Service set (802.11 network)
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, which are 48-bit labels that conform to MAC-48 conventions. Logical networks (including extended service sets) are identified by SSIDs, which serve as "network names" and are typically natural language labels.
Basic service sets (BSSs)
An infrastructure mode wireless network basic service set (BSS) consists of one redistribution point — typically an access point (WAP or AP) — together with one or more "client" stations that are associated with (i.e. connected to) that redistribution point. The operating parameters of the infrastructure-BSS are defined by the redistribution point. Stations communicate only with the redistribution point that they are associated with, and all traffic within the infrastructure-BSS is routed through/bridged by that redistribution point.
Each basic service set has its own unique identifier, a BSSID, which is a unique 48-bit identifier that follows MAC address conventions. An infrastructure-BSSID is usually non-configurable, in which case it is either preset during manufacture, or mathematically derived from a preset value such as a serial number, the MAC address of the LAN connection, etc. As with the MAC addresses used for Ethernet devices, infrastructure-BSSIDs are a combination of a 24-bit Organization Unique Identifier (OUI, the manufacturer's identity) and a 24-bit serial number. A BSSID with a value of all 1s is used to indicate the wildcard BSSID, usable only during probe requests or for communications that take place outside the context of a BSS.
From the point of view of a wireless clients, IEEE 802.11s wireless mesh networks appear as a conventional infrastructure mode topology, and are centrally configured as such. The formation of the mesh's BSS, as well as wireless traffic management (including path selection and forwarding) is negotiated between the nodes (redistribution points) of the mesh infrastructure. The mesh's BSS is distinct from the networks (which may also be wireless) used by a mesh's redistribution points to communicate with one another.
In contrast to the stations in an infrastructure-mode network, the stations in a wireless ad hoc network communicate directly with one another, i.e. without a dependence on a distribution point to relay traffic between them. In this form of peer-to-peer wireless networking, the peers form an independent basic service set (IBSS). Some of the responsibilities of a distribution point — such as defining network parameters and other "beaconing" functions — are established by the first station in an ad-hoc network. But that station does not relay traffic between the other stations; instead, the peers communicate directly with one another. Like infrastructure-BSSs, independent-BSSs also have a 48-bit MAC address-like identifier. But unlike infrastructure-BSS identifiers, independent-BSSs identifiers are not necessarily unique: the individual/group bit of the address is always set to 0 (individual), the universal/local bit of the address is always set to 1 (local), and the remaining 46 bits are randomly generated.
A basic service set should not to be confused with the coverage of an access point, known as the 'basic service area' (BSA).
Extended service sets (ESSs)
An extended service set (ESS) 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 BSS to another (within the same ESS). Extended service sets make possible distribution services such as centralized authentication and seamless roaming between infrastructure-BSSs. 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 part of an extended infrastructure. In that formal sense an independent-BSS has no extended service set. However, the network packets of both independent-BSSs and infrastructure-BSSs have a logical network service set identifier (described below), and the logical link control does not distinguish between the use of that field to name an ESS network, and the use of that field to name a peer-to-peer ad hoc network. The two are effectively indistinguishable at the logical link control layer level.
There is formally no such thing as an 'ESSID' in 802.11 standards.cf.  In 802.11 standard documents, the logical network (ESS or independent-BSS) service set identifier is simply abbreviated 'SSID',) irrespective of whether an SSID identifies an infrastructure-BSS's ESS, or the peer-to-peer network of an independent-BSS. 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 2012 edition of the 802.11 standard defines a simplified variant of UTF-8 for encoding and interpreting non-ISO basic Latin characters in SSID labels, but 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.[n 1] 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.
- To associate with a wireless network, a station must know the network's SSID. This information is either obtained from beacons broadcast by a base station (in which case a client can passively infer whether it is in range of that network), or — if no base station is advertising the SSID — a station must know the SSID beforehand by other means (e.g. from a previous configuration). When a client wishes to associate with a network, it sends the SSID in a probe request. An access point replies with a probe response if the SSID in a probe request is the wildcard SSID (SSID is zero-length) or matches a SSID that the access point supports otherwise the access point does not respond to the probe request.
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