Data transmission systems

Identifying packets carrying data for a data communication session from encrypted packets carrying data for a plurality of data communication sessions. This involves locating a header pattern of encrypted packets, the header pattern comprising a pattern of encrypted packets in which multiple encrypted packets carry substantially the same encrypted data, and identifying other encrypted packets within the same data communications session as the header. The method facilitates testing of a digital mobile phone network when traffic within the network is encrypted as the sent data may be recovered from a signal tapped at a point within the network without decrypting the data.