Atlantis Land A02-IPCAM4-W54 - Manual (Page 58)

NetCamera NVW
A02-IPCAM4-W54 59
Answer If a firewall exists on the network, port 80 is open for ordinary
data communication.
Question What is Spread Spectrum?
Answer 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).
Question What is DSSS? What is FHSS? And what are their
differences?
Answer 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.