A method and a communication system are in which a first synchronization message identifying a data transmission method with the highest-level error protection that a first device is capable of executing is transmitted from said first device to a second device. A guard monitors the site using a live 3D model, which is updated from different directions using the multiple video streams. Memory locations of the group-specific doubly linked lists are identified by a hash table that includes a unique hash table pointer for each one of the groups.