**4.2.1 Network active link selection**

To establish train-to-earth communications, OCM and TCM can communicate through different communications network physical links. These two entities communicate each other to select the active link considered most favorable for communications. Therefore, OCM and TCM are continuously monitoring all enabled network link status, and switch from one to other in two cases: (1) when active link connectivity is lost and (2) when OCM and TCM select another link to be the new active link. In these two cases, the active communication link change is transparent for final applications that do not detect connection interruptions if these link changes occur while they are transmitting.
