A coupled data network which ensures in sequence delivery of messages is disclosed. The copuled data network includes a foreign network and a home network. The foreign network includes a foreign mobile switching center and a base station, the base station including an access hub with a serving inter-working function. The home network includes a home mobile switching center, the home mobile switching center including a home inter-working function. A roaming end system is a subscriber to the home network and operates within the foreign network, a message being transportable between the roaming end system and the home inter-working function through the serving inter-working function using a protocol that ensures in sequence delivery of data packets.