## 5 Position and Opportunities for Further Research  
### Timing Predictability of Safety-Critical CPSs  
To support the development of time-critical CPSs, the research questions posed in Section 1 are refined as follows. 1. Are the existing models, languages and frameworks for the development of the CPSs expressive enough to specify the timing requirements not only on the computation and communication times but also on the ready times and ready order of the inputs that are acquired from the physical processes? 2. Are there any existing methods and techniques that can formally verify the specified timing requirements at the design time to support pre-runtime timing predictability verification of the CPSs? If the answer to Research Question 2 is \"yes\", can the identified techniques efficiently support and be integrated to the existing safety analyses? 3. Are there any existing techniques to support timing predictable run-time environment for the CPSs that can provide bounded delays with regards to the computation, communication, ready times and ready order of the sensor inputs? To answer the first research question, we explore the existing development models, languages and frameworks for CPSs. The existing works in the computation and communication parts support the specification of timing requirements, which are sufficient to support the corresponding part of time-critical CPSs. For example, the timing model in the AUTOSAR standard (used in the automotive domain) provides 21 different timing constraints that can be specified on the system. Similarly, the EAST-ADL software architecture description language and several other component models including RCM support the specification of the timing requirements [22; 46]. Another example can be seen in the avionics domain, where several existing frameworks support the specification of timing requirements [47; 48]. However, we identify that the existing techniques, methods and frameworks do not support the specification of timing requirements corresponding to the input ready times and the input ready order. There are a few recent works that discuss the input ready times and the input ready order in the context of CPSs [27]. However, the formal semantics of these terms and their corresponding timing requirements are still missing from the state of the art. This, in turn, hampers the specification of holistic or end-to-end timing requirements in time-critical CPSs, i.e., the timing requirements that constrain the delivery time of the output of an actuator corresponding to the time when a new input is generated from a sensor that is deployed in the physical process. The timing model of the AUTOSAR standard includes several timing constraints, e.g., the _Order Constraint_. This constraint is also supported by several other modelling languages such as EAST-ADL, TADL2 and RCM. We believe this constraint can be extended to support the input ready order timing requirement in the CPSs. Basically, the Order Constraint constrains an order among the occurrences of events [22; 24]. If this constraint is adapted to constrain the arrival order of the sensor values from the physical process, then it can be applied to constrain the input ready order in the CPSs. On the other hand, the semantics of the input ready times and the corresponding requirement need to be properly defined and included in the existing techniques and frameworks that are used for the development of the CPSs. The second research question is answered by exploring the existing schedulability techniques [18; 25; 49; 50; 51; 52]. The support to verify timing predictability at the design time in the computation and communication parts are quite mature. If the Order Constraint is extended to support the specification of timing requirement on the inputs arrival order in CPSs, the corresponding timing analysis can be used to verify this requirement at the design time [22]. To the best of our knowledge, the timing analysis to verify the timing requirements on the input ready times is still missing from the state of the art.