Erling Vestergaard Matthiesen

Dependability evaluation of a replication service for mobile applications in dynamic ad-hoc networks

In order to increase availability and reliability of stateful applications, redundancy as provided by replication in cluster solutions is a well-known and frequently utilized approach. For mobile services in dynamic ad-hoc networks, such replication mechanisms have to be adapted to deal with the frequently higher communication delays and with the intermittent connectivity. Dynamic clustering strategies in which the replica set is adjusted to the current network state can help to handle the network dynamicity. The paper develops a stochastic Petri net model (and its corresponding Markov chain representation) to analyze the resulting availability and replica consistency in such dynamic clusters. The numerical results are interpreted in the context of a vehicular (c2c) communication use-case and can be used to determine optimized cluster configuration parameters.

Inconsistency evaluation in a replicated IP-Based call control system

The Session Initiation Protocol has been chosen for controlling multimedia sessions in the IMS part of UMTS infrastructures. In such networks, availability is crucial and the integration of SIP with a fault-tolerant solution, often based on a replication technique, has become necessary. Because the replicated stateful servers are deployed in distributed networks, state inconsistency may be introduced. Mechanisms have been proposed, which aim at keeping the inconsistency level below a certain threshold by introducing an adaptive delay before the states are committed. The effectiveness of those adaptive mechanisms depends on the accuracy of the inconsistency evaluation during the system operation. In this context, the careful definition of a practically measurable inconsistency metric is necessary in order to benefit from those mechanisms while minimizing their impacting on performance. This paper discusses the relevance of different inconsistency definitions and suggests a common model in which the inconsistency metrics are broken down into a set of measurable and/or analytically derivable contributing factors. We analyze the validity of this evaluation approach with results obtained in a prototype implementation of a 3GPP IMS call control system integrated in a distributed fault-tolerant architecture, so-called RSerPool, for the example of instant message sessions between users.

Scalable Emulation of Dynamic Multi-hop Topologies

Communication in wireless networks is affected by uncontrollable disturbances in the channel. Effects of these disturbances are exacerbated in networks with dynamic topologies and multiple hops. Lack of control of the channel complicates testing of such networks as test conditions are hard, or impossible, to reproduce. This paper describes how to create reproducible test conditions for these networks by emulating the wireless links. Emulation is performed by a topology emulator to which end-nodes are connected using wired links. In real-time, the emulator imposes packet drops and delays onto traffic between end-nodes. The imposed properties are based on simulations of node mobility, loss and delay models. Evaluation confirms that the testbed is capable of emulating links in real time and transparent to upper layer protocols. Additionally, the impact on test results is evaluated, such as increased network delays and reduction of bandwidth when loading the emulator. Finally, an outlook on advancing capabilities and how to integrate such in the emulator is presented.

A New Selection Metric for Backup Group Creation in Inter-Vehicular Networks

Reliable service provisioning in car-to-car networks is challenging because the environment is very dynamic and network topologies are changing rapidly, hence making communication unreliable. For service-level fault-tolerance, the service needs to be replicated onto several vehicles. For state-full services with dynamically changing state, a careful choice of the replica servers is necessary due to the dynamically changing properties of the communication paths between them. This paper proposes and analyzes a heuristic metric called geo-cost that aids the selection of replica candidates based on information about speed and direction of the cars. The analysis in simulation experiments shows that the proposed heuristic is performing equally well when compared to an existing approach based on a snapshot measurement of network delays. The difference between the existing metric and geo-cost is the feasibility of geo-cost compared to network delays that are often hard to determine.

Localized Algorithms for Virtual Backbone Formation in Wireless Multi-hop Networks with unidirectional links

A virtual backbone can be utilized in various network optimizations in wireless multi-hop networks. Many distributed algorithms have been proposed for virtual backbone formation in the last decade. However, the assumption of undirected graph as the topology model in most existing work may not hold in real scenarios, where unidirectional links occur for many reasons. Moreover, in most of the existing algorithms, there is dependency among the decisions of different nodes. In this paper, we propose a new localized algorithm for the construction of a virtual backbone by forming a connected dominating and absorbent set. Besides the locality, the proposed algorithm also constructs a virtual backbone of smaller size compared to the existing algorithms.

Optimized Macro Mobility within the 3GPP IP Multimedia Subsystem

In the 3GPP IMS, macro mobility (changing IPaddress) is not supported, if the user changes access technology the user equipment (UE) will automatically get a new IP-address. After this reassignment of the IP-address the user has to re-register to the IMS and send new invites to all corresponding nodes (CNs) before the sessions can continue. This will potentially introduce a long interruption of ongoing sessions. In this paper a solution is introduced that reduces the handover delay by sharing the registration information and call states. The full register and invite flows are not necessary in that case, since the servers in the IMS already have state information about the user and sessions from the context transfer. The benefit with respect to reduced hand-over delay is quantitatively analyzed using an experimental implementation of the improved SIP mobility solution for IMS.