GUARDS Project - PDCC Technical Note, Report GUARDS/I3A4/AO/6015 version a, 11 June 1998.

Brief Overview of some Tools to Support the Modelling and Evaluation of Phased Mission Systems

A. Bondavalli*, I. Mura**

* CNUCE Istituto del CNR, Via S. Maria 36, 56126 Pisa, Italy

** Dept. of Information Engineering, Via Diotisalvi 2, 56100 Pisa, Italy


Most of the critical systems in the context of GUARDS project can be classified as PMS, in that their operational life-time (mission) is partitioned in a set of disjoint intervals (phases). Not only the activities that PMS have to perform during their mission can be completely different from phase to phase, but also the dependability requirements, the operational configuration and the environmental conditions can be subject to change from one phase to another. Due to their intrinsic complexity and dynamic structure, modelling of PMS is a challenging activity that the GUARDS consortium decided to address. Therefore part of the effort de-voted in GUARDS to dependability modelling has been spent deal with modelling methodolo-gies for Phased-Mission Systems. Two different methodologies, based on two different ap-proaches, have been proposed by PDCC in GUARDS for the dependability modelling and analysis of PMS, described in details in [1-4].
The first methodology [4], is based on a separate modelling of the different phases. The phase models are combined in a global model of the mission through an upper level model, which represents the profile of the mission. For the modelling of each phase a Generalised Stochastic Petri Net (GSPN) representation can be conveniently used, while the upper level is modelled as a Discrete-Time (DT) Markov chain. Then, PDCC introduced a new modelling approach [3], based on Deterministic and Stochastic Petri Nets (DSPN), which provides a single model of the overall phased-mission system. Still, the analytic solution of the DSPN model can be partitioned and reduced to the sequential solution of each phase, thus requiring the same computational cost as needed by the former approach.
These two modelling approaches are in some sense complementary, but can be judged as equally efficient in terms of solution cost. Both basically require for the evaluation the transient solution of a set of Markov chains, either Continuous-Time or Discrete-Time, and therefore no specific tools for this purpose are required and have been foreseen. Rather, the solution is intended to be carried out by taking advantage of the existing tools for the automated evaluation of systems dependability.
Among the off-the-shelf tools commercially available, we chose three examples, namely SURF-2 [5], UltraSAN [6], and SPNP [7], and this short note provides some information on the tools that may be used for supporting the modelling and evaluation methodologies for Phased Mission Systems developed by PDCC in the context of GUARDS. The main characteristics, like expressive power and easiness of use of the tools, together with more technical informa-tion as how to acquire the tool, which architecture supports them, etc., are described.


[1] A. Bondavalli, I. Mura and M. Nelli, "Analytical Modelling and Evaluation of Phased-Mission Systems for Space Applications," in Proc. IEEE High Assurance System Engineering Workshop (HASE'97), Washington D.C. USA, 1997, pp. to appear.

[2] A. Bondavalli, I. Mura and M. Nelli, "Analytical modelling and evaluation of the GUARDS instances: example for space applications," GUARDS Project - PDCC Activity Output (D3A4/AO/6001/C) (also GUARDS First Year Deliverable), Mar 18 1997.

[3] A. Bondavalli, I. Mura, X. Zang and K. S. Trivedi, "Dependability Modelling and Evaluation of Phased Mission Systems: a DSPN Approach," GUARDS Project - PDCC Activity Output (I3A4/AO/6010/a), 20 January 1998 1998.

[4] I. Mura and A. Bondavalli, "Hierarchical Modelling and Evaluation of Phased-Mission Systems," CNUCE-CNR Internal Report (C97-016) also GUARDS Project - PDCC Activity Output, (I3A4/AO/6007/A), November 1997.

[5] WEB address:

[6] WEB address:

[7] WEB address: