Abstract: A transmission system for sub-networks having different transmission protocols. Control codes which are assigned to a message in a source sub-network are converted at the transition with a first intermediate sub-network, into reference codes which are compatible with a reference protocol ("meta protocol"). Subsequently, control codes are assigned both to the message and to its reference codes, which control codes are compatible with the protocol in the first intermediate sub-network. At each subsequent sub-network transition, control codes are always assigned to both the message and to the reference codes, which control codes are compatible with the local protocol of the subsequent intermediate sub-network ("tunnelling"). At the interface of the last intermediate sub-network and the destination sub-network, after removing the local control codes, the reference codes are converted into control codes which are compatible with the local protocol of the destination sub-network.