11.8 IEEE 802.11 Family Operations

This section examines the operations of an IEEE 802.11 family device in an infrastructure BSS. Although very similar mechanisms exist for the independent BSS as well, this section focuses on the more ubiquitous infrastructure BSS, or just simply BSS. To obtain service in a WLAN environment, each station has to execute a few functions or services as noted in the IEEE 802.11 standards.

The scenario explored in this section is an IEEE 802.11 family user who wishes to use his or her device to access a Web server in the Internet. In the wired LAN, when properly configured, the user simply plugs into the LAN port and requests the Web page needed. An equivalent process in the IEEE 802.11 WLAN to obtain access to the Internet is examined in Figure 11-8.

Figure 11-8. IEEE 802.11 family operations.

11.8.1 The First Step: Synchronization and Scanning

The IEEE 802.11 family station ( shortened to 802.11 station) needs to first look for access points within its coverage range and synchronize with a specific BSS. There may be several IEEE 802.11 family systems in the same location, and the 802.11 station needs to lock onto a preferred or preconfigured BSS. To facilitate this, access points send out periodic information that is broadcast within their coverage area.

This periodic information is referred to as a beacon and contains information such as BSS ID (helps identify the BSS), ESS ID (identifies the ESS), PHY layer parameters, and the native clock of the access point. The 802.11 station can passively listen for all the beacons in a neighborhood and synchronize its clock with the access points. Once a list of valid beacons is obtained, the 802.11 station selects an access point based on a selection criterion (PHY layer parameters, ESS ID, and load in the system). This method of selection is known as passive scanning.

Another method is known as active scanning, where the 802.11 station proactively looks for access points in the neighborhood by sending probe requests. The access points that hear these probe requests respond with a probe response that contains broadcast information similar to the beacon frames mentioned earlier. Again, the 802.11 station selects the best BSS that fits its selection criteria.

11.8.2 Authentication

The next step is authentication, which is an integral part of the IEEE 802.11 family. However, there are two flavors of authentication. An open system authentication mechanism is the default authentication service, which always returns a successful result. It is used in situations where it is not necessary to validate the users positively.

WEP provides the second mechanism of authentication, which uses the shared key mechanism. The access point challenges the 802.11 station, and if the challenge is successful, this results in the 802.11 station being authenticated. The IEEE 802.11 family allows a station to authenticate with multiple BSSs if necessary to facilitate roaming from one BSS to another. If products with Wi-Fi certifications are purchased, shared key authentication is required and the shared keys need to be configured in the 802.11 station and the access point(s).

11.8.3 Association

Until this point, the presence of the 802.11 station is unknown to the 802.11 distributed system. To enable routing of packets to the appropriate destination, once the authentication process is completed, the 802.11 station needs to associate with the access point. This creates a logical link between the 802.11 station and the access point. If this association is accepted, the access point forwards the MAC address of the 802.11 station (which was supplied in the association request from the 802.11 station) to the distributed system.

The distributed system makes a note of the association of the 802.11 station in a specific BSS for routing of packets within the ESS. The access point sends an association response with an association identifier (AID) that is used by the 802.11 station for further communication with the access point.

11.8.4 Data Transfer

The data itself may be sent to someone within the same BSS or someone in the same ESS or outside to others as well. The distributed system helps the AP route packets to the appropriate destination. When the AP receives a packet from the 802.11 station, it checks to see if the destination is in the same BSS.

If the packet is in the same BSS, the AP forwards it to the BSS. If the destination is another BSS or the Web server, using the information provided by the distributed system, the AP forwards the data packets to another BSS or a portal to send it over the wired network.

11.8.5 Reassociation Process

One of the unique features of the IEEE 802.11 family is the ability to roam from one BSS to another in the infrastructure mode. This is referred to as roaming, where the 802.11 station may move from one BSS to another. The process of registering or establishing a new logical link with another BSS is referred to as reassociation.

The reassociation process occurs due to the movement of the 802.11 station from one access point to another. It may also occur due to changes in the radio environment or due to the increased load in the initial access point, known as load balancing. This process of reassociation allows extended coverage needed for enterprise as well as public WLAN operators that provide citywide or even larger coverage.

The reassociation process helps refresh the distributed system with the latest location of 802.11 station. The new access point receives the reassociation request message and, if allowed access, passes this information to the distributed system, which records this information for any further routing of data packets. A station may only be associated with one AP at any time.

The new access point contacts the old access point to remove old associations and obtain any queued packets. Although the mechanisms are not standardized for two access points to communicate with each other, working groups in the IEEE such as the 802.11f are working on the Inter-Access Point Protocol (referred to as IAPP) to enable a communication mechanism across multivendor access points.

11.8.6 Moving Out of Coverage of an ESS

The 802.11 station may eventually move out of coverage of one ESS, which consists of a few BSSs for providing extended coverage. When 802.11 stations move from one ESS to another, the IEEE 802.11 family has not defined any specific mechanisms to facilitate roaming. It is up to the vendors or the IT managers that employ the WLAN solution to roam across ESS.

The IEEE 802.11 family does not provide any standard mechanisms for movement across ESSs. Higher-level protocols like mobile IP may be used to forward packets to the new ESS if required. If mobile IP is not implemented, the 802.11 station may at a minimum acquire a new BSS in the new ESS by starting the whole process of scanning, selection of a new BSS, authentication, and association as described earlier. It may also use DHCP to obtain IP addresses if needed.