Vladimir Brik

Wireless device identification with radiometric signatures

We design, implement, and evaluate a technique to identify the source network interface card (NIC) of an IEEE 802.11 frame through passive radio-frequency analysis. This technique, called PARADIS, leverages minute imperfections of transmitter hardware that are acquired at manufacture and are present even in otherwise identical NICs. These imperfections are transmitter-specific and manifest themselves as artifacts of the emitted signals. In PARADIS, we measure differentiating artifacts of individual wireless frames in the modulation domain, apply suitable machine-learning classification tools to achieve significantly higher degrees of NIC identification accuracy than prior best known schemes. We experimentally demonstrate effectiveness of PARADIS in differentiating between more than 130 identical 802.11 NICs with accuracy in excess of 99%. Our results also show that the accuracy of PARADIS is resilient against ambient noise and fluctuations of the wireless channel. Although our implementation deals exclusively with IEEE 802.11, the approach itself is general and will work with any digital modulation scheme.

DSAP: a protocol for coordinated spectrum access

The continually increasing number of wireless devices operating in the unlicensed frequency bands makes the freely-available wireless spectrum a scarce commodity. Under such circumstances, efficient wireless spectrum management is necessary to minimize the effects of overcrowding and maximize quality of service. In this paper we present the design, implementation and evaluation of dynamic spectrum access protocol (DSAP), a centralized method for managing and coordinating spectrum access

A Client-Driven Approach for Channel Management in Wireless LANs

We propose an efficient client-based approach for channel management (channel assignment and load balancing) in 802.11-based WLANs that lead to better usage of the wireless spectrum. This approach is based on a “conflict set coloring” formulation that jointly performs load balancing along with channel assignment. Such a formulation has a number of advantages. First, it explicitly captures interference effects at clients. Next, it intrinsically exposes opportunities for better channel re-use. Finally, algorithms based on this formulation do not depend on specific physical RF models and hence can be applied efficiently to a wide-range of in-building as well as outdoor scenarios. We have performed extensive packet-level simulations and measurements on a deployed wireless testbed of 70 APs to validate the performance of our proposed algorithms. We show that in addition to single network scenarios, the conflict set coloring formulation is well suited for channel assignment where multiple wireless networks share and contend for spectrum in the same physical space. Our results over a wide range of both simulated topologies and in-building testbed experiments indicate that our approach improves application level performance at the clients by upto three times (and atleast 50%) in comparison to current best-known techniques.

Eliminating handoff latencies in 802.11 WLANs using multiple radios: applications, experience, and evaluation

Deployment of Voice-over IP (VoIP) and other real-time streaming applications has been somewhat limited in wireless LANs today, partially because of the high handoff latencies experienced by mobile users. Our goal in this work is to eliminate handoff latency by exploiting the potential of multiple radios in WLAN devices. Our proposed approach, called MultiScan, is implemented entirely on the client-side, and, unlike prior work, MultiScan requires neither changing the Access Points (APs), nor having knowledge of wireless network topology. MultiScan nodes rely on using their (potentially idle) second wireless interface to opportunistically scan and pre-associate with alternate APs and eventually seamlessly handoff ongoing connections. In this paper we describe our implementation of MultiScan, present detailed evaluations of its effect on handoff latency and evaluate performance gains for MultiScan-enhanced wireless clients running Skype, a popular commercial VoIP application.

A measurement study of a commercial-grade urban wifi mesh

We present a measurement study of a large-scale urban WiFi mesh network consisting of more than 250 Mesh Access Points (MAPs), with paying customers that use it for Internet access. Our study, involved collecting multi-modal data, e.g., through continuous gathering of SNMP logs, syslogs, passive traffic capture, and limited active measurements in different parts of the city. Our study is split into four components - planning and deployment of the mesh, success of mesh routing techniques, likely experience of users, and characterization of how the mesh is utilized. During our data collection process that spanned 8 months, the network changed many times due to hardware and software upgrades. Hence to present a consistent view of the network, the core dataset used in this paper comes from a two week excerpt of our dataset. This part of the dataset had more than 1.7 million SNMP log entries (from 224 MAPs) and more than 100 hours of active measurements. The scale of the study allowed us to make many important observations that are critical in planning and using WiFi meshes as an Internet access technology. For example, our study indicates that the last hop 2.4GHz wireless link between the mesh and the client is the major bottleneck in client performance. Further we observe that deploying the mesh access points on utility poles results in performance degradation for indoor clients that receive poor signal from the access points.

Towards an Architecture for Efficient Spectrum Slicing

With the increased demands for wireless spectrum, dynamic spectrum sharing is emerging as an important and powerful concept. Most research in this domain is being conducted in design of cognitive radios and on specific PHY and MAC layer challenges associated with them. However, for a dynamic spectrum sharing architecture to be viable, research is needed to resolve many other challenges, e.g., in the context of real-time spectrum management and enforcement. This paper is the first to present a study of some such important architectural considerations, driven by our ongoing design and implementation of a spectrum sharing system, called Spark. We propose some promising approaches to address these challenges, and enumerate the need and opportunities for significant future research in this domain.

Client-driven channel management for wireless LANs

With the explosive growth in the density of 802.11 access points (APs) in the form of hotspots and public/home access networks, coordinating the shared use of spectrum has become an important problem. The irregular coverage topologies present in WLANs due to the vagaries of the indoor RF environment make the channel assignment algorithms in cellular networks inapplicable [1, 2].