Yoram Ofek

Time-driven priority flow control for real-time heterogeneous internetworking

We consider real-time traffic in a heterogeneous internetworking environment with IP routers, MAC bridges, hubs, switched LANs etc. We assume that the current routing protocols remain unchanged. However in this environment, in order to provide quality of service (QoS): bandwidth, delay, constant-bounded jitter and no-loss due to congestion, we suggest a new flow control function called time-driven priority, which is an internal traffic shaping mechanism. We show how it supports two classes of connections: constant bit rate (CBR) with deterministic guarantees, and variable bit rate (VBR) with statistical multiplexing. The mechanism does not require to identify and separate the packet flows of different real-time sessions/connections inside the network. As a result, it achieves lower switching complexity when compared with other internal traffic shaping methods. As consequences of the time-driven priority mechanism we further achieve: (1) QoS parameters which are independent of the connection bandwidth, (2) QoS parameters which are independent of the existing heterogeneous internetworking asynchronous data traffic and (3) the capability for policing and securing the network QoS.

Pseudo-isochronous cell forwarding

Abstract This paper shows how to design a packet switched network, for real-time traffic, such that under full network load: (i) the end-to-end delay bound of a low-rate voice connection is minimized, (ii) the bound on the delay uncertainty or jitter is a fixed network parameter – independent of the network size and the connection rate, and (iii) the required buffer sizes (inside the network) to ensure congestion-free routing is minimized. In addition, this design can be generalized to accommodate either variable bit rate (VBR) traffic with statistical multiplexing or the integration of available bit rate (ABR) traffic 7 , 12 . The isochronous timing information (can be provided by the global positioning system (GPS) [1] ) is used for pacing the packet/cell forwarding inside the network. This means that a cell is forwarded from one switch to another not at a specific time but within a time frame of a relatively long duration as compared with the cell transmission time. This time frame is an independent network parameter, which determines the delay and jitter bounds inside the network. A study of the blocking probability of this approach is presented. (Blocking is defined as the impossibility of allocating bandwidth for a new connection while capacity is still available, but not in the proper time frames.) The study includes both analytical and simulation results, which demonstrate an important trade-off between the blocking probability and the end-to-end delay bound.

Fractional Lambda Switching Principles of Operation and Performance Issues

This article introduces fractional lambda (λ) switching (FλS) and studies its blocking issues. FλS uses a global common time reference (CTR) for implementing pipeline forwarding (PF) inside the network. A global CTR is conveniently realized with the UTC (coordinated universal time) standard. Resource reservation over an FλS network requires a schedule. As in other scheduling cases, a call may not be accepted, even though there is enough capacity, because the schedule is not available—the call is then considered blocked. This work studies the probability of call blocking as a function of link utilization. The results show that (especially if multiple-wavelength division multiplexing channels are deployed on optical links between fractional λ switches) high-link utilization can be achieved with negligible call blocking, even when the switching fabric is a Banyan network.

Fractional Lambda Switching for Flexible Bandwidth Provisioning in WDM Networks: Principles and Performance†

A new approach is introduced in this paper to make possible a flexible utilization of WDM networks using current technology. It is shown that the bandwidth made available end-to-end by a single wavelength can be simply broken up into smaller pieces, or fraction of lambda, by relying on a worldwide common time reference system, such as GPS, previously deployed for different applications. The common time reference system is used to synchronize switches and to facilitate pipeline forwarding of data units. Pipeline forwarding is a known optimal method widely used in manufacturing and computing. It is shown how this new approach, called Time Driven Switching, behaves in terms of call blocking when the basic parameters of the scheme are varied.

End-to-end delay of videoconferencing over packet switched networks

Videoconferencing is an important global application -it enables people around the globe to interact when they are far from one another. In order for the participants in a video-conference call to interact naturally, the end-to-end delay should be below human perception-about 100 ms. Since the global propagation delay can be about 100 ms, the actual end-to-end delay budget available to the system designer (excluding propagation delay) can be no more than 10 ms. We identify the components of the end-to-end delay in various configurations with the objective of understanding how it can be kept below the desired 10 ms bound. We analyze these components going step-by-step through six system configurations obtained by combining three generic network architectures with two video encoding schemes. We study the transmission of raw video and variable bit rate (VBR) MPEG video encoding over (i) circuit switching, (ii) synchronous packet switching, and (iii) asynchronous packet switching. In addition, we show that constant bit rate (CBR) MPEG encoding delivers unacceptable delay, which is on the order of the group of pictures (GOP) time interval. This study shows that having a global common time reference, together with time-driven priority (TDP) and VBR MPEG video encoding, provides adequate end-to-end delay, which is (i) below 10 ms, (ii) independent of the network instant load, and (iii) independent of the connection rate. The resulting end-to-end delay (excluding propagation delay) can be smaller than the video frame period, which is better than what can be obtained with circuit switching.

Adaptive group multicast with time-driven priority

This paper shows how to provide an adaptive real-time group multicast (many-to-many) communication service. Adaptive means that the number of nodes that transmit to the multicast group is continuously changing. In order to meet deterministic quality-of-service (QoS) requirements of a real-time group multicast, some communication resources are reserved. We show (1) how bandwidth is reserved for each multicast group and (2) how an active source can dynamically share the bandwidth allocated to this multicast group with other active group members. Quality-of-service support for a real-time multicast group is based on time-driven priority. In this scheme the time is divided into time frames of fixed duration, and all the time frames are aligned by using a common global time reference, which can be obtained from the Global Positioning System. Bandwidth is allocated to a multicast group as a whole, rather than individually to each user. The allocation is done by reserving time intervals within time frames in a periodic fashion. This type of allocation raises two problems that are studied in this paper: (1) scheduling: how time intervals are reserved to each multicast group and (2) adaptive sharing: how the active (transmitting) participants can dynamically share the time intervals that have been reserved for their multicast group. The proposed approach is based on the embedding of multiple virtual rings, one for each multicast group. By using the virtual rings, it is simple to route messages to all the participants while minimizing the bound on the buffer sizes and queueing delays. The final part of this paper introduces a scalable growth of the multicast group by adding multiple subtrees to the virtual ring.

Scalable Fractional Lambda Switching: A Testbed

This paper presents experiments on a testbed based on ultra-scalable switches realized using off-the-shelf optical and electronic components. The scalability of this switching architecture, a direct outcome of the deployment of pipeline forwarding, results-in addition to much lower cost-in the need for a smaller amount of components, and, consequently, lower power dissipation, which is key to a “greening” of the Internet. Although an all-optical architecture is demonstrated, we reached the conclusion that, given the current state of the art, a hybrid electro-optical architecture is the “best-of-breed” switch solution.

Scalable Switching Testbed not "Stopping" the Serial Bit Stream

In order to achieve ultra scalable IP packet switching it is essential to minimize stopping of the serial bit streams. In our recent experimental work we demonstrated how this can be achieved with an ultra-scalable switching architecture reaching multi-terabits per second (10-100 Tb/s) in a single chassis. The implemented testbed uses only off-the-shelf optical and electronic components. The scalability of this architecture is the direct outcome of how global time (i.e., UTC - coordinated universal time) and pipeline forwarding are utilized. The paper presents the design of a prototype switch and experimental activity with it.

Tunable laser-based design and analysis for fractional lambda switches

Fractional lambda switching (FlambdaS) is a novel approach for traffic management over all-optical networks with sub-wavelength provisioning capability. The unique characteristic of FlambdaS is the utilization of UTC (coordinated universal time) for switching with minimum or no buffers. Several central research issues are still open in FlambdaS and need to be formally defined and analyzed. In this paper, we introduce three novel switch designs that are based on the use of tunable lasers (which can be replaced in the future with wavelength converters). First, the paper presents analytical results of scheduling feasibility, which measures the total number of possible different schedules for each switch design. Then it is shown that the architecture with the highest scheduling feasibility is strictly non blocking in the space domain. Next, the paper provides a closed form analysis of the blocking probability in the time domain, which is applicable for any strictly non-space blocking switch, using combinatorics. In addition, the paper provides measures of the switching hardware complexity, which, for the strictly non-blocking architecture, has the same switching complexity as Clos interconnection network, i.e., O(Nradic(N)) where N is the number of optical channels.

Analysis of Load-Balanced Switch with Finite Buffers

Recently the Birkhoff-von Neumann load-balanced (LB) switch has become a promising switch design due to its high scalability properties and simple control. The performance of the LB switch was studied under strong assumptions such as infinite buffers and admissible traffic conditions. However, both such assumptions may be violated in multi-hop networks since admissibility requirement cannot be maintained unless some inter-switch feedback mechanism is implemented, and infinite buffers are not feasible either. This paper considers the performance of the LB switch with finite central stage buffers under both (i) admissible and (ii) inadmissible input traffic conditions. Its contributions are two folds: firstly, by means of mathematical model we demonstrate that the load-balanced switch has a non-zero cell dropping probability due to buffer overflow even under the admissible input traffic assumptions. Secondly, cell loss probabilities are even higher and large buffers are required under inadmissible traffic conditions to cope with such behavior.