In IEEE 802.11 the addressable unit is a station (STA), which is a message destination, but not (in general) a fixed location. IEEE 802.11 handles both mobile and portable stations. Mobile Stations (MSs) access the LAN while in motion, whereas a Portable Station (PS) can be moved between locations, but it is used only at a fixed location. MSs are often battery powered, and power management is an important consideration since we cannot assume that a station’s receiver will always be powered on.
IEEE 802.11 appears to higher layers [logical link control (LLC)] as an IEEE 802 LAN, which requires that the IEEE 802.11 network handles station mobility within the Medium Access Control (MAC) sublayer, and meets the reliability assumptions that LLC makes about the lower layers.
The IEEE 802.11 architecture provides a WLAN supporting station mobility transpar- ently to upper layers. The Basic Service Set (BSS) is the basic building block consisting of member stations remaining in communication. If a station moves out of its BSS, it can no longer directly communicate with other members of the BSS.
The Independent BSS (IBSS) is the most basic type of IEEE 802.11 LAN and may consist of at least two stations that can communicate directly. This LAN is formed only as long as it is needed and is often referred to as an ad hoc network. The association between an STA and a BSS is dynamic, since STAs turn on and off, come within range and go out of range.
A BSS may form the Distribution System (DS), which is an architectural component used to interconnect BSSs. IEEE 802.11 logically separates the Wireless Medium (WM) from the Distribution System Medium (DSM). Each logical medium is used for different purposes by a different component of the architecture. The IEEE 802.11 LAN architecture is specified independently of the physical characteristics of any specific implementation. The DS enables mobile device support by providing the logical services necessary to handle address-to-destination mapping and seamless integration of multiple BSSs. An Access Point (AP) is an STA that provides access to the DS by providing DS ser- vices in addition to acting as an STA. The data move between a BSS and the DS
via an AP. All APs are also STAs, and they are addressable entities. The addresses used by an AP for communication on the WM and on the DSM are not necessarily the same.
The DS and BSSs allow IEEE 802.11 to create a wireless network of arbitrary size and complexity calledExtended Service Set(ESS)network. The ESS network appears the same to an LLC layer as an IBSS network. Stations within an ESS may communicate and MSs may move from one BSS to another (within the same ESS), transparently to LLC.
A portal is the logical point at which MAC Service Data Units (MSDUs) from an integrated non-IEEE 802.11 LAN enter the IEEE 802.11 DS. All data from non-IEEE 802.11 LANs enter the IEEE 802.11 architecture via a portal, which provides logical integration between the IEEE 802.11 architecture and existing wired LANs. A device may offer both the functions of an AP and a portal, for example, when a DS is implemented from IEEE 802 LAN components.
Architectural services of IEEE 802.11 are as follows: authentication, association, deau- thentication, disassociation, distribution, integration, privacy, reassociation, and MSDU delivery. These services are provided either by stations as the Station Service (SS) or by the DS as the Distribution System Service (DSS).
The SS includes authentication, deauthentication, privacy, and MSDU delivery. The SS is present in every IEEE 802.11 station, including APs, and is specified for use by MAC sublayer entities.
The DSSs include association, disassociation, distribution, integration, and reassociation. The DSSs are provided by the DS and are accessed by an AP, which is an STA that also provides DSSs. DSSs are specified for use by MAC sublayer entities.
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The IEEE 802.11 architecture handles multiple logical media and address spaces and is independent of the DS implementation. This architecture interfaces cleanly with network layer mobility approaches.
8.4.1 IEEE 802.11a and IEEE 802.11b
IEEE 802.11a is an extension to 802.11 that applies to WLANs and provides up to 54 Mbps in the 5-GHz band. IEEE 802.11a uses an Orthogonal Frequency Division Multiplexing (OFDM) encoding scheme rather than FHSS or DSSS.
The IEEE 802.11a standard is designed to operate in the 5-GHz Unlicensed National Information Infrastructure (UNII) band. Specifically, the FCC has allocated 300 MHz of spectrum for unlicensed operation in the 5-GHz block, 200 MHz of which is at 5.15 to 5.35 MHz, with the other 100 MHz at 5.725 to 5.825 MHz. The spectrum is split into three working domains. The first 100 MHz in the lower section is restricted to a maximum power output of 50 mW (milliwatts). The second 100 MHz has 250-mW power output, and the top 100 MHz is used for outdoor applications with a maximum of 1 watt power output.
IEEE 802.11b, also referred to as 802.11 High Rate or Wi-Fi (Wireless Fidelity), is an extension to 802.11 that applies to WLANs and provides 11-Mbps transmission (with a fallback to 5.5, 2, and 1 Mbps) in the 2.4-GHz band. IEEE 802.11b uses only DSSS. IEEE 802.11b was a 1999 ratification to the original 802.11 standard, allowing wireless functionality comparable to Ethernet.
The IEEE 802.11b specification allows for the wireless transmission of approximately 11 Mbps of data at distances from several dozen to several 100 ft over the 2.4-GHz (2.4 to 2.483) unlicensed RF band. The distance depends on impediments, materials, and LOS.
IEEE 802.11b standard defines two bottom levels of OSI reference model – the Phys- ical Layer (PHY) and the Data Link Layer (MAC sublayer).
IEEE 802.11b defines two pieces of equipment, a wireless station, which is usually a PC or a Laptop with a wireless Network Interface Card (NIC), and an Access Point (AP), which acts as a bridge between the wireless stations and Distribution System or wired networks. There are two operation modes in IEEE 802.11b, Infrastructure Mode and Ad Hoc Mode.
Infrastructure Mode consists of at least one AP connected to the Distribution System. An AP provides a local bridge function for the BSS. All wireless stations communicate with the AP and no longer communicate directly. All frames are relayed between wireless stations by the AP.
An ESS is a set of infrastructure BSSs, in which the APs communicate amongst themselves to forward traffic from one BSS to another to facilitate movement of wireless stations between BSSs.
The wireless stations communicate directly with each other. Every station may not be able to communicate with every other station because of the range limitations. There are no APs in an IBSS. Therefore all stations need to be within range of each other and they communicate directly.
IEEE 802.11b defines dynamic rate shifting, allowing data rates to be automatically adjusted for noisy conditions. This means IEEE 802.11b devices will transmit at lower speeds, 5.5 Mbps, 2 Mbps, and 1 Mps under noisy conditions. When the devices move
back within range of a higher speed transmission, the connection will automatically speed up again.