Thibault Renier

Mid-Session Macro-Mobility in IMS-Based Networks

With the introduction of terminals that support multiple access technologies, macro-mobility and roaming are becoming even more important functionalities: it is expected that mobile terminals will handover between heterogeneous networks in order to extend coverage or to get connectivity to faster and/or cheaper access technologies. Access and session control must be guaranteed when roaming across different administrative domains. The 3GPP IP multimedia subsystem (IMS) is the first platform standardized towards network-independent access and session control. However, the current IMS specifications do not allow for any change of the users IP address during a session, which prevents seamless mid-session macro-handover. In this paper, we present and compare a set of solutions, implemented respectively at the session and network layers, for shortening the handover delays and/or providing session continuity in case of macro-handover in IMS-based networks. Since one of the main functions of IMS is quality of service (QoS) negotiation and the control of legacy interfaces to the access networks for QoS provisioning, special attention is given to the discussions of the QoS re-negotiation procedures during such handover cases.

Energy-efficient interference avoidance for interconnected Bluetooth personal area networks

A Bluetooth personal area network (PAN) supports wireless communications among devices within short range. Since PAN-devices are usually battery powered, the protocols applied in PAN should employ economic energy usage. Basic networking entity in Bluetooth is piconet and piconets can be interconnected into a scatternet. Since the piconets in scatternet are not mutually coordinated at the medium access layer, there is an interpiconet interference, which can seriously degrade the communication performance and cause energy wasting. In this paper we propose a distributed algorithm for interference avoidance in Bluetooth scatternets. The algorithm avoids collision by ensuring distributed consensus among piconets in case of concurrent transmission. The overhead introduced by the algorithm is low, while there are high gains in energy efficiency and throughput.

Transaction consistency in replicated SIP call control systems

The session initiation protocol (SIP) has been selected as the main call control protocol in the 3GPP IP multimedia subsystem (IMS). SIP service users and providers require fault-tolerance with high service availability and reliability. One approach is to replicate SIP call control functionality over a number of dispersed hosts. In order to allow for midcall fail-over, call states need to be replicated, but this may cause call state inconsistency. SIP transaction inconsistency is quantified here by the misread probability. Server response time is defined by the local delay when a read access to call state is to be processed. The tradeoff relationship between misread probability and read delay is exploited to design an update commit algorithm (UCA) for adaptive consistency control. The UCA iteratively updates the read delay so that the misread rate converges towards the predefined misread probability. The algorithm has a rather low complexity and can adapt to a variety of traffic types and load, inter-server link parameters, state-sharing protocols and failure/repair random processes.

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.

Distributed redundancy or cluster solution? an experimental evaluation of two approaches for dependable mobile internet services

Third generation mobile networks are offering the user access to Internet services. The Session Initiation Protocol (SIP) is being deployed for establishing, modifying, and terminating those multimedia sessions. Mobile operators put high requirements on their infrastructure, in particular – and in focus of this paper – on availability and reliability of call control. One approach to minimize the impact of server failures is to implement redundant servers and to replicate the state between them in a timely manner. In this paper, we study two concepts for such a fault-tolerant architecture in their application to the highly relevant use-case of SIP call control. The first approach is implemented as a distributed set of servers gathered in a so-called pool, with fail-over functionality assigned to the pool access protocols. The second is a cluster-based solution that normally implies that the servers are confined in a local network, but on the other hand the latter solution is completely transparent to clients accessing the service deployed in the cluster. To evaluate these two approaches, both were implemented in an experimental testbed mimicking SIP call control scenarios in 3rd generation mobile networks. An approach for measurement of various dependability and performance parameters in this experimental setting is developed and concluded with a set of preliminary results.

Client-Centric Performance Analysis of a High-Availability Cluster

High-Availability as provided by fault-tolerance mechanisms comes at the price of increased overhead due to additional processing and communication, which may be a limiting factor to service performance as perceived by the clients. In order to quantify this impact and to understand the underlying mechanisms for performance degradation, this paper presents an approach for the analysis of client-centric performance metrics in cluster-based service deployment scenarios using High-Availability Middleware. The approach is based on a combination of measurement based empiric analysis under synthetically generated load patterns and simple queueing models, that allow for the extrapolation of empiric results and are used to gain insights into the underlying causes of the empiric performance behavior. The empiric and numerical results in the paper are based on an abstracted SIP-like call control service as deployed in future version of IP-based cellular networks, running on a two-node cluster system.

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.

Optimal configuration of fault-tolerance parameters for distributed replicated server access

Server replication is a common fault-tolerance strategy to improve transaction dependability for services in communications networks. In distributed architectures, fault-diagnosis and recovery are implemented via the interaction of the server replicas with the clients and other entities such as enhanced name servers. Such architectures provide an increased number of redundancy configuration choices. The influence of a (wide area) network connection can be quite significant and induce trade-offs between dependability and user-perceived performance. This paper develops a quantitative stochastic model using stochastic activity networks (SAN) for the evaluation of performance and dependability metrics of a generic transaction-based service implemented on a distributed replication architecture. The composite SAN model can be easily adapted to a wide range of client-server applications deployed in replicated server architectures. In order to obtain insight into the system behaviour, a set of relevant environment parameters and controllable fault-tolerance parameters are chosen and the dependability/performance trade-off is evaluated.

Cross-Layer Optimization of Multipoint Message Broadcast in MANETs

Multipoint-to-multipoint message broadcast is a demanding application scenario in ad-hoc networks. Adaptive management of wireless resources is necessary to support such applications in a safety critical context. In this work we study adaptation of transmission rate and power to varying densities of ad-hoc nodes. Our approach is to construct a cross-layer model building on existing models for physical and link layers. To enable optimization in relation to metrics of end-to-end delay and message reception probability a model of flooding broadcast is proposed as a part of the cross-layer model. In a simulation study we show that adaptation of transmission power and rate can be necessary to achieve delay requirements and maximize message reception probability. Compared to simulation our cross-layer model based optimization approach generates slightly more conservative parameter settings. It is further shown how correlated losses have a significant impact on the robustness of the broadcast technique.