Flexibility: Many real-time communication systems must support different system configurations that change over time. A real-time protocol should be flexible to accommodate these changes without requiring a software modification and retesting of the operational nodes that are not affected by the change. Since the bandwidth of any communication channel is limited, there exists an upper bound on the increase in communication traffic that can be handled within the given time constraints.
Topology. The standard communication topology in distributed real-time systems is multicast, not point-to-point. The same image of an RT entity is needed at a number of different components, e.g., at the man-machine interface, at a processmodel component, and at an alarm-monitoring component. A message should be delivered to all receivers of the receiver group within a short and known time interval.
Dynamic Addition of a Partner. It should be possible to add a new communication partner dynamically. If this new partner is passive, i.e., it is only receiving messages but not sending messages, the multicast topology can support this requirement by adding the new partner to the receiver group. If the new partner is active, i.e., it is sending messages, then the communication infrastructure should provide the necessary bandwidth without violating the temporal guarantees given to the already existing partners.
Example: A communication system within a car must support different configurations of nodes, depending on customer demand. One customer might demand a car with a sunroof and automatic seats with memory, while another customer might opt for a special airconditioning system and a sophisticated anti-theft system. All possible combinations of nodes must be supported by the communication system without a need for retesting existing nodes.
Physical Structure: The physical structure of a real-time communication system is determined by technical needs and economic considerations.
Example: In the harsh environment of a plant, the physical transmission system must be more robust than in a benign office environment.
Physical Fault Isolation. The communication system should provide for the physical isolation of nodes that are placed at different locations in space, such that
common mode node failures, e.g., those caused by a lighting stroke, will not occur. The transducer circuits that link the wires to the nodes must withstand the specified high-voltage disturbances. Fiber-optic transmission channels provide for the best physical isolation.
Example: Consider an airplane with a fly-by-wire system. The nodes that form a faulttolerant unit for this critical function should be at different locations within the plane and connected by well-isolated channels, such that a high-voltage disturbance or a physical damage of a section of the plane during an incident (e.g., lightning stroke) will not result in the correlated loss of the safety-critical system function of all replicated nodes.
Low Cost Wiring. In many embedded systems, e.g., in a car or an airplane, the weight and cost of the wiring harness is substantial. The selection of the communication protocols and in particular the physical transmission layer is influenced by the desire to minimize the wiring weight and cost.
