Importance

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Abstract

A communications system including a central station and a plurality of remote stations, for example, a mobile telephone system, is disclosed in which communications channel establishing signalling transmissions between the central and remote stations are conducted in a clear (unenciphered) mode and subsequent voice transmissions between the central and remote stations are conducted in a secure (enciphered) mode. Each remote station has a unique code assigned thereto which is used by it to at least decipher incoming enciphered voice transmissions from the central station and by the central station to encipher outgoing voice transmissions to the remote station. The unique code may also be used by the remote station to encipher outgoing voice transmissions and by the central station to decipher incoming voice transmissions from the remote station. Alternatively all remote station outgoing voice transmissions may be enciphered with a code common to all remote stations which code is used by the central station for deciphering incoming voice transmissions. The central station can automatically, or in response to a specific request for secure service, switch its equipment to the secure mode upon completion of communications channel establishing signalling transmissions between it and a remote station and can control remote station switching to the secure mode by sending an enciphered voice transmission to the remote station which responds to receipt thereof by switching to the secure mode.

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First Claim