Long Le

The effects of active queue management on web performance

We present an empirical study of the effects of active queue management (AQM) on the distribution of response times experienced by a population of web users. Three prominent AQM schemes are considered: the Proportional Integrator (PI) controller, the Random Exponential Marking (REM) controller, and Adaptive Random Early Detection (ARED). The effects of these AQM schemes were studied alone and in combination with Explicit Congestion Notification (ECN). Our major results are: <ol>For offered loads up to 80% of bottleneck link capacity, no AQM scheme provides better response times than simple drop-tail FIFO queue management. For loads of 90% of link capacity or greater when ECN is not used, PI results in a modest improvement over drop-tail and the other AQM schemes. With ECN, both PI and REM provide significant response time improvement at offered loads above 90% of link capacity. Moreover, at a load of 90% PI and REM with ECN provide response times competitive to that achieved on an unloaded network. ARED with recommended parameter settings consistently resulted in the poorest response times which was unimproved by the addition of ECN.</ol.We conclude that without ECN there is little end-user performance gain to be realized by employing the AQM designs studied here. However, with ECN, response times can be significantly improved. In addition it appears likely that provider links may be operated at near saturation levels without significant degradation in user-perceived performance.We present an empirical study of the effects of active queue management (AQM) on the distribution of response times experienced by a population of web users. Three prominent AQM schemes are considered: the Proportional Integrator (PI) controller, the Random Exponential Marking (REM) controller, and Adaptive Random Early Detection (ARED). The effects of these AQM schemes were studied alone and in combination with Explicit Congestion Notification (ECN). Our major results are: For offered loads up to 80% of bottleneck link capacity, no AQM scheme provides better response times than simple drop-tail FIFO queue management. For loads of 90% of link capacity or greater when ECN is not used, PI results in a modest improvement over drop-tail and the other AQM schemes. With ECN, both PI and REM provide significant response time improvement at offered loads above 90% of link capacity. Moreover, at a load of 90% PI and REM with ECN provide response times competitive to that achieved on an unloaded network. ARED with recommended parameter settings consistently resulted in the poorest response times which was unimproved by the addition of ECN. We conclude that without ECN there is little end-user performance gain to be realized by employing the AQM designs studied here. However, with ECN, response times can be significantly improved. In addition it appears likely that provider links may be operated at near saturation levels without significant degradation in user-perceived performance.

Impact of background traffic on performance of high-speed TCP variant protocols

This paper examines the effect of background traffic on the performance of existing high-speed TCP variant protocols, namely BIC-TCP, CUBIC, FAST, HSTCP, H-TCP and Scalable TCP. We demonstrate that the stability, link utilization, convergence speed and fairness of the protocols are clearly affected by the variability of flow sizes and round-trip times (RTTs), and the amount of background flows competing with high-speed flows in a bottleneck router. Our findings include: (1) the presence of background traffic with variable flow sizes and RTTs improves the fairness of most high-speed protocols, (2) all protocols except FAST and HSTCP show good intra-protocol fairness regardless of the types of background traffic, (3) HSTCP needs a larger amount of background traffic and more variable traffic than the other protocols to achieve convergence, (4) H-TCP trades stability for fairness; that is, while its fairness is good independent of background traffic types, larger variance in the flow sizes and RTTs of background flows causes the protocol to induce a higher degree of global loss synchronization among competing flows, lowering link utilization and stability, (5) FAST suffers unfairness and instability in small buffer or long delay networks regardless of background traffic types, and (6) the fairness of high-speed protocols depends more on the amount of competing background traffic rather than its rate variability. We also find that the presence of high-speed flows does not greatly reduce the bandwidth usage of background Web traffic.

Vehicular wireless short-range communication for improving intersection safety

Vehicle-to-infrastructure and vehicle-to-vehicle communication has recently garnered considerable attention from both academia and industry because it holds the potential to improve road safety and to reduce road accidents. For this reason, V2X communication has been considered a key technology of the future Intelligent Transportation Systems. Use cases for V2X communication for enhancing intersection safety are appealing because intersections are the most complex and dangerous driving environments. This article reviews major related work and presents a communication architecture for intersection safety that supports these use cases. Our communication architecture respects the salient traits of V2X communication and fits into the overall V2X communication architecture. We perform various simulations to quantify the load of V2X communication at intersections and report the findings of our simulation study.

Performance Evaluation of Beacon Congestion Control Algorithms for VANETs

Beacon and event-driven warning messages are two important messages that are used by many Intelligent Transportation Systems (ITS) applications for improving road safety and traffic efficiency. Beacon messages are regularly broadcast by each vehicle to notify other vehicles of its presence and its status. Event-driven warning messages are emitted when a vehicle or the road infrastructure discerns a critical situation. When congestion occurs, it is necessary to reduce the beacon load to keep a certain amount of the available bandwidth for the event-driven messages because they carry time- critical information of high importance. This paper considers three beacon congestion control algorithms that control the beacon load below a certain threshold by adjusting the transmit rate and/or transmit power for the beacon messages. We perform a large set of simulations to evaluate these algorithms. Our simulation results reveal promising results for these algorithms.

An MPEG performance model and its application to adaptive forward error correction

We present a general analytical model for predicting the reconstructed frame rate of an MPEG stream. Our model captures the temporal relationships between I-, P, and B-frames but is independent of the channel and media characteristics. We derive an adaptive FEC scheme from the general model and verify it by comparing it to the results of a simulation. The prediction error of the model compared to the simulation for a wide array of parameter values is less than 5%. We then use the derived adaptive FEC scheme to study the optimal rate allocation (i.e., between generating a higher frame rate or increasing the protection for a lower frame rate) when equation-based TCP rate control is used to couple packet rates to channel characteristics such as round trip time and packet loss probabilities. Surprisingly, we find that optimal protection levels for I- and P-frames are relatively static as loss rates increase from 1% to 4% while changes in the frame type pattern are used to ameliorate the effects of the increased loss. The study demonstrates how our model can be used to reveal joint source/channel coding tradeoffs and how they relate to encoding and transmission parameters.

Cross-layer optimization made practical

Limited resources and time-varying nature of wireless ad hoc networks demand optimized use of resources across layers. Cross-layer optimization (CLO) for wireless networks, an approach that coordinates protocol behaviors at different layers with a goal to maximize a utility function, has received considerable attention lately. However, most existing work remains as theory and no practical CLO based on utility optimization exists today. The main difficulties in implementing theoretical CLO designs arise often from impractical assumptions about the characteristics of the wireless medium and also from computational and communication overhead of proposed solutions to achieve or approximate the optimality. In contrast, many existing practical approaches for CLO (not necessarily utility optimization) are rather ad hoc in nature and developed mostly based on intuitions. Thus, a clear gap between theory and practice in CLO exists. This paper addresses this dichotomy to close the gap by taking an optimal solution from utility-based CLO and applying practical approximation to enable a practical implementation in a wireless mesh network where nodes are statically positioned in an ad hoc fashion. We focus on the utility of maximizing throughput. We identify the impractical or computationally-intensive components of a theoretically-derived optimal throughput-maximizing solution and then propose, in most cases, practical approximation with O(1) complexity for MAC, scheduling, routing and congestion control. The result is a practical CLO solution that approximates the theoretically-derived optimal solution, but achieves much improved performance over existing practical CLO implementations.

Field operational tests for cooperative systems: a tussle between research, standardization and deployment

After a decade of research and technology development, road cooperative systems based on vehicle-to-vehicle and vehicle-to-roadside infrastructure communication are currently in a trial phase. Major field operational tests (FOTs) are carried out to verify the operation of cooperative systems in real environments and assess the impact of applications on road safety, traffic efficiency as well as driver behavior and user satisfaction. Standards to achieve interoperability are developed and a potential introduction of a cooperative system is prepared. An FOT is exposed to various requirements from research, standards and deployment that are -- at least partially -- adverse to each other. We study the dependencies for the case of the DRIVE~C2X project, a pan-European FOT for cooperative system. The paper puts the technologies used in the FoT, particularly focusing on communication, into the context of research activities for cooperative systems. We show that the FOT is based on technologies for a minimal cooperative system that is ready to be introduced and sustainably deployed. Further, we identify research concepts and technologies that did not find their way into the basic cooperative system yet and discuss potential directions for future enhancement of the minimal system.

CAR-2-X Communication SDK - A Software Toolkit for Rapid Application Development and Experimentations

The CAR-2-X Communication SDK is a software implementation of a networking protocol stack for car-to-car and car-to-infrastructure communication with IEEE 802.11 radio technology. The core of the SDK is a routing protocol that provides wireless ad hoc and multi-hop communication using geographical positions for addressing and packet forwarding. The routing protocol scheme is enhanced by various features to meet the requirements of safety and infotainment applications in vehicular environments. The SDK can be executed in various hardware environments, such as virtual machines, software development systems and embedded hardware for automotive and road side units. The SDK enables rapid development of safety and infotainment applications for CAR-2-X communication by providing state- of-the-art communication protocols and well-defined application programming interfaces (APIs). The advanced features and a high grade of configurability makes the SDK an ideal software platform for experimentation and evaluation of CAR-2-X com- munication systems in laboratory environments and real world testbeds. In this paper we describe functionalities and features of the CAR-2-X Communication SDK and present the SDKs imple- mentation aspects for the protocols and API. Then, we present a framework for benchmarking and evaluation of CAR-2-X communication protocols. The CAR-2-X Communication SDK is available under the conditions of a software license and provided

Distributed rate control algorithm for VANETs (DRCV)

This paper presents Distributed Rate Control for VANETs (DRCV), a distributed light-weight congestion control algorithm tailored for safety messages. DRCV monitors and estimates channel load and controls the packet rate of outgoing periodic packets. A new approach called Fast Drop is adopted to promptly drop the rate of periodic packets when event-driven safety packets are detected. Simulations show the effectiveness of DRCV in increasing packet reception probabilities and achieving efficient channel usage.

V2X Communication and Intersection Safety

Vehicle-to-vehicle and vehicle-to-infrastructure communication (V2X communication) has great potential to increase road and passenger safety, and has been considered an important part of future Intelligent Transportation Systems (ITS). Several R&D projects around the world have been investigating various aspects of V2X communication. Some of these projects focus on specific issues of V2X communication for intersection safety (communication-based intersection safety) because intersections are the most complex driving environments where injury and fatal accidents occur frequently. In this paper, we discuss the technical details of V2X communication and discuss how it can be used to improve intersection safety.