Layer 2 ISDN Connection Initialization

The TE device and the telephone network initialize a Layer 2 connection for ISDN as follows:

The TE device and the telephone network exchange Receive Ready (RR) frames, to indicate that they are available for data transmission. A call cannot be set up if either the TE device or telephone network does not transmit RR frames.
If both ends of the ISDN connection are available to receive data, the TE device sends an Unnumbered Information (UI) frame to all devices on the ISDN link.
When it receives the UI frame, the network responds with a message containing a unique terminal endpoint identifier (TEI) that identifies the endpoint on the ISDN link for all subsequent data transmissions.
When the TE device receives the TEI message, it sends out a call setup message.
The network sends an acknowledgement of the call setup message.
When the TE device receives the acknowledgement, a Layer 2 connection is initialized on the ISDN link.