined for IEEE 802.11 standard. Both standards were approved by the IEEE in September 1999.
The maximum data rate supported by the IEEE 802.11b standard is 11 Mbps. It also supports additional data rates of 1, 2, and 5.5 Mbps besides the 11 Mbps. The standards also provide a sliding mechanism to traverse among these different data rates. The lower rates of 1 and 2 Mbps are backward compatible with the same 1 and 2 Mbps in the IEEE 802.11 standard, when using the same RF mechanism. Unlike the IEEE 802.11 standard, which supports multiple RF and infrared mechanisms, the IEEE 802.11b standard only supports one RF PHY layer mechanism. The IEEE 802.11b standard operates in the same 2.4-GHz ISM band that the IEEE 802.11 standard operates in as only the RF mechanism is employed. With higher data rates, several applications are possible with the IEEE 802.11b standard. This includes applications that the IEEE 802.11 standard is used for as well as newer applications, such as streaming applications and transmission of large data blocks without adversely affecting the transmission delay in the system. As the IEEE 802.11b standard is backward compatible with the IEEE 802.11 standard when using one flavor of the PHY layer, introduction or overlay of the IEEE 802.11b standard devices in an IEEE 802.11 standard system should be possible, where the IEEE 802.11b standard device can operate in 1 or 2 Mbps if desired. 11.3.3 IEEE 802.11a FeaturesAs mentioned earlier, a project was initiated in the IEEE 802 working group to create a new high-speed wireless LAN standard known as IEEE 802.11a. One of the more obvious reasons is the need for higher data rates as compared to the IEEE 802.11 standard and the IEEE 802.11b standard. Another reason for the creation of the IEEE 802.11a standard was the fact that many countries, including the United States, had allocated a new unlicensed band in the 5-GHz range for use, which is not as prone to interference as the 2.4 GHz that the IEEE 802.11 standard and IEEE 802.11b standards were using. The fact that more bandwidth was available (up to 300 MHz available in the 5-GHz band as opposed to a maximum of 79 MHz available) in the 5-GHz range necessitated a fresh look at technologies that could be used for extending services to include applications such as voice, image, and video services besides the tried and tested packet data services. The IEEE 802.11a standard was completed in the same time frame as the IEEE 802.11b standard (September 1999), and devices using this technology are coming out at this time. A new technology is used in IEEE 802.11a standard, which is very different (and incompatible as well) with the other standards. The data rates for the IEEE 802.11a standard can go as high as 54 Mbps achieving the necessary data rates for all types of applications. The IEEE 802.11a standard provides a wireless LAN with data payload communication capabilities of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. The support of transmitting and receiving at data rates of 6, 12, and 24 Mbps is mandatory. The spectrum that the IEEE 802.11a standard operates is in the unlicensed 5-GHz range. Several countries around the world created additional spectrum in the 5-GHz range. For example, the U.S. government released 300 MHz of spectrum in the Unlicensed National Information Infrastructure (UNII) band in the range of 5.15-5.25, 5.25-5.35, and 5.725-5.825 GHz. Many other countries have released up to 200MHz in this band. An increase in the data rates opens the door for a wide variety of applications, including delay-sensitive (or referred to in the standards as time-bound applications) such as voice and video. IEEE 802.11a standard is not backward compatible with the IEEE 802.11 standard and the IEEE 802.11b standards and, therefore, care should be taken to decide on the technology that best suits the requirements of WLAN deployment. A comparison of the various IEEE WLAN standards, including those that are being created during the publication of this book, is shown in the following table:
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