Traditional system-oriented dependability metrics like reliability and availability do not fully reflect the impact of system failure-repair behavior in service-oriented environments. The telecommunication systems community prefers to use Defects Per Million (DPM), defined as the number of calls dropped out of a million calls due to failures, as a user-perceived dependability metric. In this paper, we provide new formulation for the computation of the DPM metric for a system supporting Voice over IP functionality using the Session Initiation Protocol (SIP). We evaluate different replication schemes that can be used at the SIP application server. They include the effects of software failure, failure detection, recovery mechanisms, and imperfect coverage for recovery mechanisms. We derive closed-form expressions for the DPM taking into account the transient behavior of recovery after a failure. Our approach and underlying models can be readily extended to other types of service-oriented environments.

IEEE 1609.4 protocol defines a channel switching mechanism to enable a single radio operating efficiently on multiple channels to support both safety and non-safety services. Basic safety message (BSM) is transmitted only through the control channel at regular intervals. In this paper, we propose an analytic model based on interacting semi-Markov process (SMP) to evaluate both Medium Access Control (MAC) and application level performance and reliability of safety message broadcasting incorporating the impact of multi-channel operations. Message service time distribution is derived using Laplace-Stieltjes transform based on the proposed model. Due to the cyclic interactions between the SMP models for vehicles contending the same channel, fixed-point iteration is used to obtain converged solutions. Subsequently, MAC and application-level performance and reliability metrics are derived based on the converged solutions. Channel fading with path loss is taken into consideration in this derivation. The analytic models are validated through extensive simulations in NS2 to verify the effectiveness and accuracy of our proposed fixed-point iteration based model decomposition. The effects of channel switching mechanism and channel fading are also evaluated by comparing with other analytic models.

In modern IT systems, data backup and restore operations are essential for providing protection against data loss from both natural and man-made incidents. On the other hand, data backup and restore operations can be resource-intensive and lead to performance degradation, or may require the system to be offline entirely. Therefore, it is important to properly choose backup and restore techniques and policies to ensure adequate data protection while minimizing the impact on system availability and performance. In this paper, we present an analytical modeling approach for such a purpose. We study a file service system that undergoes periodic data backups, and investigate metrics concerning system availability, data loss and rejection of user requests. To obtain the metrics, we combine a variety of model types, including Markov chains, queuing networks and Stochastic Reward Nets, to construct a set of analytical models that capture the operational details of the system. We then compute the metrics of interest under different backup/restore techniques, policies, and workload scenarios. The numerical results allow us to compare the effects of different backup/restore techniques and policies in terms of the tradeoff between protective power and impact on system performance and availability.

This paper develops a new analytic model for the performance and reliability analysis of safety-related message broadcast in vehicular ad hoc networks (VANETs) at rural intersections. First, a semi-Markov process (SMP) model for characterizing channel behavior of IEEE 802.11 based VANET is deployed to interact with the M/G/1 queue through fixed-point iteration. Then, the mean transmission delay between two nodes is derived. Furthermore, given two communicating nodes placed at the same intersection, the node reception probability that a node successfully receives the broadcast message from a sending node is computed. Consequently, the packet reception ratios are derived through integration of the node reception probabilities over the intended range of the sending node. The analytical model takes intersection geometry, IEEE 802.11 backoff counter process, hidden terminal problem, and non-saturated message arrival into account. From the obtained numerical results under various network parameters, the new model is validated and new observations are obtained.

In this paper, we propose an accurate and efficient analytic model to evaluate the impact of channel fading on multi-hop safety message disseminations in vehicular ad hoc networks (VANET). A multi-hop relay strategy is also proposed by setting distance-based timers (the longer the distance, the shorter the timer) so that the farther receiver has the higher priority for it to rebroadcast the message. Important performance and reliability metrics are derived and analyzed for a thorough understanding of the safety message transmission behavior. Extensive simulations are conducted in Matlab to verify the correctness of our proposed model under realistic network parameter settings.

To support both safety and non-safety applications in vehicular communications, IEEE 1609.4 protocol defines a channel switching mechanism to enable a single radio to operate efficiently on multiple channels. Basic safety message is transmitted only through control channel at regular intervals. In this paper, an analytic model based on semi-Markov process is proposed to evaluate MAC level performance and reliability of safety message dissemination incorporating the impact of the multi- channel operations. Safety message service time distribution is derived using Laplace-Stieltjes transform based on the proposed model. Subsequently, fixed-point iteration is used to obtain converged solutions. MAC level performance metrics are then derived. Channel fading with path loss is taken into account in this derivation. The analytic models are validated through simulations in NS2 to verify the effectiveness and accuracy of the proposed model.

In this paper, an analytic model is proposed for the performance evaluation of vehicular safety related services in the dedicated short range communications (DSRC) system on highways. The generation and service of safety messages in each vehicle is modeled by a generalized M/G/1 queue. The overall model is a set of interacting M/G/1 queues, one queue for each vehicle. The interaction is that the server is shared as it is the contention medium. To make the model scalable, we use semi-Markov process (SMP) model to capture the shared server's behavior from one tagged vehicle's perspective, where the medium contention and back off behavior for this vehicle and influences from other vehicles are considered. Furthermore, this SMP interacts with the tagged vehicle's own M/G/1 queue through fixed-point iteration. The proof for the existence, uniqueness and convergence of the fixed point is provided. Based on the fixed-point solution, performance indices including mean transmission delay, packet delivery ratio (PDR), and packet reception ratio (PRR) are derived. Analytic-numeric results are verified through extensive simulations under various network parameters. Compared with the existing models, the proposed SMP model facilitates the impact analysis of hidden terminal problem on the PDR and PRR computation in a more precise manner.

Data backup operation is an essential part of common IT system administration to protect against data loss caused by any storage failures, human errors, or disasters. Lost data can be recovered from the backed up data if it exists. Since the backup and restore operations accrue downtime overhead or performance degradation, they have to be designed to ensure the data reliability while minimizing the performance and availability overhead. In this paper, we study the impacts of different backup policies on availability measures such as storage availability, system availability, and user-perceived availability. Backup and restore operations are designed using SysML Activity diagrams that are automatically translated into Stochastic Reward Net (SRN) to compute the availability measures. Our numerical results show the effectiveness of the combination of full backup and partial backup in terms of user-perceived data availability and data loss rate. Furthermore, the sensitivity ranking can help improve the availability measures.

IEEE- and ASTM-adopted Dedicated Short Range Communications (DSRC) vehicle safety-related communication services, which require reliable and fast message delivery, usually demand broadcast communications in vehicular ad hoc networks (VANETs). In this paper, we propose and justify a distributive robust scheme for DSRC one-hop safety-critical services. The new scheme enhances broadcast reliability using dynamic receiver-oriented packet repetitions and mini-slot within DIFS in IEEE 802.11 for one-hop emergency warning message dissemination. In addition, we investigate the reliability and performance of the proposed broadcast scheme for DSRC VANET safety-related services on highway analytically and by simulations. The analytic model accounts for the impact of the beacon message broadcast and the fading channel conditions on the reliability and performance.