There are many situations in which an additional network interface-or two-can provide benefits to a mobile user. Additional interfaces can support parallelism in network flows, improve handoff times, and provide sideband communication with nearby peers. Unfortunately, such benefits are outweighed by the added costs of an additional physical interface. Instead, virtual interfaces have been proposed as the solution, multiplexing a single physical interface across more than one communication endpoint. However, the switching time of existing implementations is too high for some potential applications, and the benefits of this approach to real applications are not yet clear. This paper directly addresses these two shortcomings. It describes a link-layer implementation of a virtual 802.11 networking layer, called Juggler, that achieves switching times of approximately 3 ms, and less than 400 mus in certain conditions. We demonstrate the performance of this implementation on three application scenarios. By devoting 10 percent of the duty cycle to background tasks, Juggler can provide nearly instantaneous handoff between base stations or support a modest sideband channel with peer nodes, without adversely affecting foreground throughput. Furthermore, when the client issues concurrent network flows, Juggler is able to assign these flows across more than one AP, providing significant speedup when wired-side bandwidth from the AP constrains end-to-end performance.

Mobile devices cannot rely on a single managed network, but must exploit a wide variety of connectivity options as they travel. We argue that such systems must consider the derivative of connectivity--the changes inherent in movement between separately managed networks, with widely varying capabilities. With predictive knowledge of such changes, devices can more intelligently schedule network usage.

This paper presents Virgil, an automatic access point discovery and selection system. Unlike existing systems that select access points based entirely on received signal strength, Virgil scans for all available APs at a location, quickly associates to each, and runs a battery of tests to estimate the quality of each AP's connection to the Internet. Virgil also probes for blocked or redirected ports, to guide AP selection in favor of preserving application services that are currently in use. Results of our evaluation across five neighborhoods in three cities show Virgil finds a usable connection from 22% to 100% more often than selecting based on signal strength alone. By caching AP test results, Virgil both improves performance and success rate. Our overhead is acceptable and is shown to be faster than manually selecting an AP with Windows XP.

Mobile devices are vulnerable to theft and loss due to their small size and the characteristics of their common usage environment. Since they allow users to work while away from their desk, they are most useful in public locations and while traveling. Unfortunately, this is also where they are most at risk. Existing schemes for securing data either do not protect the device after it is stolen or require bothersome reauthentication. Transient Authentication lifts the burden of authentication from the user by use of a wearable token that constantly attests to the user's presence. When the user departs, the token and device lose contact and the device secures itself. We show how to leverage this authentication framework to secure all the memory and storage locations on a device into which secrets may creep. Our evaluation shows this is done without inconveniencing the user, while imposing a minimal performance overhead