David Wetherall

TIMELY: RTT-based Congestion Control for the Datacenter

By: 
Radhika Mittal, Vinh The Lam, Nandita Dukkipati, Emily Blem, Hassan Wassel, Monia Ghobadi, Amin Vahdat, Yaogong Wang, David Wetherall, David Zats
Appears in: 
CCR August 2015

Datacenter transports aim to deliver low latency messaging together with high throughput. We show that simple packet delay, measured as round-trip times at hosts, is an effective congestion signal without the need for switch feedback. First, we show that advances in NIC hardware have made RTT measurement possible with microsecond accuracy, and that these RTTs are sufficient to estimate switch queueing. Then we describe how TIMELY can adjust transmission rates using RTT gradients to keep packet latency low while delivering high bandwidth.

Wi-fi backscatter: internet connectivity for RF-powered devices

By: 
Bryce Kellogg, Aaron Parks, Shyamnath Gollakota, Joshua R. Smith, David Wetherall
Appears in: 
CCR August 2014

RF-powered computers are small devices that compute and communicate using only the power that they harvest from RF signals. While existing technologies have harvested power from ambient RF sources (e.g., TV broadcasts), they require a dedicated gateway (like an RFID reader) for Internet connectivity. We present Wi-Fi Backscatter, a novel communication system that bridges RF-powered devices with the Internet. Speci´Čücally, we show that it is possible to reuse existing Wi-Fi infrastructure to provide Internet connectivity to RF-powered devices.

Tool Release: Gathering 802.11n Traces with Channel State Information

By: 
Daniel Halperin, Wenjun Hu, Anmol Sheth, and David Wetherall
Appears in: 
CCR January 2011

We are pleased to announce the release of a tool that records detailed measurements of the wireless channel along with received 802.11 packet traces. It runs on a commodity 802.11n NIC, and records Channel State Information (CSI) based on the 802.11 standard. Unlike Receive Signal Strength Indicator (RSSI) values, which merely capture the total power received at the listener, the CSI contains information about the channel between sender and receiver at the level of individual data subcarriers, for each pair of transmit and receive antennas.

Predictable 802.11 Packet Delivery From Wireless Channel Measurements

By: 
Daniel Halperin, Wenjun Hu, Anmol Sheth, and David Wetherall
Appears in: 
CCR October 2010

RSSI is known to be a fickle indicator of whether a wireless link will work, for many reasons. This greatly complicates operation because it requires testing and adaptation to find the best rate, transmit power or other parameter that is tuned to boost performance. We show that, for the first time, wireless packet delivery can be accurately predicted for commodity 802.11 NICs from only the channel measurements that they provide. Our model uses 802.11n Channel State Information measurements as input to an OFDM receiver model we develop by using the concept of effective SNR.

A "Gen 2" RFID Monitor Based on the USRP

By: 
Michael Buettner and David Wetherall
Appears in: 
CCR July 2010

We have developed a low cost software radio based platform for monitoring EPC Gen 2 RFID traffic. The Gen 2 standard allows for a range of PHY layer configurations and does not specify exactly how to compose protocol messages to inventory tags. This has made it difficult to know how well the standard works, and how it is implemented in practice. Our platform provides much needed visibility into Gen 2 systems by capturing reader transmissions using the USRP2 and decoding them in real-time using software we have developed and released to the public.

Public Review By: 
A. Chaintreau

As a new area editor of CCR, imagine how comfortable I was to receive a paper that deals partly with some aspect of physical layer, and contains two acronyms unfamiliar to me in the title. However, after reading the paper and getting some expert advice, it became quite clear that this paper is worth communicating to our community. Here is why:
Radio Frequency IDentification (RFID) is meant to be cheap, but conducting RFID research is not. Indeed, the motivation for this standard is, among other things, to bring computing from expensive powered devices to low cost computer chips. However, when it comes to addressing some of its performance issues, monitoring becomes critical and requires today expensive signal analyzers. This paper describes a novel approach to monitor RFID, which was implemented in a well known software defined radio architecture, (briefly) validated, and can be downloaded and reused. This is both a useful tool and a significant step to bring more research in RFID.
One possible application of this monitoring tool is to reverse engineer commercial products, to know which choices are made when the standard left some fine-grained behavior undefined. It is demonstrated in the paper by describing how several functions, such as for example rate adaptation, are run in a commercial RFID reader. One could argue, as some reviewers did, that this paper is simply describing a translation tool. However, all reviewers agreed that this work can be influential as the ease and availability of such monitoring equipment allows for the first time (1) to analyze issues like different uplink error rates and capture effects, (2) to analyze RFID performance evaluations in various environments.
The main fault of this paper, once some points of presentation have been clarified, is to leave us frustrated not to see this new tool applied to one of these new issues. We believe that making this tool available, and including in CCR a description of its functioning with preliminary validation outweighs this drawback. Hopefully this will also help our community to address these issues more quickly.

802.11 with Multiple Antennas for Dummies

By: 
Daniel Halperin, Wenjun Hu, Anmol Sheth, and David Wetherall
Appears in: 
CCR January 2010

The use of multiple antennas and MIMO techniques based on them is the key feature of 802.11n equipment that sets it apart from earlier 802.11a/g equipment. It is responsible for superior performance, reliability and range. In this tutorial, we provide a brief introduction to multiple antenna techniques. We describe the two main classes of those techniques, spatial diversity and spatial multiplexing. To ground our discussion, we explain how they work in 802.11n NICs in practice.

Understanding and Mitigating the Impact of RF Interference on 802.11 Networks

By: 
Ramakrishna Gummadi, David Wetherall, Ben Greenstein, and Srinivasan Seshan
Appears in: 
CCR October 2007

We study the impact on 802.11 networks of RF interference from devices such as Zigbee and cordless phones that increasingly crowd the 2.4GHz ISM band, and from devices such as wireless camera jammers and non-compliant 802.11 devices that seek to disrupt 802.11 operation. Our experiments show that commodity 802.11 equipment is surprisingly vulnerable to certain patterns of weak or narrow-band interference.

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