The following application note demonstrates the configuration of the ELPRO 645M-4 cellular modem utilizing the GRE Tunnelling feature. Generic Routing Encapsulation (GRE) is a tunnelling protocol which will encapsulate a wide variety of network layer protocols inside a point to point link. There are some benefits of using GRE in that it will allow multicast packets to traverse across a network and the routing of multiple subnets. When using a Public Cellular network, it is recommended that some form of security be used to protect the data from eavesdropping.