WLAN Operation

The coverage area of an AP is called the Basic Service Set (BSS); other names for the BSS are microcell and cell. The identifier of the BSS is called the BSS identifier (BSSID).


If a single cell does not provide enough coverage, any number of cells can be added to extend the range to an extended service area (ESA). It is recommended that the ESA cells have 10 to 15 percent overlap to allow remote users to roam without losing RF connections. If VoIP is implemented in the wireless network, it is recommended that the ESA cells have a 15 to 20 percent overlap. As discussed earlier, bordering cells should be set to different nonoverlapping channels for best performance.


A Service Set Identifier (SSID) is an identifier or name of a WLAN.

An SSID on an AP and on an associated client must match exactly. APs broadcast their SSIDs in a beacon, announcing their available services; clients associate with a specific SSID or learn the available SSIDs from the beacon and choose one with which to associate.

APs can be configured not to broadcast a particular SSID, but the SSID is still sent in the header of all the packets sent and thus is discoverable by wireless survey tools. Therefore, configuring the AP not to broadcast an SSID is not considered a strong security mechanism by itself. This feature should be combined with some of the stronger mechanisms discussed in the next section.

KEY Roaming occurs when a wireless client moves from being associated to one AP to another POINT AP—from one cell to another cell—within the same SSID. Roaming is explored in the "Mobility in a Cisco Unified Wireless Network" section later in this chapter.

APs can have up to 16 SSIDs; VLANs are extended to the wireless network by mapping VLANs to SSIDs.

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