Naveen Santhapuri

Routing with opportunistically coded exchanges in wireless mesh networks

Network coding is known to improve network throughput by mixing information from different flows and conveying more information in each transmission. Recently there have been some proposals for applying network coding to wireless mesh networks leveraging the broadcast nature of wireless transmissions. These approaches exploit coding opportunities passively while forwarding packets but they do not proactively change routing of flows to create more coding opportunities. In this paper, we attempt to investigate the extent of performance gain possible when routing decisions are made with the awareness of coding. We first define the expected number of coded transmissions for a successful exchange of packets between two nodes through an intermediate node. We then formulate optimal routing with coding, given the topology and traffic, as a linear programming problem. We conduct a preliminary evaluation of coding-aware routing and show that it offers significant gain particularly when there are many long distance flows.

Successive interference cancellation: a back-of-the-envelope perspective

Successive interference cancellation (SIC) is a physical layer capability that allows a receiver to decode packets that arrive simultaneously. While the technique is well known in communications literature, emerging software radios are making practical experimentation feasible. This motivates us to study the extent of throughput gains possible with SIC from a MAC layer perspective. Contrary to our initial expectation, we find that the gains from SIC are not easily available in many realistic situations. Moreover, we observe that the scope for SIC gets squeezed by the advances in bitrate adaptation, casting doubt on the future of SIC based protocols.

Successive Interference Cancellation: Carving Out MAC Layer Opportunities

Successive interference cancellation (SIC) is a PHY capability that allows a receiver to decode packets that arrive simultaneously. While the technique is well known in communications literature, emerging software radio platforms are making practical experimentation feasible. This motivates us to study the extent of throughput gains possible with SIC from a MAC layer perspective and scenarios where such gains are worth pursuing. We find that contrary to our initial expectation, the gains are not high when the bits of interfering signals are not known a priori to the receiver. Moreover, we observe that the scope for SIC gets squeezed by the advances in bitrate adaptation. In particular, our analysis shows that interfering one-to-one transmissions benefit less from SIC than scenarios with many-to-one transmissions (such as when clients upload data to a common access point). In view of this, we develop an SIC-aware scheduling algorithm that employs client pairing and power reduction to extract the most gains from SIC. We believe that our findings will be useful guidelines for moving forward with SIC-aware protocol research.

Predicting length of stay at WiFi hotspots

Todays smartphones provide a variety of sensors, enabling high-resolution measurements of user behavior. We envision that many services can benefit from short-term predictions of complex human behavioral patterns. While enablement of behavior awareness through sensing is a broad research theme, one possibility is in predicting how quickly a person will move through a space. Such a prediction service could have numerous applications. For one example, we imagine shop owners predicting how long a particular customer is likely to browse merchandise, and issue targeted mobile coupons accordingly - customers in a hurry can be encouraged to stay and consider discounts. Within a space of moderate size, WiFi access points are uniquely positioned to track a statistical framework for user length of stay, passively recording metrics such as WiFI signal strength (RSSI) and potentially receiving client-uploaded sensor data. In this work, we attempt to quantity this opportunity, and show that human dwell time can be predicted with reasonable accuracy, even when restricted to passively observed WiFi RSSI.

Order matters: transmission reordering in wireless networks

Modern wireless interfaces support a physical-layer capability called Message in Message (MIM). Briefly, MIM allows a receiver to disengage from an ongoing reception and engage onto a stronger incoming signal. Links that otherwise conflict with each other can be made concurrent with MIM. However, the concurrency is not immediate and can be achieved only if conflicting links begin transmission in a specific order. The importance of link order is new in wireless research, motivating MIM-aware revisions to link-scheduling protocols. This paper identifies the opportunity in MIM-aware reordering, characterizes the optimal improvement in throughput, and designs a link-layer protocol for enterprise wireless LANs to achieve it. Testbed and simulation results confirm the performance gains of the proposed system.

I²MIX: Integration of Intra-Flow and Inter-Flow Wireless Network Coding

Wireless network coding has been shown to reduce the number of transmissions by exploiting the broadcast nature of the wireless medium. Multiple packets may be encoded into a single packet when their respective next hops have enough information to decode them. Previous research has shown that packets belonging to different flows may be encoded (interflow coding) when they are passing through a common router. Similarly, it has also been shown that coding packets of the same flow (intra-flow coding) may provide better reliability by not relying on the reception of any single packet. In this work, we first present IMIX, an intra-flow wireless network coding scheme which has the potential to save transmissions and therefore improve network throughput. We then propose I2 MIX, the first design to the best of our knowledge, that can benefit from integrating inter and intra flow wireless network coding. Finally, we show through trace based evaluations that I2 MIX can reduce the total number of transmissions by 21-30%.

Known Interference Cancellation: Resolving Collisions Due to Repeated Transmissions

Reception of duplicate packets by a node in a wireless network is a common occurrence. Reasons for repeated transmissions range from broadcast flooding to multicast streaming to unicast forwarding. These repeated transmissions may also get involved in collisions like other original transmissions. We argue that when one of the colliding packets is previously overheard, its interference can be cancelled to decode the other packet. In other words, when a receiver overhears a packet, it becomes effectively immune to the interference caused by the packets subsequent transmission. We refer to this as known interference cancellation (KIC). In this paper, we identify the scenarios in which KIC is applicable. We then implement KIC on USRP/GnuRadio testbed to demonstrate its feasibility and conduct QualNet simulations to illustrate its potential performance gain.

On Spatial Reuse and Capture in Ad Hoc Networks

Neighbors of both the transmitter and the receiver must keep quiet in a 802.11 wireless network as it requires bidirectional exchange, i.e., nodes reverse their roles as transmitters and receivers, for transmitting a single DATA frame. To reduce role reversals and to improve spatial reuse, a piggybacked acknowledgment based approach has been proposed to enable concurrent transmissions. Recent findings on physical layer capture show that it is possible to capture a frame of interest in the presence of concurrent interference and that the SINR threshold is dependent on the relative order in which the frame and the interference arrive at the receiver. In this paper, we show that it is possible to exploit capture and increase concurrent transmissions in wireless adhoc networks. We develop a distributed channel access scheme and demonstrate that it offers significant throughput gain particularly at lower data rates.

Selection of Bit-Rate for Wireless Network Coding

Network coding is known to improve throughput by mixing information from different flows and conveying more information in each transmission. Recently some proposals have demonstrated the benefits of applying network coding to wireless networks with broadcast transmissions. It is expected that the opportunities for coding and the corresponding gains depend on the bit-rate chosen for determining routes and transmitting packets. However, the previous work on wireless network coding assumed a fixed rate and did not explicitly account for the interaction between rate selection and coding gain. In this paper, we define a new metric, expected coded time (ECT), that measures the total time needed by a node to deliver two packets to their receivers given the bit-rate for transmitting coded packets. We then investigate how the optimal bit-rate for coded packets differs from that for transmission of native packets individually. We also study the performance of network coding under different fixed bit-rates for the whole network. Our evaluation shows that 11 Mbps is the best default fixed rate for MIT Roofnet and 5.5 Mbps is mostly the optimal rate to transmit coded packets when the ideal individual bit-rate for each receiver is different.

A Dual Authentication Protocol for IEEE 802.11 Wireless LANs

In this paper, we first identify a vulnerability of IEEE 802.11 wireless LANs in which a compromised access point can still authenticate itself to a wireless station and gain control over the connection, and show that the current IEEE 802.11i standard does not address this problem. We then propose a new protocol that can counter this attack by providing dual authentication for both a wireless station and its corresponding access point at connection setup time using the authentication server. We also consider roaming situations and present a roaming authentication protocol. Finally, we show that our protocol is in conformance with the requirements of the IEEE 802.11i standard, and show that it performs no worse with respect to communication time than IEEE 802.11i using a prototype implementation of each protocol