Dchannel

D-channel is a signaling channel used in ISDN and related digital telecommunication networks. It carries the control information needed to establish, manage, and terminate calls, as well as certain network-management messages. This channel is distinct from the B channels, which carry user data.

In ISDN, two common interface types determine how D-channel signaling is used. Basic Rate Interface (BRI) provides two B channels and one D-channel, with the D-channel operating at 16 kbps for signaling. Primary Rate Interface (PRI) increases capacity and varies by region: in North America on T1 lines, PRI typically uses 23 B-channels plus one 64 kbps D-channel; in Europe on E1 lines, PRI uses 30 B-channels plus one 64 kbps D-channel. The D-channel transmits signaling messages using LAPD (a HDLC-like data-link protocol) at the data-link layer, while higher-layer call-control messages are carried by Q.931 on the D-channel.

The D-channel is responsible for call setup, maintenance, and teardown, including numbers such as called party identification and other signaling features. It may also carry lightweight data in some configurations. In practice, D-channel signaling enables coordination between user equipment and the network, handling control tasks independently of the bearer B-channels.

In modern networks, ISDN signaling has largely declined in favor of IP-based signaling systems (e.g., SIP). Nevertheless, the D-channel concept remains relevant for legacy equipment, interworking gateways, and hybrid systems that connect ISDN to contemporary networks. Some network architectures still route or translate D-channel signaling when interoperability with legacy SS7 or ISDN infrastructure is required.