Windows XP Goes Wireless

One of Windows XP's new features is enhanced support for 802.11b wireless networking. While Microsoft was advancing XP through its prerelease versions, the company took every opportunity to show off the features that would supposedly make XP an ideal platform for 802.11 wireless LANs (WLANs). With the release version of XP Professional in hand, I decided to test the OS's WLAN functionality.

But before I share my findings, I want to provide a little background about WLANs and discuss the challenges they present to organizations that deploy them.

A Little Background
Although wireless networking might seem to be a new development in corporate environments, the technology has been around for several years. The recent popularity of WLANs is partially due to the efforts of the IEEE, which published several standards that harnessed the industry's collective expertise and presented a unified model for WLAN technology. The result was a boom in the availability of 802.11 devices that provided high bandwidth and low prices.

When the IEEE published the 802.11 standard for WLANs in 1997, the maximum available wireless bandwidth was 2Mbps. The first wave of wireless NICs cost more than $500 each, and access points (APs)—devices that function as a combination hub for wireless clients and bridge to link wired and wireless networks—cost more than $1200. In 1999, the IEEE ratified the 802.11b standard and ushered in 11Mbps wireless throughput capability. The performance boost, combined with an average 80 percent drop in the cost of wireless hardware, made WLANs an attractive technology for businesses of all sizes—a development that Microsoft watched with interest.

Even though 802.11b wireless networking has great potential, this young technology remains a work in progress. The two areas in which the standard needs the most work are roaming and security. Not surprisingly, Microsoft has focused on these two areas in its efforts to integrate wireless capabilities into XP.

The Roaming Challenge
The ability to roam your corporate campus with an untethered portable device would seem to be a core allure of 802.11 wireless networking, but the 802.11 standard doesn't specify how roaming should work. The standard addresses functionality on only the Physical and Data Link layers (Layers 1 and 2) of the standard Open System Interconnection (OSI) model, whereas roaming functionality involves the Network layer (Layer 3). Therefore, vendors have developed proprietary methods to enable their devices to roam seamlessly, as end users expect. In an attempt to stem a proliferation of proprietary roaming solutions, the IEEE drafted 802.11f, a best-practices guideline for wireless roaming. Although this guideline has helped keep roaming solutions on similar paths, some vendors (e.g., NetMotion Wireless, NetSeal Technologies, ReefEdge, Ecutel) offer solutions that enhance wireless networking by providing especially robust roaming and security capabilities.

Every roaming scenario is unique, but Figure 1, page 50, shows a general representation. To join a network, a wireless device must associate with an AP on that network. The device then periodically scans the available range of wireless channels for other AP signals. If the device detects a stronger signal from another AP—a common event as a device moves out of the range of one AP and into the range of another—the device switches channels and reassociates to the new AP. Many vendors use Inter-Access Point Protocol (IAPP) to make this channel-switching process transparent. In a scenario such as the one in Figure 1, each AP contains an association table to track wireless devices that the AP is hosting, as well as active sessions with hosts on the network. As the wireless device moves from one AP to another, the APs share this information through IAPP and ensure that all data to and from the wireless device is properly routed. A common limitation to this seamless experience occurs when the wireless device crosses network boundaries where IP addresses must change. In XP, Microsoft offers a solution to this limitation.

When a wireless device roams across a network boundary, it must clear two hurdles. First, the end user typically must initiate an association with an AP on the new network. To do so, the user enters the new network's name (aka the Service Set Identifier—SSID) and possibly an encryption key. Second, because the wireless device's TCP/IP configuration isn't valid for the new network, the end user must manually change the TCP/IP settings or initiate a configuration renewal with a DHCP server. This process is a hassle for the power user, a potential showstopper for the average user, and a support headache for the administrator. Enter XP's Zero Configuration for Wireless.

Zero Configuration for Wireless automates the process of configuring and authenticating roaming wireless devices. In a nutshell, Zero Configuration for Wireless can detect available wireless networks, automatically configure network settings, and establish an association to an AP—in most cases, without any user intervention. You can configure profiles that contain any necessary encryption keys and other authentication to connect to WLANs automatically and by order of preference. XP can even detect the absence of an AP and initiate ad hoc or peer-to-peer networking with another 802.11b device. (Microsoft specifies that, to enjoy the functionality I just described, you need 802.11b NICs that are compatible with Zero Configuration for Windows. At the time of this writing, Agere Systems, Cisco Systems, and Symbol Technologies offer such NICs, and Intersil and 3Com plan to release compatible products by the time this article reaches you.)

To test Zero Configuration for Wireless, I used a Dell Latitude 600 portable laptop and APs that Intel, Intermec, and Dell supplied. I also tested the feature on a pair of Compaq DeskPro EN desktop systems, using Intel's LAN PCI Carrier—a PCI card containing a PC Card slot—to plug in an 802.11b wireless NIC.

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