Packet Radio Network
The Packet Radio Network was a program set up by DARPA in 1973 to explore the possibility of an ad-hoc data network using wireless links. It was perhaps most important as one of several physical network technologies being explored by DARPA, the need to interconnect which caused DARPA to work on internetworking, which led to the development of the TCP/IP protocol family, used in today's Internet.
The network included 'Packet Radio Units' (PRUs), which included all the actual radio electronics, along with a microprocessor to implement the protocol used for controlling the transmission of packets between PRUs. These are connected via radio in an ad-hoc network, structured as a tree (in connectivity terms, not geography) based on a special node, the 'station' (in the prototype, a PDP-11/40 running ELF, connected to a PRU), which is responsible for overall monitoring and control of the network (called PRNET).
Hosts connected to PRUs by use of an 1822 interface, as used to connect to ARPANET IMPs. (HDLC was much later substituted for 1822 as the physical channel for talking to the later version of the PRUs.)
However, rather than the Host-to-IMP Protocol used by the IMPs, PRUs used an alternative, the 'Channel Access Protocol' (CAP), which also includes some of the control mechanisms of the PRNET. CAP packets held up to 116 16-bit words of user data, and an 11 word CAP header. CAP, at a high level, operates between the overall source and destination of a given packet, built on top of hop-by-hop operation. On top of CAP, there was another layer, the 'Station to PRU Protocol' (SPP), which operated between PRUs and the station, for high-level overall network control functions.
Major participants included BBN, which produced the station, and the gateways to connect the PRNET to other networks; SRI, which created the Terminal Interface Unit as a test user host for the PRNET; and Rockwell International's Collins division, which was responsible for the Packet Radio Units.
- Robert E. Kahn, Steven A. Gronemeyer, Jerry Burchfiel, Ronald C. Kunzelman, Advances in Packet Radio Technology, Proceedings of the IEEE (Volume: 66, Issue: 11, Nov. 1978), pp. 1468-1496 (online precis available here)
- M. Beeler, SPP Definition, PRTN No. 177, ARPA Packet Radio Network working document, April 1976
- Army Packet Radio Network Protocol Study - III The ARPA Packet Radio Network, pp. 8-14
- The DARPA Packet Radio Network Protocols - from 1987, describes a relatively late version of the system
- DARPA Packet Radio Network
- Internetwork Experiments with the Bay Area Packet Radio Network
- DSP - source code (in MACRO-11) for SPP in the Terminal Interface Unit