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Publications by Type: PhD Theses

Impact of Network Effects on Application Quality
Citation key M-INEAQ-12
Author Mehmood, Amir
Year 2012
Month July
School Technische Universit├Ąt Berlin
Abstract The ubiquity of high speed Internet access, proliferation in the adoption of mobile devices, and the popularity of content-rich applications have brought a new dimension to the Internet landscape. Indeed, high speed residential connectivity and mobile wireless access have changed user expectations. The broadband access technologies (i.e., DSL,WiFi, 3/4G, and LTE), and smart mobile devices (i.e., Android, iPhone, iPad, etc.), have enabled users to interactively browse, stream videos (i.e., YouTube and Netflix), play online games, and share content for social networking. All trends together have caused a fundamental change in how users interact with the Internet. Any adverse impact due to high Internet traffic, heterogeneous access, and application protocol mix on flows of different applications can result in sub par network performance and unsatisfactory user experience. Understanding the relation between network performance and user perception is thus crucial for application designers, network, and service providers. In this thesis, we endeavor to explore the impact of emerging network effects on different applications both from network performance and user experience point of view. Studies of network performance and user experience require a multi-purpose heterogeneous testbed that supports a variety of networking conditions commonly present in today┬┐s Internet. We propose the design and architecture of QoE-Lab. The main features of QoE-Lab include 1) Next Generation Mobile Networks (NGMN), i.e., WiFi and 3G UMTS, 2) access/ backbone network emulation, and 3) virtualization. It provides services like traffic generation, topology emulation, and high-precision cross-layer monitoring. We describe two Quality of Experience (QoE) case studies to show the benefits of the QoE-Lab testbed framework. Next, we perform a sensitivity study of the packet loss process within a router for different network load levels, flow size distributions, and buffer sizes. We compare the loss process for TCP and UDP flows at different link utilizations and buffer sizes. We highlight the importance of understanding the flow-level properties of the traffic, e.g., packet loss under different networking conditions and their consequences on application performance, i.e., flow-happiness. We find that packet losses do not affect all flows similarly. Depending upon the network load and the buffer sizes, some flows either suffer significantly more drops or significantly less drops than the average loss rate. Based on anonymized packet level traces from more than 20,000 DSL lines, server logs from a large content distribution network (CDN), and publicly available backbone traces, we investigate the flow-level performance of popular applications across a range of size-based flow classes. We use retransmissions, throughput, and round-trip-times (RTTs) as key flow performance metrics. We compare these metrics under different network loads, DSL link capacities, and for up/downstream directions. We show that irrespective of the direction, flows are severely impacted by events related to network load and application behavior. We also find that, in general, this impact (as measured by our performance metrics) differs markedly across the different flow classes. In particular, contrary to popular belief, small flows from all applications, which make up the majority of flows, experience significant retransmissions, while very large flows, although small in number, experience very limited retransmissions. In terms of application-related performance, we observe that especially when compared to HTTP, P2P flows suffer from continuously high retransmissions. As for the root cause of these retransmissions, we identify the access part of the network as the main culprit and not the network core. Further, we focus on the impact of networking conditions due to the adoption of heterogeneous wireless access technologies such as WiFi and 3G UMTS. These technologies have different physical layer characteristics and impose different networking conditions on flows. We study the impact of network handovers, codec switchover, and packet loss on VoIP applications. We compare our subjective test results with the wideband Perceptual Evaluation of Speech Quality (PESQ) prediction model. PESQ is often used as a standard for new generations of smart phones for quality assurance. We find that the WB-PESQ model underestimates the auditory quality in certain NGMN conditions, e.g., for wideband to narrowband speech codec switching. In addition, we explore the impact of access networks, network handovers, video codecs and codecs changeover, video bit-rate, and bit-rate switching. Our work highlights the bottlenecks in video delivery over NGMNs. We find that network handovers have mostly a negative impact on user perception even if the video transmission is not affected by packet loss. In addition, the choice of video codec influences the video quality. While H.264 provides higher overall quality in WiFi networks, MPEG-4 improves user experience in 3G UMTS. Moreover, changing the video codec during a lossless transmission generally degrades the user experience. Finally, we study the impact of NGMN conditions on web streaming video. We aim to understand how different access networks influence transport protocol (TCP) metrics and impact web video streaming quality. We complement the QoE estimations with network Quality of Service (QoS) parameters such as throughput, delay, and transport layer statistics. Our results show that 1) video QoE remains stable in WiFi even with high packet loss, 2) QoE in 3G UMTS is sensitive to packet loss even for low loss rates due to high variations in the network QoS, namely, throughput and delay, 3) the decrease in QoE and QoS in 3G UMTS is due to its negative interactions with the aggressive congestion control of CUBIC TCP, and 4) handover from WiFi to 3G UMTS degrades QoE.
Bibtex Type of Publication Dissertation
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