Virtual Circuit Research Articles

Reliability of Computational Evaluation and Indeterminacy of Fundamental Error Characteristics in Virtual Measurement Circuits

We present a methodology for conveniently obtaining analytic forms to estimate the reliability of metrological characteristics of results obtained from various virtual measurement circuits. We compare approaches to determination of reliability for metrological characteristics by using a computational method and expressions for computation of measurement indeterminacy.

Multimedia file allocation on VC networks using multipath routing

The problem of allocating high-volume multimedia files on a virtual circuit network with the objective of maximizing channel throughput (and minimizing data transmission time) is addressed. The problem is formulated as a multicommodity flow problem. We present both the optimal and suboptimal solutions to the problem using novel approaches.

A DAVIC-based video-on-demand system over IP networks

This paper describes the design and implementation of a video-on-demand system which is DAVIC compliant so as to ensure interoperability across vendors, applications and services. The system is based on a client-server architecture comprising the service gateway, the video server and the clients (which are the set-top units). Interoperability is accomplished through the use of DAVIC stipulated CORBA-based Digital Storage Media-Command and Control (DSM-CC) protocols and ATM pre-established permanent virtual circuits. The former is adopted for communication among the different system entities over the network while the latter handles the real-time audio/video transportation. The proposed system has successfully completed an interoperability test conducted with Panasonic Singapore Laboratories, JVC Singapore and the Nanyang Technological University over the Singapore ONE ATM Testbed attesting to its compliance to DAVIC's interoperability standards.

Simulation of Modified Backpressure Flow Control Scheme Incorporating Dynamic Buffer Allocation at Nodes in a Queueing Network

Congestion in networks occurs due to varied traffic characteristics. There are several congestion control techniques based on buffer avallibllity, burstiness of traffic, round trip delay, timeout of packet, and flow control. The backpressure is a flow control scheme In which the heavily loaded node builds pressure on the downstream node in the virtual circuit (connection oriented network). The pressure ultimately reaches the source and the source throttles its rate of transmission. The throughput of network using this scheme and also the pressure at nodes are not satisfactory. This Is due to static allocation of buffers at the intermediate nodes during virtual circuit setup. The paper presents a modified backpressure scheme incorporating dynamic buffer allocation at Intermediate nodes and Input buffer limit at the source node In a virtual circuit. In this scheme buffers are allocated dynamically based on the Incoming traffic at the Intermediate nodes. Input buffer limit scheme controls the local packets entering the network from the source. These changes in backpressure scheme have resulted in fair Increase In the network throughput and reduction in pressure at intermediate nodes. The technique is simulated and the results are given in the paper.

Greedy online algorithms for routing permanent virtual circuits

We analyze the competitive ratio of two greedy online algorithms for routing permanent virtual circuits in a network with arbitrary topology and uniform capacity links. We show that the competitive ratio of the first algorithm, with respect to network congestion, is in and where m is the number of links in the network, 𝒟 is the maximum ratio, over all requests, of the length of the longest path for the request to the length of the shortest path for the request, and ℒ︁ is the ratio of the maximum-to-minimum bandwidth requirement. We show that the competitive ratio of the second greedy algorithm is in Ω(d + log(n −d)) and min{O(d log n), when the optimal route assignment is pairwise edge disjoint, where n is the number of network nodes and d is the length of the longest path that can be assigned to a request. It is known that the optimal competitive ratio for this problem is Θ(log n). Aspnes et al. designed a Θ(log n) competitive online algorithm that computes an exponential function of current congestion to make each decision. The greedy online algorithms, although not optimal, make each decision more quickly and still have good competitive ratios in many nontrivial situations. © 1999 John Wiley & Sons, Inc. Networks 34: 136–153, 1999

Greedy online algorithms for routing permanent virtual circuits

We analyze the competitive ratio of two greedy online algorithms for routing permanent virtual circuits in a network with arbitrary topology and uniform capacity links. We show that the competitive ratio of the first algorithm, with respect to network congestion, is in and where m is the number of links in the network, 𝒟 is the maximum ratio, over all requests, of the length of the longest path for the request to the length of the shortest path for the request, and ℒ︁ is the ratio of the maximum-to-minimum bandwidth requirement. We show that the competitive ratio of the second greedy algorithm is in Ω(d + log(n −d)) and min{O(d log n), when the optimal route assignment is pairwise edge disjoint, where n is the number of network nodes and d is the length of the longest path that can be assigned to a request. It is known that the optimal competitive ratio for this problem is Θ(log n). Aspnes et al. designed a Θ(log n) competitive online algorithm that computes an exponential function of current congestion to make each decision. The greedy online algorithms, although not optimal, make each decision more quickly and still have good competitive ratios in many nontrivial situations. © 1999 John Wiley & Sons, Inc. Networks 34: 136–153, 1999

Providing heterogeneous quality of service bounds for correlated video traffic at a multiplexor

Network support for variable bit-rate video needs to consider (i) properties of workload induced (e.g., significant auto-correlations into far lags and heterogeneous marginal distributions), and (ii) application specific bounds on delay-jitter and statistical cell-loss probabilities. The objective of this paper is to present a quality-of-service solution for such traffic at each multiplexing point in a network. Heterogeneity in both offered workload and quality-of-service requirements are addressed. A per-virtual-circuit framing structure and a pseudo earliest-due-date cell dispatcher are introduced to provide guaranteed delay-jitter bounds. Heterogeneous jitter-bounds are supported through software controlled frame-sizes which may be independently set for each virtual-circuit. The framing structure is a generalization of per-link framing introduced by Golestani. The proposed framing structure eliminates correcting for phase mismatches between incoming frames and outgoing frames, necessary in per-link framing. This results in reduction in end-to-end delay bound and buffer requirements, and a simpler implementation. Strong auto-correlations typically seen in video traffic make equivalent bandwidth computations for heterogeneous cell-loss bounds intractable. To address this, the framing strategy is combined with an active cell-discard mechanism with prioritized cell-dropping, the latter utilizing the history of dropped cells and target cell-loss bounds for each virtual circuit. Upper bounds on the equivalent bandwidth needed to support a given workload with a target quality of service are developed. These are validated through numerical and simulation results from variable bit-rate MPEG-I video traces.

Undergraduate surgical teaching utilizing telemedicine.

To evaluate the concept of a new telemedicine orientated educational application by exposing junior medical students to surgical teaching via video-conferencing from the operating theatre and comparing this to the traditional method currently employed, which requires the presence of students in the operating room. To determine student satisfaction with this novel method and identify deficiencies associated with traditional surgical teaching. Students were connected by video-conferencing equipment 2 Mbit/s permanent virtual circuits that provided two-way picture and voice communication. Traditional tutoring involved students attending the operating theatre in set numbers. Video recordings were made during the telemedicine transmission for highlighting important factors relevant to the operative procedure. A questionnaire was used to assess the quality of time spent and information obtained by the students. Telemedicine centre and operating theatre, St Mary's Hospital, London. Junior clinical students. The median score for surgical teaching utilizing video-conferencing was 9 (scale 0-10) compared to 5 for traditional operating theatre surgical teaching. All 46 (100%) subjects indicated a willingness to return for the telemedicine influenced method of tutoring compared to 65% of students exposed to the conventional method. Our early experience with telemedicine assisted surgical teaching indicate high acceptance and satisfaction rates by clinical students. Further studies to evaluate the objective gain in knowledge associated with this method is required prior to its implementation in any future medical curriculum.

Performance issues in VC-merge capable switches for multiprotocol label switching

In a multiprotocol label switching (MPLS) domain, ATM label-switching routers (LSRs) are potentially capable of providing the highest forwarding capacity in the backbone network. Virtual circuit (VC) merging is a mechanism in an ATM-LSR that allows many IP routes to be mapped to the same VC label and provides a scalable mapping method that can support thousands of destinations. VC merging requires reassembly buffers so that cells belonging to different packets intended for the same destination do not interleave with each other. In this paper, the impact of VC merging on the buffering requirement for the reassembly buffers is investigated. We propose a realistic architecture that supports VC merging. We study the performance of this architecture using an analytic approach and using simulation driven by empirical Internet packet-size distribution. At the cell level, our main finding indicates that VC merging incurs a minimal overhead compared to non-VC merging, in terms of additional buffering. Moreover, the overhead decreases as utilization increases or as the traffic becomes more bursty with longer dependence. The finding has important practical consequences since routers and switches are dimensioned for high utilization and stressful traffic conditions. At the packet level, VC merging generally achieves a higher goodput than non-VC merging with EPD for the same buffer size. We also study the delay performance and find that the additional delay due to VC merging is insignificant at high speed.

A virtual circuit deflection protocol

We propose a communication protocol, called the virtual circuit deflection (VCD) protocol, which combines some of the individual characteristics of virtual circuit switching and deflection routing. An advantage of the VCD protocol over previous (datagram) deflection schemes is that deflections in the former occur on a per session basis (or a per subsession basis, if sessions need to be split to find adequate capacity on the outgoing links), while in the latter, they occur on a per packet basis. This makes packet resequencing at the destination considerably easier to accomplish in the VCD protocol than in datagram deflection schemes. The VCD protocol exploits the storage arising from the high bandwidth-delay product of optical fibers to provide lossless communication with little buffering at the switches and without the need for advance reservations. This makes it particularly suitable for networks that use optical switching, where buffers are expensive to implement with current optical technology. We present a simple implementation of the VCD protocol for such networks, which requires only limited buffering, accomplished through the use of a minimal number of optical delay lines. We also analyze the performance of the protocol for the Manhattan Street network topology by using new analytical models. In particular, we examine the effect of the traffic load and the network size on the throughput and the length of the paths followed by the sessions, and compare the analytical results obtained with corresponding simulation results. The results indicate that the VCD protocol is efficient under both light and heavy traffic conditions, especially when the link capacities are large compared to the basic rate of individual sessions, as is expected to be the case in future multigigabit networks.

Linear-complexity algorithms for QoS support in input-queued switches with no speedup

We present several fast, practical linear-complexity scheduling algorithms that enable provision of various quality-of-service (QoS) guarantees in an input-queued switch with no speedup. Specifically, our algorithms provide per-virtual-circuit transmission rate and cell delay guarantees using a credit-based bandwidth reservation scheme. Our algorithms also provide approximate max-min fair sharing of unreserved switch capacity. The novelties of our algorithms derive from judicious choices of edge weights in a bipartite matching problem. The edge weights are certain functions of the amount and waiting times of queued cells and credits received by a virtual circuit. By using a linear-complexity variation of the well-known stable-marriage matching algorithm, we present theoretical proofs and demonstrate by simulations that the edge weights are bounded. This implies various QoS guarantees or contracts about bandwidth allocations and cell delays. Network management can then provide these contracts to the clients. We present several different algorithms of varied complexity and performance (as measured by the usefulness of each algorithm's contract). While most of this paper is devoted to the study of soft guarantees, a few hard guarantees can also be proved rigorously for some of our algorithms. As can be expected, the provable guarantees are weaker than the observed performance bounds in simulations. Although our algorithms are designed for switches with no speedup, we also derive upper bounds on the minimal buffer requirement in the output queues necessary to prevent buffer overflow when our algorithms are used in switches with speedup larger than one.

On-Line Resource Management with Application to Routing and Scheduling

We propose a framework to model on-line resource management problems based on an on-line version of positive linear programming. We consider both min cost problems and max benefit problems and propose logarithmic competitive algorithms that are optimal up to a constant factor. The proposed framework provides a general methodology that applies to a wide class of on-line problems: shop scheduling, packet routing, and in general a class of packing and assignment problems. Previously studied problems as on-line multiprocessor scheduling and on-line virtual circuit routing can also be modeled within this framework.

ATM-based TH-SSMA network for multimedia PCS

Personal communications services (PCS) promise to provide a variety of information exchanges among users with any type of mobility, at any time, in any place, through any available device. To achieve this ambitious goal, two of the major challenges in the system design are: (i) to provide a high-speed wireless subsystem with large capacity and acceptable quality-of-service (QoS) and (ii) to design a network architecture capable of supporting multimedia traffic and various kinds of user mobility. A time-hopping spread-spectrum wireless communication system called ultra-wide bandwidth (UWB) radio is used to provide communications that are low power, high data rate, fade resistant, and relatively shadow free in a dense multipath environment. Receiver-signal processing of UWB radio is described, and performance of such communications systems, in terms of multiple-access capability, is estimated under ideal multiple-access channel conditions. A UWB-signal propagation experiment is performed using the bandwidth in excess of 1 GHz in a typical modern office building in order to characterize the UWB-signal propagation channel. The experimental results demonstrate the feasibility of the UWB radio and its robustness in a dense multipath environment. A ATM network is used as the backbone network due to its high bandwidth, fast switching capability, flexibility, and well-developed infrastructure. To minimize the impact caused by user mobility on the system performance, a hierarchical network-control architecture is postulated. A wireless virtual circuit (WVC) concept is proposed to improve the transmission efficiency and simplify the network control in the wireless subsystem. The key advantage of this network architecture and WVC concept is that the handoff can be done locally most of the time, due to the localized behavior of PCS users.

Considerations on preestablished tree rerouting handoff protocols for wireless ATM PCN

Handoff in an ATM-based wireless network is an important and hard problem. Mobile virtual circuit (MVC) and virtual connection tree (VCT) represent an approach of preestablished tree rerouting. However, a major error is found in the MVC based handoff scheme. Possible remedies to it are suggested. Comparisons between MVC and VCT are made and limitations of VCT are discussed. For a better solution, an architecture for an ATM-based wireless PCN is proposed and a simple but efficient handoff scheme based on this architecture is described. Analysis shows that it shares the advantages of VCT while overcoming the latter's limitations.

A DAVIC-based video-on-demand system over ATM networks

This paper describes the design and implementation of a video-on-demand system which is DAVIC compliant so as to ensure interoperability across vendors, applications and services. The system is based on a client-server architecture comprising the service gateway, the video server and the clients (which are the set-top units). Interoperability is accomplished through the use of DAVIC stipulated CORBA-based Digital Storage Media-Command and Control (DSM-CC) protocols and ATM pre-established permanent virtual circuits. The former is adopted for communication among the different system entities over the network while the latter handles the real-time audio/video transportation. The proposed system has successfully completed an interoperability test conducted with Panasonic Singapore Laboratories, JVC Singapore and the Nanyang Technological University over the Singapore ONE ATM Testbed attesting to its compliance to DAVIC's interoperability standards.

Support QoS in IP over ATM

An integrated service internet running real-time and multimedia applications is rapidly becoming a reality. Meanwhile, ATM technology is appearing in the marketplace. It is an important problem to integrate ATM networks into this integrated service internet. One of the approaches, classical IP over ATM, is now widely deployed, effectively solving the problem of internetworking and interoperability. A key remaining issue is to provide quality-of-service (QoS) guarantees for internet traffic running through ATM subnets. This paper describes a priority scheme, named User Priority, for providing an IP integrated service with QoS over ATM switched virtual circuits (SVCs) to obtain better performance of packet delivery. The User Priority is defined as a three-bit field which uses a type of service (TOS) field in the IP datagram header. This yields eight different service classes with a value of 7 for the highest priority and 0 for the lowest priority. Class 6 and 7 services are for real-time traffic and have their own VCs. Packets with Classes 0 through 5 are sent an aggregate VC. This method is different from the IP over ATM scheme where only one VC is set up between two communicating IP hosts. Thus, packets can be treated differently according to their priorities such that they can enjoy the various QoS guarantees provided by ATM networks. It is backward-compatible with existing IP implementations. These newer options need only to be implemented on the end systems that want to take advantage of them.

Load Profiling Based Routing for Guaranteed Bandwidth Flows

AbstractTo support the stringent Quality of Service (QoS) requirements of real‐time (e.g. audio/video) applications in integrated services networks, several routing algorithms that allow for the reservation of the needed bandwidth over a Virtual Circuit (VC), established on one of several candidate routes, have been proposed. Traditionally, such routing is done using the least‐loaded concept, and thus results in balancing the load across the set of candidate routes. In this paper, we propose the use of load profiling as an attractive alternative to load balancing for routing guaranteed bandwidth VCs (flows). Load profiling techniques allow the distribution of “available” bandwidth across a set of candidate routes to match the characteristics of incoming VC QoS requests. We thoroughly characterize the performance of VC routing using load profiling and contrast it to routing using load balancing and load packing. We do so both analytically and via extensive simulations of multi‐class traffic routing in Virtual Path (VP) based networks. Our findings show that for routing guaranteed bandwidth flows in VP networks, load profiling is desirable as it reduces VP bandwidth fragmentation, which increases the likelihood of accepting new VC requests. This fragmentation could be particularly harmful when the granularity of VC requests is large. Typically, this occurs when a common VC is established to carry the aggregate traffic flow of many high‐bandwidth real‐time sources. For VP‐based networks, our simulation results show that our load‐profiling VC routing scheme performs better or as well as the traditional load‐balancing VC routing in terms of revenue under both skewed and uniform workloads. Furthermore, load‐profiling routing improves routing fairness by proactively increasing the chances of admitting high‐bandwidth Hows.

A new concept the ‘virtual circuits’ and its application in large-scale network analysis with tearing

A new concept the ‘virtual circuit’ is raised in this article. Its application in large-scale network analysis with tearing is presented. This new approach unifies three different tearing methods, i.e., the branch tearing, the node tearing and the hybrid tearing. Based on the virtual circuit, the tearing equation with a standard Bordered Block Diagonal form can be established directly. The approach presented in this article is applicable to any large-scale network analysis. Copyright © 1999 John Wiley & Sons, Ltd.

Designing an ATM switch for extreme reliability

Lucent Technologies PacketStar ATM Core Switch is designed to deliver the high reliability required in public telecommunications networks. It achieves this reliability through use of a variety of strategies including duplication of hardware and software components, isolation of groups of components to minimize the effect of problems, fault recovery hierarchies, and others. As a result, cell loss is very minimal during problems, and permanent virtual circuits are unaffected even if the main switch control system goes down.

IP flow identification for IP traffic carried over switched networks

Today much of the Internet backbone traffic is carried over switched networks. Switching within these backbone networks is done primarily by using permanent virtual circuits to interconnect internal routers. To improve efficiency, schemes have been suggested that involve identifying IP data flows at the edge of the switched network, and then carrying these flows over a virtual connection through the switched network. To interface with the Internet as it is today, there must be a way to identify flows that are long enough to economically warrant setting up a switched connection. Shorter flows would still be routed on a segment by segment basis. In this paper we look at the effectiveness of several flow identification algorithms, and compare them to the effectiveness of an optimum scheme. We recommend a flow definition based on client–server connections (C–S flow) that results in the lowest SVC setup rate and allows QoS to be applied based on the application.