Cisco Aironet 1520 Series User Manual download pdf (Page 2)

Each set of bands 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 are subject to greater

attenuation from solid objects.

802.11 Modulation Techniques

The IEEE 802.11 standard makes provisions for the use of several different modulation techniques to encode the

transmitted data onto the RF signal. These modulation techniques are used to enhance the probability of the

receiver correctly receiving the data and thus reducing the need for retransmissions. The techniques vary in their

complexities and robustness to RF signal propogation impairments.

Direct-Sequence Spread Spectrum

The direct-sequence spread spectrum (DSSS) 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 spread spectrum (FHSS) 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

400 milliseconds. For the 1- and 2-Mb FHSS systems, the hopping pattern must include 75 different channels, and

must use every channel before reusing any one. For wide-band frequency hopping (WBFH) systems, which permit

up to 10-Mb data rates, the rules require the use of at least 15 channels, and they cannot overlap. With only 83

MHz of spectrum, WBFH limits the systems to 15 channels, thus causing scalability issues.

In every case, for the same transmitter power and antennas, a DSSS system will have greater range, scalability,

and throughput than an FHSS system. For this reason, Cisco has chosen to support only direct-sequence systems

in the spread spectrum products.

Orthogonal Frequency Division Multiplexing

The orthogonal frequency division multiplexing (OFDM) used in 802.11a and 802.11g data transmissions offers

greater performance than the older direct-sequence systems. In the OFDM system, each tone is orthogonal to the

adjacent tones and therefore does not require the frequency guard band needed for direct sequence. This guard

band lowers the bandwidth efficiency and wastes up to 50 percent of the available bandwidth. Because OFDM is

composed of many narrow-band tones, narrow-band interference degrades only a small portion of the signal, with

little or no effect on the remainder of the frequency components.

Antenna Properties and Ratings

An antenna gives the wireless system three fundamental properties - gain, direction, and polarization. Gain is a

measure of increase in power. Direction is the shape of the transmission pattern. A good analogy for an antenna

is the reflector in a flashlight. The reflector concentrates and intensifies the light beam in a particular direction

similar to what a parabolic dish antenna would do to a RF source in a radio system.

Antenna gain is measured in decibels, which is a ratio between two values. The gain of a specific antenna is

compared to the gain of an isotropic antenna. An isotropic antenna is a theoretical antenna with a uniform three-

dimensional radiation pattern (similar to a light bulb with no reflector). dBi is used to compare the power level of a

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Cisco Aironet 1520 Series User Manual Page 2