## 4 IEC61850 Multicast Communication for Wide-Area Applications
### WAN Communication in Wide-Area Applications  
In wide-area applications, R-SV/R-GOOSE messages are sent as IP packets with the multicast address, IP-multicast traffic, over a WAN communication network. This WAN communication can be accomplished over a private IP-based network (Figure 5) or acquired as a service from the Internet Service Provider (Figure 6). In Figure 5, the IP-Multicast sender sends R-SV/R-GOOSE messages in IP packets with a specific multicast address as the destination IP address. These IP-multicast packets are exchanged between substation networks by multicast routers (capable of routing IP-multicast) located across the WAN. There are two main protocols [33] associated with the IP-Multicast: Protocol Independent Multicast (PIM) and Internet Group Management Protocol (IGMP). PIM defines a multicast mode for directing multicast traffic between multicast routers. In each multicast router, PIM is responsible for forwarding the received IP-multicasts traffic to the appropriate outgoing router interfaces. Therefore, PIM creates a multicast tree, i.e., a single path between the sender router (FHR) and the group of receiver routers (LHRs). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Communication Protocol** & **Communication Model** & **Communication Type** & **Message Multicast Address** & **OSI Model Layer** & **Communication Network** \\ \hline SV/GOOSE & Publish-Subscribe & Multicast & Ethernet-multicast & Layer 2 & LAN \\ \hline R-SV/R-GOOSE & Publish-Subscribe & Multicast & IP-multicast & Layer 3 & WAN, LAN \\ \hline Routable-Tunneled SV/GOOSE & Publish-Subscribe & Multicast &
\begin{tabular}{c} IP-multicast and \\ Ethernet-multicast \\ \end{tabular} & Layer 3 and Layer 2 & WAN, LAN \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of IEC61850 multicast messages. Figure 5: IP-Multicast communication from Substation 1 to Substation 2 and 3 via private utility network. The IGMP operates between the IP-Multicast Receiver and the local router (LHR) so that the receiver joins a multicast group that is represented by a multicast destination IP address. Accordingly, IP-Multicast receivers send IGMP join messages to their adjacent LHRs to establish specific multicast group (G) membership, i.e., to receive IP packets with a specific destination IP address. There are three versions of IGMP which are backwards compatible. IGMPv3 is used for R-SV/R-GOOSE communication. In IGMPv3, IP-Multicast receivers can not only join a specific group (G) but can also form a specific source (S). This message is sent from the receiver to the adjacent LHR in the form of IGMPv3 join (S, G) where S is the IP address of the sender, and G is the IP-multicast address. This enables the PIM Source Specific Multicast (PIM SSM) forwarding model over the WAN. This provides efficient bandwidth utilization because the IP-multicast messages are only forwarded to those IP-multicast receivers that signal group membership from a specific source. In Figure 5, WAN communication is administered by a utility company. This requires building and maintaining a dedicated IP-network over large geographical areas, in which all the multicast routers should be configured for multicast routing. The advantage of using a private network is that the utility has full control of the network traffic. However, implementing and maintaining your own WAN is expensive for a utility company. This challenge can be overcome by utilizing the existing networking infrastructure (public IP network) for WAN communication, as shown in Figure 6.  
The public IP network should be able to deliver high quality Internet connection among remote substations at a reasonable cost. Recent advances in cellular technologies (e.g., 5G communication) provide high-speed Internet (IP-based) access with wide coverage. This has made cellular technology a feasible candidate for data transmission in wide-area applications.