2
The IEEE 802.11 protocols are implemented in the firmware
so that file transfers or database access can begin
immediately.
Direct Sequence Spread Spectrum Approach
The use of spread spectrum techniques for wireless
computer communications is widely accepted because of
its robustness against multipath effects and interference
from intentional or unintentional radiators. The use of
spread spectrum techniques in the ISM frequency band
also allows products to be deployed without the need for an
FCC license.
The two main methods by which spread spectrum
communications can be achieved are Direct Sequence
Spread Spectrum (DSSS) and Frequency Hopping Spread
Spectrum (FHSS). This wireless LAN PC card uses the
DSSS technique. DSSS transmission has the best
performance in terms of multipath immunity and jamming
rejection. In an office environment, jamming sources are
likely to be unintentional such as emissions from
microwave ovens. Even though unintentional, they pose a
threat to the communications network. Direct sequence
techniques are superior to frequency hopping systems in
this case because FHSS gains its immunity to jamming by
avoiding the location of a single tone jammer (such as
other FHSS users). When collisions occur, data is lost.
With a DSSS system, the despreading function in the
receiver gives immunity to jamming by spreading the
interfering energy by the Pseudo Random Number (PN)
code over the whole bandwidth. This selective despreading
attenuates the jamming power while despreading the
desired signal.
In the office environment, multipath effects may degrade
network communications. Direct sequence techniques offer
better protection than slower frequency hopping systems in
the presence of multipath interference. With frequency
hopped systems, if the hopper jumps to a frequency where a
null resides, then data is lost until the next hop. Multipath
signals can be thought of as a special case of unintentional
jamming. In the DSSS approach, nulls resulting from
multipath fading only eliminate a fraction of the signal power
since the bandwidth in the DSSS case is very large. A
significant amount of energy still remains in the signal and
effective despreading still occurs. The probability of burst
errors is reduced significantly.
An often overlooked factor when comparing IEEE 802.11
compliant DSSS and FSSS implementations, is the
achievable data rate. A frequency hopping occupied
bandwidth of 1MHz as specified by the FCC acts as a
limitation when using data rates beyond 2Mbps. A similar
bandwidth limitation has not been imposed when using the
direct sequence implementation. In the new 802.11 high
data rate (11Mbps) standard utilizing Complementary Code
Keying (CCK) modulation, the 5-1/2 times increase in data
rate has been achieved in the same 17MHz bandwidth! This
is accomplished by encoding 6 bits of data in one out of a
possible 64 orthogonal PN spreading sequences. More
information on the new high data rate standard may be found
in Applications Note AN9850 “Complementary Code Keying
Made Simple” which may be found on the Intersil Web Site.
Installation of HWB3163 Windows 95/98
Drivers
PRISM Test Utility (PTU) Software
Installation
NOTE: Perform after Windows Driver Installation.
Step 1. With the PRISM PCMCIA card removed, boot your
PC under Microsoft Windows 95 or 98.
Step 2. Once your system has booted and is idle, insert
PRISM II Driver for Windows, Disk #1 into the “A”
Floppy Drive. On the Desktop, left click on
<Start> -> <RUN> then type A:SETUP <Enter>
Follow the on-screen instructions. Accept all defaults.
Step 3. When the preliminary installation is complete,
insert the wireless LAN PC card in the lower
PCMCIA slot (see Hardware Installation below).
Step 4. Windows should automatically recognize that the
card has been inserted. It then displays a dialog box
titled “New Hardware Found”.
Step 5. Insert PRISM II Driver for Windows, Disk #1 into
the floppy drive. Tell Windows that the driver is
located on drive “A”. Accept all defaults.
Step 6. Follow the on-screen instructions to complete
installation of the driver. When complete, the NDC
driver icon should appear in the system area on the
desktop (computer monitor with antenna). Clicking on
this icon enables setting of channel, mode, etc.
Step 7. If operating in the Pseudo IBSS mode, you must
assign a unique IP address to the computer in
order for the card to be operable. Left click on
<Start> -> <Settings> -> <Control Panel> .
Double click on Network. Select TCP/IP ... PRISM
IEEE 802.11 PC Card .... and click on Properties.
Select the IP Address tab. Click on Obtain an IP
Address. Enter a valid IP address. Enter a valid
Subnet Mask (suggest 255 255 255 0). Click
on OK.
Step 1. Insert the PTU disk #1 into the floppy drive.
Step 2. On the Desktop, left click on <Start> -> <RUN>
then type A:setup <Enter>. Follow the on-screen
instructions. Accept all defaults. When the
installation is complete, an icon should
automatically appear on the Desktop.
Application Note 9864
