来源：http://www.wireless-center.net/Wireless-Internet-Technologies-and-Applications/1874.html
这里是其中的一段。
原文：
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.
译文：
IEEE 802.11协议族的站点（简称802.11站点）需要先寻找到它的接收范围内的接入点并且与一个特定的基本服务集（BSS）同步。可能在同一个地方会有多个IEEE 802.11协议族的系统，而802.11站点需要锁定到一个优选的或者预配置的基本服务集。为了帮助实现这一点，接入点定时在它们的信号范围内发送广播信息。
这个定时发送的信息被称为信标（Beacon），包含一些信息，例如基本服务集标识（用来区分基本服务集），扩展服务集标识（ESS ID，用来区分扩展服务集），物理层参数，接入点的时钟。802.11站点可以被动地接收附近所有这些信标帧并且将自己的时钟与接入点同步。一旦获取了一个可用的信标帧列表，该802.11站点就会依据一个选择标准（物理层参数，扩展服务集标识，系统负载）来选择一个接入点。 这种选择方法被称为被动扫描。
另一种方法被称为主动扫描，在这种方式中，802.11站点主动向周围发送探测请求（Probe Request）帧以寻找接入点。接听到这些探测请求的接入点会回复一个探测响应（Probe Response）帧，其中包含了与前面提到信标帧类似的广播信息。同样，该802.11站点依据自己的选择标准来选择最适合的基本服务集。