HF packet radio link layer protocol design

Efficient packet communications over the HF medium require a data link protocol that is specifically designed for the HF environment. On a given link, the HF channel exhibits wide variations in channel capacity due to time varying propagation characteristics. Little has been previously reported concerning the study of data link protocols for HF packet communications. The work reported in the present paper concerns the investigation of several major issues involved in the design of an HF data link protocol. These issues include adaptivity as a mechanism to cope with varying channel capacity, the avoidance of collisions on a half-duplex radio channel and the efficient utilization of single-tone modems. The basis of the investigations was the measurement of link layer data throughput across a simulated HF link. All measurements were performed using MIL-STD-188-110A single-tone modems (1) and a pair of HF channel simulators. Throughput measurements were made for different protocol, HF modem and channel settings and compared to determine the impact of various parameters of interest. An experimental approach based upon the use of HF channel simulators was adopted since the interactions of the protocol, modem and channel are not easily dealt with by analytical means. The stationary nature of the channel model ensures repeatability of the tests and provides a convenient method for analysis and evaluation. Representative results of over-the-air testing are also presented