A new generation of ATM switches with more sophisticated cell queueing and scheduling capabilities are becoming available. We discuss an effective approach to connection admission control (CAC) for the real-time variable-bit-rate (rtVBR) service category in an ATM switch with per-VC queueing and scheduling. The quality of service (QoS) requirements for rtVBR are given in terms of the cell loss ratio (CLR) and the cell delay variation (CDV). Given the rtVBR traffic descriptor and QoS parameters, we compute the capacity C/sub rtVBR/ required to support all the ATM virtual circuits (VCs) in the rtVBR category while meeting their QoS requirement. As part of the process of computing C/sub rtVBR/, we also compute a per-VC effective bandwidth used to program the scheduler. We also characterize the admissible region in the 2-class case and show that it is made up of delay-dominant and loss-dominant regions.

This paper investigates the packet delay variation and packet loss probabilities to be expected in typical applications of voice trunking using ATM adaptation layer type 2 (AAL2) and the consequences these may have, such as the need for sequence numbers in voice packet headers. Simulations have been run for the case of 32-kb/s ADPCM encoded voice across a physical link whose bandwidth is 1/4, 1/2, or 1 times DS1 (1.536 Mb/s). The results show the number of calls that can be admitted while meeting the ninety-ninth percentile queuing delay objective of 20 ms or less. The delay budget that can be allocated to packet queuing delay at access is about 20 ms, given a one-way end-to-end delay budget of 100 to 150 ms for conversational voice. The number of calls is also limited so that the ratio of late or lost packets does not exceed 10/sup -3/. Voice quality is known to be tolerant to packet losses of up to one in 100. When the packet delay variation is significant, as it threatens to be in this situation, sequence numbers can help a receiver to detect and to recover from anomalies-lost, early, or late packets-and make fewer reconstruction errors than if sequence numbers were absent or ignored. Three circumstances are identified where packet slips can occur with ripple effects. An analysis of these cases leads to lower bounds on the sequence number modulus. The appropriate value of the sequence number modulus depends on the desired call load and performance objectives, and can be deduced from the results presented.

Asynchronous transfer mode (ATM) adaptation layer 2 (AAL2) has been designed for efficient transport of voice, fax, and voiceband data (VBD) traffic over an ATM virtual circuit. The protocol helps achieve low latency and high bandwidth efficiency while applying suitable compression methods on voice/VBD/fax calls and silence elimination on voice calls. We analyze the performance and capacity of an ATM multiplexer based on AAL2 adaptation. We assume that embedded adaptive differential pulse code modulation (ADPCM) is used to compress voice, and silence elimination is used to achieve statistical multiplexing gain. The embedded ADPCM coding scheme allows selective dropping of less significant bits of voice during congestion in the ATM/AAL2 multiplexer. We compare the call capacities of voice multiplexers with and without bit dropping (BD). The performance models and results presented are based on fairly general assumptions and can be used for traffic engineering and call admission control in land-line or wireless ATM systems for a variety of voice/voiceband compression algorithms. A generalized algorithm for call admission control is also described.

There have been numerous studies on congestion control for the ABR service in ATM networks. These studies typically focus on the performance and fairness of the algorithms and make simplistic assumptions regarding the switch architecture and the link scheduling. One central issue of these studies has been the computation of the fair share of the link bandwidth. On the other hand, newer generation of ATM chipsets and switches now implement per-VC queueing and scheduling that is capable of providing flow isolation as well as fair sharing of the link bandwidth among contending connections. As a result, ABR congestion control algorithms can now focus on solving the congestion control problem without unnecessarily being burdened by fairness considerations. In this paper, we take advantage of the per-VC queueing/scheduling capability of the new generation of ATM switches and develop an ABR rate-based congestion control algorithm. In contrast to most algorithms that appeared in the literature which are heuristics-based, this algorithm extends previous work (see IEEE/ACM Trans. on Networking, vol.1, no.6, p.693-707, 1993) using a control-theoretic approach and takes advantage of the per-VC queue length information to achieve a simple to implement and yet complete control of the stability, rate of convergence, and performance of ABR service. Simulation results confirm the excellent performance and fairness characteristics achieved by the algorithm.

Integrated Network Design Tools (INDT) is a suite of tools being developed at Bell Labs. This suite of tools is built on a flexible software architecture based on C++ and can provide optimal network design for a mix of private line, switched voice and switched multimedia services using a mix of circuit and ATM switches and cross connects and facilities with different bandwidth granularity. INDT also has the ability to design, in a standalone mode or in conjunction with logical designs, the core transport network using mesh and/or ring topologies and using PDH or SONET/SDH products. The transport network may be designed over existing fiber routes and capacities or the tool may be used to help decide the fiber layout. We focus on a new capability within INDT, viz., SONET ring design. We present a high level description of the various submodules in the ring network design and provide an overview of the main factors that influenced our algorithm development. We also present sample studies of our approach to intra-ring load balancing as well as our economic quantification of penalties due to dual ring interconnect (DRI) and other features of interest.

In this paper, we consider the performance of a node in a high speed network which is subject to a threshold type of feedback control policy.We obtain the loss probability and throughput at the node as a function of round-trip propagation delay and buffer size.These parameters are critical in high speed networks.We show that the simple feedback control policy is effective only when the ratio of propagation delay to job transmission time is not too large.Otherwise, a simple static "open-Icmp" control policy, which does not rely on constant exchanges of state information, can perform better.

An important part of a distributed system design is the choice of a load sharing or global scheduling strategy. A comprehensive literature survey on this topic is presented. We propose a taxonomy of load sharing algorithms that draws a basic dichotomy between source-initiative and server-initiative approaches. The taxonomy enables ten representative algorithms to be selected for performance evaluation. A performance metric called the Q-factor (quality of load sharing) is defined which summarizes both overall efficiency and fairness of an algorithm and allows algorithms to be ranked by performance. We then evaluate the algorithms using both mathematical and simulation techniques. The results of the study show that: i) the choice of load sharing algorithm is a critical design decision; ii) for the same level of scheduling information exchange, server-initiative has the potential of outperforming source-initiative algorithms (whether this potential is realized depends on factors such as communication overhead); iii) the Q-factor is a useful yardstick; iv) some algorithms, which have previously received little attention, e.g., multiserver cyclic service, may provide effective solutions.

The VAX/VMS process schedule is briefly described. A simple priority-driven round-robin queuing model is then constructed to analyze the behavior of the time-critical processes of VAX/VMS under such a schedule.