Technical data

sec mode 4 - V22bis recommendation (takes 7-10 seconds); in this case, the modems

establish a connection faster than under V34, but if there is a lot of distortion in the

communication channel, the modems may lose synchronization (and secure connection

will be broken); this mode is convenient for transmission of short messages over poor-

quality communication channels.

The secure mode of operation of the telephone is intended for a guaranteed protection

of telephone conversation against the any third party who may have access to the telephone

channel between the users. This mode can be used only when both ends of the line are

equipped with AncVoice Coder - 2400 phone sets.

Protection principles are based on conversion of a voice signal into a digital form,

with subsequent compression to the exchange rate of 2400 bits per second, using a parametric

coding such as LPC10E, or compression to the exchange rate of 4800 bits per second using

the CELP algorithm. Transmission of signal over the channel is performed by a built-in the

modem, with preliminary linear coding or transpositioning.

Operation of the AncVoice Coder - 2400 unit in the secure mode is enabled for one of

the following cases:

operation with only one internal key, which is automatically generated by each phone set

every time when it is switched in the secure mode;

operation with two keys: the internal key and a key located in the nonvolatile memory of a

Touch Memory button, pre-recorded for both subscribers during a key generation session

done on a single phone set;

operation with three keys - the internal and external keys are supplemented with an 8-digit

(*, #, PAUSE keys may be used) password key, dialed from the numeric keypad;

Operation with two keys – the internal key and the password, dialed from the keypad.

Use of additional keys provides an opportunity to built different

organizational/technical versions of a secure telephone network. Many of such options may be

based on the use of several Touch Memory devices for each phone set, with different keys for

different users. In this case, there is an additional possibility to authenticate the subscriber of

the secure telephone network.

A required amount of external nonvolatile memory Touch Memory devices can be

purchased from the manufacturer.

It is important to keep in mind that:

When the phone is in the secure mode, all conventional services keys on the keypad are

locked, and the only working key is the "Open" key for switching into the open mode of

operation without having to break the connection;

When working with three keys, the first one to be entered is the Touch Memory button,

then – the digital password key, entered from the keypad (the third key, internal, is

automatically generated by the phone sets between which the secure connection is being

established).

The procedure of generation and recording of private keys on Touch Memory external

nonvolatile memory devices has to be done simultaneously for Touch Memory buttons of

all subscribers who are going to use Touch Memory as their keys for the establishment of

secure communications between each other in the future.

The skeleton diagram of operation of a telephone set AncVoice Coder - 2400 is shown

in a fig. 1.

The flow chart of AncVoice Coder – 2400 operation is shown on fig. 1.