Jie Hui

Discovering fine-grained RRC state dynamics and performance impacts in cellular networks

To conserve power while ensuring good performance on resource-constrained mobile devices, devices transition between different Radio Resource Control (RRC) states in response to network traffic and according to parameters specific to network operators. As RRC states significantly affect application power consumption and performance, it is important to understand how RRC state timers interact with network traffic patterns. In this paper, we show that the impact of RRC states on performance is significantly more complex and diverse than found in previous work. To do so, we introduce an open-source tool that allows the impact of RRC states on network and application performance to be measured in a robust and accurate manner on unmodified user devices, and deploy the tool in 23 countries around the world to test a broad range of cellular network technologies. We detect previously unknown performance problems which increase network latencies by up to several seconds and for LTE, can increase packet losses by an order of magnitude. Through an in-depth cross-layer analysis of several carriers, we examine the lower-layer causes of these problems. We determine that the highly complex state transitions of certain carriers, and in particular poor interactions between state demotions and network traffic, can lead to substantial, unexpected latencies.

Performance Characterization and Call Reliability Diagnosis Support for Voice over LTE

To understand VoLTE performance in a commercial deployment, in this paper we conduct the first comprehensive performance characterization of commercially deployed VoLTE, and compare with legacy call and over-the-top (OTT) VoIP call. We confirm that VoLTE excels in most metrics such as audio quality, but its call reliability still lags behind legacy call for all the three major U.S. operators. We propose an on-device VoLTE problem detection tool, which can capture new types of problems concerning audio quality with high accuracy and minimum overhead, and perform stress testing on VoLTE calls reliability. We discover 3 instances of problems in the early deployment of VoLTE lying in the protocol design and implementation. Although the identified problems are all concerned with the immature LTE coverage in the current deployment, we find that they can cause serious impairment on user experience and are urgent to be solved in the developing stage. For example, one such instance can lead to up to 50-second-long muting problem during a VoLTE call! We perform in-depth cross-layer analysis and find that the causes are rooted in the lack of coordination among protocols designed for different purposes, and invalid assumptions made by protocols used in existing infrastructure when integrated with VoLTE. We summarize learnt lessons and suggest solutions.

Understanding RRC state dynamics through client measurements with mobilyzer

Understanding how network and application behavior patterns impact client performance on mobile devices is a difficult yet important problem to solve. Often, we are most interested in the performance experienced by end users, but accurately and effectively measuring performance on uncontrolled mobile devices in the wild continues to be a challenging problem. In this paper, we have developed a tool to allow us to more accurately characterize RRC states using client-based measurements. To do so, we have made use of and contributed to an open-source framework called Mobilyzer, which facilitates collecting complex network measurements on client devices without impacting the user. We have deployed our tool on unmodified devices in 23 countries worldwide to directly measure the impact of RRC state machine configurations on individual packets and network protocols. Demonstrating the value of client-based measurements, our large-scale measurement study allowed us to uncover previously unknown performance problems that can increase network latency by several seconds and increase packet losses by an order of magnitude.