Hardware manual

Introduction 1
Nov 98
Introduction
Because wireless LAN technology is still new and dynamic, Proxim has created this guide to outline
considerations for both the writer to address in wireless review "write-ups" and the reviewer or IT manager to
address in wireless comparison testing.
To an end user, well designed wireless LANs provide equivalent functionality to wired networks and seamless
access to network resources without the need to be physically tethered. To an IT manager, wireless LANs
should be as simple, or simpler, to install and administrate. While ideal goals, the technical challenges
associated with wireless technology make attainment more difficult than it might first seem. This document
will discuss the background and describe tests that will determine the robustness and suitability of a wireless
LAN product for mobile applications.
Technology overview
The de-facto communication standard for wireless LANs is spread spectrum, a radio technique which
broadcasts signals over a range of frequencies, thus avoiding the lack of interference immunity often
associated with a narrowband system. Users today can choose from two very different spread spectrum RF
technologies for 2.4 GHz wireless LANs, Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping
Spread Spectrum (FHSS).
DSSS
DSSS avoids excessive power concentration by spreading the signal over a wider frequency band. Each bit of
data is mapped into a pattern of "chips" by the transmitter. At the destination the chips are mapped back into
a bit, recreating the original data. Transmitter and receiver must be synchronized to operate properly.
The ratio of chips per bit is called the "spreading ratio". A high spreading ratio increases the resistance of the
signal to interference. A low spreading ratio increases the net bandwidth available to a user.
Several DSSS products in the market allow users to deploy more than one channel in the same area. They
accomplish this by separating the 2.4 GHz band into multiple sub-bands, each of which contains an
independent DSSS network. Because DSSS truly spreads across the spectrum, the number of independent,
non-overlapping channels in the 2.4 GHz band is small, typically there are only three. Only a very limited
number of co-located networks can therefore operate without interference.
FHSS
FHSS spreads the signal by operating on one frequency for a short duration, "hopping" to another frequency
for another short burst and so on. The source and destination of a transmission must be synchronized so they
are on the same frequency at the same time. The hopping pattern (frequencies and order in which they are
used) and dwell time (time at each frequency) are restricted by most regulatory agencies. For example, the
FCC requires that 75 or more frequencies be used and a maximum dwell time of 400 ms.
Because of its frequency-agile nature, FHSS is more immune to outside interference than DSSS. Unlike a
DSSS radio which is stationary at a preselected frequency, an FHSS radio cannot be blocked by a single