The Communications Group of the Faculty of Computer Science,
University of New Brunswick, Fredericton, NB, Canada is
currently involved in the evaluation and application of data link
protocols for harsh error environments, such as cellular telephone
links, VLF (very low frequency) links and satellite links.
Participating in this research and development work were
(in alphabetical order) Dr. Aziz Al-Zoman (formerly Ph.D. student),
John M. Dedourek (Professor), Dr. Bernd J. Kurz (Professor).
The standard HDLC protocols have been developed for
communications media with low error rates and reasonably small
delay x bandwidth products. If used in harsh error environments,
the standard HDLC protocols suffer greatly from frequent time-outs
and, thus, exhibit unacceptably small link efficiencies and
throughput rates. This behaviour is due mainly to the small
window sizes and the strict retransmission mechanisms.
We have developed a new version of HDLC which
improves on the above mentioned shortcomings of standard HDLC
versions. A larger permissible window size and more flexible
errored-frame signalling and retransmission mechanisms are the
design keys for improved performance in harsh error environments
and for large delay x bandwidth products. In addition, the new
HDLC version retains the standard frame formats with only changes
as to the interpretation and use of certain bit groups. This approach
offers compatibility with standard frame structures and should
facilitate the protocol's acceptance.
For pre-implementation evaluation purposes, the novel robust
HDLC Protocol has been tested out in our Protocol Simulation Testbed
with realistic error types and statistics. Recently completed work
revealed that our novel HDLC version can increase the link performance
over cellular telephone type 'harsh error channels' with long propagation
delays by about a factor of 3 over the best published HDLC versions
(an increase from 15% to 45%).
Project Status: Started: November 1991
Completed: April 1995
Al-Zoman, Dedourek, Kurz: Modelling Cellular Radio Channels for Asynchronous Data Transmission, International Journal of Modelling and Simulation, IASTED, Anaheim, Calgary, Zurich, Vol.17, N0.1, March 1997, pp. 1-5 Al-Zoman, Dedourek, Kurz: A Study of Data Communication over Non-Conventional Channels, Proc. APICS, Halifax, Nov 1992, pp. 78-87 Al-Zoman, Dedourek, Kurz: Performance of Data Link Protocols over Cellular Networks, Proc. 2nd Int'l. Conf. on Telecommuni- cation Systems and Analysis, Vanderbuilt Univ., Nashville, TN, March 1994, pp. 328-334 Al-Zoman, Dedourek, Kurz: Modelling Cellular Radio Channels for Asynchronous Transmission, Proc. Int'l. Conf. on Modelling and Simulation and Control SIM'94, IASTED, Ottawa, May 1994, pp. 51-54 Al-Zoman, Dedourek, Kurz: Automatic Retransmission Rather than Automatic Repeat Request, Proc. 94 Int'l. Conf. on Network Protocols, IEEE, IEEE Comp. Soc. Press, Boston, Mass., Oct 1994, pp. 48-55 Al-Zoman: Performance Evaluation of Data Communications Protocols Used with Non-Conventional Channel Media, Ph.D. thesis, Fac. of Comp. Science, Univ. of New Brunswick, Fredericton, N.B., Canada, April 1995
Back to: Kurz's Research Page | Kurz's Home Page
Last revised: 6 June 1997 by Bernd Kurz bjkurz@unb.ca