Capacity improvement is one of the principal challenges in wireless networking. We present a link-layer protocol called Slotted Seeded Channel Hopping, or SSCH, that increases the capacity of an IEEE 802.11 network by utilizing frequency diversity. SSCH can be implemented in software over an IEEE 802.11-compliant wireless card. Each node using SSCH switches across channels in such a manner that nodes desiring to communicate overlap, while disjoint communications mostly do not overlap, and hence do not interfere with each other. To achieve this, SSCH uses a novel scheme for distributed rendezvous and synchronization. Simulation results show that SSCH significantly increases network capacity in several multi-hop and single-hop wireless networking scenarios.

Cooperative task management can provide program architects with ease of reasoning about concurrency issues. This property is often espoused by those who recommend "event-driven" programming over "multithreaded " programming. Those terms conflate several issues. In this paper, we clarify the issues, and show how one can get the best of both worlds: reason more simply about concurrency in the way "event-driven" advocates recommend, while preserving the readability and maintainability of code associated with "multithreaded" programming. We identify the source of confusion about the two programming styles as a conflation of two concepts: task management and stack management. Those two concerns define a two-axis space in which "multithreaded" and "event-driven" programming are diagonally opposite; there is a third "sweet spot" in the space that combines the advantages of both programming styles. We point out pitfalls in both alternative forms of stack management, manual and automatic, and we supply techniques that mitigate the danger in the automatic case. Finally, we exhibit adaptors that enable automatic stack management code and manual stack management code to interoperate in the same code base.

We study a fundamental yet under-explored facet in wireless communication – the width of the spectrum over which transmitters spread their signals, or the channel width. Through detailed measurements in controlled and live environments, and using only commodity 802.11 hardware, we first quantify the impact of channel width on throughput, range, and power consumption. Taken together, our findings make a strong case for wireless systems that adapt channel width. Such adaptation brings unique benefits. For instance, when the throughput required is low, moving to a narrower channel increases range and reduces power consumption; in fixed-width systems, these two quantities are always in conflict. We then present SampleWidth, a channel width adaptation algorithm for the base case of two communicating nodes. This algorithm is based on a simple search process that builds on top of existing techniques for adapting modulation. Per specified policy, it can maximize throughput or minimize power consumption. Evaluation using a prototype implementation shows that SampleWidth correctly identities the optimal width under a range of scenarios. In our experiments with mobility, it increases throughput by more than 60 % compared to the best fixed-width configuration. Categories and Subject Descriptors:

Submit paper summaries online to cs262(at)cory.eecs.berkeley.edu before the start of each class. CS262a is the first semester of a year-long sequence on computer systems research, including operating systems, database systems, and internet infrastructure systems. The goal of the course is to cover a broad array of research topics in computer systems, and to engage you in top-flight systems research. The first semester is devoted to basic thematic issues and underlying techniques in computer systems, while the second semester goes deeper into topics related to scalable, parallel and distributed systems. The class is based on a discussion of important research papers, and a research project. Prerequisites: The basic prerequisite is to pass the entrance exam in class. It will cover undergraduate operating systems material at the level of Berkeley's CS162, and will be pass/fail. It will not be difficult if you've had an undergraduate course, though you may need to "page in " the material. The test will have no effect on your grade; it is purely for admission, to ensure that everyone has a solid foundation, and in turn a high level of discussion. The test is on the second day of class, Friday August 29th. Exams, Papers, etc. The main work of this class is to read steadily and deeply, while working toward a group research project of publishable quality. Each student will be individually responsible for writing up a short