Cisco WRTP54G User Manual
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Wireless-G Broadband Router with 2 Phone Ports
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Appendix A: Troubleshooting
Frequently Asked Questions
acknowledgment from its original access point, it undertakes a new search. Upon finding a new access point, it
then re-registers, and the communication process continues.
What is ISM band?
The FCC and their counterparts outside of the U.S. have set aside bandwidth for unlicensed use in the ISM
(Industrial, Scientific and Medical) band. Spectrum in the vicinity of 2.4 GHz, in particular, is being made
available worldwide. This presents a truly revolutionary opportunity to place convenient high-speed wireless
capabilities in the hands of users around the globe.
What is Spread Spectrum?
Spread Spectrum technology is a wideband radio frequency technique developed by the military for use in
reliable, secure, mission-critical communications systems. It is designed to trade off bandwidth efficiency for
reliability, integrity, and security. In other words, more bandwidth is consumed than in the case of narrowband
transmission, but the trade-off produces a signal that is, in effect, louder and thus easier to detect, provided that
the receiver knows the parameters of the spread-spectrum signal being broadcast. If a receiver is not tuned to
the right frequency, a spread-spectrum signal looks like background noise. There are two main alternatives,
Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS).
What is DSSS? What is FHSS? And what are their differences?
Frequency-Hopping Spread-Spectrum (FHSS) uses a narrowband carrier that changes frequency in a pattern
that is known to both transmitter and receiver. Properly synchronized, the net effect is to maintain a single
logical channel. To an unintended receiver, FHSS appears to be short-duration impulse noise. Direct-Sequence
Spread-Spectrum (DSSS) generates a redundant bit pattern for each bit to be transmitted. This bit pattern is
called a chip (or chipping code). The longer the chip, the greater the probability that the original data can be
recovered. Even if one or more bits in the chip are damaged during transmission, statistical techniques
embedded in the radio can recover the original data without the need for retransmission. To an unintended
receiver, DSSS appears as low power wideband noise and is rejected (ignored) by most narrowband receivers.