Multiple data message format
The Multiple Data Message Format (MDMF) is used to transmit short messages on the analog telephone connection line when on-hook. The best known is the transmission of the caller's number ( CLIP function).
Not to be confused with data transfer during a call .
implementation
The transmission takes place with an RS-232 -compatible signaling with 1200 baud, 8 data bits, 1 stop bit. For this purpose, the binary states 0 (also called space) and 1 (mark) are represented by two different frequencies; this is frequency shift keying . The exact frequencies are set out in the standard, but research has shown that there are different implementations. Therefore, an appropriately equipped end device (telephone) has to make do with different frequencies. A header is added to the actual data, which enables the frequencies to be automatically detected in the receiver.
| Binary state | ETSI standard EN 300 659-3 | Bellcore GR-30 core | Germany (measured) |
|---|---|---|---|
| 1 (Mark) | 2130 Hz | 1200 Hz | 1400 Hz |
| 0 (space) | 2750 Hz | 2200 Hz | 1950 Hz |
Example of transmission with CLIP
In Germany and most European countries, this signal transmission takes place in low amplitude between the first and second ring. Depending on the country, the call time is also transferred or not, in Germany the transfer is not common. The following picture emerges.
Here is a sound sample for the number 1234567890.
Normally you do not hear this tone because the office immediately interrupts the signal transmission when picking up the telephone receiver (creating the loop current).
Decoding
A decoder is woken up by the first ring. It can store the energy of the alternating ringing voltage and use it for the subsequent decoding. Then he expects a sequence of changing bits (0-1-0-1 ...), which corresponds to the seamless RS-232 transmission of the character "U" (0x55). From this sequence, the decoder forms the frequency average as a future decision criterion.
Then the decoder expects a constant, lower frequency. Then the decoding of the user data can begin: With each frequency increase above the frequency threshold determined at the beginning, an RS-232 start bit is recognized, the reading of the data bits of each character takes place at 1200 steps per second (baud). Gaps (consisting of 1 bits) are permitted between the individual characters, but this is unusual.
The individual data bytes added with the checksum must result in a zero byte. The data packet consists of an outer frame (with the identifier 0x80 for "call preparation") and any number of inner frames, of which the picture shows the two commonly used. The identifier for the caller's number is 0x02, that for suppression is 0x04. Detailed information about the possibilities can be found in the ETSI standard. Based on the length specifications of the sub-packets, the decoder software can easily skip unknown sub-packets and thus ignore them.
Single data message format
The package structure with SDMF differs from MDMF in that the length of the inner data block is "missing"; thus there can only be one inner data block. A practical implementation of SDMF is not known.
Web links
- ETSI EN 300 659-3 V1.3.1 (2001-01) (PDF; 204 kB) Access and Terminals (AT); Analogue access to the Public Switched Telephone Network (PSTN); Subscriber line protocol over the local loop for display (and related) services; Part 3: Data link message and parameter codings
- Analogue on-hook data transmission . (PDF; 14 kB) Amendment to Section 10 and New Section 12 of 1996 Telecom Public Switched Telephone Network (PSTN) Analogue Line Interface, Telecom New Zealand
- Sivannarayana Nagireddi: VoIP Voice and Fax Signal Processing Verlag John Wiley & Sons, Hoboken NJ 2008, p. 187 ff