MODULE IIIIEEE 802.11 Wireless LAN Standard

IEEE 802.11 Wireless LAN StandardPhysical LayerThree physical media are defined in the

original 802.11 standard:Direct sequence spread spectrum

(DSSS)operating in the 2.4-GHz ISM band, at

data rates of 1 Mbps and 2 Mbps.Frequency-hopping spread spectrum

(FHSS)operating in the 2.4-GHz ISM band, at

data rates of 1 Mbps and 2 Mbps.Infrared

at 1 Mbps and 2 Mbps operating at a wavelength between 850 and 950nm

IEEE 802.11 Wireless LAN StandardDirect Sequence Spread Spectrum

Up to three non overlapping channels, each with a data rate of 1 Mbps or 2 Mbps, can be used in the DSSS scheme.

Each channel has a bandwidth of 5 MHz.

Frequency-Hopping Spread SpectrumFHSS system makes use of a multiple

channels, with the signal hopping from one channel to another based on a pseudonoise sequence.

In the case of the IEEE 802.11 scheme, I-MHz channels are used.

IEEE 802.11 Wireless LAN StandardInfrared

The IEEE 802.11 infrared scheme is omnidirectional rather than point to point.

A range of up to 20 m is possible.The modulation scheme for the 1-Mbps

data rate is known as 16-PPM (pulse position modulation).

IEEE 802.11 Wireless LAN StandardIEEE 802.11 FHSSIEEE 802.11 FHSS uses the frequency-

hopping spread spectrum (FHSS) methodFHSS uses the 2.4-GHz ISM band.The band is divided into 79 subbands of 1

MHz (and some guard bands).A pseudorandom number generator

selects the hopping sequence.The modulation technique in this

specification is either two-level FSK or four-level FSK with I or 2 bitslbaud, which results in a data rate of 1 or 2 Mbps,

IEEE 802.11 Wireless LAN Standard

IEEE 802.11 Wireless LAN StandardIEEE 802.11 DSSSIEEE 802.11 DSSS uses the direct

sequence spread spectrum (DSSS) methodDSSS uses the 2.4-GHz ISM band.The modulation technique in this

specification is PSK at 1 Mbaud/s.The system allows 1 or 2 bitslbaud (BPSK

or QPSK), which results in a data rate of 1 or 2 Mbps,

IEEE 802.11 Wireless LAN Standard

IEEE 802.11 Wireless LAN StandardIEEE 802.lla OFDMIEEE 802.Ila OFDM describes the

orthogonal frequency-division multiplexing (OFDM) method for signal generation in a 5-GHz ISM band

All the subbands are used by one source at a given time.

Sources contend with one another at the data link layer for access.

The band is divided into 52 subbands, with 48 subbands for sending 48 groups of bits at a time and 4 subbands for control information.

Dividing the band into subbands diminishes the effects of interference

IEEE 802.11 Wireless LAN StandardIf the subbands are used randomly, security

can also be increased.OFDM uses PSK and QAM for modulation.

The common data rates are 18 Mbps (PSK) and 54 Mbps (QAM).

IEEE 802.llb DSSSIEEE 802.11 b DSSS describes the high-

rate direct sequence spread spectrum (HRDSSS) method for signal generation in the 2.4-GHz ISM band.

HR-DSSS is similar to DSSS except for the encoding method, which is called complementary code keying (CCK).

IEEE 802.11 Wireless LAN StandardCCK encodes 4 or 8 bits to one CCK

symbol.To be backward compatible with DSSS, HR-

DSSS defines four data rates: 1,2, 5.5, and 11 Mbps.

The first two use the same modulation techniques as DSSS.

The 5.5-Mbps version uses BPSK and transmits at 1.375 Mbaudls with 4-bit CCK encoding.

The II-Mbps version uses QPSK and transmits at 1.375 Mbps with 8-bit CCK encoding.

IEEE 802.11 Wireless LAN Standard