In defense of wireless carrier sense

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2009.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references.Carrier sense, or clear channel assessment (CCA), is widely used in wireless medium access control (MAC) protocols as the means to arbitrate access and regulate concurrency, striking a balance between interference protection and spatial reuse. Criticized widely in the literature, carrier sense has been subject to many replacement attempts with sophisticated and complex alternatives, yet it remains extremely popular. Is the search for a superior alternative justified? In this thesis, we develop a physically motivated theoretical model for average case carrier sense behavior in the two-sender case, based upon radio propagation theory and Shannon capacity. We argue from our model that common notions about carrier sense, such as the hidden and exposed terminal phenomena, are inherently misleading in the context of adaptive bitrate, casting in black and white terms effects that often cause only mild reduction in throughput. The frequency of severe misbehavior is low. We also demonstrate that it is possible to choose a fixed sense threshold which performs well across a wide range of scenarios, in large part due to the role of the noise floor. The noise floor has a significant effect on fairness as well. Using our model, we show that, when implemented well, average-case carrier sense performance is surprisingly close to optimal. We conclude with experimental results from our indoor 802.11 testbed, which corroborate these claims.by Micah, Z. Brodsky.S.M

Distributed Shared State with History Maintenance

Shared mutable state is challenging to maintain in a distributed environment. We develop a technique, based on the Operational Transform, that guides independent agents into producing consistent states through inconsistent but equivalent histories of operations. Our technique, history maintenance, extends and streamlines the Operational Transform for general distributed systems. We describe how to use history maintenance to create eventually-consistent, strongly-consistent, and hybrid systems whose correctness is easy to reason about

World Wide Web Without Walls

Today's Web is built upon a particular symbiotic relationship betweensites and users: the sites invest capital to create and market a setof features, and users gain access to the sites often in exchange fortheir data (e.g., photos, personal information, creative musings,etc.). This paper imagines a very different Web ecosystem, in whichusers retain control of their data and developers can justify theirexistence without hoarding user data

WikiDo

Not formally publishedThe Internet has allowed collaboration on an unprecedented scale. Wikipedia, Luis Von Ahn’s ESP game, and reCAPTCHA have proven that tasks typically performed by expensive in-house or outsourced teams can instead be delegated to the mass of Internet computer users. These success stories show the opportunity for crowdsourcing other tasks, such as allowing computer users to help each other answer questions like “How do I make my computer do X?”. Such a system would reduce IT cost, user frustration, and machine downtime. The current approach to crowd-sourcing IT tasks, however, only allows users to collaborate on generating text. Anyone who goes through the process of searching help wikis and user forums hoping to find a solution for some computer problem knows the inefficacy and the frustration accompanying such a process. Text is ambiguous and often incomplete, particularly when written by non-experts. This paper presents WikiDo, a system that enables the mass of non-expert users to help each other answer how-to computer questions by actually performing the task rather than documenting its solution.National Science Foundation (U.S.) (grant IIS-0835652

Toward Secure Services from Untrusted Developers

We present a secure service prototype built from untrusted,contributed code.The service manages private data for a variety of different users, anduser programs frequently require access to other users' private data.However, aside from covert timing channels, no part of the service cancorrupt private data or leak it between users or outside the systemwithout permission from the data's owners.Instead, owners may choose to reveal their data in a controlled manner.This application model is demonstrated by Muenster, a job searchwebsite that protects both the integrity and secrecy of each user's data.In spite of running untrusted code, Muenster and other services canprevent overt leaks because the untrusted modules are constrained bythe operating system to follow pre-specified security policies, whichare nevertheless flexible enough for programmers to do useful work.We build Muenster atop Asbestos, a recently described operating systembased on a form of decentralized information flowcontrol

Synthetic morphogenesis : space, time, and deformation

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2014.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 127-133).Synthetic biology has presented engineers with a fascinating opportunity: can we understand the principles of our origins { animal embryonic development - by re-engineering it in the laboratory? I investigate, from an engineer's perspective, some of problems that arise in developing geometric form in a deformable substrate. More abstractly, I attack the problem of establishing spatial patterns, when rearranging and deforming parts of the system is inherent to the process. Deformable, foam-like cellular surfaces are developed as a model for embryonic epithelia (polarized cellular sheets), one of the principal tissue types in early animal development. I explore ways in which simple agent programs running within the individual cells can collectively craft large-scale structures. The mechanical properties of the substrate prove crucial to the patterning process. In such a distributed, heterogeneous substrate, little can be assumed about the progress of time. In one branch of my work, I develop patterning techniques where convergence is transparently and locally detectable, drawing insights from clockless digital circuits and casting the problem as distributed constraint propagation. In another branch of work, I avoid the problem of timing by making all patterns self- correcting. In self-correcting patterning, I attempt to understand "canalization" - how development is naturally robust to perturbations. I formulate a model for regional patterning, inspired by regeneration experiments in developmental biology, and using mathematical principles from classical models of magnetic domains and phase separation. The problem again becomes a form of distributed constraint propagation, now using soft constraints. I explore some of the resulting phenomena and then apply the mechanism to crafting surface geometries, where self-correction makes the process robust to both damage and self-deformation. I conclude with a look at how this naturally leads to an example of partial redundancy { multiple systems that partly but not completely overlap in function - yielding confusing responses to the effects of virtual knock-out experiments, reminiscent of the confusing behavior of knock-out experiments in biology.by Micah Z. Brodsky.Ph. D