Wireless LAN 802.11 Architecture The standard defines two kinds of services: the basic services set (BSS) and theextended service set (ESS). Basic service Set IEEE 802.11 defines the basic service set (BSS) at the building block of wireless LAN.A basic service set is made of stationary or mobile wireless stations and a possiblecontrol base station known as the Access Point (AP). The BSS without an AP is a stand-alone network and cannot find data to other BSSS.It is what is called an ad-hoc architecture. In this architecture, stations can form anetwork without the need of an AP ; they can locate each other and agree to be part ofBSS. Station Types IEEE 802.11 defines three types of stations on their mobility in a wireless LAN: no-transition, BSS-transition, and ESS-transition. No-Transition mobility: A station with no-transition mobility (riot moving) or movingonly inside a BSS. BSS-Transition mobility: A station with BSS-transition mobility can move from oneBSS to another, but the movement is confined inside one ESS. ESS- Transition mobility. A station with ESS-transition mobility can move from oneBSS to another. However, IEEE802.11 does not guarantee that communication iscontinuous during the move. Physical layer. Defines specification for the conversion of bits to signal in the physicallayer. Upper Layer 802.11 infrared 802.11 FHSS 802.11 DSSS 802.11a OFDM 802.11b HR-DSSS 802.11g OFDM IEEE 802.11 FHSS FHSS describes the frequency-hopping spread spectrum (FHSS) method for signalgeneration in a 2.4 GHz ISM band.
It uses PSK at 1 M baud is 15. The system allows 1 or 2 bits/s and two-level FSK orfour-level FSK which results in a data rate of 1 or 2 Mbps. IEEE 802.11 DSSS describes the (direct sequence spread spectrum (DSSS)) Method forsignal generation in a 2.4 GHz ISM band. Band DSSS uses a 2.4 GHZ ISM band. Thebit sequence assuming entire band. Modulation and Data rate. The modulation techniques in this specification is PSK atMbaud/s. The system allows 1 or 2 bits/baud (BPSK or QPSM), which results in adata rate of 1 or 2 Mbps. IEEE 802.1 1a OFDM IEEE 802.11 a OFDM describes the orthogonal frequency density multiplexing (OFDM)method for signal generation in a 5 GHz band. OFDM: OFDM is the same as FDM, with one major difference. All the subband areused by one source at a given time. Sources contend with one another at the data linklayer for access. Modulation and data rate. OFDM uses PSK and QAM for modulation. The commondata rates as 18 Mbps (PSK) and 54 Mbps (QAM) Quadrature amplitudes modulation(QAM). It is combination of ASK& QSK IEEE 802.11b HR DSSS High Rate directsequence spread spectrum (DSSS). HR-DSS Complementary Code Keying (CCK) URencodes 4 or 8 bits to the (CK symbol). Band the specification uses a 2.4 GHz ISMband. IEEE 802.11g OFDM This relatively new specification use OFDM with 2.4 GHz ISM band. The complexmodulation techniques achieve a 54 Mbps data rate.