A novel hierarchical and parallel model for wireless network simulations andits design using PtolemyII infrastructure
Jens Voigt, Dresden University of Technology, Dresden, Germany

In this presentation, the course of interactions in network level simulations of radio access networks is investigated.

An object oriented model of a radio access network is investigated for interactions that occur between objects during the runtime of a simulation. Analyzed ordering relationships give reasons for the application of different, even concurrent models of computation.

A radio access network could be shown to be decomposable into a hierarchical model. Interactions within a lower level of the hierarchy occur on the outside as atomic.

In the highest level of the hierarchy, interactions between objects of network elements and an object of the transmission channel as well as interactions for instantiation of network elements are dealt with. An overall cyclic repetition of these interactions emerged during the analysis. Here, in general more than one network element interacts with the transmission channel at point in the cycle of interactions. This is an implicit sign of concurrency in the model.

By means of the interaction cycle, the interaction alphabets of the objects in the highest level of the hierarchy as well as the ordering relationships between their interactions is well known before the simulation is started. Hence, there is no run-time scheduling necessary. This allows for parallel simulations of the radio access network without the necessity of pessimistic or optimistic run-time synchronization algorithms of parallel discrete-event simulations.

Instead, the a-priori known course of interactions within the interaction cycle itself can be used to maintain a causal simulation time in parallel working active objects on the highest level in the hierarchy. Here, the known interactions alphabets of the objects allow the use of rendezvous interactions, which are based on the idea of formal process calculi.

Objects that model different instances of network elements of one type are composed into one partition of the parallel software. Using this network element type partitioning strategy, the partitions of the network elements are loosely coupled via the partition of the transmission channel and only use naturally occurring interactions themselves to ynchronize parallel working parts of the software without any additional effort. Furthermore, limitations of classic partitioning strategies like division into geographical parts or groups of physical radio resources can be circumvented. This is especially important for radio network level simulations of third generation CDMA networks.

The interactions of objects inside one partition of single network elements are controlled within the second level of the hierarchy by means of discrete events. This is due to the randomness of occurrence of these mostly traffic model based network element internal interactions.

Only some of the internal interactions in a network element are to be synchronized with interactions of other network elements and the transmission channel in order to guarantee an overall causal simulation run. Thus, a global ordering relationship between all interactions in an object oriented model of a radio access network does not exist. This is a further sign of concurrency in the model.

The entire model is implemented on top of UC Berkeley's PtolemyII software infrastructure.

A performance gain of the simulations depending on the number of used processors, the number of software partitions, and the traffic load of the simulations could be proven.