## 4 Proposed MRGM Framework  
Figure 1 shows the proposed DDS-based communication framework for multicast R-GOOSE message (MRGM) over the WAN. It also contains two algorisms for GOOSE message transition from the first substation (Substation A) to the second one (Substation B) which are assumed to be in two different local area networks (LAN). As can be seen in this figure, the DDS is selected as a data-centric communication middleware for GOOSE data exchange between two participants in two substations. The IEC 61850 defines a real-time publish-subscribe (RTPS) protocol for GOOSE data exchange within the wall of a substation. In this protocol, the GOOSE message is published by the GOOSE publisher to the local network with specific format including sequence number (SqNum), status number (StNUm), GOOSE dataField, and an APPID (It is the ID of the published message which is checked by subscribers). Once the message is published, subscribers with the same APPID instantly subscribe to the message and praise the message to take an appropriate action. The rate of sending GOOSE messages varies due to the system condition. In normal operating mode, where there is no need for protection relays' actions, GOOSE messages are sent periodically to test the communication system. It usually happens every one second.  
Figure 1: Proposed data-centric communication framework for multicast routable generic object-oriented substation event messages (MRGM).  
The high-frequency cascading messages are sent due to protection action requirement because of fault detection in the system or control commands. In this situation, as shown in Figure 2, a train of GOOSE messages are sent to the network starting with a very high frequency and continue to lower frequency rates to make sure that the messages are received by appropriate subscribers to cover the possibility of missing samples. The standard has defined a 4 ms time span for GOOSE message delivery within the data link (Layer 2 of the OSI model). However, the published GOOSE messages could be needed to be subscribed by other IEDs in other substations or by the control/monitoring center. For this purpose, GOOSE messages should be transferred from the data layer (Layer 2 of OSI model) to the network layer (Layer 3 of OSI model) and routed to their destinations in the system over the internet network. The standard has defined a 10 ms time span for routable GOOSE (R-GOOSE) messages.  
As mentioned before, specific network settings are required to address this fast delivery over the WAN, therefore, in this paper we propose to deploy the DDS standard for R-GOOSE messages as it covers all the required communication needs. Furthermore, it provides the multicast feature of R-GOOSE messages at the network layer, the same as GOOSE messages at the data layer. This means that several subscribers could receive a published GOOSE message simultaneously and take proper actions immediately. This feature is very helpful in advanced protection systems. In [30], authors have presented an advanced protection scheme for a modern power system, which are experiencing a high level of fault current beyond the breaking capacity of circuit breakers. The proposed advanced protection scheme is mainly based on R-GOOSE messages over the WAN and assumes that a multicast R-GOOSE message framework could deliver the messages to its destination in less than 10 ms. This paper is addressing this framework and demonstrating its capabilities through experimental results. The proposed MRGM algorithm includes three main steps starting with GOOSE publisher in the first network, routing the message over the WAN, and subscribing it within the second network.