Wee Lum Tan

Analytical Models and Performance Evaluation of Drive-thru Internet Systems

Drive-thru Internet systems are multiple-access wireless networks in which users in moving vehicles can connect to a roadside access point (AP) to obtain Internet connectivity for some period of time as the vehicles pass through the APs coverage range. In order to evaluate the type of communication services and the quality-of-service that these systems can provide, in this paper, we investigate the data communication performance of a vehicle in Drive-thru Internet systems. In particular, we derive analytical models with tractable solutions to characterize the average and the distribution of the number of bytes downloaded by a vehicle by the end of its sojourn through an APs coverage range, in the presence of other vehicles contending for the same APs resources. Our models are able to quantify the impact of road traffic density, vehicle speed, service penetration rate, APs transmission range and the corresponding bit rate, on the amount of data downloaded by an individual vehicle. In terms of analysis technique, we map the study of our vehicular data downloading process into the transient analysis of a series of Markov reward processes. Our use of Markov reward model is novel in the sense that we only select from the corresponding Markov chain, a subset of relevant sample paths that matches the required behavior of our vehicular flow model. We also validate our proposed analytical models through extensive simulations, driven by empirical vehicular traffic traces. We believe our work offers a unique analytical framework based on which the interplay between vehicular traffic parameters and a vehicles data communication performance in a Drive-thru Internet system can be studied and optimized in a systematic, quantitative manner.

Automated analysis of AODV using UPPAAL

This paper describes an automated, formal and rigorous analysis of the Ad hoc On-Demand Distance Vector (AODV) routing protocol, a popular protocol used in wireless mesh networks. We give a brief overview of a model of AODV implemented in the UPPAAL model checker. It is derived from a process-algebraic model which reflects precisely the intention of AODV and accurately captures the protocol specification. Furthermore, we describe experiments carried out to explore AODVs behaviour in all network topologies up to 5 nodes. We were able to automatically locate problematic and undesirable behaviours. This is in particular useful to discover protocol limitations and to develop improved variants. This use of model checking as a diagnostic tool complements other formal-methods-based protocol modelling and verification techniques, such as process algebra.

A process algebra for wireless mesh networks

We propose a process algebra for wireless mesh networks that combines novel treatments of local broadcast, conditional unicast and data structures. In this framework, we model the Ad-hoc On-Demand Distance Vector (AODV) routing protocol and (dis)prove crucial properties such as loop freedom and packet delivery.

Sequence numbers do not guarantee loop freedom: AODV can yield routing loops

In the area of mobile ad-hoc networks and wireless mesh networks, sequence numbers are often used in routing protocols to avoid routing loops. It is commonly stated in protocol specifications that sequence numbers are sufficient to guarantee loop freedom if they are monotonically increased over time. A classical example for the use of sequence numbers is the popular Ad hoc On-Demand Distance Vector (AODV) routing protocol. The loop freedom of AODV is not only a common belief, it has been claimed in the abstract of its RFC and at least two proofs have been proposed. AODV-based protocols such as AODVv2 (DYMO) and HWMP also claim loop freedom due to the same use of sequence numbers. In this paper we show that AODV is not a priori loop free; by this we counter the proposed proofs in the literature. In fact, loop freedom hinges on non-evident assumptions to be made when resolving ambiguities occurring in the RFC. Thus, monotonically increasing sequence numbers, by themselves, do not guarantee loop freedom.

Efficient topology discovery in software defined networks

Software Defined Networking (SDN) is a new networking paradigm, with a great potential to increase network efficiency, ease the complexity of network control and management, and accelerate the rate of technology innovation. One of the core concepts of SDN is the separation of the networks control and data plane. The intelligence and the control of the network operation and management, such as routing, is removed from the forwarding elements (switches) and is concentrated in a logically centralised component, i.e. the SDN controller. In order for the controller to configure and manage the network, it needs to have up-to-date information about the state of the network, in particular its topology. Consequently, topology discovery is a critical component of any Software Defined Network architecture. In this paper, we evaluate the cost and overhead of the de facto standard approach to topology discovery currently implemented by the major SDN controller frameworks, and propose simple and practical modifications which achieve a significantly improved efficiency and reduced control overhead. We have implemented our new topology discovery approach on the widely used POX controller platform, and have evaluated it for a range of network topologies via experiments using the Mininet network emulator. Our results show that our proposed modifications achieve an up to 45% reduction in controller load compared to the current state-of-the-art approach, while delivering identical discovery functionality.

A rigorous analysis of AODV and its variants

In this paper we present a rigorous analysis of the Ad hoc On-Demand Distance Vector (AODV) routing protocol using a formal specification in AWN (Algebra for Wireless Networks), a process algebra which has been specifically tailored for the modelling of Mobile Ad Hoc Networks and Wireless Mesh Network protocols. Our formalisation models the exact details of the core functionality of AODV, such as route discovery, route maintenance and error handling. We demonstrate how AWN can be used to reason about critical protocol correctness properties by providing a detailed proof of loop freedom. In contrast to evaluations using simulation or other formal methods such as model checking, our proof is generic and holds for any possible network scenario in terms of network topology, node mobility, traffic pattern, etc. A key contribution of this paper is the demonstration of how the reasoning and proofs can relatively easily be adapted to protocol variants.

Evaluation of parameterised route repair in AODV

One of the key challenges for routing protocols in wireless multi-hop networks is to deal with link failures, and to repair the routes in these situations. In the Ad-hoc On Demand Distance Vector (AODV) protocol, routes can either be repaired by re-establishing a new route from scratch starting from the source node (Source Repair), or they can be locally repaired by the node that detects the link break along the end-to-end path (Local Repair). In some situations Source Repair will lead to better performance, in other situations Local Repair will be the more appropriate choice. In this work, we explore a flexible, parameterised approach in deciding on which of these two route repair strategies to use in the event of a link break. We define a Local Repair Threshold parameter that determines how far along the end-to-end path that a link break needs to occur in order to initiate Local Repair, as opposed to Source Repair. Our simulation results show that the optimal choice of the Local Repair Threshold, in terms of Packet Delivery Ratio, depends on the network load. We show that a flexible, parameterised and adaptive approach to choosing the Local Repair Threshold, can improve the Packet Delivery Ratio by up to 37% (in absolute terms), compared to the approach employed by standard AODV. We also show a significant potential improvement of up to 18% over the route repair strategy employed by the Dynamic On demand MANET (DYMO) routing protocol, which is based on AODV.

Multi-Source–Destination Distributed Wireless Networks: Pareto-Efficient Dynamic Power Control Game With Rapid Convergence

A game-theoretic method for transmit power control across multi-source-destination distributed wireless networks is proposed, which is viable for any number of source-destination pairs, with any number of players (or sources). A dynamic noncooperative repeated game is proposed to optimize both packet delivery ratio (PDR) and transmit power considering a realistic signal-to-interference-plus-noise ratio (SINR) model of the wireless channel. Here, the sources, which are players, transmit concurrently and, thus, have imperfect information about the actions of other players. The game accounts for a limited set of discrete values for transmit power, and the game can be applied in static, quasi-static, and slow-fading channels. If the SINR is feasible, each game stage has a subgame perfect equilibrium, and the game requires fewer iterations to converge to a Pareto-efficient outcome than other appropriate techniques such as SINR discrete power balancing and multiobjective power optimization. In this context, a novel accurate PDR model is given in terms of a compressed exponential function of inverse SINR, which is a function that is realistic for many IEEE 802.11-type implementations of various packet sizes and data rates, and facilitates a tractable analysis and implementation of this dynamic game.

Experimental evaluation of measurement-based SINR interference models

In 802.11-based wireless networks, the ability to accurately predict the impact of interference via the use of an interference model is essential to better and more efficient channel assignment algorithms and data routing protocols. Recently, there have been several works that proposed new interference models utilizing the well-known concept of signal-to-interference-plus-noise ratio (SINR). Using active measurements, these models construct a profile that maps either the measured received signal strength (RSS) or the computed SNR or SINR values at a receiver, to its packet delivery ratio (PDR) performance. The profile is then used by the models to predict the PDR performance in more complex scenarios involving multiple interferers. While comparison with other basic models (e.g. hop-based and distance-based) have been made in these works, there has as yet been no comprehensive comparison on the accuracy of these measurement-based SINR interference models. In this paper, we systematically evaluate the performance of three measurement-based SINR interference models in predicting the interference impact on the successful reception of packets. Our evaluations cover various interference scenarios with both 802.11 and non-802.11 interferers, in experiments carried out in both our conducted testbed and an over-the-air testbed. Our results show that an interference model that utilizes an SINR profile can accurately predict the PDR performance with a maximum root-mean-square error (RMSE) of 10.8% across all our evaluations. In contrast, interference models that rely on the SNR profile and the RSS profile perform poorly, with a maximum RMSE of 61.7% and 66.1% respectively.

Time-based and low-cost bandwidth estimation for IEEE 802.11 links

This paper presents a practical and low-cost approach to estimate the maximum achievable wireless link bandwidth based on the prevailing link conditions in an IEEE 802.11 network. This approach works by observing the number of bits successfully delivered over a link, divided by the corresponding channel occupancy time. The method is passive and does not introduce any extra overhead on the channel, and is based on the timing model of the IEEE 802.11 MAC layer. All required parameters are obtained locally at the sending node from the MAC layer. We evaluate the accuracy of the proposed method via a full implementation and experiments on our conducted testbed, which provides a more controlled environment and increased repeatability of experiments, compared to over-the-air wireless testbeds. The experiment results show that our method can accurately estimate the link bandwidth with both saturated and low traffic load across links with different transmission rates and link qualities. In addition, results also show that our method is able to track the link bandwidth as its link quality changes.