Marjan Bozinovski

Maximum availability server selection policy for efficient and reliable session control systems

There has been a rapid growth of services based on session control. Session-based services comprise multimedia conferences, Internet telephone calls, instant messaging, and similar applications consisting of one or more media types such as audio and video. Deployment examples include session control services as part of the IP multimedia subsystem (IMS), in the third-generation mobile networks. High service dependability in session control systems is achieved by introducing redundancy, e.g., through reliable server pooling (RSerPool) or clustering. Namely, session control servers are multiplied in server sets. Performance of such replicated session control servers is quantified by transaction control time. Thus, reducing transaction control time enhances performance. Server selection policies (SSP) are crucial in achieving this goal. The maximum availability (MA) SSP is proposed to improve session control performance in scenarios with server and communication failures. Based on a status vector, MA aims at maximizing the probability of successful transaction with the current transmission, thereby minimizing the average number of attempted servers until success. MA is applicable in a broad range of IP-based systems and services, and it is independent of the fault-tolerant platform. A simple protocol extension is proposed in order to integrate MA into the RSerPool fault-tolerant architecture. In addition, an analytic model is derived based on certain system model assumptions. Analytic and simulation results show that transaction control time is considerably reduced with MA as opposed to when using traditional round robin.

Performance evaluation of a SIP-based state-sharing mechanism

The Session Initiation Protocol (SIP) has been selected to manage sessions within the 3GPP IP-based multimedia subsystem (IMS). SIP sessions have to be highly available and reliable. This paper presents a performance evaluation of a novel state-sharing algorithm for providing reliable SIP services. The novelty is a result of sharing SIP (application-protocol) states as opposed to transport-protocol states. The most important performance parameters are evaluated in terms of the system variables such as server failure and repair time distributions, call arrival and call duration distributions, and link parameters, i.e., delay and bandwidth. The considered performance parameters are availability, reliability and dependability. Concerning the state transport mechanism, the results show that TFTP significantly outperforms FTP with respect to the overall performance.

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.

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.

A state-sharing mechanism for providing reliable SIP sessions

The session initiation protocol (SIP) is the main signaling protocol in the 3GPP IP multimedia subsystem (IMS). The SIP sessions in IMS have to be highly reliable. A state-sharing algorithm based on file transfer protocols such as FTP and TFTP, is proposed to increase session reliability. Theoretical analysis and experimental scenarios showed that TFTP outperforms FTP.

Novel strategy for call admission control in mobile cellular network

This paper presents a novel history-based policy for call admission control (CAC) in mobile cellular networks, which is based on the statistics of the past systems behavior. This algorithm can be used as an extension of the conventional guard channel (CGC) scheme or it can perform independently of CGC. It shows improvements compared to CGC by reducing the new call blocking probability, increasing the carried traffic, while the forced call termination probability is kept almost constant. These results are confirmed via simulation of a one-dimensional system. The proposed policy is a fractional guard policy and introduces a variable threshold for guard channels. Each cell builds cumulative statistics from its operating history and accepts a new call only when predefined upper bound of handoff failure probability is not exceeded. A great advantage of this scheme stems from the fact that its generic form is independent of cellular network topology.

Adaptive Call Admission Control for Wireless Multimedia Network

There is a tremendous growth of broadband multimedia services in mobile cellular networks. Call admission control (CAC) is the most significant issue in quality of service (QoS) provisioning for these services. We propose an adaptive CAC, which upgrades the upper limit (UL) scheme and is verified through the simulation. The proposed algorithm allows each cell to accept a new call if the estimated average differential of each multiclass handoff failure probability is nonpositive. The generic form of the algorithm is independent of cellular network structure.

Distributed adaptive admission control in mobile multimedia network

There is a fast deploying of high-speed multimedia services in mobile cellular networks. Call admission control (CAC) is the most significant issue in quality of service (QoS) provisioning for these services. This paper proposes an adaptive CAC which demonstrates improvements compared to the upper limit (UL) scheme by reducing new call blocking probability, increasing the carried traffic, while the forced call termination probability is kept almost unchanged. A new call request is accepted if the estimated average handoff failure-to-attempt ratio of each traffic class in the cell is not greater than a corresponding predefined QoS value. The basic form of the algorithm is independent of cellular network structure.