Call encryption in a TETRA network

摘要

An encryption apparatus (200) comprises an elapsed time processor (205) which determines an elapsed time from a transmission of a previous encryption synchronisation message such as a synchronisation initialisation value. Encryption synchronisation messages are transmitted by replacing user data transmissions, e.g. by slot stealing. A priority processor (207) determines a first priority of the current user data transmission and a time threshold function (209) provides a corresponding first time. A transmit unit (201) transmits a new encryption synchronisation message when the elapsed time exceeds the first time threshold. The apparatus (200) further comprises a function controller (211) which is operable to modify the function between the first priority and the first time threshold during the call. The invention is particularly applicable to TETRA and may provide a dynamic trade-off between the speed of encryption re-synchronisation and late entry and the audio quality degradation.