This paper focuses on routing for vehicles getting access to infrastructure either directly or via multiple hops though other vehicles. We study Routing Protocol for Low power and lossy networks (RPL), a tree-based routing protocol designed for sensor networks. Many design elements from RPL are transferable to the vehicular environment. We provide a simulation performance study of RPL and RPL tuning in VANETs. More specifically, we seek to study the impact of RPL's various parameters and external factors (e.g., various timers and speeds) on its performance and obtain insights on RPL tuning for its use in VANETs.

Call Admission Control (CAC) is recognized as one of the key strategies to achieve satisfactory QoS support for VoIP over IEEE 802.11 WLANs. However, most of the prior CAC solutions for VoIP over WLAN are centralized, and the few distributed CAC schemes proposed so far do not account for issues such as the loss of channel time due to medium contention and the coexistence of VoIP traffic with background traffic such as TCP data flows. In this paper, we describe a distributed CAC scheme which aims at addressing these issues by leveraging on channel monitoring techniques, thus being readily implementable in today's consumer devices. The proposed scheme is thoroughly evaluated by means of testbed experiments as well as network simulations in different scenarios, and it is shown to yield a better CAC performance with respect to prior solutions.

Nowadays, mobile users wish to use their multi-interface mobile devices to access the Internet through network points of attachment (PoA) based on heterogeneous wireless technologies. They also wish to seamlessly change the PoAs during their ongoing sessions to improve service quality and/or reduce monetary cost. If appropriately handled, multihomed mobile nodes offer a potential solution to this issue. In this paper, we present an improvement of SIP mobility (pre-call plus mid-call mobility) to support seamless mobility of multihomed mobile nodes in heterogeneous wireless networks. Pre-call mobility is extended to associate user identifier (i.e. SIP URI) and interface identifiers (i.e. IP addresses). The multiple addresses of a mobile device are weighted by the user to create a priority list in the SIP server so as to guarantee resilient reachability of mobile nodes and to avoid unnecessary signaling through wireless links, thus saving radio resources. Then, three variations of mid-call mobility, called hard, hybrid and soft procedures, are also proposed. Their main aim is to minimize, or even avoid, packet losses during interface switching at the mobile node. The proposed solutions have been implemented in a wireless heterogeneous testbed composed of 802.11 WLAN plus 3.5 cellular network, which are fully controlled and configurable. The testbed has been used to study the performance and the robustness of the three proposed mid-call mobility procedures.

Characterizing the number and type of transmitters occupying a given frequency band is a critical aspect of spectrum sensing specifically and cognitive radio generally. We present an analytical framework based on probability to identify the number of frequency hopping transmitters of one specific type in a band of interest, and show that the probability mass functions associated with the different potential number of transmitters quickly becomes Gaussian as the number of channel observations increases. Simulation results confirm that the approach can lead with high probability to a correct decision regarding the number of interferers. Thus, the methods here can serve as a valuable complement to other spectrum sensing approaches.

Throughput scaling and optimal hop distance of interference-limited wireless networks have been well characterized in literature. For some emerging wireless networks, throughput may be more limited by battery energy rather than by interference. In characterizing throughput scaling and optimal hop distance of such power-limited networks, prior work have invoked a zero signal processing energy assumption, which led to the belief that whispering to the nearest neighbor (WtNN, with the average number of hops per source destination pair increasing with increasing node density) achieves the optimal throughput scaling. We show that this belief must be modified for power-limited networks when signal processing energy is not an insignificant factor. In fact, for a power-limited network with nodes uniformly randomly distributed in a bounded region and in the limit of interference-free operation, taking Theta(1) (i.e. does not increase or decrease with increasing node density) number of hops is throughput, energy, and delay optimal, achieving Theta(1) pairwise throughput, energy per bit, and packet delay under uniform traffic, whereas WtNN is strictly suboptimal, achieving a pairwise throughput of O(logradic( n/ n)), which decreases with increasing node density. In addition, we show that a constant characteristic hop distance of dcharsimultaneously achieves the pairwise throughput scaling and minimum network energy consumption for random networks.

Wireless networks that do not require pre-deployed infrastructures are ideal for applications such as disaster relieve, search and rescue, and small tactical unit operations. However, since the type of information exchange in these applications typically have stringent delay requirements, existing ad hoc wireless networks that provide best-effort service cannot satisfy the service demands. We propose a proactive mobile wireless network paradigm - one that proactively maintains network connections in order to provide assured delivery for applications with high quality of service (QoS) demands. The proactive techniques include node localization, trajectory prediction, judicious insertion of helper nodes, and helper node trajectory control In addition to presenting an architectural framework for proactive mobile wireless networks, we describe two helper node trajectory control schemes. Through simulations, we show that with proper trajectory control, even a few helper nodes can significantly improve network connectivity under typical operating scenarios.

Traditional ad hoc wireless networks designed for best effort performance lack provisions to maintain network connectivity, particularly if nodes are mobile. For applications that require continual and timely service, current commercial grade ad hoc wireless networks cannot satisfy these demands. We study several architecture options that proactively deploy additional fixed or mobile infrastructure nodes (we call helper nodes) to help maintain user network connectivity. Our goal is to answer the questions of when and how helper nodes should be used. Using random line networks as a point of departure, we compare random and strategic helper node deployment schemes and show that at a nominal normalized user node density of N ~ 1, the number of helper nodes needed under random helper deployment can be more than 10 times the number needed under strategic deployment. At lower TV, this factor increases significantly without bound. Based on the expected number of helper nodes needed for different values of N, we delineate scenarios where each scheme is a sensible architecture choice.

Existing slotted channel access schemes in wireless networks assume that slot boundaries at all nodes are synchronized. In practice, relative clock drifts among nodes cause slot misalignment over time and can result in catastrophic data loss in such systems. We propose a simple network-wide slot synchronization scheme (Slot-Sync and Slot-Resync) suitable for duty-cycling wireless sensor networks. The proposed scheme attains high accuracy by circumventing dominant sources of error inherent in traditional time synchronization protocols. At moderate duty-cycle frequencies, the proposed scheme also has the unique advantage of eliminating re-synchronization (re-sync) overhead completely, thereby achieving slot re-sync essentially for free. We provide an energy efficient slot guard time and re- sync interval design for the proposed scheme and analyze several spanning tree structures for slot-alignment message propagation. In addition, we derive upper bounds on the synchronization (sync) error for a family of trees. Through simulations, we compare the spanning trees we propose to those used for time sync in literature and show up to 80% reduction in sync error and up to 70% reduction in energy needed for slot-alignment message propagation by choosing appropriate tree structures.

It is widely believed that "whispering to the nearest neighbor" (ie. taking a large number of short hops) is the throughput optimal routing scheme for all ad-hoc wireless networks. We show that for power-limited networks where processing energy is not an insignificant factor, instead of taking short hops, there exists a characteristic hop distance that, along with load balancing, can attain high throughput, even for networks with a small number of nodes. In fact, the "whisper to the nearest neighbor" routing scheme leads to strictly suboptimal uniform throughput on the order of O(radic(inN/N)) when processing energy is considered. We show that taking thetas(1) number of hops is throughput, energy, and delay optimal, achieving thetas(1) uniform capacity under uniform traffic. These results are contrary to previous results on the capacity of power-limited wireless networks under a zero-processing energy assumption, which showed that capacity increases with increasing number of nodes. Hence, failing to account for processing energy not only leads to suboptimal routing schemes, but also leads to inflated optimism about the capacity scaling behavior of power-limited wireless networks.

Since battery energy is often a premium resource in wireless sensor networks, many systems attempt to conserve battery energy by turning the transceivers off in a process referred to as duty-cycling. In [4-5], the authors proposed two different random duty-cycling protocols utilizing pseudo random number generators to efficiently exchange ON/OFF schedules among neighboring nodes for transmission coordination. We propose two simple transmission schemes that are superior to the performance of the schemes in [4-5] in terms of energy consumption and a macro network metric termed delivery capacity. The delivery capacity measures the maximum long term average unicast traffic delivery capability of a network. Through analyses and simulations, we present a detailed energy and delivery capacity tradeoff study of the transmission schemes in [4-5] and the schemes we propose. We show that one of our proposed schemes is near-optimal in energy consumption for low traffic loads. In addition, we provide insights on the appropriate ON/OFF duty-cycling parameters that can achieve the optimal energy and delivery capacity tradeoff curve.

Many wireless network studies pertain to the derivation of throughput bounds and cross-layer optimization for energy conservation. From a practical perspective, often a network must meet the competing objective of attaining high throughput while conserving energy for prolonged operation. The tradeoff between these two important network metrics has not been adequately addressed in the literature. A further deficiency in existing research lies in the suppression of non-distance-dependent processing energy in energy models, which can lead erroneously to routing on paths with large number of hops. We model processing energy explicitly, and show that for a particular implementation of wireless network, the average path power is a monotonically increasing, piecewise linear function of throughput. Furthermore, we quantify a network region size threshold such that in a small region relative to the threshold, direct transmission routing is both energy conserving and throughput achieving. For larger regions, path power optimal routing may not achieve the maximum throughput. Higher throughput may be achieved with suboptimal routing schemes that lead to higher marginal power increase as the throughput increases

Satellite network architecture plays an important role in the success of a satellite business. For future commercial broadband data satellite networks integrated with the terrestrial network, satellite network topology, link capacity, and routing have major impacts on the cost of the network and the amount of revenue the network can generate. To find the most cost-effective satellite network topology, we propose a unified mathematical framework using a two-stage stochastic programming formulation. The solution to the stochastic programming formulation gives optimal link capacities and an optimal routing strategy for different network topologies, taking into account uncertainties in long-term aggregate traffic statistic estimation. Using a simple satellite network example, we show the feasible topology regions for three different satellite topologies and show that, for some parameter values, the hybrid topology is more cost effective than nonhybrid topologies. In the limit of high traffic rejection cost, stochastic dimensioning reduces to static dimensioning. We study worst case static dimensioning for a general geosynchronous earth orbit satellite network and show the feasible topology regions, as well as effective cost comparisons for different topologies. We conclude with a discussion on network cost and architectural flexibility relating to satellite network design.

For future commercial broadband data satellite networks, satellite network topology, link capacity, and routing have major impacts on the cost of the network and the amount of revenue the network can generate. We propose a unified mathematical framework using a two-stage stochastic programming technique to assist network designers in selecting the most cost-effective network topology for data applications. The solution to the stochastic programming formulation gives optimal link capacities and an optimal routing strategy for different network topologies, taking into account uncertainties in longterm aggregate traffic statistics estimation. In particular, we give the optimal topology selection criteria for a general GEO satellite network and identify regions where a hybrid satellite-terrestrial topology is the most cost effective.