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Chapter 4Simulation and analytical modelling p• Traffic generators are built-in objects used for simulating traffic sources. Thegenerated traffic stream has packet length and a packet inter-arrival distribution,which is selected from a set of supported probability distributions. Queues andProcessors are objects and are highly specifiable. Queues are used to simulatebuffers of communications links and protocol behaviour. Example statistics that aqueue can collect are current/average queue size, queuing delay and queue overflow.Each queue object might spawn multiple sub-queues in which packets can be stored.Processors have the capability of packet manipulation but no ability to store packetsand can be used as packet sinks or as traffic generators, when the traffic streamscannot be modelled using the predefined probability distribution.• Process domain: The last level is called the Process domain (Figure 4.1c). At thislevel the functionality of each queue or processor object is defined using a FiniteState Machine (FSM). States and transitions graphically define the progression of aprocess in response to events. In general, each FSM can define private statevariables and can make calls to code in user provided libraries. FSM are dynamicand can be spawned by other FSMs during simulation in response to specific events.Dynamics of FSM dramatically simplify specification of protocol that manages ascalable number of resources or sessions. Finally, each state of a process modelcontains ‘Proto-C’ code, supported by an extensive library of functions designed forprotocol programming.4.2.2 Common Simulation Framework (CSF)Although OPNET models are accompanied by an extensive library of predefinedcommunications protocols covering all seven layers of the Open System Interconnection(OSI) Reference Model, no MAC protocols are available that are suitable for modellingbi-directional CATV networks. Thus, the high demand for evaluating emergingprotocols such as the IEEE 802.14 and DOCSIS, have motivated MIL3 (now OPNETTechnologies Inc.) and Cablelabs to develop the Common Simulation Framework(CSF) [77], which provides the basic building blocks of a typical CATV network fordelivering data applications without defining the functionality for the MAC or layersabove.4-4