Hao Han, Yunxin Liu, Guobin Shen, Yongguang Zhang, Qun Li
Wi-Fi tethering (i.e., sharing the Internet connection of a mobile phone via its Wi-Fi interface) is a useful functionality and is widely supported on commercial smartphones. Yet existing Wi-Fi tethering schemes consume excessive power: they keep the Wi-Fi interface in a high power state regardless if there is ongoing traffic or not. In this paper we propose DozyAP to improve the power efficiency of Wi-Fi tethering. Based on measurements in typical applications, we identify many opportunities that a tethering phone could sleep to save power. We design a simple yet reliable sleep protocol to coordinate the sleep schedule of the tethering phone with its clients without requiring tight time synchronization. Furthermore, we develop a two-stage, sleep interval adaptation algorithm to automatically adapt the sleep intervals to ongoing traffic patterns of various applications. DozyAP does not require any changes to the 802.11 protocol and is incrementally deployable through software updates. We have implemented DozyAP on commercial smartphones. Experimental results show that, while retaining comparable user experiences, our implementation can allow the Wi-Fi interface to sleep for up to 88% of the total time in several different applications, and reduce the system power consumption by up to 33% under the restricted programmability of current Wi-Fi hardware.
Public Review uploaded by lzhong:
This public review was prepared by Kameswari Chebrolu
Wi-Fi tethering is a functionality supported on many smart-phones to share the phone's Internet access (e.g. 3G connection) with other devices via the phone's WiFi interface. Basically the smart phone acts as a software WiFi access point. However the standard implementations that come with the smart-phone are highly power inefficient or not very user friendly. This is a very practical problem in need of a solution. The paper is the first to look at this issue and presents a new design DozyAP, that permits duty-cycling of the WiFi-interface of the access point to conserve smartphone battery. While there has been considerable focus on client side duty-cycling, access point side duty-cycling is relatively new. The authors motivate the opportunities available for duty-cycling at the access point by measuring the traffic pattern of various commonly used applications. They show that speed discrepancy between cellular and Wi-Fi access, content fetching and consumption gap, all contribute to sleeping opportunities. They then design a sleep request-response protocol between the software AP and the clients, where all involved agree on a valid sleep schedule. The entire DozyAP system was implemented on commercial smartphones and evaluated using real-user traces. The system provides up to 33% overall power savings with little overhead (increases latency by less than 5%).
The proposed solution however does fall short on a few counts. The system design is fairly straightforward, an application of known techniques. Absolute performance improvement is not as compelling, ranging in power savings from as little as 3% to 33% depending on the application scenario. This is to some extent due to the restricted programmability of WiFi hardware. The above results are for a single client running one application at a time. The addition of a second client or another application bring down the power savings considerably. The authors do not consider clients or simultaneous applications beyond 2. DozyAP also needs upgradation of the client software, which may not be feasible in dumb WiFi clients.
This work is a first step in solving this important and practical problem. There is good scope for further improvement, as identified by the authors themselves as part of the discussion in the paper. Hopefully, future solutions will build upon this work and address the above shortcomings.
We appreciate the valuable review comments of Prof. Kameswari Chebrolu.
The current Wi-Fi tethering implementations on existing smartphones are very power inefficient. They keep a smartphone running in the high-power state even without any on-going traffic, thus significantly waste energy. DozyAP provides a simple and effective solution to this practical problem.
The power-saving ability of our implementation is largely limited by the restricted programmability of the Wi-Fi hardware of existing smartphones. When Wi-Fi tethering is enabled, there is no way to put the Wi-Fi hardware into sleep. In our implementation, we turn off the Wi-Fi hardware to save power, which leads to significant energy overhead as shown in Figure 11. We expect that future Wi-Fi firmware can open more programming interfaces to control the power states of the Wi-Fi hardware so that DozyAP can save much more power.
DozyAP is the first work on power-efficient Wi-Fi tethering. We agree with Prof. Kameswari Chebrolu on its shortcomings. There are multiple possible ways to further improve it and we expect that more solutions will be developed to address the shortcomings of our work.