Huahui Wu

Adjusting forward error correction with quality scaling for streaming MPEG

Packet loss can severely impact streaming video quality. Repair techniques protect streaming video from packet loss but at the price of a reduced effective transmission rate when streaming a flow in a capacity constrained situation. This paper proposes an algorithm that optimizes the choice of Forward Error Correction (FEC) to repair packet loss for streaming MPEG videos under a capacity constraint with quality scaling. An analytic model is developed to estimate the video quality of streaming MPEG given a quality scaling level and a specific FEC strength. Given network conditions in terms of packet loss rate, the model searches the total variable space to find the combination of FEC and scaling that yields the optimal quality under the capacity constraint. Analysis over a range of network conditions indicates that adjusting FEC with quality scaling provides significant performance improvement.

Inferring Queue Sizes in Access Networks by Active Measurement

Router queues can impact both round-trip times and throughput. Yet little is publicly known about queue provisioning employed by Internet services providers for the routers that control the access links to home computers. This paper proposes QFind, a black-box measurement technique, as a simple method to approximate the size of the access queue at last mile router. We evaluate QFind through simulation, emulation, and measurement. Although precise access queue results are limited by receiver window sizes and other system events, we find there are distinct difference between DSL and cable access queue sizes.

Adjusting forward error correction with temporal scaling for TCP-friendly streaming MPEG

New TCP-friendly constraints require multimedia flows to reduce their data rates under packet loss to that of a conformant TCP flow. To reduce data rates while preserving real-time playout, temporal scaling can be used to discard the encoded multimedia frames that have the least impact on perceived video quality. To limit the impact of lost packets, Forward Error Correction (FEC) can be used to repair frames damaged by packet loss. However, adding FEC requires further reduction of multimedia data, making the decision of how much FEC to use of critical importance. Current approaches use either inflexible FEC patterns or adapt to packet loss on the network without regard to TCP-friendly data rate constraints. In this article, we analytically model the playable frame rate of a TCP-friendly MPEG stream with FEC and temporal scaling, capturing the impact of distributing FEC within MPEG frame types with interframe dependencies. For a given network condition and MPEG video encoding, we use our model to exhaustively search for the optimal combination of FEC and temporal scaling that yields the highest playable frame rate within TCP-friendly constraints. Analytic experiments over a range of network and application conditions indicate that adjustable FEC with temporal scaling can provide a significant performance improvement over current approaches. Extensive simulation experiments based on Internet traces show that our model can be effective as part of a streaming protocol that chooses FEC and temporal scaling patterns that meet dynamically-changing application and network conditions.

Measuring queue capacities of IEEE 802.11 wireless access points

While queue capacities have a direct impact on loss and latency during congestion, and wireless networks continue to spread in university, corporate and home networks, little is publicly known about the queue capacities of wireless access points (APs). This paper presents and deploys the Access Point Queue (APQ) methodology for externally estimating the queue capacity for a wireless AP. APQ determines the AP saturation point, measures the baseline delay, induces the saturation rate to measure the delay with a full AP queue and computes the queue capacity. APQ is deployed to determine the queue capacities of three commercial class and four residential class APs. The wireless AP queue capacities are shown to be packet-based and to range from 50 packets to over 350 packets. The fact that queue capacities vary so much among devices targeted for the same network configuration suggests future work to determine the most appropriate queue capacity.

Tools and Techniques for Measurement of IEEE 802.11 Wireless Networks

With the growing popularity of wireless local area networks (WLANs) has come an increased need for effective measurements of real-world WLANs and their applications. This paper presents tools and techniques for measuring IEEE 802.11 WLANs. The techniques include details on setting up a PC as a wireless access point and building a wireless sniffer while the tools include programs for measuring link, network and application layer traffic. The tools are all open-source software available for download and the techniques all use open-source software and off-the-shelf hardware components. Together, these tools and techniques facilitate WLAN performance analysis across network layers in a flexible, accurate and cost-effective manner. To illustrate the usefulness of these tools and techniques for gathering WLAN measurements three case studies are presented: a streaming video session showing cross-layer performance; network characteristics of a wireless hand-held game; and measurements of access point queue size. Research employing these tools can yield more accurate WLAN models and more realistic evaluation of proposed WLAN changes in a network testbed.

Application, network and link layer measurements of streaming video over a wireless campus network

The growth of wireless LANs has brought the expectation for high-bitrate streaming video to wireless PCs. However, it remains unknown how to best adapt video to wireless channel characteristics as they degrade. This paper presents results from experiments that stream commercial video over a wireless campus network and analyze performance across application, network and wireless link layers. Some of the key findings include: 1) Wireless LANs make it difficult for streaming video to gracefully degrade as network performance decreases; 2) Video streams with multiple encoding levels can more readily adapt to degraded wireless network conditions than can clips with a single encoding level; 3) Under degraded wireless network conditions, TCP streaming can provide higher video frame rates than can UDP streaming, but TCP streaming will often result in significantly longer playout durations than will UDP streaming; 4) Current techniques used by streaming media systems to determine effective capacity over wireless LAN are inadequate, resulting in streaming target bitrates significantly higher than can be effectively supported by the wireless network.

On combining temporal scaling and quality scaling for streaming MPEG

Temporal Scaling and Quality Scaling are both widely-used techniques to reduce the bitrate of streaming video. However, combinations and comparisons of Temporal and Quality Scaling have not been systematically studied. This research extends previous work to provide a model for combining Temporal and Quality Scaling, and uses an optimization algorithm to provide a systematic analysis of their combination over a range of network conditions and video content. Analytic experiments show: 1) Quality Scaling typically performs better than Temporal Scaling, with performance differences correlated with the motion characteristics of the video. In fact, when the network capacity is moderate and the loss rate is low, Quality Scaling performs nearly as well as the optimal combination of Quality and Temporal Scaling; 2) when the network capacity is low and the packet loss rate is high, Quality Scaling alone is ineffective, but a combination of Quality and Temporal Scaling can provide reasonable video quality; 3) adjusting the amount of Forward Error Correction (FEC) provides significantly better performance than video streaming without FEC or video streaming with a fixed amount of FEC.

Performance analysis of the intertwined effects between network layers for 802.11g transmissions

While the canonical behavior of todays home Internet users involves several residents concurrently executing diverse Internet applications, the most common home configuration is a single external connection into a wireless access point (AP) that promises to provide concurrent high-bandwidth Internet access for multiple hosts through a wireless local area network (WLAN). Recent research has attempted to assess the performance impact of hosts with weak wireless connectivity upon the other WLAN hosts by employing measurement studies or analytic models that focus primarily on wireless channel characteristics. This paper examines the intertwined effects on performance of the user applications, the network protocol and the wireless channel characteristics via carefully designed experiments that leverage previously developed network measurement tools. The study provides empirical evidence that suggests the overall performance of a WLAN is not only determined by the individual wireless channel qualities associated with each host, but also by the interaction of the various network layers with respect to transmission contention, queuing at the access point, transport protocol, and behavior of the specific applications. These results imply that effective WLAN performance modeling needs to include details on multiple network layers.

Demonstration of adjusting forward error correction with quality scaling for TCP-friendly streaming MPEG

The growth in the power and connectivity of the Internet has sparked an even larger growth in streaming media. The sheer number of possible users and applications at any point in time raises the probability of streaming multimedia flows encountering congestion. To overcome short-term congestion and avoid long-term congestion collapse, there is a growing consensus that Internet applications must be TCP-Friendly, with proposed approaches to detect and punish non-TCP friendly flows. Unlike TCP, new TCP-friendly streaming media protocols refrain from retransmissions to avoid delay and jitter, but they are susceptible to quality degradation from packet loss. While multimedia applications can tolerate some data loss, excessive packet loss during congestion yields unacceptable media quality. Since video encoding involves interframe dependencies to achieve high compression rates, the random dropping of packets by routers can seriously degrade video quality. For example, as little as 3% MPEG packet loss can cause 30% of the frames to be undecodable. Streaming media flows often utilize lower latency repair approaches, such as Forward Error Correction (FEC), in conjunction with TCP-Friendly protocols to deliver streaming applications over the Internet. However, FEC requires redundant repair data to be added to the original video stream. Current approaches use either apriori, static FEC choices or adapt FEC to perceived packet loss on the network without regard to TCP-Friendly data rate constraints. When a streaming video operates within TCP-Friendly bitrate limits, adding FEC will reduce the effective transmission rate of the original video content.