In ISDN, the D-channel ( English : data channel ) is the channel for transmitting control information ( signaling ), in particular for establishing and clearing the connection . A signaling protocol used in ISDN is, for example, DSS1 . The transmission of the control information via the D channel is secured with the LAPD protocol . Unlike the B channel, the D channel is not intended for direct communication between two subscribers , but is primarily used for communication with the exchange . It has a data transfer rate of 16 kbit / s for basic connections and 64 kbit / s for primary rate connections . Signaling between the exchanges takes place on the so-called central signaling channel (64 kbit / s) on which the ITU-T signaling system no. 7 is used.
There is a constant connection to the exchange via the D-channel, which is more powerful than is necessary for the pure exchange of control information. The remaining capacity can be used, for example, for Datex-P connections (up to 9600 bit / s). In the past, Deutsche Telekom was also able to use this channel to get information when an email was received on a T-Online account (which is no longer possible today for reasons of competition law ).
When establishing a connection , in addition to the call number , other information such as a subaddress or - to a limited extent - character strings can be sent via the D channel , which are passed on to the called party . This makes it possible to transfer complex information, including files, through repeated dialing attempts without setting up a connection (subject to a charge). For example, there was a Linux project that used exactly this function and could be used for a very slow but free data transfer . The Linefire program uses the method described to transmit text messages. Since this places a heavy load on the telephone network , the functions required for this have been switched off at many telephone companies.
The signaling information transmitted on the D channel can be read and logged with an ISDN monitor .
- ITU-T I.412 ISDN User-Network Interfaces - Interface Structures and Access Capabilities.
- 3GPP TS 45.002: Multiplexing and multiple access on the radio path.