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Chapter 4Simulation and analytical modelling pThe IA TX Controller is a spawned state machine (see Figure 4.11) or child process tothe INA Manager. Its purpose is to receive either MAC messages or data messages fromits parent and transmit them on the downstream channel as MPEG2 frames. The IA TXController process interacts with the bus object (to_bus_tx) via a statistical wire thatinforms when the bus is free (Figure 4.9b). When the downstream bus is idle and thereare MAC messages or data packets to transmit, the IA TX Controller serves first theMAC messages in a First In First Out (FIFO) order, and assembles up to three MACmessages into an MPEG2 frame, which is then delivered to the downstream channel.Figure 4.11 – INA TX Controller - FSM.4.4 Analytical modellingThe analytical evaluation of CATV protocols, and by implication DVB/DAVIC, provesvery difficult due to the increased complexity involved in such protocols, as reviewed inSection 1.2.3.2. The downstream channel, which is a unidirectional broadcast channelused only by the INA, can be easily modelled as a simple delay queue with a constantservice rate. However, the modelling of the upstream channel is complicated due tohybrid multiple random access-reservation MAC.4-14