SSID (Service Set Identifier)

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.

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