The packet pair technique aims to estimate the capacity of a path (bottleneck bandwidth) from the dispersion of two equal-sized probing packets sent back-to-back. It has also been argued that the dispersion of longer packet bursts (packet trains) can estimate the available bandwidth of a path. This paper examines such packet pair and packet train dispersion techniques in depth. First we demonstrate that, in general, packet pair bandwidth measurements follow a multimodal distribution, and explain the causes of multiple local modes. The path capacity is a local mode, often different than the global mode of this distribution. We illustrate the effects of network load, cross traffic packet size variability, and probing packet size on the bandwidth distribution of packet pairs. We then switch to the dispersion of long packet trains. The mean of the packet train dispersion distribution corresponds to a bandwidth metric that we refer to as Average Dispersion Rate (ADR). We show that the ADR is a lower bound of the capacity and an upper bound of the available bandwidth of a path. Putting all pieces together, we present a capacity estimation methodology that has been implemented in a tool called pathrate. We report on our experiences with pathrate after having measured hundreds of Internet paths over the last three years.

Packet dispersion techniques and capacity estimation

For two decades automakers have been developing technology that could make vehicles go farther on a gallon of gasoline. But instead, they have chosen pep and size — making vehicles like the new Murano accelerate faster than cars like the old Mustang, and making them bigger.

The average vehicle, which 25 years ago accelerated to 60 miles an hour in 14.4 seconds, now does it in 9.9 seconds, a pace once typical only of sporty or luxury cars like Camaros and Jaguars. And vehicle weight now averages about 4,100 pounds, up from about 3,200 in the early 1980's, as many buyers switched to larger, roomier cars or to sport utility vehicles and minivans, and as automakers added safety equipment.

Buyers like the extra zoom and room, but these have come at a cost: average fuel economy has fallen slightly over the last two decades. The government's new standards for light trucks like S.U.V.'s, published yesterday, will require an 8.1 percent increase in miles per gallon over the four model years from 2008 through 2011.

But over the longer term, significant improvement appears to be lagging. As scientists and engineers look for ways to satisfy the fast-growing demand for energy and to slow climate change, many of them say that fundamental changes are needed in the way fuel is produced and consumed.

Automakers Use New Technology to Beef Up Muscle, Not Mileage

The really hard question—and one that we do not answer as well as we might have in Who Controls the Internet? -- is whether new technologies empower individuals or organizations more.

Who Controls the Internet? and An Army of Davids By Jack Goldsmith, Glenn Reynolds, and Tim Wu

After completing this self-contained course on server-based Internet applications software, students who start with only the knowledge of how to write and debug a computer program will have learned how to build web-based applications on the scale of Amazon.com. Unlike the desktop applications that most students have already learned to build, server-based applications have multiple simultaneous users. This fact, coupled with the unreliability of networks, gives rise to the problems of concurrency and transactions, which students learn to manage by using the relational database system.

After working their way to the end of the book, students will have the skills to take vague and ambitious specifications and turn them into a system design that can be built and launched in a few months. They will be able to test prototypes with end-users and refine the application design. They will understand how to meet the challenge of extreme business requirements with automatic code generation and the use of open-source toolkits where appropriate. Students will understand HTTP, HTML, SQL, mobile browsers, VoiceXML, data modeling, page flow and interaction design, server-side scripting, and usability analysis.

Software Engineering for Internet Applications

Technology matters, writes David Nye, because it is inseparable from being human. We have used tools for more than 100,000 years, and their central purpose has not always been to provide necessities. People excel at using old tools to solve new problems and at inventing new tools for more elegant solutions to old tasks. Perhaps this is because we are intimate with devices and machines from an early age--as children, we play with technological toys: trucks, cars, stoves, telephones, model railroads, Playstations. Through these machines we imagine ourselves into a creative relationship with the world. As adults, we retain this technological playfulness with gadgets and appliances--Blackberries, cell phones, GPS navigation systems in our cars.

We use technology to shape our world, yet we think little about the choices we are making. In Technology Matters, Nye tackles ten central questions about our relationship to technology, integrating a half-century of ideas about technology into ten cogent and concise chapters, with wide-ranging historical examples from many societies. He asks: Can we define technology? Does technology shape us, or do we shape it? Is technology inevitable or unpredictable? (Why do experts often fail to get it right?)? How do historians understand it? Are we using modern technology to create cultural uniformity, or diversity? To create abundance, or an ecological crisis? To destroy jobs or create new opportunities? Should "the market" choose our technologies? Do advanced technologies make us more secure, or escalate dangers? Does ubiquitous technology expand our mental horizons, or encapsulate us in artifice?

These large questions may have no final answers yet, but we need to wrestle with them--to live them, so that we may, as Rilke puts it, "live along some distant day into the answers."

"A deeply informed historian who writes with impressive clarity, David Nye persuades us in Technology Matters that we should ask the kind of life-shaping questions about technology that we customarily pose about politics and economics. He does not finally answer the timely questions that he explicates, but provokes us to search for our own answers."
--Thomas P. Hughes, author of Human-Built World: How to Think about Technology and Culture

Technology Matters

Open source software is considered by many to be a novelty and the open source movement a revolution. Yet the collaborative creation of knowledge has gone on for as long as humans have been able to communicate. CODE looks at the collaborative model of creativity -- with examples ranging from collective ownership in indigenous societies to free software, academic science, and the human genome project -- and finds it an alternative to proprietary frameworks for creativity based on strong intellectual property rights.

Intellectual property rights, argues Rishab Ghosh in his introduction, were ostensibly developed to increase creativity; but today, policy decisions that treat knowledge and art as if they were physical forms of property actually threaten to decrease creativity, limit public access to creativity, and discourage collaborative creativity. "Newton should have had to pay a license fee before being allowed even to see how tall the 'shoulders of giants' were, let alone to stand upon them," he writes.

The contributors to CODE, from such diverse fields as economics, anthropology, law, and software development, examine collaborative creativity from a variety of perspectives, looking at new and old forms of creative collaboration and the mechanisms emerging to study them. Discussing the philosophically resonant issues of ownership, property, and the commons, they ask if the increasing application of the language of property rights to knowledge and creativity constitutes a second enclosure movement -- or if the worldwide acclaim for free software signifies a renaissance of the commons. Two concluding chapters offer concrete possibilities for both alternatives, with one proposing the establishment of "positive intellectual rights" to information and another issuing a warning against the threats to networked knowledge posed by globalization.

Rishab Aiyer Ghosh is Program Leader at the International Institute of Infonomics at Maastricht University. He was one of the founders and is the current managing editor of First Monday, the peer-reviewed Internet journal.

Endorsements

"CODE is a mature and sophisticated exploration of the most important issues related to creativity in the digital age. The broad mix of scholars, offering extraordinarily insightful perspectives, makes this collection essential for understanding this critically important set of questions."
--Lawrence Lessig, Stanford Law School, author of Free Culture

CODE

Digital technology has changed the way we interact with everything from the games we play to the tools we use at work. Designers of digital technology products no longer regard their job as designing a physical object--beautiful or utilitarian--but as designing our interactions with it. In Designing Interactions, award-winning designer Bill Moggridge introduces us to forty influential designers who have shaped our interaction with technology. Moggridge, designer of the first laptop computer (the GRiD Compass, 1981) and a founder of the design firm IDEO, tells us these stories from an industry insider's viewpoint, tracing the evolution of ideas from inspiration to outcome. The innovators he interviews--including Will Wright, creator of The Sims, Larry Page and Sergey Brin, the founders of Google, and Doug Engelbart, Bill Atkinson, and others involved in the invention and development of the mouse and the desktop--have been instrumental in making a difference in the design of interactions. Their stories chart the history of entrepreneurial design development for technology.

Moggridge and his interviewees discuss such questions as why a personal computer has a window in a desktop, what made Palm's handheld organizers so successful, what turns a game into a hobby, why Google is the search engine of choice, and why 30 million people in Japan choose the i-mode service for their cell phones. And Moggridge tells the story of his own design process and explains the focus on people and prototypes that has been successful at IDEO--how the needs and desires of people can inspire innovative designs and how prototyping methods are evolving for the design of digital technology.

Designing Interactions is illustrated with more than 700 images, with color throughout. Accompanying the book is a DVD that contains segments from all the interviews intercut with examples of the interactions under discussion.

Interviews with:
Bill Atkinson • Durrell Bishop • Brendan Boyle • Dennis Boyle • Paul Bradley • Duane Bray • Sergey Brin • Stu Card • Gillian Crampton Smith • Chris Downs• Tony Dunne • John Ellenby • Doug Englebart • Jane Fulton Suri • Bill Gaver • Bing Gordon • Rob Haitani • Jeff Hawkins • Matt Hunter • Hiroshi Ishii • Bert Keely • David Kelley • Rikako Kojima • Brenda Laurel • David Liddle • Lavrans Løvlie • John Maeda • Paul Mercer • Tim Mott • Joy Mountford • Takeshi Natsuno • Larry Page • Mark Podlaseck • Fiona Raby • Cordell Ratzlaff • Ben Reason • Jun Rekimoto • Steve Rogers • Fran Samalionis • Larry Tesler • Bill Verplank • Terry Winograd • Will Wright

Award-winning designer Bill Moggridge is a founder of IDEO, one of the most successful design firms in the world and one of the first to integrate the design of software and hardware into the practice of industrial design. He has been Visiting Professor in Interaction Design at the Royal College of Art in London, Lecturer in Design at the London Business School, member of the Steering Committee for the Interaction Design Institute in Ivrea, Italy, and is currently Consulting Associate Professor in the Joint Program in Design at Stanford University.

Designing Interactions

Innovative uses of global and local networks of linked computers make new ways of collaborative working, learning, and acting possible. In Group Cognition Gerry Stahl explores the technological and social reconfigurations that are needed to achieve computer-supported collaborative knowledge building--group cognition that transcends the limits of individual cognition. Computers can provide active media for social group cognition where ideas grow through the interactions within groups of people; software functionality can manage group discourse that results in shared understandings, new meanings, and collaborative learning. Stahl offers software design prototypes, analyzes empirical instances of collaboration, and elaborates a theory of collaboration that takes the group, rather than the individual, as the unit of analysis.

Stahl's design studies concentrate on mechanisms to support group formation, multiple interpretive perspectives, and the negotiation of group knowledge in applications as varied as collaborative curriculum development by teachers, writing summaries by students, and designing space voyages by NASA engineers. His empirical analysis shows how, in small-group collaborations, the group constructs intersubjective knowledge that emerges from and appears in the discourse itself. This discovery of group meaning becomes the springboard for Stahl's outline of a social theory of collaborative knowing. Stahl also discusses such related issues as the distinction between meaning making at the group level and interpretation at the individual level, appropriate research methodology, philosophical directions for group cognition theory, and suggestions for further empirical work.

Group Cognition

Decades before the Internet, ham radio provided instantaneous, global, person-to-person communication. Hundreds of thousands of amateur radio operators--a predominantly male, middle- and upper- class group known as "hams"--built and operated two-way radios for recreation in mid-twentieth century America. In Ham Radio's Technical Culture, Kristen Haring looks at ham culture: why so many adopted this technical hobby and how the pastime helped them form identity and community.

Ham radio required solitary tinkering with sophisticated electronics equipment, often isolated from domestic activities in a "radio shack," yet thrived on fraternal interaction. Conversations on the air grew into friendships, and hobbyists gathered in clubs or met informally for "eyeball contacts." Within this community, hams developed distinct values and practices with regard to radio, creating what Haring calls a "technical culture." Outsiders viewed amateur radio operators with a mixture of awe and suspicion, impressed by hams' mastery of powerful technology but uneasy about their contact with foreigners.

Drawing on a wealth of personal accounts found in radio magazines and newsletters and on technical manuals, trade journals, and government documents, Haring describes how ham radio culture rippled through hobbyists' lives. She explains why hi-tech employers recruited hams and why electronics manufacturers sought out these specialty customers. She discusses hams' position within the military and civil defense during the Second World War and the Cold War as well as the effect of the hobby on family dynamics. By considering ham radio in the context of technical hobbies--model building, photography, high-fidelity audio, and other popular pursuits--Haring shows what experiences were shared by people who took up various technologies for leisure and how their perspectives influenced attitudes toward technology beyond hobby communities.

Kristen Haring is a historian of science and technology who lives in New York City. An article on which a chapter of Ham Radio's Technical Culture is based received the IEEE Life Members' Prize in Electrical History from the Society for the History of Technology.

Ham Radio's Technical Culture

Software platforms are the invisible engines that have created, touched, or transformed nearly every major industry for the past quarter century. They power everything from mobile phones and automobile navigation systems to search engines and web portals. They have been the source of enormous value to consumers and helped some entrepreneurs build great fortunes. And they are likely to drive change that will dwarf the business and technology revolution we have seen to this point. Invisible Engines examines the business dynamics and strategies used by firms that recognize the transformative power unleashed by this new revolution--a revolution that will change both new and old industries.

The authors argue that in order to understand the successes of software platforms, we must first understand their role as a technological meeting ground where application developers and end users converge. Apple, Microsoft, and Google, for example, charge developers little or nothing for using their platforms and make most of their money from end users; Sony PlayStation and other game consoles, by contrast, subsidize users and make more money from developers, who pay royalties for access to the code they need to write games. More applications attract more users, and more users attract more applications. And more applications and more users lead to more profits.

Invisible Engines tells this story through the lens of the companies that have mastered this platform-balancing act. It offers detailed studies of the personal computer, video game console, personal digital assistant, smart mobile phone, and digital media software platform industries, focusing on the business decisions made by industry players to drive profits and stay a step ahead of the competition. Shorter discussions of Internet-based software platforms provide an important glimpse into a future in which the way we buy, pay, watch, listen, learn, and communicate will change forever. An electronic version of this book is available under a Creative Commons license.

Invisible Engines