Frequency Hopping Spread Spectrum (FHSS) and Infrared (IR) are two of the
several available PHY specifications. IR and FHSS are not widely used today.
DSSS and OFDM are the most common technologies currently in use today.
FHSS
The 802.11 standard defines a set of FH channels that are
evenly spaced across the 2.4- GHz band. The number of channels, as with DSSS,
depends on geography. There can be up to 79 channels in North America and most
of Europe, and 23 channels in Japan. The exact frequency range varies slightly
by location.
The FHSS PMD transmits PPDUs by hopping from channel to
channel, according to a particular pseudo-random hopping sequence that
uniformly distributes the signal across the operating frequency band. After the
hopping sequence is set in an AP, stations will automatically synchronize to
the correct hopping sequence. Three sets of valid hopping sequences are
defined.
The PMD uses a two-level Gaussian frequency shift key (GFSK)
modulation to transmit at 1 Mbps. A four-level GFSK modulation sine is used for
transmitting at 2 Mbps. Stations operating at 2 Mbps must also be able to
operate at 1 Mbps.
Infrared (IR) PHY
The IR PHY uses
near-visible light in the 850-nm to 950-nm range for signaling. This is similar
to the spectral usage of common consumer devices such as infrared remote
controls, as well as other data communications equipment, such as Infrared Data
Association (IrDA) devices. Unlike many other infrared devices, the IR PHY is
not directed. This means that the receiver and transmitter do not have to be
aimed at each other and do not need a clear line-of-sight. This permits easier
construction of a wireless WLAN system. A pair of conformant infrared devices
would be able to communicate in a typical environment at a range up to about 10
m (33 ft). This standard allows more sensitive receivers, which may increase
range up to about 20 m (66 ft).
The IR PHY relies on both reflected infrared energy and line-of-sight
infrared energy for communications. Most designs anticipate that all of the
energy at the receiver is reflected energy. This reliance on reflected infrared
energy is called diffuse infrared transmission.
The IR PHY will operate
only in indoor environments. Infrared radiation does not pass through walls,
and is significantly attenuated passing through most exterior windows. This
characteristic can be used to contain an IR PHY in a single physical room such
as a classroom or conference room. Different LANs using the IR PHY can operate
in adjacent rooms separated only by a wall without interference and without the
possibility of eavesdropping.
There are currently no worldwide frequency
allocation or bandwidth allocation regulatory restrictions on infrared
emissions. The emitter, typically a light-emitting diode (LED), and detector,
typically a PIN diode used in infrared communications, are relatively
inexpensive at the infrared wavelengths specified in the IR PHY and at the
electrical operating frequencies required by this PHY
.