Outline of Research

ATM (Asynchronous Transfer Mode) was designed for very-low error environments, such as 
fibre optic cables (BER approx 10E-12), and high-speed applications. For this reason the 
ATM layers do not usually perform error recovery but delegate this task to higher 
layers, such as the Transport layer (ISO-4) in an end-to-end manner. When using ATM in 
harsh-error environments, such as wireless networks (cellular, PCS, satellite) this 
assumption is violated and results in extremely low performance. In addition, the long 
propagation delay of satellite channels compounds this effect of end-to-end error recovery.

   This project addresses the poor performance of ATM in a harsh-error environment and 
develops a protocol stack to combat the underlying problems. A link layer is added 
below the ATM layer and the AAL layer is fitted with an error control mechanism. This 
stack organization allows frequent errors at the physical layer to be corrected at the 
link layer (if it exists) in a node-to-node fashion before they reach the ATM layer. 
Infrequent cell loss at the ATM layer, or errors propagated from the physical layer 
(if the link layer does not exist), are corrected at the AAL layer in and end-to-end 
fashion. For some applications both error control mechanisms may be implemented, for 
others only one or the other.




   The distribution of the error control over the two layers above and below the ATM 
layer is investigated for varying communication environments (error rate, propagation 
delay). For both error control layers various protocol types, such as stop-wait and 
windowing protocols with GBN and SREJ are evaluated. The optimal arrangement is found 
and the performance is determined.

   It was found that for harsh-error cellular wireless environments the error control 
procedures at both layers (data link and AAL) are needed with a stop-wait protocol at 
the data link layer and a windowing protocol at the AAL layer providing the best 
performance. For a satellite channel with lower BER a single layer error control at 
the AAL layer is sufficient to recover from errors caused by both the physical medium 
and the ATM cell loss. In addition, it was found that the implementation of a data link 
layer below the ATM layer has the effect of a non-stationary but error-free network and 
poses particular problems for the timers (for emergency time-out procedures) at the AAL 
layer's error control procedure.

   Participating in the research were Dr. Bernd J. Kurz (professor), with the 
assistance of Mr. Yi Wang, former MCS graduate student.

Project Status: Started:   May 1997
                Completed: August 1998

Relevant Publications

"Performance Analysis of ATM in Wireless Environments", by Yi Wang, MCS thesis, 
Faculty of Computer Science, University of New Brunswick, August 1998

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Last revised: 1 December 1998 by Bernd Kurz bjkurz@unb.ca