Figure 3. Roamable Multi-Ap Infrastructure Network - Draytek Vigor520 User Manual

they automatically select the MAC address of one BSS member as an identifier for the group;
this is the BSSID.
It has become customary to refer to a wireless device's MAC address as its BSSID even
when the address is not being used to identify an active BSS.
A BSS that includes an access point is often referred to as a cell.
SSID/Domain Identifier
A Service Set Identifier (SSID) is a shared name, usually assigned by a network
administrator, that identifies wireless devices that are allowed to communicate with each
other. This is one way of providing basic security on a wireless network. An SSID can be up
to 32 characters long, and can include letters and numbers.
Giving multiple access points the same SSID and encryption settings allows mobile stations
that also share those settings to "roam" from one AP to another. When at least one AP is
among the devices sharing an SSID, the name may be referred to as an Extended Service Set
Identifier (ESSID).
An SSID is sometimes called a "domain name," but it is unrelated to the domain names used
to identify machines and networks on the Internet. Other terms for SSID are domain ID, net
ID, network name, extended network ID, and wireless network name. The set of devices that
an SSID identifies is sometimes called a communication domain or wireless domain.
Figure 3 shows a common wireless network setup. If the APs share SSID and encryption
settings, mobile devices that also share those settings can roam among them.

Figure 3. Roamable multi-AP infrastructure network

Roaming
When APs covering adjacent areas have the same SSID and encryption settings, a mobile
device that also shares those settings can communicate through the APs continuously while
moving from one coverage area to another. This is known as roaming. When one AP's
signal begins to weaken, the mobile device automatically searches for another AP that it is
authorized to "associate with" (that is, connect to); when the second AP's signal quality is
better than that of the first, the station automatically switches over.
4