JPL's Wireless Communication Reference Website

Chapter: Network Concepts and Standards

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).



JPL's Wireless Communication Reference Website Jean-Paul M.G. Linnartz, 1993, 1995.