IEEE 802.11 Operating Modes

IEEE 802.11 Operating Modes

IEEE 802.11 defines the following operating modes:

• Infrastructure mode

• Ad hoc mode

Regardless of the operating mode, a Service Set Identifier (SSID), also known as the wireless network name, identifies the wireless network. The SSID is a name configured on the wireless AP (for infrastructure mode) or an initial wireless client (for ad hoc mode) that identifies the wireless network. The SSID is periodically advertised by the wireless AP or the initial wireless client using a special 802.11 MAC management frame known as a beacon frame.

Infrastructure Mode

In infrastructure mode, there is at least one wireless AP and one wireless client. The wireless client uses the wireless AP to access the resources of a traditional wired network. The wired network can be an organization intranet or the Internet, depending on the placement of the wireless AP.

A single wireless AP supporting one or multiple wireless clients is known as a Basic Service Sets (BSS). A set of two or more wireless APs connected to the same wired network that defines a single logical network segment bounded by a router (also known as a subnet) is known as an Extended Service Sets (ESS). The distribution system is the wired network that connects the wireless APs in an ESS to each other and to the larger wired network. An ESS is shown in Figure 1-4.

Figure 1-4. Infrastructure mode and an ESS.

When a wireless network adapter is turned on, it begins to scan across the wireless frequencies for wireless APs and other wireless clients. (Scanning is an active process in which the wireless adapter sends Probe-Request frames on all channels of the ISM frequency range and listens for the Probe-Response frames sent by wireless APs and other wireless clients.) After scanning, a wireless adapter chooses a wireless AP with which to associate. This choice is made automatically by using the SSID of a known or preferred wireless network and the wireless AP with the best signal strength (the highest signal-to-noise ratio). Next, the wireless client negotiates the use of a logical wireless port with the chosen wireless AP. This process is known as association.

The wireless client s configuration settings determine whether the wireless client prefers to connect with infrastructure or ad hoc mode networks. By default, a Windows XP or Windows Server 2003 wireless client prefers infrastructure mode wireless networks over ad hoc mode wireless networks.

If the signal strength of the wireless AP is too low, if the error rate is too high, or if instructed by the operating system, the wireless client scans for other wireless APs to determine whether a different wireless AP can provide a stronger signal to the same wireless network. If so, the wireless client negotiates a connection with that wireless AP. This process is known as reassociation.

NOTE
Every 60 seconds, Windows XP and Windows Server 2003 instruct the wireless network adapter to scan for the available wireless APs.

Reassociation with a different wireless AP can occur for many different reasons. For example, the signal can weaken because the wireless client moves away from the wireless AP or the wireless AP becomes congested with too much other traffic. The automatic switching by wireless clients to less congested wireless APs tends to distribute the load of wireless network traffic across the available wireless APs, increasing the performance for other wireless clients.

As a wireless client moves its physical location, it can associate and reassociate from one wireless AP to another, maintaining a continuous connection during physical relocation. For example, for Transmission Control Protocol/Internet Protocol (TCP/IP), a wireless client is assigned an Internet Protocol (IP) address when it connects to the first wireless AP. When the wireless client roams within the ESS, it creates wireless connections with other wireless APs, but keeps the same IP address because all the wireless APs are on the same subnet.

When the wireless client roams to a different ESS, however, the IP address configuration is no longer valid. For a Windows XP or Windows Server 2003 wireless client, a reassociation is interpreted as a media disconnect/connect event. This event causes Windows XP and Windows Server 2003 to perform a Dynamic Host Configuration Protocol (DHCP) renewal for the IP configuration. Therefore, for reassociations within the ESS, the DHCP renewal refreshes the current IP address configuration. When the Windows XP or Windows Server 2003 wireless client reassociates with a wireless AP across an ESS boundary, the DHCP renewal process obtains a new IP address configuration that is relevant for the logical IP subnet of the new ESS.

Ad Hoc Mode

In ad hoc mode, wireless clients communicate directly with each other without the use of a wireless AP, as shown in Figure 1-5.

Figure 1-5. Wireless clients in ad hoc mode.

Ad hoc mode is also called peer-to-peer mode. Wireless clients in ad hoc mode form an Independent Basic Service Set (IBSS). One of the wireless clients, the first wireless client in the IBSS, takes over some of the responsibilities of the wireless AP. These responsibilities include the periodic beaconing process and the authentication of new members. This wireless client does not act as a bridge to relay information between wireless clients.

Ad hoc mode is used to connect wireless clients together when there is no wireless AP present. The wireless clients must be explicitly configured to use ad hoc mode. There can be a maximum of nine members in an ad hoc 802.11 wireless network.