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Chapter 4Simulation and analytical modelling p4.2.1 OPNET modellingIn order to have an insight of how simulation models are implemented in OPNET, inthis section an overview of the hierarchical design used by this simulation package ispresented. OPNET models are based on a hierarchical three-level structure as illustratedin Figure 4.1.• etwork domain: The top-most level is the etwork domain (see Figure 4.1a). Therole of this domain is to define the topology of the communication network, thecommunication entities called nodes and their interconnection (using bus, point-topointor radio links). Based on these basic building blocks, more complex modelscan be developed.• ode domain: The next level is called the ode domain (Figure 4.1b). This leveldefines the functionality of each communication device that can be deployed andinterconnected to the network, (e.g. routers, bridges, terminals, switches, etc). Eachnode consists of traffic source generators, processors, queues and varioustransmitters or receivers allowing a node to be attached to communication links inthe network.a ) N e tw o rk d o m a inT e rm in a lT e rm in a lS e rve rP ro to - C C o d ein e a ch sta teS e rve rB rid g e /R o u te r/S w itc hW o rk sta tio nc ) P ro c e s s d o m a in (F S M )b ) N o d e d o m a ind e fa u ltF ra m e _ W a itin ginitS in kp ro c es sorT ra fficg e n e ra to rfrm _ wdef_ o ffS p a c in g _ E la p s e dR e ceiverM a cQ u e u eT ra ns m itte rF ra m e _ W a itin g _ L o w & &! C h a n n e l_ B u s ybsy_w! F ra m e _ W a itin g & &!C h a n n e l_ B u syd e fa u ltd e fa u ltspacingB u s y _ L o wfree_ wD e fe r p ro c e ssorF ra m e _ W a itin g _ L o wB u s y _ H ig hd ef_o nd e fa u ltFigure 4.1 – Stage diagram of OPNET simulation models.4-3