Wireless Ethernet Bridge
31
Instant Wireless
TM
Series
30
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 devel-
oped by the military for use in reliable, secure, mission-critical communica-
tions 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 transmis-
sion, 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.
Would the information be intercepted while transmitting on air?
Instant Wireless products feature two-fold protection in security. On the hard-
ware side, as with Direct Sequence Spread Spectrum technology, it has the
inherent security feature of scrambling. On the software side, Instant Wireless
products offer the encryption function (WEP) to enhance security and access
control. Users can set it up depending upon their needs.
What is ad-hoc mode?
When a wireless network is set to ad-hoc mode, the wireless-equipped com-
puters are configured to communicate directly with each other. The ad-hoc
wireless network will not communicate with any wired network.
What is infrastructure mode?
When a wireless network is set to infrastructure mode, the wireless network is
configured to communicate with a wired network through a wireless access
point.
What is roaming?
Roaming is the ability of a portable computer user to communicate continu-
ously while moving freely throughout an area greater than that covered by a sin-
gle wireless network access point. Before using the roaming function, the
workstation must make sure that it is the same channel number as the wireless
network access point of the dedicated coverage area.
To achieve true seamless connectivity, the wireless LAN must incorporate a
number of different functions. Each node and wireless network access point,
for example, must always acknowledge receipt of each message. Each node
must maintain contact with the wireless network even when not actually trans-
mitting data. Achieving these functions simultaneously requires a dynamic RF
networking technology that links wireless network access points and nodes. In
such a system, the user’s end node undertakes a search for the best possible
access to the system. First, it evaluates such factors as signal strength and qual-
ity, as well as the message load currently being carried by each wireless net-
work access point and the distance of each wireless network access point to the
wired backbone. Based on that information, the node next selects the right
wireless network access point and registers its address. Communications
between end node and host computer can then be transmitted up and down the
backbone.
As the user moves on, the end node’s RF transmitter regularly checks the sys-
tem to determine whether it is in touch with the original wireless network
access point or whether it should seek a new one. When a node no longer
receives acknowledgment from its original wireless network access point, it
undertakes a new search. Upon finding a new wireless network access point, it
then re-registers, and the communication process continues.
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