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172 Chapter 4 • Communication Security: Wireless

Two methods of synchronizing wireless devices are:

■ Frequency hopping spread spectrum (FHSS)

■ Direct sequence spread spectrum (DSSS)

Frequency Hopping Spread Spectrum

As the name implies, FHSS works by quickly moving from one frequency to
another according to a psuedorandom pattern.The frequency range used by the fre-
quency hop is relatively large (83.5 MHz), providing excellent protection from
interference.The amount of time spent on any given frequency is known as dwell
time and the amount of time it takes to move from one frequency to another is
known as hop time. FHSS devices begin their transmission on one frequency and
move to other frequencies according to a pre-defined psuedorandom sequence and
then repeat the sequence after reaching the final frequency in the pattern. Hop time
is usually very short (200 to 300 Ìs) and not significant relative to the dwell time
(100 to 200 ms). In general, the longer the dwell time, the greater the throughput
and the more susceptible the transmission is to narrowband interference.
The frequency hopping sequence creates a channel, allowing multiple channels
to coexist in the same frequency range without interfering with each other.As
many as 79 FCC-compliant FHSS devices using the 2.4 GHz ISM band can be
co-located together. However, the expense of implementing such a large number of
systems limits the practical number of co-located devices to well below this
number.Wireless networks that use FHSS include HomeRF and Bluetooth, which
both operate in the unlicensed 2.4 GHz ISM band. FHSS is less subject to EM
interference than DSSS, but usually operates at lower rates of data transmission
(usually 1.6 Mbps, but can be as high as 10 Mbps) than networks that use DSSS.

Bluetooth
Head of the Class… 79 different frequencies within a radio band. Unlike 802.11 networks
Bluetooth uses the same 2.4 GHz frequency that the IEEE 802.11 wireless
networks use but, unlike those networks, Bluetooth can select from up to

where the wireless client can only be associated with one network at a
time, Bluetooth networks allow clients to be connected to seven net-
works at the same time. However, one of the main reasons that
Bluetooth never succeeded like the 802.11 standard did is because of its
low bandwidth capabilities and a lack of range.

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