A Field Guide to Wireless LANs for Administrators and Power Users

There are two different usage models for IEEE 802.11, namely infrastructure Basic Service Set (BSS or ESS) and independent Basic Service Set (IBSS). Even though both kinds of BSS could be called IBSS, the "I" in IBSS means "independent." IBSS mode is also known informally as "ad hoc" mode, and in this mode, STAs that are near enough to each other may communicate directly with each other, station-to-station, without needing to use an AP. In IBSS mode, all the STAs must be able to hear each other, so hidden nodes are impossible by assumption.

An Extended Service Set is formed by the concatenation of two or more infrastructure BSSs that share the same SSID. Infrastructure BSS mode is often referred to as simply "BSS," or "ESS" mode. Communications between stations within an infrastructure BSS, or between one node in one BSS and another node in a different BSS within a single ESS is facilitated by at least one special intervening node known as an AP. In order to get a frame between two wireless stations, it may be necessary for the frame to cross more than one AP, but always at least one AP, unless the WLAN is in IBSS mode and therefore has no AP.

An AP advertises its presence by issuing periodic "Beacon" frames that allow the wireless stations to detect its presence. Before data may be transmitted by a client, an "association" must be negotiated between the client and the AP. If an AP is present, all traffic between WLAN clients must go through the AP. Beacons are also used in IBSS mode, but in IBSS mode, each STA takes turns sending the Beacon. The BSSID is determined by the first STA to create the IBSS mode BSS with a given SSID. In an infrastructure mode BSS, the BSSID is the MAC address of the STA in the AP (every AP is logically a STA as well).

The AP provides essential management functions in the WLAN, but the cost of an AP is that every frame between WLAN clients on the same BSS must traverse the RF medium twice even if they are within range of each other's radios thereby reducing the effective throughput of the medium to about half of the theoretical maximum bit transmission rate.^[22] On the plus side, the AP (or set of APs) enables communication between stations that would otherwise be too far apart for their own radios and antennas to hear each other, and thus the AP can prevent STAs that are mutually out of range from interfering with a STA that is in range of both of those STAs. The presence of the AP, and the protocols used to gain access to the medium, allow for the presence of so-called "hidden" nodes in an infrastructure BSS. No node is hidden from the AP, but some nodes can be out of range of other nodes.

^[22] Ad hoc mode works fine for stations located within range of each other, but an AP is required if connectivity beyond the WLAN is desired. The Portal function enables this connectivity. APs that are deployed primarily to facilitate communication between wired and wireless stations probably aren't impeding the station-to-station traffic on the WLAN, since most traffic on such WLANs will probably be going to, or coming from, the wired side of the AP.

Besides forwarding traffic amongst the WLAN clients, the AP usually also includes a "Portal" function that enables traffic from the WLAN's clients to be bridged to a wired LAN, typically an Ethernet. The IEEE 802.11 term for the wired infrastructure that connects APs together is the "Distribution System" (DS). The Portal function is optional…it's possible to have a wireless station act as an AP and provide no access to a DS. The combination of the AP and its associated Portal acts like any other mixed-media LAN bridge, in that it learns which MAC addresses are reachable via which of its interfaces.

Among other things, the Portal function makes sure that frames transmitted onto the Ethernet are never larger than 1518 total octets (inclusive of Ethernet's 14-octet header and its 4-octet trailer (Frame Check Sequence, or FCS), and also translates the MAC header format so that frames from stations on the WLAN can be recognized by stations on the Ethernet, and vice versa. For example, IP packets that are encapsulated in LLC/SNAP on the WLAN need to be translated into DIX (Classic Ethernet) format on the Ethernet side. If an IEEE 802.11 frame had an IEEE 802.1Q-2003 VLAN header, the Portal function could translate that frame into an Ethernet frame with an appropriately formatted IEEE 802.1Q header set.

Conversely, DIX-encapsulated Ethernet packets that are received by the Portal in the AP from the DS need to be converted to LLC/SNAP-encapsulated frames before they are transmitted onto the WLAN. If an LLC-encapsulated frame arrives from the Ethernet side, then the only transformation of the frame is that the MAC sub-layer portion of the IEEE 802.3 header (the first 14 octets) will be replaced by the necessary IEEE 802.11 MAC sub-layer header, probably 24 octets long.