Appendix E. ITU-T Messages The table in this appendix summarizes messages and the purpose of each. is the layer 3 protocol of the subscriber signaling system used for ISDN and is known as Digital Subscriber Signaling System No. 1 (DSS 1). | Appendix E. ITU-T Messages The table in this appendix summarizes messages and the purpose of each. is the layer 3 protocol of the subscriber signaling system used for ISDN and is known as Digital Subscriber Signaling System No. 1 DSS 1 . It employs a message set that is made for interworking with SS7 s ISDN User Part ISUP . As such the message set maps to the ISUP message set. Table E-1. Messages Binary Code Message Name Purpose Call Establishment Messages 0 0 0 0 0 0 0 1 Alerting Direction Called User Network and Network Calling User The called user is being alerted that is the B party s phone is ringing. 0 0 0 0 0 0 1 0 Call Proceeding Direction Called User Network or Network Calling User Call establishment is taking place and as such no more call establishment signaling is accepted. 0 0 0 0 0 1 1 1 Connect Direction Called User Network and Network Calling User The B party has accepted the call that is has answered the phone. 0 0 0 0 1 1 1 1 Connect Acknowledge Direction Network Called user or Calling User Network Message sent to indicate that the called user has been awarded the call. If sent to the network by the calling user the message allows symmetrical call-control procedures. 0 0 0 0 0 0 1 1 Progress Direction User Network or Network- User Sent to indicate the progress of a call in the event of interworking or in relation to the provision of in-band information patterns for example announcements . 0 0 0 0 0 1 0 1 Setup Direction Calling user Network and Network Called User Initial message sent to initiate a call. 0 0 0 0 1 1 0 1 Setup Acknowledge Direction Called user Network or Network- Calling User Indicates that call establishment is underway but additional information might be requested. Call Information Phase Messages 0 0 1 0 0 1 1 0 Resume Direction User Network Sent request to resume a previously suspended call. 0 0 1 0 1 1 1 0 Resume Acknowledge Direction Network User Indicates to the user that the request to resume a .
