Cisco 10-ft Low Loss Cable AIR-CAB010LL-N= Manual De Usuario
Los códigos de productos
AIR-CAB010LL-N=
Reference Guide
© 2009 Cisco Systems, Inc. All rights reserved. This document is Cisco Public Information.
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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 are
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 400 milliseconds. 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 83 MHz of spectrum,
it limits the systems to 15 channels, thus 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.
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 DS systems. In the OFDM system, each
tone is orthogonal to the adjacent tones and therefore does not require the frequency guard band
needed for DS. This guard band lowers the bandwidth efficiency and wastes up to 50 percent of
the available bandwidth. Because OFDM is composed of many narrowband tones, narrowband
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 to a RF source in a radio system.
Antenna gain ratings are in decibels which is a ratio between two values. An antenna rating is
typically to the gain of an isotropic or dipole 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 given antenna to the theoretical isotropic antenna. The U.S.
FCC uses dBi in its calculations. An isotropic antenna is said to have a power rating of 0 dB; for
example, zero gain/loss when compared to itself.