In IEEE 802.11 wireless local area networking standards (including Wi-Fi), a service set is a group of wireless network devices that are operating with the same networking parameters.
Service sets are arranged hierarchically, basic service sets (BSS) are units of devices operating with the same medium access characteristics (i.e. radio frequency, modulation scheme etc.), while extended service sets (ESS) are logical units of one or more basic service sets on the same logical network segment (i.e. IP subnet, VLAN etc.).
There are two classes of basic service sets: those that are formed by infrastructure mode redistribution points (access points or mesh nodes), and those that are formed by independent stations in a peer-to-peer ad hoc topology.
Basic service sets are identified by BSSIDs (basic service set identifiers), which are 48-bit labels that conform to MAC-48 conventions. Logical networks (including extended service sets) are identified by SSIDs (service set identifiers), which serve as “network names” and are typically natural language labels.
The SSID is broadcast by stations in beacon packets to announce the presence of a network.
Unlike basic service set identifiers, SSIDs are usually customizable. These SSIDs can be zero to 32 octets (32 bytes) long, and are, for convenience, usually in a natural language, such as English. The 802.11 standards prior to the 2012 edition did not define any particular encoding/representation for SSIDs, which were expected to be treated and handled as an arbitrary sequence of 0–32 octets that are not limited to printable characters.
The IEEE 802.11-2012 defines a tag that the SSID is UTF-8 encoded and when interpreting could contain any non-ISO basic Latin characters within it. Wireless network stacks must still be prepared to handle arbitrary values in the SSID field.
Since the contents of a SSID field are arbitrary, the 802.11 standard permits devices to advertise the presence of a wireless network with beacon packets in which the SSID field is set to null.] A null SSID (the SSID element’s ‘length’ field is set to zero) is called a “wildcard SSID” in IEEE 802.11 standards documents, and as a “no broadcast SSID” or “hidden SSID” in the context of beacon announcements, and can be used, for example, in enterprise and mesh networks to steer a client to a particular (e.g. less utilized) access point. A station may also likewise transmit packets in which the SSID field is set to null; this prompts an associated access point to send the station a list of supported SSIDs.