## 5 Communication Dependability
### Real-Time Requirement in Wide-Area Applications  
WAN communication imposes additional delays to R-SV/R-GOOSE exchanges between sender and receiver devices. This communication must be real-time, i.e., sufficiently fast to achieve the objectives of the wide-area application. In the electricity grid, three classes (Local, Wide-Area, and Remote) can be defined for monitoring, protection, and control applications. The wide-area applications aim to bridge the gap between the Local and Remote classes, as depicted in Table 2.  
In the Local class, the focus is on a system with localized operation by internal integration of the devices, for example, highly time-critical electrical protection functions carried out by the IEDs inside substations. With Remote class applications, the emphasis is on the system as a whole, and takes in the Distribution System Operator (DSO) or Transmission System Operator (TSO) levels through the Vertical integration of the power system components to the control center. While SCADA is used for power system monitoring/control in the Remote class, the Energy Management System (EMS) and the Distribution Management System (DMS) are applied for less time-critical grid monitoring and management applications.  
The Wide-Area class is based on a network-wide view of the system utilizing Horizontal integration, i.e., coordination between IEDs located in remote substations. In this class, the real-time requirement value is in the range of 10-1000 ms. The exact value is application-specific and determined by the algorithm used in wide-area application: for example, 50 ms to 500 ms for synchrophasor applications [2], 100 ms in the case of GOOSE-based Logic Selectivity [26], and 300 ms for Communication-based LOM [27]. Thus, the communication latency of R-SV/R-GOOSE over WAN has to be less than this real-time requirement to ensure the correct operation of the wide-area application. This latency time is equal to the transmission time of messages over WAN, as well as their processing times in the routers and in other possible networking devices.  
It should be mentioned that R-SV/R-GOOSE communication over WAN supports [30] IP Class of Traffic (CoT) that can reduce delays in processing IP-multicast communication by routers. IP CoT enables Quality of Service for prioritizing IP-multicast traffic (R-SV/R  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Class** & **Integration** & **Scope** & **Participants** & **Operation Level** & \begin{tabular}{c} **Time** \\ **Constraint** \\ \end{tabular} & **Response Time** \\ \hline Local & Internal & Substation & IED & Substation Level & Highly time-critical & \textless{}10 ms \\ Wide-Area & Horizontal & Inter-Substation & IED, PMU, PDC & Coordinated Level & Time-critical & 10–1000 ms \\ \hline Remote & Vertical & Control Center & \
\begin{tabular}{c} SCADA, \\ EMS, DMS \\ \end{tabular} & TSO/DSO Level &
\begin{tabular}{c} Lesser \\ time-critical \\ \end{tabular} & \textgreater{}1 min \\ \hline \hline \end{tabular}
\end{table}
Table 2: Monitoring, protection, and control classes in power system.  
GOOSE messages) in the routers by defining IP priority tagging. This results in faster processing of multicast traffic and, subsequently, smaller processing delays in the routers.