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Radio Technologies
In the mid 1980s, the FCC modified Part 15 of the radio spectrum regulation, which governs unlicensed devices. The
modification-authorized wireless network products to operate in the Industrial, Scientific, and Medical (ISM) bands using
spread spectrum modulation. This type of modulation had formerly been classified and permitted only in military products.
The ISM frequencies are in three different bands, located at 900 MHz, 2.4 GHz, and 5 GHz. This document covers only the
2.4GHz band.
The ISM bands typically allow users to operate wireless products without requiring specific licenses, but this will vary in some
countries. In the U.S., there is no requirement for FCC licenses. The products themselves must meet certain requirements to be
certified for sale, such as operation under 1 watt transmitter output power (in the U.S.) and maximum antenna gain or EIRP ratings.
Radio Frequency Fundamentals
Radio Frequency (RF) engineering is a very complex field of study and far too involved to explain in detail here.
2.4GHz Spectrum
The Cisco Aironet 350 Series uses RF spectrum in the 2.4 GHz unlicensed ISM band. In the U.S., three bands are defined as
unlicensed and known as the ISM bands (Industrial, Scientific, and Medical). The ISM bands are as follows:
• 900 MHz (902-928MHz)
• 2.4 GHz (2.4 - 2.4835 GHz)—IEEE 802.11b
• 5 GHz (5.15-5.35 and 5.725-5.825 GHz)—IEEE 802.11a, HIPERLAN/1 and HIPERLAN/2. This band is also known
as the UNII band.
Each range has different characteristics. The lower frequencies exhibit better range, but with limited bandwidth and hence lower
data rates. The higher frequencies have less range and subject to greater attenuation from solid objects.
Direct Sequence Spread Spectrum
The Direct Sequence (DS) Spread Spectrum approach involves encoding redundant information into the RF signal. Every data bit
is expanded to a string of chips called a chipping sequence or Barker sequence. The chipping rate as mandated by the U.S. FCC is
10 chips at the 1 and 2 Mbps rates and 8 chips at the 11 Mbps rate. So, at 11 Mbps, 8 bits are transmitted for every one bit of data.
The chipping sequence is transmitted in parallel across the spread spectrum frequency channel.
Frequency Hopping Spread Spectrum
Frequency Hopping (FH) Spread Spectrum uses a radio that moves or hops from one frequency to another at predetermined times
and channels. The regulations require that the maximum time spent on any one channel is 400mS. For the 1- and 2-Mb FH systems,
the hopping pattern must include 75 different channels, and must use every channel before reusing any one. For the Wide Band
Frequency Hopping (WBFH) systems, that permit up to 10-Mb data rates, the rules require use of at least 15 channels, and they
cannot overlap. With only 83MHz of spectrum, it limits the systems to 15 channels, thereby causing scalability issues.
In every case, for the same transmitter power and antennas, a DS system will have greater range, scalability and throughput than
an FH system. For this reason Cisco has chosen to support only DS systems in the Spread Spectrum products.
