Caller ID

The Multiple Data Message Format ( MDMF ) is used for transmission of short messages on the analog phone line connection in the on-hook state. Best known is the transmission of the caller's number (CLIP function).

Not to be confused with the data transfer during the conversation.

Implementation

The transfer takes place with an RS -232 -compatible signaling with 1200 baud, 8 data bits, 1 stop bit. To this end, the binary states are 0 ( also known as Space ) and 1 (Mark) represented by two different frequencies, it is frequency shift keying. The exact frequencies are defined in the standard, but studies have shown that there are different implementations. Therefore, an appropriately equipped device must ( phone) with different frequencies get along. The real data a header ( header) is attached, which allows automatic detection of the frequencies in the receiver.

Example of transmission in CLIP

In Germany and most European countries these signal transmission is carried out in low amplitude between the first and second ring. Depending on the country the call time is transmitted or not, in Germany the transmission is uncommon. This results in the following picture.

And then listen to this; the number is anonymous 1234567890th

This tone you normally do not get to hear because the office when picking up the phone ( the production of the loop current ) to stop immediately the signal transmission.

Decoding

A decoder is awakened by the first ring. He can also use the energy of the AC ringing voltage for the subsequent decoding. Thereafter, this expects a sequence of alternating bits ( 0-1-0-1 ... ), which corresponds to the gapless 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, higher -lying frequency. Then the decoding of user data can begin: Each frequency drops below the initially determined frequency threshold a RS -232 start bit is detected, and the reading of the data bits of each character is done with 1200 step per second ( baud). Between the individual characters are gaps ( of 1- bits) allowed but uncommon.

The data bytes checksum added together must result in a zero byte. The data packet consists of an outer frame ( with the ID 0x80 for call preparation) and any number of inner frame, from which the image showing two commonly used. The identifier for the caller's number is 0x02, the 0x04 for the suppression. Details of the options are available in the ETSI standard. Based on the lengths of the sub- packets, the decoder software easily skip unknown sub- packages and thus ignore.

Single Data Message Format

The packet structure at SDMF differs from the preceding by the "missing" length specification of the inner data block; thus there can only be an internal data block.

Of no practical implementation is known.