IS organisations may decide to purchase 802.11a combined with 802.11g (designated as 802.11a/g or 802.11a/b/g), but they should be aware of necessary software upgrades, as other 802.11 supporting standards (for example, 802.11i) force design changes. These upgrades should not be onerous as long as the base client machine has sufficient processing power and bandwidth, which should be checked before purchase. Upgradeable access points can be purchased with 802.11b now, and their open slot filled at a later date.
It is important to note that 802.11a requires card bus technology to handle the greater throughput of the network. Although card bus technology is available on notebooks, many handheld devices do not yet support this feature.
The frequencies supported in both the access points must span the entire range of valid 5GHz bands -- 5.125GHz to 5.250GHz, 5.250GHz to 5.350GHz, 5.470GHz to 5.750GHz and 5.725GHz to 5.825GHz -- as well as several other country-specific bands. Failure to do this will prevent the detection of rogue access points (access points installed without IS organisation knowledge or permission). This problem is evident in the most-popular access point, the Cisco 1200, which supports only the 5.2GHz and 5.3GHz bands.
Although 802.11a at 5GHz is often cited as having less range than 802.11g at 2.4GHz, this issue is generally irrelevant because we recommend that WLANs be deployed in cells with a 30-metre (100 feet) radius (subject to a site survey). At this density level, our calculations show that, although it may not provide the absolute top performance of a pure 802.11g network (possibly a 10 percent difference), it still delivers approximately three times the overall performance of an 802.11b network. The difference in range shouldn't result in coverage holes. Additional issues also make 802.11g less attractive.
802.11b -- For the foreseeable future, 802.11b will be the lowest-common-denominator wireless network. It can be deployed either as 802.11b or through the backward-compatibility capability of 802.11g. In many cases, it will provide all that is necessary, and will be particularly important to handheld devices lacking the bus interface to support higher speeds.
802.11g -- The 802.11g wireless network is the most controversial physical layer. It supports the higher link rates of 802.11a at 54Mbps, operates within the same three-channel 2.4GHz band as 802.11b and provides backward support for 802.11b. However, when in the presence of 802.11b clients, the 54Mbps-link-rate performance of 802.11g degrades to about 60 percent of the specification. This is because timing parameters must be increased to accommodate 802.11b traffic. 802.11g also leaves the network operating in the increasingly crowded 2.4GHz ISM band.
Enterprises that want more throughput should move to 802.11a instead of 802.11g, because they can operate at a less-congested frequency with many more channels. We do not recommend the use of 802.11g in enterprise environments. Even so, 802.11g is increasingly replacing 802.11b. Deployment of 802.11g may be inevitable, but planning for future applications should be directed toward 802.11a.
Enterprises should plan on an aggregate bandwidth for any physical layer of approximately half of the quoted link rate. This bandwidth is shared among the users attached to a particular access point and will be affected by the number of users and their workloads. Furthermore, interference from other access points and radiators can affect bandwidth. To determine whether a technology will be appropriate, examine applications and the user layout. To size the network, keep in mind that most users run Microsoft Office, email and browsing, and are generally satisfied with the 200Kbps performance they receive at home over asymmetric digital subscriber line technology.
Regardless of which physical layer choices are made, network deployments may wish to designate certain bands along the 5GHz spectrum for various uses (akin to what might be done with virtual LANs). For example, it may be preferable to reserve channels at 5.2GHz (where available) that are designed for WLAN use for future VoIP applications instead, whereas deployment at 5.8GHz -- another industrial, scientific and medical band like 2.4GHz -- may make devices more susceptible to uncontrolled interference from consumer products.
Location of access points:
All deployments require an initial site survey. These surveys can range from walking around with a WLAN-enabled handheld to using sophisticated automated layout software. Many enterprise WLAN equipment providers offer services. Newer, automated tools are also available.
All access points and radios should be located at common installation points. Common points are locations where both 2.4GHz and 5GHz technologies are co-located (either as a result of dual radio access points or via side-by-side deployments). Although this approach sacrifices some of the capability of each physical technology, it can significantly reduce deployment and maintenance costs. It is best to deploy access points in the ceiling or high in the building, depending on product and application design. The exact location can be altered by using more-capable antennas, such as 6DBi, that provide greater range but a potential loss of density.
Security:
Many methods are available to secure the WLAN. There are pre-standard 802.11/Wi-Fi choices meant for behind-the-firewall deployments such as WPA and the forthcoming WPA2 (using AES encryption), based on early releases of 802.11i, which will be the future IEEE standard for WLAN authentication and link encryption. As an alternative, enterprises have chosen to place their WLAN in a separate subnet -- virtual LAN -- in the "demilitarised zone" (DMZ) or completely outside the firewall. When placed in the DMZ or on the Internet boundary, enterprises have used traditional VPN technology to secure the connection. If the path toward 802.11i is chosen, the key selection issue is how authentication is performed. The supported 802.11i/WPA/WPA2 option is 802.1x, but this offers so many choices that interoperability is difficult when employing multiple vendors, due to the variety of Extensible Authentication Protocols that are supported but not required.
Key facts
WLAN deployment requires the consideration of many factors. The three most important factors are application requirements; the size, location and staffing level of the area to be covered; and a solid understanding of vendor offerings.
