8.2.3 CTM burst structure

26.2263GPPCellular text telephone modemGeneral descriptionRelease 17TS

For the CTM signals, a synchronous transmission is used, which is organized in bursts. A burst is initiated as soon as there are bits available coming from the block "UTF‑8 character encoding". At the beginning of each burst a preamble is transmitted, which can be used at the receiving side for the synchronization of the deinterleaver and the error correction. Each burst is kept active as long as the block "Mute + AddResyncInfo" is able to transmit bits to the interleaver. During the whole burst, the synchronism is kept, i.e. the bits are transmitted at a fixed rate of 400 bit/s, according to a duration of 5 ms per pair of two bits.

The generation of the preamble at the beginning of each burst is described in Section 8.2.5. Since the preamble is located at the interleaver’s dummy elements, the transmission of the preamble does not introduce any additional delay. Before the first bits of a new burst are passed from the interleaver to the CTM modulator, a sequence {0,0, 1,0, 1,1, 0,1} should be passed to the modulator. This sequence triggers the modulator to generate a sequence of four tones with the frequencies 400 Hz, 800 Hz, 1000 Hz, and 600 Hz, to simplify the initial synchronization of the demodulator at the receiving side.

The CTM burst is kept active as long as the block "Mute + AddResyncInfo" is able to transmit bits to the interleaver. If the bit‑stream towards the interleaver is running out, because there are no more characters that have to be transmitted, one <IDLE> character (see Section 9.1) is sent from the block "UTF‑8 character encoding" to the FEC error protection. This insertion of <IDLE> characters can be repeated up to four times, if there are still no regular characters available at the text input of the CTM transmitter. The burst is terminated if five <IDLE> characters have been transmitted consecutively without any regular characters in between.

For the termination of the burst, it must be guaranteed that all bits, which are still stored in the buffers of the FEC error protection and the interleaver, are transmitted to the far-end side.