IEEE 802.11

IEEE 802.11 is a set of standards carrying out wireless local area network (WLAN) computer communication in the 2.4, 3.6 and 5 GHz frequency bands. They are implemented by the IEEE LAN/MAN Standards Committee (IEEE 802).

IEEE 802.11a-1999

IEEE 802.11a-1999 or 802.11a is an amendment to the IEEE 802.11 specification that added a higher data rate of up to 54 Mbit/s using the 5 GHz band. It has seen widespread worldwide implementation, particularly within the corporate workspace. The amendment has been incorporated into the published IEEE 802.11-2007 standard.

802.11 is a set of IEEE standards that govern wireless networking transmission methods. They are commonly used today in their 802.11a, 802.11b, 802.11g and 802.11n versions to provide wireless connectivity in the home, office and some commercial establishments.

802.11 Architecture

The 802.11 logical architecture contains several main components: station (STA), wireless access point (AP), independent basic service set (IBSS), basic service set (BSS), distribution system (DS), and extended service set (ESS). Some of the components of the 802.11 logical architecture map directly to hardware devices, such as STAs and wireless APs. The wireless STA contains an adapter card, PC Card, or an embedded device to provide wireless connectivity. The wireless AP functions as a bridge between the wireless STAs and the existing network backbone for network access.

An IBSS is a wireless network, consisting of at least two STAs, used where no access to a DS is available. An IBSS is also sometimes referred to as an ad hoc wireless network.

A BSS is a wireless network, consisting of a single wireless AP supporting one or multiple wireless clients. A BSS is also sometimes referred to as an infrastructure wireless network. All STAs in a BSS communicate through the AP. The AP provides connectivity to the wired LAN and provides bridging functionality when one STA initiates communication to another STA or a node on the DS.

An ESS is a set of two or more wireless APs connected to the same wired network that defines a single logical network segment bounded by a router (also known as a subnet).

The APs of multiple BSSs are interconnected by the DS. This allows for mobility, because STAs can move from one BSS to another BSS. APs can be interconnected with or without wires; however, most of the time they are connected with wires. The DS is the logical component used to interconnect BSSs. The DS provides distribution services to allow for the roaming of STAs between BSSs.

Architecture
Architecture is designed to support a network where mobile station is responsible for the decision making.
Advantages are :

-very tolerant of faults in all of the WLAN equipment.
-eliminates any possible bottlenecks a centralized architecture would introduce.
Architecture has power-saving modes of operation built into the protocol to prolong the battery life of mobile equipment without losing network connectivity.

Components
Station the component that connects to the wireless medium. Supported services are authentication, deauthentication, privacy, and delivery of the data.
Basic Service Set A BSS is a set of stations that communicate with one another. A BSS does not generally refer to a particular area, due to the uncertainties of electromagnetic propagation. When all of the stations int the BSS are mobile stations and there is no connection to a wired network, the BSS is called independent BSS (IBSS). IBSS is typically short-lived network, with a small number of stations, that is created for a particular purpose. When a BSS includes an access point (AP), the BSS is called infrastructure BSS.

When there is a AP, If one mobile station in the BSS must communicate with another mobile station, the communication is sent first to the AP and then from the AP to the other mobile station. This consume twice the bandwidth that the same communication. While this appears to be a significant cost, the benefits provided by the AP far outweigh this cost. One of them is, AP buffers the traffic of mobile while that station is operating in a very low power state.

Extended Service Set (ESS) A ESS is a set of infrastructure BSSs, where the APs communicate among themselves to forward traffic from one BSS to another and to facilitate the movement of mobile stations from one BSS to another. The APs perform this communication via an abstract medium called the distribution system (DS). To network equipment outside of the ESS, the ESS and all of its mobile stations appears to be a single MAC-layer network where all stations are physically stationary. Thus, the ESS hides the mobility of the mobile stations from everything outside the ESS.

Distribution System the distribution system (DS) is the mechanism by which one AP communicates with another to exchange frames for stations in their BSSs, forward frames to follow mobile stations from one BSS to another, and exchange frames with wired network.

Services
- Station Services: Authentication, De-authentication, privacy, delivery of data
- Distribution Services: Association, Disassociation, Reassociation, Distribution, Integration

Station Services Similar functions to those that are expected of a wired network. The wired network function of physically connecting to the network cable is similar to the authentication and de-authentication services. Privacy is for data security. Data delivery is the reliable delivery of data frames from the MAC in one station to the MAC in one or more other station, with minimal duplication and minimal ordering.

Distribution Services provide services necessary to allow mobile stations to roam freely within an ESS and allow an IEEE 802.11 WLAN to connect with the wired LAN infrastructure. A thin layer between MAC and LLC sublayer that are invoked to determine how to forward frames within the IEEE 802.11 WLAN and also how to deliver frames from the IEEE 802.11 WLAN to network destinations outside of the WLAN.