Time-triggered protocol

CAN is the serial communication network used in most intelligent SAS products. Future systems such as SbW will need fault-tolerant networks (FTU), which provide the necessary safety level for those systems, that is, being predictable and deterministic. There are two major protocols in development, TTP (time triggered protocol [32]) and Flexray, supported by two different consortia of car manufacturers and system suppliers. VW/Audi, PSA, Renault, Delphi, Visteon, United Technologies, Airbus, Honeywell, AMS C C Electronics, and NEC are in the TTP consortium. DC, BMW, CM, Bosch, Philips, digital dna, Motorola, and partners are the Flexray group [30, 31]. 

If a global time base is available (either directly or via a protocol that bounds relative drift between the set of system clocks), then it is relatively straightforward to build fault-tolerant services on either the time-triggered or event-triggered models. 

MACA-BI—Multiple access with collision avoidance by invitation (Talucci, 1997). In MACA-BI, the receiver polls a prospective sender by transmitting a ready-to-receive (RTR) packet. (This is an example of a receiver-initiated protocol.) In order to perform the polling in a timely fashion the receiver is required to correctly predict the traffic originated by the sender. Periodic traffic makes this task easier. In case either the data buffer or the delay at the terminal increases above a certain threshold this terminal may trigger a... 

Gated imaging without table movement requires a new way of thinking in terms of protocol selection. The smooth transition between prospective triggering and retrospective reconstruction offers the user complete freedom in tailoring the scan to the clinical question. For each patient, the protocol is optimized balancing the required cardiac phases, temporal resolution, and the radiation dose. Patient comfort is also increased due to the very short breath hold times and no distracting table movement. 

Pulsation of the aorta causes artifacts, especially in the aortic root and ascending aorta. ECG-triggered data acquisition helps to significantly reduce these artifacts and therefore plays an important role in the examination of unclear chest pain or thoracic aortic dissection (Fig. 23.1). Furthermore, this technique enables the evaluation of coronary arteries and can replace invasive clinical diagnostics in some cases. Flowever, detailed protocols of the thoracic aorta are normally based on protocols of coronary artery CT. Due to their specifications, they lead to longer acquisition times than those of standard protocols and do not properly visualize the abdominal aorta. This can be overcome by an ECG-gated acquisition of the thoracic aorta and a change to the standard protocol for the abdominal aorta. In order to achieve a sufficient contrast in the abdominal aorta, the time delay to modify the examination protocol should be minimized (Fig. 23.2). 

In practice this is achieved with timers that trigger the respective transmissions. However, this approach may perform poorly, with high significant packet losses during certain periods of time, herein called critical periods, independently of the network load. This is the case in the presence of interfering external periodic traffic with approximately coherent periods. In this case, given the fixed TDMA round structure, the protocol will continue triggering transmissions at the same time as the interference source, until the clock drifts eventually set those instants apart (Fig. 2). 

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