Data is transferred between mobile or fixed end devices connected to respective base stations. A first session identifier (session ID) is selected from a first array for sessions originating from an initiating end device. Data is transferred as a plurality of packets each of which contains the first session ID to a remote end device connected to a remote base station, the remote base station being configured to: allocate a second session ID from a second array, map to one another the said first and second session IDs, and, a forward the said second session ID to the remote end device. In this way, the mapping between session IDs need not be stored in the end devices, allowing for a session to be handed over from one end device to another end device where both end devices are connected to the same base station.