Frame Packing in Real-Time Communication

A common computational model in distributed embedded systems is that the nodes exchange signals via a network. Most often a signal represents the state of some physical device and has a signal size ranging from a single bit up to a few bytes. Furthermore, each signal typically has a deadline requirement. The communication networks used are often based on a broadcast bus where fixed or variable sized frames are transmitted. The amount of data that can be transmitted in each frame is almost always bigger than the size of a signal. Thus, from a resource perspective it would be desirable if each frame could transport several signals. In this paper we investigate how to assign signals to periodic frames with the objective function to minimise the network bandwidth requirement while not violating specified deadlines. This problem is NP-hard, but can for most typical applications be solved efficiently by using simple heuristics. The effectiveness of our algorithm is demonstrated by applying it to signal sets derived from automotive applications for a CAN based system and for the newly developed, low cost and low speed, Local Interconnect Network (LIN). The results can be of great use in cost sensitive embedded systems such as car control systems, where the used hardware, communication networks and nodes (typically micro-controllers), have to be highly utilised to keep the production cost at a minimum level.