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Nowadays VoIP has become an evolutionary technology in telecommunications. Hence it is very important to study and enhance its dependability attributes. In this paper, an analytical dependability model for VoIP is proposed. The study is focused on analyzing the combined effects of resource degradation and security breaches on the Quality of Service (QoS) of VoIP, to enhance its overalldependability. As a preventive maintenance policy to prevent or postpone software failures which cause resource degradation, software rejuvenation is adopted. The dependability model is analyzed using semi-Markov process which captures the effects of non-Markovian nature of the time spent at various states of the system. The steady-state as well as the time dependent analysis of the dependability model ispresented. The steady state results are obtained analytically, whereas the time-dependent results are obtained from simulation. Also, the analytical model is validated via simulation. The model analysis using a numerical example indicates the feasibility of our approach. Various dependability attributes such as availability, reliability and confidentiality are also obtained. A comparative study isalso done between our proposed model and the existing models.

1. Introduction
Voice over Internet Protocol (VoIP), also known as Internet telephony, is the technology that enables people to use the Internet as the transmission medium for voice communications.
It is a technology that allows to make voice calls using abroad-band Internet connection instead of a regular (or analog) phone line.It has been evolving quite rapidly in the telecommunication area in recent years as it provides long distance calls at a very low cost [1]. Hence, it is essential forwell-designed VoIP networks to be reliable and safe, to meet certain quality-of-service (QoS) requirements, and to provide its services in a timely manner, in the wake of resource degradation and also in the context of intrusions,attacks and accident failures in a hostile environment. Therefore, the main focus of this paper is on the QoS of VoIP in the case of resource degradation and security breaches to improve its availability, reliability and safety. The QoS issues (availability, reliability, security) addressed in this paper can be considered as a dependability issue. Dependability is global concept that subsumes theusual attributes of availability, reliability, security, integrity, maintainability, etc. The consideration of security brings in concerns for confidentiality, in addition to availability and reliability.

Network dependability is an important issue for service providers, vendors, and users. A lot of research has been done on the area of network dependability. A survey on the existing model-basedtechniques for evaluating system dependability is done in [4], and it is summarized that how these techniques can be extended to evaluate system security. A comparative study on the analytical models of computer system dependability and security is carried out in [5]. The paper [6] outlines the details of how redundancy may be implemented by making enhance- ments to the basic IEEE 802.11 channelaccess protocol. In this paper, the authors have presented the reliability, availability and survivability analysis of the two configurations to evaluate the dependability of the network under study, and compared them with the scheme with no redundancy. In [7], a comprehensive dependability analysis of WLAN for system dependability mea- surement, modeling and evaluation is presented, and methods aresuggested to increase the communication link reliability and availability of wireless LANs. An integrated solution to increase the dependability of wireless mesh networks (WMNs) is proposed in [8]. The approach in this paper combines network coverage planning on the physical layer, bandwidth management on the link layer and live network monitoring to improve the reliability, availability and...