DARPA Packet Radio Network

This article is still in construction!

Motivation

Much of today‘s wireless technology (see Wireless Networks) is based on the inspiration, ideas and concepts that have been developed at the DARPA PRNet Project. The fundamental problems of mobility and adhoc networking are still the same as 30 years ago, so it is worth having a look on the first solutions which finally led to what we know as wireless networks today.

Overview

In 1973, the Defense Advanced Research Projects Agency (DARPA) initiated research on the feasibility of using packet-switched radio communications to provide reliable computer communications. This development was motivated by the need to provide computer network access to mobile hosts and terminals, and to provide computer communications in a mobile environment. The DARPA PRNET has evolved through the years (1973-1987) to be a robust, reliable, operational experimental network. Capabilities have been increased step by step as new algorithms were developed and implemented. During 14 years of development the DARPA PRNET has been in daily use (experimental purposes) for about 10 years. The DARPA PRNET projects includes network devices, routing protocols and protocols for automatic distributed network management.

Hardware

The Packet Radio (PR) was the main component of a PRNet.

Communication

As one can tell from the name packet radios use radio frequency technology to transmit and receive data. The implmented packet radios support omnidirectional, spreadspectrum, half-duplex transmission and reception at 400kbit/s and 100kbit/s rates. They implement the physical, data link and network layer (OSI model).

The devices implement chip modulation at a 12.8 mchips/s rate which produces a direct-sequence, spread-spectrum waveform using a pseudo-noise sequence. This pseudo-noise sequence is a function of 56bit-Key,25bit-Slottime and a 32bit-qualifier. DES (Data Encryption Standard) produces the pseudo-noise-code (packets are encoded). If slot times are used to produce the code the network needs to be time-synchonized.

20 frequencies between 1718.4MHz and 1840.0MHz are available for the channel selection. Channel-hopping is possible on a packet-by-packet basis with 3ms settling time. Also attenuation to the signal can be changed on a packet-by-packet basis (adaptive power control) from 0dB to 24dB in 8-dB Steps to effectively control connection quality and power consumption.

Error Handling

Forward Error Correction „repairs“ received packets that are "broken" and so prevents retransmission. To achieve this repair capability convolutional codes & sequential decoding are applied (half-rate coding).

Cyclic Redundancy Checksum (32bit) is used to provide a "good" error detection. If a packet fails, it is discarded and the receiving node waits for retransmission, which follows automatically if there is no acknowledgement.

Burst noise is mitigated by interleaving

Routing and Network Management

The routing protocols used in PRNet are designed to enable reliability, speed and correctness and thus include network management facilities. Routing can be split up into two main protocols: forwarding and transmission.

Network Management

Automatic and distributed network management results from the use of the protocols above. The result of this network management is that the DARPA PRNet can be installed and deployed quickly and easily. Almost nothing needs to be configured. Devices need to be put into their locations and powered on, then the network initialises itself.

See also

Wikipedia on DARPA

Wikipedia on Packet Radio