**3.1. Task model**

Chen *et al.* [3] gives a formal definition of system model of a typical distributed systems consisting of a set of processors and a set of tasks. A distributed system are characterized as follows. A set of processors Ω = {*S*1, *S*2, ..., *Sm*} where Ω is the processor set, *Si* is the *i* − *th* processor and *m* is the total number of processors. In this model, all processors are assumed to be identical to assure same execution time for each task on different processors. It is also supposed that enough processors are provided. A set of primary copy of real-time tasks Φ = {*τ*1, *τ*2, ..., *τn*}, where *τ<sup>i</sup>* = {*ci*, *pi*} , *i* = {1, 2, ..., *n*}. Here Φ is the set of tasks, *τ<sup>i</sup>* is the *i* − *th* task, *n* is the number of tasks which are periodic, independent and preemptive, *ci* denotes the execution time of *τi*, *pi* denotes the period of task *τi*.
