D channel is a telecommunications term which refers to the ISDN

Integrated Services Digital Network

Integrated Services Digital Network is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network...

 channel in which the control and signalling information is carried.

The bit rate

Bit rate

In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time....

 of the D channel of a basic rate interface

Basic rate interface

Basic Rate Interface is an Integrated Services Digital Network configuration intended primarily for use in subscriber lines similar to those that have long been used for plain old telephone service...

 is 16 kbit/s, whereas it amounts to 64 kbit/s on a primary rate interface

Primary rate interface

The Primary Rate Interface is a standardized telecommunications service level within the Integrated Services Digital Network specification for carrying multiple DS0 voice and data transmissions between a network and a user....

.

For DSS1 signalling, the D channel layer 2 protocol is Q.921 also called LAPD and it carries Layer 3 messages according to Q.931

Q.931

ITU-T Recommendation Q.931 is ISDN's connection control protocol, roughly comparable to TCP in the Internet Protocol stack. Q.931 doesn't provide flow control or perform retransmission, since the underlying layers are assumed to be reliable and the circuit-oriented nature of ISDN allocates...

protocol.