[NCLUG] Wireless cards and Linux

[John L. Bass]

802.11 wireless networks are designed to operate in two completely different modes,
the first is with an Access Point (AP), the other is as a collection of peers (Adhoc).
The AP mode requires that all clients be able to reach the AP, since no peer-peer
conversations are allowed, and the AP generally plays the role of an ethernet bridge,
and wireless repeater/bridge. Mfgr's claim AP mode gives you twice the range over
Adhoc mode, in that two clients can talk that are on opposite sides of the maximum
range from the AP. So AP's are normally placed in the center of a coverage area,
or multiple AP's in slightly overlapping coverage areas with a wired backbone, the
later allows roaming between AP in a large area.

Adhoc mode is not quite as limiting as first blush, but does require that more
interesting network technologies be built to span past the range of two devices.
For a collection of devices with static locations, a larger network is relatively
easy to build with host routes on a single subnet, or possibly with a collection
of subnets and routes (static or dynamic). Adding roaming devices into an Adhoc
network is quite a bit more challenging, but possible using dynamic routing (gated)
and dynamic address re-assignment (DHCP). Adhoc roaming is relatively easy to do
with some simple scripts using linux, impossible to a bit tougher with WinDoze
(requires forcing pcmcia eject/insertion processes completely from scripts).

Both of the above are completely independent of the radio technology used by the
802.11 device. There are two completely different radio technologies used, FHSS
(Breezecom and Webgear products) and DSSS (Lucent/Prism chipsets used by a large
number of companies like Lucent, Apple, Aironet). The FHSS radios hop (pseudo)
randomly between 26 channels that are 1 MHz wide with a 3MHz spacing, using
standard modulation techniques. Channels which are busy (have interference) are
just avoided.  DSSS radios uses an advanced military spread spectrum technique
where a single 10 MHz wide carrier is rapidly shifted +/- 5 MHz off the channel
center frequency several times per bit. This produces channels which are 22 Mhz
wide, and every bit has energy across the entire channel width. Using advanced
FFT techniques, the channel is broken into a number of slices, normalized to the
noise floor, and are then added togather, recovering a signal which may have
average energy in the spectrum at or below the noise floor. Each technology has
it's advantages, and disavantages - each working in environments where the other
has difficulty.

The 802.11b spec, extends the original 802.11 DSSS spec from 1-2 mbps, to
include 5.5 and 11 mbps encoding rates in the 2.4Ghz band. Other additions
to the 802.11 spec include 5.7Ghz band operation and standardizing roaming
between multiple AP's of different vendors (using the same radio technology).

There are also non-802.11 devices in the band using variations of the above
technologies. There are DSSS phones and video extenders, which use a narrower
channel/carrier. There are FHSS phones, and other devices too, with a variety
of channel/hopping configurations. Plus newer very low power data devices using
the Bluetooth and Home RF standards.

A lot of choices, all which have a huge matrix of interoperability and co-location
compatability issues.

John