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
"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 firstname.lastname@example.org