A service set identifier (SSID) is a sequence of characters that uniquely names a wireless local area network (WLAN). An SSID is sometimes referred to as a "network name." This name allows stations to connect to the desired network when multiple independent networks operate in the same physical area.
Each set of wireless devices communicating directly with each other is called a basic service set (BSS). Several BSSs can be joined together to form one logical WLAN segment, referred to as an extended service set (ESS). A Service Set Identifer (SSID) is simply the 1-32 byte alphanumeric name given to each ESS.
For example, a departmental WLAN (ESS) may consist of several access points (APs) and dozens of stations, all using the same SSID. Another organization in the same building may operate its own departmental WLAN, composed of APs and stations using a different SSID. The purpose of SSID is to help stations in department A find and connect to APs in department A, ignoring APs belonging to department B.
Each AP advertises its presence several times per second by broadcasting beacon frames that carry the ESS name (SSID). Stations can discover APs by passively listening for beacons, or they can send probe frames to actively search for an AP with the desired SSID. Once the station locates an appropriately-named AP, it can send an associate request frame containing the desired SSID. The AP replies with an associate response frame, also containing SSID.
Some frames are permitted to carry a null (zero length) SSID, called a broadcast SSID. For example, a station can send a probe request that carries a broadcast SSID; the AP must return its actual SSID in the probe response. Some APs can be configured to send a zero-length broadcast SSID in beacon frames instead of sending their actual SSID. However, it is not possible to keep an SSID value secret, because the actual SSID (ESS name) is carried in several frames.