Abstract-The current hest-effort Internet cannot readily provide the service guarantees that V O W applications often require. Path snitching can potentially address this problem without requiring new network mechanisms, simply hy leveraging the robustness to performance variations available from connectivity options such as multi-homing and overlays. In this paper, we evaluate the effectiveness and benefits of path switching in improving the quality of VOW applications, and demonstrate its feasihility through the design and implementation of a prototype gateway. We argue for an application-driven path switching system that accounts for both network path characteristics and application-specific factors (e.g., codec algorithms, playout buffering schemw)+ We also develop an application path quality estimator based on the ITU-T E-model for voice quality assessment, and an applicationdriven path switching algorithm that dynamically adapts the time scales over which path switching decisions are made to maximize voice quality. Through network emulation and experiments over a wide-area multi-homed testbed, we show that, with sufficient path diversity, path switching can yield meaningful improvemenb in voice quality. Hence by exploiting the inherent path diversity of the Internet, application-driven path switching is a viable option in providing quality-of-service to applications. The consolidation of the Internet as the de facto communication infrastructure, the increasing availability of broadband access, and the emergence of integrated applications have all contributed to finally propelling Voice over IP (VoIP) to the forefront. However, VoIP like many other real-time applications requires minimum service guarantees that go beyond the best-effort structure of today's IP networks. In particular, even if modern codecs are capable of some level of adaptation and error concealment, VoIP quality remains sensitive to performance degradation in the network. Ensuring that the network is capable of meeting those service requirements is. therefore, key to the acceptance of VoIP as the technology of choice for voice transmission. There are two possible approaches for realizing this goal that are complementary but embody different perspectives on how to deliver service guarantees. The first possibility involves adding service differentiation mechanisms to the network itself, and configuring them to dedicate resources and delivering service guarantees to VoIP traffic. The main disadvantage of this approach is in the added complexity it imposes on the network, and, more important, the difficulty of deploying it ubiquitously across a network of the size of the Intemet. Consequently, networkbased service differentiation has so far been most successful of bandwidth on an access link to ensure a sufficient share to VoIP traffic. The problem of delivering robust end-to-end service guarantees, therefore, remains and has motivated the development of other, more application-centric solutions. In particular and in keeping with the original end-to-end principke of the Intemet [l], another altemative is to rely on the diversity of connectivity options often available, and let applications decide how to best use them [ZI, I3It [41, [51. Specifically, in this paper we explore the extent to which voice quality can be improved, when VoIP traffic is able to dynamically select from among several paths based on their performance. The expectation is that unless all paths are simultaneously experiencing poor performance, a path switching mechanism that consistently picks the best path should offer meaningful improvements in voice quality. Obviously. there are many potential obstacles to realizing such a goal. They range from the difficulty of predicting future path performance, to the challenge of mapping network performance to voice quality. to the added complexity of implementing the required functionality. In this paper, we carry out a comprehensive investigation aimed at demonstrating and quantifying the extent to which path diversity is capable of delivering strong performance guarantees to VoIP traffic. Path switching as introduced in this paper is not a new concept? and has been previously investigated in a number of different settings. For instance, commercial solutions, such as [6], [7], monitor the quality of paths offered by different network providers and use that information to select the best provider. Similarly, the work of [SI, [9] demonstrated the use of a similar principle in the context of overlay networks, where the detection of performance degradation triggers the use of different overlay paths. Our earlier work [lo] illustrated the benefits of path switching in improving performance when both multiple providers and overlay paths were available. More recently and probably most relevant to this paper, Skype 11 11 introduced a similar idea in their peer-to-peer VoIP system, which maintains multiple connections for a single session and dynamically chooses the one that is best suited at the time. In this paper, we expand on these earlier works in two main directions. First, we explore the application fpvef benefits that dynamic path switching affords to VOW traffic, and develop a simple scheme for realizing those benefits. Second, we quantify the magnitude of those benefits in a variety of environments and demonstrate the feusibilirq. of our proposed scheme through an operational prototype. The paper demonstrates the feasibility of providing reasonably stable performance guarantees to VoIP traffic, simply by leveraging the path diversity that the Internet offers, and without changes to either the network itself or the application software clients. When coupled with a mechanism that estimates 2268 0-7803-8968-9/05~0.00 (C)ZOOS IEEE the voice call quality supported by each path, our prototype gateway was found to deliver meaningful improvements across a broad range of network conditions. This was demonstrated using both a network emulator that allowed us to experiment with a variety of network impairments in a controlled fashion, and a wide-area testhed involving three sites connected by both multiple providers and several overlay palhs. The prototype gateway is lightweight and capable of delivering voice quality that closely approximates its optimal feasible value h a t can only be achieved if assuming a priori howledge of path quality variations and always selecting the best path at every instant. We believe that the use of simple techniques such as those described in this paper can help the current best-effon Intemet accommodate the more stringent quality requirements of applications such as VoIP. The remainder of this paper is organized as follows. Section I1 provides a high-level description of the architecture of our path switching system, and explains our design choices. Section I11 is devoted to a detailed discussion of how path quality is estimated. In Section IV, we study the various issues related to designing a path switching mechanism and developing a practical scheme that adapts to changes in path characteristics. Performance results are presented in Section V that demonstrates the effectiveness of path switching for a broad range of emulated environments and our wide-area testbed. Finally, Section VI summarizes our findings and points to several extensions we are currently considering. Exploiting path diversity to improve the quality of applications such as V o P raises several challenges. First, the perceived quality of a voice call depends on many factors, and not merely network factors such as packer delay or loss. Furthermore, the relation between network performance and voice quality is often non-trivial, determined partly by applicationspecific parameters such as codec algorithms. playout buffer size, error concealment mechanisms, etc. In other words, simply selecting the path with the best network performance is not adequate. This calls for an applicarion-driven path switching system that takes into account both network performance and appIication-specific parameters. Such a system needs to i) measure and predict network performance of various paths, ii) map measuredpredicted network performance into application quality estimates, and iii) make judicious path switching decisions based on the estimatcd application quality. Finally, implementation must be efficient and scalable in order to be deployable on an Internet scale, ideally without requiring modifications to the network or the applications. The above observations and requirements motivate the design of application-driven path switching (APS) gateways. which reside at the edge of access networks, between end hosts and the larger Intemet. The APS gateways perform dynamic path switching using application-specific quality estimation derived from measured network path performance. While path switching decisions are application-driven, the operations of these gateways are transparent to end hosts and applications. A typical application scenario of our path switching system is shown in Fig. i : a VoIP service provider has multiple paths available (e.g., by connecting to several ISP's) to carry the traffic from its customers. At each end of the communication, an APS gateway is used to dynamically determine which path should be used to forward voice packets. The goal of path switching is to maintain the best possible network conditions for voice data transfer so that the application quality perceived by a user can be improved. Before we describe the key components of our APS gateway, we first briefly discuss the various factors that affect VoIP call quality and describe how call quality is typically assessed. This provides the necessary background as well as justification for the design of our APS
In REF , Tao et al. present a practical solution to improve VoIP quality by using application driven path-switching gateways (APS) located at the edge of the access networks, between the end-hosts and the Internet.
Improving VoIP quality through path switching
Proceedings IEEE 24th Annual Joint Conference of the IEEE Computer and Communications Societies.
Proceedings IEEE 24th Annual Joint Conference of the IEEE Computer and Communications Societies.
Computer Science