Packet Radio
Data packet switching was developed in the mid-1960's.
The ARPANET, established in 1969, was one of the first applications.
The ALOHANET operated at the University of Hawaii was the
first packet radio network. The development of packet radio was taken up
by US researchers, mostly sponsored by military agencies.
Ham amateurs began to use packet radio in 1978. A
group of amateurs in Vancouver developed
the Terminal Node Controller (TNC) in 1980.
Terms like 'packet radio' or 'packet
broadcasting' seldom refer to the typical propagation features of realistic
radio media, but rather to the (purely architectural or
information-theoretical) notion of maximum connectivity among all terminals in a multi-user network.
Spread Spectrum Packet Radio
Perhaps as a result of the strong research sponsoring by military agencies,
much research emphasis was put on hostile
interference and strategies for network survivability.
Less effort was put to combat self-
interference systems due to multipath delays and the random signal fluctuations in mobile radio
channels. The choice for spread spectrum rules seem to have been influenced
by experiences from military research.
In the 1970's little was published in the open literature
on interference-limited system design and communication over problematic
channels. The experimental use of satellite links with their nearly perfect
Gaussian noise-limited (AWGN) channels did not stimulate much consideration
of real channel impairments - except imperfect (hard-limiting) satellite
amplifiers and jamming by an adversary, where appropriate. If terrestrial
networks were considered, these were often appropriate to a tactical
battlefield scenario, with geographically distributed store-and-forward
repeater nodes linked by random paths with fixed, but unknown losses. The
desired packet communication modes were generally of the multi-hop type,
designed to maximize the progress of packets in particular directions.
Burst Packet Transmission over Fading Channel
As a consequence of this strong research tradition, many researchers still
intuitively expect the significant propagation impairments of typical
terrestrial UHF/VHF mobile channels to reduce the moderate theoretical
throughput of contention ("collision-type") protocols. For computer networks
with cable links between terminals and hosts, uncoordinated transmissions
indeed run the risk of conflicting with each other, which results of the loss
of all messages involved. However, coinciding packets sent over a radio
channel with very different ground wave losses or instantaneous fading levels
do not necessarily all annihilate each other, given capabilities of the
receiver to capture a strong packet. Therefore, throughput expressions for
'poor' mobile channels indicate a higher capacity than intuitively suggested
by the classical studies of contention protocols in 'ideal' AWGN channels.
Intelligent processing of received signals, containing both wanted signals
and interference with partly known properties, can further enhance the
performance of wireless multi-user networks.
Packet Systems in ISM bands
The FCC part 15.247 approach (regarding ISM bands)
is a good example of interference-limited system
design. One particular form of spread spectrum, frequency-hopping, was first
patented in an electronic-warfare context, to prevent target deception by
interference to radio-guided torpedos. The specialized military expertise
only gradually becomes available for commercial use, so much may still be
learned from the (often classified) archives of Electronic Warfare. But one
lesson of this environment is clear: optimum interference-limited system
design requires gaining knowledge about 'your' interferers, and exploiting
it. Most often, this knowledge gives a statistical description of the probably
biased behavior of interference signals. This is in contrast to the approach
in noise-limited systems, where Gaussian noise is know to be the utmost
unpredictable type of signal. In a cooperative interference-limited
environment, a priori knowledge of the other party's behavior can be used to
the mutual benefit of both interferer and victim. The subject is one of great
research interest. Successful system designs in
the interference-limited environment now heavily rely on diversity, i.e., the
receiver attempts to observe the transmit signal in as many ways as possible.
Such multiple observations can be made, e.g. with differences in time,
frequency, or location of the antenna.
Amateur Packet Radio
Ham radio amateurs developed their own packet radio system.
The
Terminal Node Controller (TNC) control the operation of a Ham packet radio station.
It partitions messages into
data packets and handles the transmit and receive protocols, including
error detection and retransmission of lost messages.
Moreover, amateur packet radio stations can relay messages from
and to other amateurs, similar to multi-hop military PR systems.
This allows for larger range of communication.
Mostly, 1200 or 2400 bit/s telephone modem-type signals are
used for local VHF and UHF communications using
typical ham transceivers designed for speech communications.
1200 bps Frequency Shift Keying is widely used
in the 2 meter band, i.e., at 144-148 MHz.
Long distance shor wave communication is done at 300 bit/s.
Higher speeds can be used at VHF, UHF and
micr wave frequencies, but they require direct modulation methods.
Typically, telephone communication
programs are adapted for packet radio.
The radio protocol is called AX.25 and is based on the X.25 packet switching protocol
for wireline data communications. Carrier Sense Multiple Access (CSMA)
is used to avoid interference among stations sharing the same radio channel for their
burst transmissions.
More recently, Internet Transmission Control
Protocol/Internet Protocol (TCP/IP) are becoming popular.
It supports the FTP (File
Transfer Protocol), SMTP (Simple Mail Transport Protocol), Telnet (Remote
terminal protocol), and NNTP (Net News Transfer Protocol).
