The Cable Guy - November 2002

The Windows XP Wireless Zero Configuration Service

By The Cable Guy

The Wireless Zero Configuration (WZC) service, available in Windows XP, dynamically selects the wireless network to which to attempt connection, based either on your preferences or on default settings. This includes automatically selecting and connecting to a more preferred wireless network when it becomes available. If none of the preferred wireless networks are found nearby, the WZC service configures the wireless adapter so that there is no accidental connection until the wireless client roams within the range of a preferred network.

The wireless network adapter and its Network Driver Interface Specification (NDIS) driver must support NDIS Object Identifiers (OIDs) that are used to query and set device and driver behavior. The wireless network adapter scans for available wireless networks and passes the network names (also known as Service Set Identifiers or SSIDs) to the WZC service.

Wireless Zero Configuration Service User Interface

The user interface for the WZC service consists of the following:

• Wireless Networks tab
• Messages in the notification area
• Wireless Network Connection dialog box
• Services snap-in

Wireless Networks tab

The user interface for configuring the way in which the WZC service automatically connects to a wireless network is the Wireless Networks tab, available from the properties of a wireless adapter connection in Network Connections. The Wireless Networks tab for Windows XP Service Pack 1 (SP1) is shown in the following figure.

The Wireless Networks tab only appears for wireless adapters that support the WZC service.

Windows XP (prior to Windows XP SP1) uses the same Wireless Networks tab with the same settings. The properties of a wireless adapter for Windows XP (prior to Windows XP SP1) contain an Authentication tab. In Windows XP SP1, the Authentication tab appears with the properties of a wireless network (dialog box not shown).

On the Wireless Networks tab, you can view and configure the following:

• Use Windows to configure my wireless network settings Select this check box when you want the WZC service to automatically configure your wireless settings. If you have third-party wireless software that you want to use, clear this check box. This option is enabled by default.
• Available networks The list of wireless networks that are within range of your wireless adapter. To view or change the configuration of a specific wireless network and add it to the list of preferred networks, click it in the list of available networks, and then click Configure. To force the wireless adapter to initiate a new scan of wireless networks within range, click Refresh.
• Preferred networks The list, by order of preference, of wireless networks with which the wireless client will attempt to connect and authenticate. To add a new wireless network that does not appear in the Available networks list, click Add. To remove a wireless network, click Remove. To configure the settings of a wireless network to which you are connecting, click Properties.
• Advanced To configure advanced wireless settings that are independent of the wireless networks to which you are connecting, click Advanced. You can use these advanced settings to specify whether you want to connect only to infrastructure or ad hoc wireless networks, and whether you want to attempt to automatically connect to wireless networks that are not in your preferred list (by using the Automatically connect to non-preferred networks setting, which is disabled by default).

Messages in the notification area

If there is a condition that requires either user notification or intervention, the WZC service prompts you with a message in the notification area of your desktop, as shown in the following figure.

Wireless Network Connection dialog box

When the WZC service determines that you must select from the list of available networks, it prompts you with the "One or more wireless networks are available" message, as shown in the previous figure. When you click on the message, the Wireless Network Connection dialog box is displayed, allowing you to select an available network with which to attempt a connection. When the WZC service successfully connects to the selected network for the first time, it is automatically added to the top of the list of preferred networks, grouped by type of network (ad hoc or infrastructure mode).

The Wireless Network Connection dialog box for Windows XP SP1 is shown in the following figure.

Windows XP (prior to Windows XP SP1) uses a Connect to Wireless Network dialog box. The Wireless Network Connection dialog box in Windows XP SP1 contains an Enable IEEE 802.1x authentication for this network check box, which is not located on the Windows XP (prior to Windows XP SP1) Connect to Wireless Network dialog box.

Services snap-in

You can use the Services snap-in (available in Administrative Tools), to view the current status of and stop, start, or restart the WZC service (named Wireless Zero Configuration). You can also manage the WZC service from the command prompt by using the Net command. For example, to stop the WZC service, type net stop "Wireless Zero Configuration" at a command prompt.

How the WZC service Works

For the initial scan of available networks, the WZC service performs the following process:

1. The WZC service attempts to connect to the preferred networks in the list of available networks, in the preferred networks preference order.
2. If there are no successful connections, the WZC service attempts to connect to the preferred networks that do not appear in the list of available networks, in the preferred networks preference order.
3. If there are no successful connections and there is an ad hoc network in the list of preferred networks that is available, the WZC service tries to connect to it.
4. If there are no successful connections and there is an ad hoc network in the list of preferred networks that is not available, the WZC service configures the wireless network adapter to act as the first node in the ad hoc network.
5. If there are no successful connections to preferred networks and there are no ad hoc networks in the list of preferred networks, the WZC service determines the Automatically connect to non-preferred networks setting.
6. If Automatically connect to non-preferred networks is enabled, the WZC service attempts to connect to the available networks the order in which they were sensed by the wireless adapter. If all connection attempts fail, the WZC service creates a random network name and places the wireless network adapter in infrastructure mode. After this, the wireless adapter is not connected to any wireless network but continues to scan for preferred wireless networks every 60 seconds. This behavior prevents the Windows XP wireless client from accidentally connecting to a wireless network that does not appear in the list of preferred networks. You are then prompted with the "One or more wireless networks are available" message in the notification area.
7. If Automatically connect to non-preferred networks is disabled, the WZC service creates a random network name and places the wireless network adapter in infrastructure mode. You are then prompted with the "One or more wireless networks are available" message in the notification area.

For subsequent scans, the WZC service determines if there are any changes in the wireless environment that require switching the connection. If the Windows XP wireless client is already connected to a wireless network and there is no other preferred network higher in the preference list that has not been attempted already, the WZC service maintains the existing connection. If the Windows XP wireless client is already connected to a wireless network but a more preferred wireless network becomes available, the WZC service disconnects from the currently connected wireless network and attempts to connect to the more preferred wireless network.

The operation of the WZC service provides the following:

• The first time a wireless adapter is added to a computer running Windows XP, the WZC service prompts you with the "One or more wireless networks are available" message in the notification area, which leads you to select a wireless network by using the Wireless Network Connection dialog box. After you select a wireless network and the connection is successful, the selected network is automatically added as a preferred network, and you are no longer prompted with the "One or more wireless networks are available" message whenever you are within range of it.

  For an organization, this is the typical process for configuring the initial connection to a private wireless network. After the initial configuration, the WZC service connects (then maintains the connection to) the organization wireless network.

  When you take your laptop computer to an airport or another location with public wireless access, the WZC service first attempts to connect to your preferred network. When that connection attempt fails, you are prompted again with the "One or more wireless networks are available" message to connect to the public access wireless network.

• If there are two preferred wireless networks and the most preferred one is not initially available, the WZC service configures a wireless connection to the next most preferred network. When the most preferred network eventually becomes available, the WZC service automatically switches the wireless client connection to it after the next scan.

• If there are no preferred networks in the list of those available, the WZC service attempts to configure connections to the preferred networks in their configured order. This is done in case the access points for the wireless network are configured to prohibit the beaconing of their network name (SSID).

For More Information

For more information about wireless support in Windows XP, consult the following resources:

• Microsoft Wireless Networking Web site
• IEEE 802.11b Wireless Networking Overview (The Cable Guy column - March 2002)

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