Locating a Wireless LAN

When you install, configure, and finally start up a wireless LAN client device such as a USB client or PCMCIA card, the client will automatically “listen" to see if there is a wireless LAN within range. The client is also discovering if it can associate with that wireless LAN. This process of listening is called scanning. Scanning occurs before any other process, since scanning is how the client finds the network.

There are two kinds of scanning: passive scanning and active scanning. In finding an access point, client stations follow a trail of breadcrumbs left by the access point. These breadcrumbs are called service set identifiers (SSID) and beacons. These tools serve as a means for a client station to find any and all access points.


Service Set Identifier

The service set identifier (SSID) is a unique, case sensitive, alphanumeric value from 2- 32 characters long used by wireless LANs as a network name. This naming handle is used for segmenting networks, as a rudimentary security measure, and in the process of joining a network. The SSID value is sent in beacons, probe requests, probe responses, and other types of frames. A client station must be configured for the correct SSID in order to join a network. The administrator configures the SSID (sometimes called the ESSID) in each access point. Some stations have the ability to use any SSID value instead of only one manually specified by the administrator. If clients are to roam seamlessly among a group of access points, the clients and all access points must be configured with matching SSIDs. The most important point about an SSID is that it must match EXACTLY between access points and clients.


Beacons

Beacons (short for beacon management frame) are short frames that are sent from the access point to stations (infrastructure mode) or station-to-station (ad hoc mode) in order to organize and synchronize wireless communication on the wireless LAN.


Passive Scanning


Passive scanning is the process of listening for beacons on each channel for a specific period of time after the station is initialized. These beacons are sent by access points (infrastructure mode) or client stations (ad hoc mode), and the scanning station catalogs characteristics about the access points or stations based on these beacons. The station searching for a network listens for beacons until it hears a beacon listing the SSID of the network it wishes to join. The station then attempts to join the network through the access point that sent the beacon. Passive scanning is illustrated in Figure 7.1. In configurations where there are multiple access points, the SSID of the network the station wishes to join may be broadcast by more than one of these access points. In this situation, the station will attempt to join the network through the access point with the strongest signal strength and the lowest bit error rate.

Stations continue passive scanning even after associating to an access point. Passive scanning saves time reconnecting to the network if the client is disconnected (disassociated) from the access point to which the client is currently connected. By maintaining a list of available access points and their characteristics (channel, signal strength, SSID, etc), the station can quickly locate the best access point should its current connection be broken for any reason.

Stations will roam from one access point to another after the radio signal from the access point where the station is connected gets to a certain low level of signal strength. Roaming is implemented so that the station can stay connected to the network. Stations use the information obtained through passive scanning for locating the next best access point (or ad hoc network) to use for connectivity back into the network. For this reason, overlap between access point cells is usually specified at approximately 20-30%. This overlap allows stations to seamlessly roam between access points while disconnecting and reconnecting without the user’s knowledge.


Active Scanning


Stations send this probe frame when they are actively seeking a network to join. The probe frame will contain either the SSID of the network they wish to join or a broadcast SSID. If a probe request is sent specifying an SSID, then only access points that are servicing that SSID will respond with a probe response frame. If a probe request frame is sent with a broadcast SSID, then all access points within reach will respond with a probe response frame, as can be seen in Figure 7.2.

The point of probing in this manner is to locate access points through which the station can attach to the network. Once an access point with the proper SSID is found, the station initiates the authentication and association steps of joining the network through that access point.

The information passed from the access point to the station in probe response frames is almost identical to that of beacons. Probe response frames differ from beacons only in that they are not time-stamped and they do not include a Traffic Indication Map (TIM).

The signal strength of the probe response frames that the PC Card receives back helps determine the access point with which the PC card will attempt to associate. The station generally chooses the access point with the strongest signal strength and lowest bit error rate (BER). The BER is a ratio of corrupted packets to good packets typically determined by the Signal-to-Noise Ratio of the signal. If the peak of an RF signal is somewhere near the noise floor, the receiver may confuse the data signal with noise.