Radiocommunications equipment with a security calls mode, and extension unit forming part of such equipment

DETAILED DESCRIPTION The invention thus proposes a radiocommunications equipment comprising a radio terminal adapted to calls with a public mobile telephony network supporting voice transmission channels and data channels, the radio terminal comprising a data interface for the connection of a data input/output unit allowing calls on the data channels of the network.
According to the invention, the equipment further comprises, as a data input/output unit, an extension unit having an external appearance similar to that of a radiocommunications terminal and comprising at least a microphone, an earphone, a vocoder and a ciphering module.
In a security call mode, the vocoder forms, from the signal coming from the microphone, a binary flow which the ciphering module processes in order to produce data sent to the data interface for transmission on a data channel, and data presented at the data interface after reception on a data channel are deciphered by the ciphering module in order to produce a binary flow which the vocoder decodes in order to control the earphone.
Another aspect of the invention relates more particularly to the extension unit of this equipment.
The invention thus exploits the presence of data channels in certain radiotelephony networks.
The confidential calls use these data channels in a transparent manner.
The data thus exchanged represent voice transmission signals and are ciphered and deciphered at each of the ends of the call.
A third party monitoring the path of the call without knowing the coding method, the ciphering method and the ciphering keys used has no access to the voice transmission signals.
The functions necessary for the security calls mode are grouped in an extension unit of a existing radio terminal.
This avoids having to design, for this specific application, the whole of a radio terminal compatible with the public network.
This results in a significant limitation in the cost price of the equipment.
The economic advantage is further enhanced by the use of an ordinary radio terminal benefitting from the mass effect brought about by the development of digital cellular networks such as the GSM