Sanjit K. Kaul

Real-time status: How often should one update?

Increasingly ubiquitous communication networks and connectivity via portable devices have engendered a host of applications in which sources, for example people and environmental sensors, send updates of their status to interested recipients. These applications desire status updates at the recipients to be as timely as possible; however, this is typically constrained by limited network resources. In this paper, we employ a time-average age metric for the performance evaluation of status update systems. We derive general methods for calculating the age metric that can be applied to a broad class of service systems. We apply these methods to queue-theoretic system abstractions consisting of a source, a service facility and monitors, with the model of the service facility (physical constraints) a given. The queue discipline of first-come-first-served (FCFS) is explored. We show the existence of an optimal rate at which a source must generate its information to keep its status as timely as possible at all its monitors. This rate differs from those that maximize utilization (throughput) or minimize status packet delivery delay. While our abstractions are simpler than their real-world counterparts, the insights obtained, we believe, are a useful starting point in understanding and designing systems that support real time status updates.

Effect of Antenna Placement and Diversity on Vehicular Network Communications

In this paper we present empirical results from a study examining the effects of antenna diversity and placement on vehicle-to-vehicle link performance in vehicular ad hoc networks. The experiments use roof- and in-vehicle mounted omni-directional antennas and IEEE 802.11a radios operating in the 5 GHz band, which is of interest for planned inter-vehicular communication standards. Our main findings are two-fold. First, we show that radio reception performance is sensitive to antenna placement in the 5 Ghz band. Second, our results show that, surprisingly, a packet level selection diversity scheme using multiple antennas and radios, multi-radio packet selection (MRPS), improves performance not only in a fading channel but also in line-of-sight conditions. This is due to propagation being affected by car geometry, leading to the highly non-uniform antenna patterns. These patterns are very sensitive to the exact antenna position on the roof, for example at a transmit power of 40 mW the line-of-sight communication range varied between 50 and 250 m depending on the orientation of the cars. These findings have implications for vehicular MAC protocol design. Protocols may have to cope with an increased number of hidden nodes due to the directional antenna patterns. However, car makers can reduce these effects through careful antenna placement and diversity.

Minimizing age of information in vehicular networks

Emerging applications rely on wireless broadcast to disseminate time-critical information. For example, vehicular networks may exchange vehicle position and velocity information to enable safety applications. The number of nodes in one-hop communication range in such networks can be very large, leading to congestion and undesirable levels of packet collisions. Earlier work has examined such broadcasting protocols primarily from a MAC perspective and focused on selective aspects such as packet error rate. In this work, we propose a more comprehensive metric, the average system information age, which captures the requirement of such applications to maintain current state information from all other nearby nodes. We show that information age is minimized at an optimal operating point that lies between the extremes of maximum throughput and minimum delay. Further, while age can be minimized by saturating the MAC and setting the CW size to its throughput-optimal value, the same cannot be achieved without changes in existing hardware. Also, via simulations we show that simple contention window size adaptations like increasing or decreasing the window size are unsuitable for reducing age. This motivates our design of an application-layer broadcast rate adaptation algorithm. It uses local decisions at nodes in the network to adapt their messaging rate to keep the system age to a minimum. Our simulations and experiments with 300 ORBIT nodes show that the algorithm effectively adapts the messaging rates and minimizes the system age.

Real-time status updating: Multiple sources

We examine multiple independent sources providing status updates to a monitor through a first-come-first-served M/M/1 queue. We formulate a status-age timeliness metric and find the region of feasible average status ages for a pair of updating sources. In the presence of interfering traffic with a given offered load, we show the existence of an optimal rate at which a source should generate its updates.

Status updates through queues

Anytime, anywhere network connectivity, together with portable sensing and computing devices have led to applications in which sources, for example people or environmental sensors, send updates of their status, for example location, to interested recipients, say a location service. These applications desire status updates at the recipients to be as timely as possible; however, this is typically constrained by limited network resources. We employ a time-averaged age metric for characterizing performance of such status update systems. We use system abstractions consisting of a source, a service facility and monitors, with the model of the service facility (physical constraints) a given. While prior work examined first-come-first-served (FCFS) queues, this paper looks at the queue discipline of last-come-first-served (LCFS). We explore LCFS systems with and without the ability to preempt the packet currently in service. For each we derive a general expression for system age and solve for the average age a Poisson source can achieve given memoryless service. Specifically, when preemption is allowed, we evaluate how the source would share the service facility with other independent Poisson sources.

ParkNet: a mobile sensor network for harvesting real time vehicular parking information

This paper describes the architecture and design of ParkNet, a mobile sensor network consisting of vehicles, which collects and disseminates real-time information about the availability of parking spaces in urban areas. We outline the broad challenges in real-time data collection and consumption by a mobile sensor network, as well as the design issues specific to ParkNet. Sensor nodes in ParkNet are a combination of privately owned and city owned vehicles and employ low cost ultrasonic sensors to detect the presence of vacant parking spots as they drive by. We present early results on the performance of our sensor platform.

Creating wireless multi-hop topologies on space-constrained indoor testbeds through noise injection

To evaluate routing protocols on a controlled indoor wireless testbed, the radio range must be compressed so that larger multi-hop topologies can be mapped into a laboratory-size area. We propose noise injection as a more flexible option than hardware attenuation and consider methods for mapping real world wireless network topologies onto the testbed. Our experimental results show that additive white Gaussian noise effectively reduces the radio range, without the need for hardware attenuation and careful shielding of wireless cards. We performed experiments for a free space propagation environment. By selecting node positions through an automated procedure, we were able to create a 5-node/4-hop string topology and a random partially connected 6-node topology in a 8m by 8m area with off-the-shelf IEEE 802.11 hardware

Towards large-scale mobile network emulation through spatial switching on a wireless grid

Experimentation with large mobile networks is notoriously tedious and expensive. We present the architecture and work-in-progress implementation of the m-ORBIT testbed, a mobility emulator using spatial switching, which facilitates mobile system experiments with 802.11a/b/g wireless network interfaces. The emulator does not require any physically moving parts---it emulates mobility by switching over an array of 128 spatially distributed radios. Instead of using hardware antenna switches, we implement spatial switching in software over Gigabit Ethernet links to the radio nodes. Preliminary results support the scaling of this approach to a large number of radios at relatively low cost. Packet error rate measurements also indicate that an experimenter can create multi-hop topologies by injecting additive white Gaussian noise into the environment. We demonstrate through an Ad hoc On Demand Distance Vector routing case study how this emulator enables mobile systems experiments and plan to make the emulator available for remote access by the research community.

Detecting MS initiated signaling DDoS attacks in 3G/4G wireless networks

The hierarchical architecture of present day cellular data networks implies that a large number of base stations depend on a small number of core network elements for essential services (including Internet connectivity). If a mobile botnet launches a distributed signaling attack on one or more core network elements (e.g., gateway), a large number of subscribers would experience service degradation. In this work, we propose a new detector that examines a subset of IP packets transmitted by a mobile station (MS) to determine its infection status. Service providers can install this detector anywhere in the data path, i.e., MS, Base Station (BS), gateway, etc., to detect and quarantine infected terminals. The proposed algorithm was trained using one week of IP packet traces generated by 62 different smartphones. Results indicate that this method can detect most types of signaling attacks with more than 0.9 detection probability and less than 0.1 false alarm probability.

GeoMAC: Geo-backoff based co-operative MAC for V2V networks

This paper presents a preliminary design of Geo-MAC, a MAC protocol that exploits spatial diversity in highly mobile wireless networks. It aims to achieve low latency and high reliability, goals that are intrinsic to the success of many envisioned vehicular safety applications. Conventional MAC layers address reliability through ARQ mechanisms that re-transmit messages from the source, if earlier transmissions were not acknowledged. These schemes essentially exploit temporal diversity since retransmissions are only likely to succeed if the channel has improved. GeoMAC exploits spatial diversity, by allowing other nearby nodes to opportunistically forward and retransmit messages. Through a geo-backoff mechanism it uses geographic distance to the destination as a heuristic to select the forwarder most likely to succeed. This mechanism does not require nodes to monitor channel state or position of their neighbors, except for approximate node density, thus enabling their use in highly mobile networks with low coordination overhead. The performance of GeoMAC is evaluated via trace-driven ns2 simulation using packet error measurements from a freeway environment. GeoMac leads to lower delay jitter combined with up to 50% packet delivery rate gains, compared to the AODV and GPSR routing protocols, which also take advantage of nearby nodes for packet forwarding. Spatial diversity is also shown to better utilize available channel opportunities than ARQ mechanisms.