An abundance of relatively low-priced but easily configurable products has propelled wireless LAN (WLAN) technology—specifically 802.11b WLAN technology—from early adopters to mainstream users. The allure of flexibility, convenience, and cost savings tempts companies to deploy buildingwide or campuswide WLANs. However, the race to the WLAN market has yielded a variety of products, not all of which are created equal. But with some thought and planning, you can find the right products to create a truly effective WLAN that spans your campus and mobilizes your employees. Let's discuss new WLAN technologies and how you can put them to work in your environment. I describe Access Point (AP) technology, how APs work with roaming and associations in your WLAN, and how to select and deploy these devices at your site.
A typical WLAN infrastructure consists of multiple APs hard-wired to a LAN to form a transparent bridge for wireless clients. Wireless clients are client devices (i.e., laptops, desktops, or PDAs) that have compatible wireless access cards and use a radio protocol at a set frequency to communicate. APs generally provide a transparent way to join a wireless device to a wired network. When a wireless client connects and authenticates to an AP, the client can request an IP address and access network resources.
AP Technology
Most 802.11b APs use Direct Sequence Spread Spectrum (DSSS), a technology the US military developed that's particularly resistant to interference and interception. This technology operates in the 2.4GHz Industrial, Scientific, and Medical (ISM) frequency band, which supports 11MHz to 22MHz channels (three of these channels—1, 6, and 11—don't overlap). The 802.11b technology supports half-duplex data transfer rates of 1Mbps, 2Mbps, 5.5Mbps, and 11Mbps. For comparison, 802.11a uses Orthogonal Frequency Division Multiplexing (OFDM), operates in the 5GHz frequency band, and supports up to 54Mbps. OFDM supports eight channels that don't overlap.
The 802.11b technology is slower than 802.11a, but has been around much longer, and the equipment is less expensive and more readily available than newer 802.11a products. These two standards aren't compatible, so you can't mix 802.11a and 802.11b wireless devices.
If you use APs, you must configure your WLAN for infrastructure mode (as opposed to peer-to-peer—P2P, or ad hoc—mode), which means that network clients connect to an AP for all communication and not directly to other wireless clients. In infrastructure mode, a client can move from AP to AP depending on the client device's roaming capabilities.
In a typical configuration, the AP supports multiple wireless clients connecting to a wired LAN, as Figure 1 shows. Each AP connects directly to the LAN, typically with a Category 5 cable. Multiple APs extend the WLAN mesh and let mobile users roam across the facility or campus, switching APs as necessary. In this configuration, the users are always one wireless hop from the physical LAN.
Some APs act as a wireless bridge to transparently connect two physical LAN networks, as Figure 2 shows. (Some vendors sell network devices dedicated to this purpose.) An example of this configuration is to connect two nearby buildings where a terrestrial line (e.g., T1, Ethernet) isn't feasible or desirable. Because wireless bridges often span long distances, they can benefit from high-gain directional antennae. Depending on the AP model, wireless clients might not be permitted to connect to an AP when it's in this mode. Also, because of the high traffic that bridges handle, they often include filters or other capabilities to better handle broadcast and multicast traffic among the physical networks.
An AP that connects to another AP as its primary node (as opposed to connecting to the physical LAN) operates as a repeater, as Figure 3, page 52, shows. You can use a repeater to extend the range of your coverage area. However, because the AP must receive and retransmit data, the throughput is halved for every repeater in the chain. A repeater is good for extending range, but it doesn't maximize throughput.
The Internet Engineering Task Force (IETF) is working on a proposed mobile IP standard (for more information about this standard, see Request for Comments—RFC—3344 at ftp://ftp.isi.edu/in-notes/rfc3344.txt). Mobile IP is a modification to the TCP/IP stack that assigns two IP addresses, a home IP and a care-of IP, to the wireless client. The OS and applications bind to the machine's home IP address, and this address doesn't change. The care-of IP address is associated with the subnet of the AP to which the wireless client is connected. This second IP changes dynamically depending on which AP the client is connected to. However, applications running on the network device continue to run under the wireless client's unchanging home IP. Someday, a mobile IP might let you travel through a city or a state and transparently switch ISPs without dropping your network connection.
Roaming and Associations
When you start a wireless client, it locates and associates itself to the best AP. The client, using an often-proprietary radio protocol, distinguishes the best AP by its signal quality. Vendors define signal quality differently, but criteria typically include signal strength and AP load (but not necessarily proximity). Movement away from the associated AP often adversely affects the signal quality between the wireless client and that AP, which causes the radio protocols' roaming support to instruct the client to reassociate with a different AP.
Roaming is the feature that lets you move from AP to AP without dropping your network connection. For example, you might bring your laptop to the conference room to take notes during a meeting. In the conference room, your wireless client associates with a nearby AP and periodically confirms the quality of the signal. As you travel back to your office, moving away from the conference room, the signal quality might degrade to the point that your wireless client will automatically associate with a new AP that has a stronger signal. This handoff occurs at the physical layer, and the reassociation with the new AP should occur without dropping packets or losing your network connection. If your network doesn't have strong roaming support, your client might associate with an AP and stay with it regardless of signal quality—even to the point of disconnection. In such cases, you can usually force a manual reassociation by restarting the wireless network card. Roaming is often a particularly weak area of interoperability among AP models from different vendors; you might find that you can't roam from one vendor's AP to another without forcing a manual reassociation (and thereby breaking the network connection). For more information about choosing APs, see the sidebar "AP Selection."
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