Robust HDLC Protocol for Harsh Error Environment


Outline of Research

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

Relevant Publications

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


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Last revised: 6 June 1997 by Bernd Kurz bjkurz@unb.ca >/html>