Bharat T. Doshi

Queueing systems with vacations—a survey

Queueing systems in which the server works on primary and secondary (vacation) customers arise in many computer, communication, production and other stochastic systems. These systems can frequently be modeled as queueing systems with vacations. In this survey, we give an overview of some general decomposition results and the methodology used to obtain these results for two vacation models. We also show how other related models can be solved in terms of the results for these basic models. We attempt to provide a methodological overview with the objective of illustrating how the seemingly diverse mix of problems is closely related in structure and can be understood in a common framework.

Fast restoration of ATM networks

Asynchronous transfer mode (ATM) is now well recognized as the fundamental switching and multiplexing technique for future broadband ISDN. As these networks will be increasingly relied upon for providing a multitude of integrated voice, data, and video services, network reliability is a key concern. There are several intrinsic features of ATM networks that could potentially be exploited to provide improved restoration techniques, beyond those established for synchronous transfer mode (STM) networks, such as digital cross-connect restoration or self-healing rings. These features include ATM cell level error detection, inherent rate adaptation and nonhierarchical multiplexing. The authors explore the use of these features in developing fast restoration strategies for ATM networks. In particular, they address: (1) ATM error detection capabilities for enhanced failure detection, (2) network rerouting strategies, (3) spare capacity allocation, and (4) network control architecture and related implementation aspects. Their findings suggest that fast network span failure detection and bandwidth-efficient rerouting capabilities can be combined to develop restoration strategies for ATM networks with significantly greater performance-cost ratios when compared to existing STM network restoration strategies. >

Analysis of a two phase queueing system with general service times

We derive the transforms of the queue length at departure epochs, queue length in steady state and sojourn time of an arbitrary customer for a queueing system in which each customer receives two services: The first service is part of a batch service and the second given individually to the members of a batch. Our results generalize the recent work of Krishna and Lee in this journal.

A broadband multiple access protocol for STM, ATM, and variable length data services on hybrid fiber-coax networks

This paper presents a broadband multiple access protocol for bidirectional hybrid fiber-coax (HFC) networks. The Adaptive Digital Access Protocol (ADAPt+) supports a full range of subscriber services via HFC networks with tree and branch topologies. The protocol efficiently accommodates different circuit- and packet-based access modes, such as synchronous transfer mode (STM), asynchronous transfer mode (ATM), and inherently variable length (VL) native data (for example, the Internet protocol [IP]). ADAPt+ allocates the available bandwidth efficiently and adapts to the changing traffic mix. In addition, the paper describes the medium access control (MAC) protocol for upstream and multiplexing/demultiplexing for downstream communication, its applicability to STM, ATM, and other native data applications, and its performance with respect to throughput, latency, and bandwidth efficiency. While discussed in the context of an HFC network, many aspects of ADAPt+ have relevance to wireless, fiber to the curb (FTTC), and fiber to the home (FTTH).

Error and flow control performance of a high speed protocol

The performance of the SNR protocol of A. N. Netravali et al. (1990) is studied when it is implemented for end-to-end flow and error control. Using a combination of analysis and simulation, the efficiency with which this protocol uses the network bandwidth and its achievable throughput is evaluated as a function of certain network and protocol parameters. The protocol is enhanced by introducing two windows to decouple the two functions of receiver flow control and network congestion control. This enhancement and the original protocol are compared with go-back-N (GBN) and one-at-a-time-selective-repeat (OSR) retransmission procedures, are shown to have significantly higher throughput for a wide range of network conditions. As an example, for a virtual circuit with 60-ms roundtrip delay and 10/sup -8/ bit error rate, in order to deliver 500 Mb/s throughput, both the GBN and OSR require a raw transmission bandwidth of approximately 800 Mb/s, whereas SNR with two windows needs slightly higher than 500 Mb/s raw bandwidth. Periodic exchange of state can also provide a variety of measures for congestion control in a timely and accurate fashion. >

Conditional and unconditional distributions for M/G/1 type queues with server vacations

M/G/1 queues with server vacations have been studied extensively over the last two decades. Recent surveys by Boxma [3], Doshi [5] and Teghem [14] provide extensive summary of literature on this subject. More recently, Shanthikumar [11] has generalized some of the results toM/G/1 type queues in which the arrival pattern during the vacations may be different from that during the time the server is actually working. In particular, the queue length at the departure epoch is shown to decompose into two independent random variables, one of which is the queue length at the departure epoch (arrival epoch, steady state) in the correspondingM/G/1 queue without vacations. Such generalizations are important in the analysis of situations involving reneging, balking and finite buffer cyclic server queues. In this paper we consider models similar to the one in Shanthikumar [11] but use the work in the system as the starting point of our investigation. We analyze the busy and idle periods separately and get conditional distributions of work in the system, queue length and, in some cases, waiting time. We then remove the conditioning to get the steady state distributions. Besides deriving the new steady state results and conditional waiting time and queue length distributions, we demonstrate that the results of Boxma and Groenendijk [2] follow as special cases. We also provide an alternative approach to deriving Shanthikumars [11] results for queue length at departure epochs.

Overview of INDT-a new tool for next generation network design

We describe a new integrated network design tool (INDT) being developed in AT&T Bell Labs. This tool includes a variety of network design algorithms that encompass virtually every modern network design scenario. INDT is intended for designing private line networks, switched voice networks, integrated private line and switched voice networks and integrated ATM based multimedia networks. All these versions come in two flavors: (i) a desert start version that designs unconstrained networks assuming no embedded base and (ii) a constrained version that takes the embedded network base as input and produces a constrained optimal network design that allows maximum reuse of the embedded network.

Communication protocols for high speed packet networks

Abstract With the advent of very high speeds, cleaner transmission media and richer mix of applications in packet networks (LANs, MANs, WANs), protocols in the network and in the end systems are undergoing many changes. In this paper, we discuss these changes at functional level. The emphasis is on the motivation and functionality. Particular attention is given to the functions of switching and multiplexing, error/loss recovery, receiver flow control and network congestion control. Both the network and the end system protocols are discussed here with a somewhat greater emphasis on the latter.

Performance of an in-call buffer-window reservation/allocation scheme for long file transfers

The authors evaluate the performance of both the fixed buffer allocation (FBA) and the adaptive buffer allocation (ABA) schemes, in which the network nodes are allowed to offer less than the requested buffer size. The performance measures of interest are the blocking probability, file transfer delay, and the adaptation speed for ABA for a given buffer size and the offered load. The authors develop and analyze a quasi-birth-death model of the ABA scheme (with exponential file lengths and negligible delay in carrying out reservation and cancellation procedures). In particular, they develop a recursive computational scheme exploiting the structure of the underlying model. This is supplemented by a first-passage time analysis to evaluate the transient behavior of the control strategy. The authors use both analytic and simulation methods. The results demonstrate that the ABA schemes provide significant advantages over the FBA scheme if the parameters are appropriately chosen. They also provide guidelines on the choice of these parameters. >

Fast, scalable, and distributed restoration in general mesh optical networks

Service providers are demanding transport network solutions that can accommodate exponential traffic growth and, at the same time, provide novel services such as point-and-click provisioning of very high bandwidth circuits, optical bandwidth service management, fast protection and restoration, and bandwidth on demand. It is becoming increasingly clear that the required scalability (terabits/s to petabits/s) and cost structure can only be provided by transparent optical cross connects (OXCs). The challenge, then, is to make the optical network consisting of OXCs, dense wavelength division multiplexers (DWDMs), and optical add/drop multiplexers (OADMs) dynamic and intelligent. A major aspect of this intelligence is fast provisioning and restoration. In this paper, we present a fast, scalable, and distributed solution for optical layer restoration in general mesh-type optical networks, which is being implemented as part of the Optical Navigator System (ONS) residing in Lucents LambdaRouter product. The key ingredients to our solution are a fast and scalable restoration strategy, a fast and scalable connection setup strategy, a contention-free wavelength assignment strategy, and a fast and reliable data communications network to exchange signaling messages. We also introduce novel concepts of demand bundling and optical virtual paths that ensure that restoration performance scales with network and traffic volumes. Together these components provide an intelligent optical networking solution that not only guarantees restoration times within few hundreds of milliseconds, but also achieves scalability.