Q-series documents, controlled by the International Telecommunication Union (ITU), define the network Layer. The Q.931 document defines the Layer 3 protocol that serves as the connection control protocol for ISDN signaling--it is used primarily to manage the initiation, maintenance, and termination of connections over a digital network.
The Q signaling (QSIG) protocol is based on the Q.931 standard and is used for ISDN communications in a Private Integrated Services Network (PISN). The QSIG protocol makes it possible to pass calls from one circuit switched network, such as a PBX or private integrated services network exchange (PINX), to another. QSIG messages are, essentially, a subset of Q.931 messages that ensure the essential Q.931 FACILITY-based functions successfully traverse the network regardless of the various hardware involved.
Q.931 tunneling over Cisco IOS SIP gateways was introduced as the ability to transparently tunnel only QSIG messages--the FACILITY-based Q.931 messages. Beginning with Cisco IOS Release 12.4(15)XZ and Cisco IOS Release 12.4(20)T, tunneling of all Q.931 messages (SETUP, ALERTING, CONNECT, and RELEASE COMPLETE messages in addition to FACILITY-based messages) is supported on Cisco IOS SIP gateways. However, for clarity, the descriptions and examples in this document focus primarily on QSIG messages.