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With the advent of the open systems interconnection philosophy, interworking between satellite and ground-based data networks is rapidly becoming important. X.25 LAPB/HDLC is an important standard in this field. It defines a connection between two data switching nodes. This paper examines how a low capacity (-kb/s) anti-jam (A/J) satellite circuit may carry a X.25 LAPB/HDLC data link.It has been shown that use of an unmodified A/J modem employing convolutional coding gives a particularly poor performance in the presence of jamming. A study of the properties which govern the performance of a packet link has led to some recommendations for a special 'packet mode' for anti-jam modems and to some suggested extensions to the data link protocol. These recommendations have been implemented in a simulator and this has revealed a performance far superior to an unmodified system. The recommendations are being implemented in a trials system, developed at the University of Aberdeen, for RSRE Defford.

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