more than spread spectrum

[Phil Karn]

This idea of setting up your own independent network isn't as far fetched
as it sounds, at least in a relatively local area with critical mass.

The router nodes already exist, in the form of my own TCP/IP code for
the PC, or any of several other packages including the various freeware
UNIX clones.

Radio transmission equipment operating under Part 15 of the FCC rules
is already available. Under Part 15, you don't need a license; the
radio transmitter does have to be "type accepted" by the FCC but since
you buy that as a prepackaged box you don't have to worry about it.

The section of Part 15 that is particularly interesting is either
15.247 or 15.249 (can't remember which).  It allows you to run up to 1
watt of power, quite a bit for an unlicensed service, on any of
several "ISM" (aka "garbage") bands as long as you use spread spectrum
with a minimum processing gain of 10dB. The most popular is 902-928
Mhz, with another band in the vicinity of 2450 Ghz coming up fast.
Ahthough most Part 15 equipment is designed primarily for use within
office environments, it can be and is used over point-to-point paths
of 5-10 km with the proper directional antennas.

Although Part 15 users are not allowed to cause harmful interference
and must accept interference from other users (microwave ovens
generally operate on 2450 Mhz), in practice these nets seem to work
pretty well if they're properly engineered. Besides, the inherent
distributed redundancy available in a packet network should be able to
compensate for momentary outages due to interference.

Unlike the amateur service, which has some particularly draconian
"acceptable use" policies (despite a recent liberalization, encryption
is still illegal), there are *no* restrictions on the use of Part 15
equipment. Encryption is not only legal, some boards have hardware
encryption support (e.g., the NCR Wavelan has a DES chip). This
particular board operates at 2 megabits/sec on the 902-928 Mhz band
with 250 mW of power.

There is an even more interesting development in the works: "data
PCS".  In essence, this is "Part 15" style operation on dedicated
spectrum, i.e., new spectrum in the 1.8-2.2 Ghz band that will not
have to be shared.  The specific intent of data PCS is to allow users
to build their own ad-hoc networks without having to rely on the facilities
of (and pay money to) carriers such as telephone companies.

Phil