Weigao Xie

A stochastic model for beaconless IEEE 802.15.4 MAC operation

IEEE 802.15.4 is a popular choice for MAC/PHY protocols in low power and low data rate wireless sensor networks. In this paper, we develop a stochastic model for the beaconless operation of IEEE 802.15.4 MAC protocol. Given the number of nodes competing for channel access and their packet generation rates, the model predicts the packet loss probability and the packet latency. We compared the model predictions with NS2 simulation results and found an excellent match between the two for a wide range of the packet generation rates and the number of competing nodes in the network.

A Performance Analysis of Point-to-Point Routing along a Directed Acyclic Graph in Low Power and Lossy Networks

The ROLL working group at IETF is currently designing RPL, the Routing Protocol for Low power and lossy networks. RPL is based on organizing the nodes in a network in one or more directed acyclic graphs(DAGs) rooted at the popular/default destinations. The DAG structure naturally supports multipoint-to-point routing towards the DAG root and point-to-multipoint routing from the root towards the nodes. The point-to-point (P2P) routing mechanism specified in RPL requires a packet to travel upwards along the DAG until it is turned downwards by a node that knows the downward route along the DAG to the packet’sdestination. This up and down routing along the DAG may result in significantly suboptimal P2P routes. In this paper, we quantify the difference in terms of routing cost between DAG-based P2P routes and the shortest (or minimum cost) P2P routes available in a sample network topology and make the case for a better P2P routing solution in RPL.

Routing Loops in DAG-Based Low Power and Lossy Networks

Directed Acyclic Graphs (DAGs), rooted at popular/default destinations, have emerged as a preferred mechanism to provide IPv6 routing functionality in large scale low power and lossy networks, that include wireless sensor networks and those based on power line communication. A DAG maintains its acyclic nature by requiring that each DAG node must have a higher ’rank’ than any of its DAG parents. While a node may decrease its DAG rank safely, increasing its DAG rank to add a new parent may result in a routing loop if the new parent is also a descendant in the DAG. In this paper, we first study via simulations the time required by the network to converge to a stable, loop-free state following a rank increase operation and the number of routing messages generated (the network ’churn’) during this time. Then, we describe the precautionary measures that can be used to avoid routing loops and evaluate via simulations how these measures affect the time and churn involved in reaching a stable state following a rank increase operation.

Impact of IEEE 802.11n Operation on IEEE 802.15.4 Operation

Wireless communication among sensor devices, enabled by IEEE 802.15.4 protocol, is increasingly replacing the existing wired technology in a wide range of monitoring and control applications. IEEE 802.15.4 networks typically operate in 2.4GHz ISM band, which is used by popular IEEE 802.11 (WiFi) networks as well. An overlap between the channels used by IEEE802.15.4 and IEEE 802.11 networks may adversely impact the operation of IEEE 802.15.4 since it is a low power protocol using a small channel width compared to the transmitted power levels and channel width used by IEEE 802.11. Avoiding channel overlap between IEEE 802.15.4 and IEEE 802.11 networks will become difficult once proposed IEEE 802.11n technology becomes popular. This is because an IEEE802.11n network may use two 20MHz wide channels for its operation, rather than a single 20MHz channel as is the case with other IEEE 802.11 PHY layers. In this paper, we evaluate, via test bed experiments, the impact of IEEE 802.11n operation on IEEE 802.15.4 performance when there is an overlap between the channels used by IEEE 802.15.4 and IEEE 802.11n networks.

Evaluating the Impact of Signal to Noise Ratio on IEEE 802.15.4 PHY-Level Packet Loss Rate

In this paper, we evaluate the impact of signal to noise ratio (SNR) on the PHY-level packet loss rates on IEEE 802.15.4 links under the additive white Gaussian noise and Rayleigh fading models. We show that IEEE 802.15.4 PHY-level packet loss rate has a step-like response to the SNR deterioration. In other words, the packet loss rate is largely unaffected by SNR deterioration as long as SNR is more than a threshold. However, even a small deterioration in SNR beyond this threshold causes the packet loss rate to approach1. This result implies that SNR may not serve as a fine-granularity metric to indicate the reliability of an IEEE 802.15.4 link.

Scheduling routing table calculations to achieve fast convergence in OSPF protocol

Fast convergence to topology changes is now a key requirement in routing infrastructures while reducing the routing protocol’s processing overhead continues to be as important as before. In this paper, we examine the problem of scheduling routing table updates in link state routing protocols. Commercial routers typically use a hold time based scheme to limit the number of routing table updates as new LSAs arrive at the router. The hold time schemes limit the number of routing table updates at the expense of increased delay in convergence to the new topology, which is clearly not acceptable any more. We analyze the performance of different hold time schemes and propose a new approach to schedule routing table updates, called LSA Correlation. Rather than using individual LSAs as triggers for routing table updates, LSA Correlation scheme correlates the information in the LSAs to identify the topology change that led to their generation. A routing table update is performed when a topology change has been identified. The analysis and simulation results presented in this paper suggest that the LSA Correlation scheme performs much better than the hold time based schemes for both isolated and large scale topology change scenarios.

AntSens: An Ant Routing Protocol for Large Scale Wireless Sensor Networks

Large scale wireless sensor networks, consisting of thousands of nodes spread over a large geographical area, are being planned for a number of monitoring and control applications. The individual nodes in such networks would be severely limited in their capabilities - they will have limited memory and CPU resources and would typically run on batteries. A suitable routing protocol for large wireless sensor networks should scale well with the size of the network in terms of the control packet, memory and processing overhead. Dynamic link/node characteristics, such as PHY/MAC loss rates and battery levels, in such networks require continuous monitoring of the quality of existing routes and frequent discovery of new routes. The routing protocol should allow distribution of traffic load through out the network to avoid artificial congestion, and hence high MAC level loss rates, in parts of the network. The existing routing protocols do not meet all the criteria mentioned above. Hence, we propose a new routing protocol meeting the criteria mentioned above. The proposed protocol, called AntSens, may be considered as belonging to the category of ant routing protocols that are based on the manner ants discover and remember the best routes to the food location.

Dynamic Reconfiguration in Beaconless IEEE 802.15.4 Networks under Varying Traffic Loads

IEEE 802.15.4, a MAC/PHY protocol for low power and low data rate wireless networks, is emerging as the popular choice for various monitoring and control applications. Depending on the application, the traffic load on an IEEE 802.15.4 network may vary over a wide range. The performance of the protocol, measured in terms of the packet loss probability and the packet latency, depends upon the prevailing traffic load as well as the configuration of IEEE 802.15.4 MAC parameters. Static configuration of IEEE 802.15.4 MAC parameters, good for a particular traffic load range, may result in bad performance when the traffic load changes beyond that range. In this paper, we present a scheme that can be used by an IEEE 802.15.4 node to dynamically adjust its MAC configuration based on the observed loss rate and latency suffered by recent packets generated by the node. The performance benefits of the proposed scheme are illustrated via NS2 simulations.

IEEE 802.15.4 modifications and their impact

IEEE 802.15.4 is a popular choice for MAC/PHY protocols in low power and low data rate wireless sensor networks. In this paper, we suggest several modifications to beaconless IEEE 802.15.4 MAC operation and evaluate their impact on the performance via stochastic modeling and simulations. We found that the utility of these modifications is strongly dependent on the traffic load on the network. Accordingly, we make recommendations regarding how these modifications should be used in view of the prevalent traffic load on the network.

Improving OSPF Dynamics on a Broadcast LAN

In this article, the authors propose modifications to OSPFs interface state machine to reduce the time/processing requirements of the leader election process in a broadcast local-area network (LAN) environment. The proposed modifications are based on dynamic adjustment of wait time duration rather than using a static value. They compare the original and modified state machines both analytically as well as via testbed experiments.