9 CTM negotiation and inband signaling

26.2263GPPCellular text telephone modemGeneral descriptionRelease 17TS

An inband signaling procedure for CTM negotiation is specified. It shall be used to verify that CTM functionality is (still) available at the far-end side.

A control character may be inserted into the character stream at any position, i.e. control characters may be preceded or followed by text characters. The CTM receiver shall detected and remove these control characters from the text stream.

9.1 Definition of control characters

The following two control characters and functions are defined:

<ENQUIRY>, value 0x05. Used to request a CTM acknowledgment burst from the far‑end side (at the other side of the radio link) in order to detect, whether it is able to support CTM signals.

<IDLE>, value 0x16. Used as filler when the continuation of the ongoing burst is wanted, but no character is available for transmission, e.g. because the user makes a small pause in typing.

9.2 CTM availability negotiation

A CTM device shall permanently monitor its CTM receiver input for the potential occurrence of a CTM burst.

As soon as text input is detected for the first time by a CTM transmitter, or if it is in any other way detected that text conversation is required, the CTM availability negotiation shall be initiated. A CTM burst shall be generated that shall contain the <ENQUIRY> character at the beginning and may contain further regular text characters, control characters or none.

When a CTM burst is received the first time or a CTM burst is received with an <ENQUIRY> character, the CTM device shall immediately respond with another CTM burst to acknowledge that CTM is supported. This CTM acknowledgement burst might be empty, respectively might contain only <IDLE> characters, if there are no regular characters to transmit. If a CTM burst is already ongoing, then no additional CTM burst is required.

If the initiator of the CTM enquiry burst does not receive any CTM burst within 1320 ms after the start of the transmission, it shall repeat the enquiry burst for a maximum of 2 times. If there is still no acknowledgement received, then it shall assume that CTM is not available at the far end and shall go into transparent mode in both directions, until a potential CTM burst is received eventually. If the alternative implementation of Figure 1b is used and CTM negotiation fails, then the text conversion user interface shall convert text into traditional text telephone signals rather than CTM and shall have the capability of converting traditional text telephone signals received from the network back into text.

The enquiry burst can be used at any time during the session to verify that CTM availability is intact. This can be done periodically, or after detection of an event that may indicate that CTM support is lost at the other end. One of such events may be the loss of synchronization due to switching of the voice path to another CTM device.