to be treated and handled as an arbitrary sequence of 032 octets that are not limited to printable characters. IEEE Std 802.11-2012 defines a flag to express that the SSID is UTF-8-encoded and could contain any Unicode text. Wireless network stacks must still be prepared to handle arbitrary values in the SSID field. Since the contents of an 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. Once a device has associated with a basic service set, for efficiency, the SSID is not sent within packet headers; only BSSIDs are used for addressing. Extended service set An extended service set (ESS) is a wireless network, created by multiple access points, which appears to users as a single, seamless network, such as a network covering a home or office that is too large for reliable coverage by a single access point. It is a set of one or more infrastructure basic service sets on a common logical network segment (i.e. same IP subnet and VLAN). Key to the concept is that the participating basic service sets appear as a single network to the logical link control layer. Thus, from the perspective of the logical link control layer, stations within an ESS may communicate with one another, and mobile stations may move transparently from one participating basic service set to another (within the same ESS). Extended service sets make possible distribution services such as centralized authentication. From the perspective of the link layer, all stations within an ESS are all on the same link, and transfer from one BSS to another is transparent to logical link control. The basic service sets formed in wireless ad hoc networks are, by definition, independent from other BSSs, and an independent BSS cannot therefore be