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.

QSIG messages are tunneled by encapsulating them as a MIME body in a SIP INVITE message on the OGW. Then, the MIME body is extracted from the SIP message by the TGW at the other end of the SIP network. To tunnel QSIG messages to a TGW on another network, configure and use a SIP-to-SIP Cisco Unified Border Element connection between each network over which the SIP INVITE must travel to reach the TGW. This tunneling process helps preserve all QSIG capabilities associated with a call or call-independent signal as it travels to its destination.

The following events make it possible to tunnel QSIG messaging across a SIP network:

• The ingress gateway (OGW) receives a QSIG call (or signal) establishment request (a SETUP message) and generates a corresponding SIP INVITE request.

• A corresponding SIP INVITE message is created and will contain the following:

  • A Request-URI--message part containing a destination derived from the called party number information element (IE) in the QSIG SETUP message. The destination can be the egress (TGW or the Cisco Unified Border Element) for exiting the SIP network or it can be the required destination, leaving SIP proxies to determine which gateway will be used.
  • A From header--message header containing a uniform resource identifier (URI) for either the OGW or calling party itself.
  • A Session Description Protocol (SDP) offer--a message part proposing two media streams, one for each direction.
  • A Multipart-MIME body--message part containing the tunneled QSIG data.
• In addition to normal user agent (UA) handling of a SIP response, the OGW performs a corresponding action when it receives a SIP response, as follows:

  • OGW receives 18x response with tunneled content--identifies the QSIG message (FACILITY, ALERTING, or PROGRESS) and sends a corresponding ISDN message.
  • OGW receives 3xx , 4xx , 5xx , or 6xx final response--attempts alternative action to route the initial QSIG message or clears the call or signal using an appropriate QSIG cause value (DISCONNECT, RELEASE, or RELEASE COMPLETE). When the OGW receives a valid encapsulated QSIG RELEASE COMPLETE message, the OGW should use the cause value included in that QSIG message to determine the cause value.

Note	You should expect a SIP 415 final response message (Unsupported Media Type) if the user agent server (UAS) is unable to process tunneled QSIG or Q.931 messages.

• • OGW receives a SIP 200 OK response--performs normal SIP processing, which includes sending an ACK message. Additionally, the OGW will encapsulate the QSIG message in the response to the PSTN side and will connect the QSIG user information channel to the appropriate media streams as called out in the SDP reply.

Note	A nonzero port number for each media stream must be provided in a SIP 200 OK response to the OGW before the OGW receives the QSIG CONNECT message. Otherwise, the OGW will behave as if the QSIG T301 timer expired.

• The TGW sends and the OGW receives a 200 OK response--the OGW sends an ACK message to the TGW and all successive messages during the session are encapsulated into the body of SIP INFO request messages. There are two exceptions:

  • When a SIP connection requires an extended handshake process, renegotiation, or an update, the gateway may encapsulate a waiting QSIG message into a SIP re-INVITE or SIP UPDATE message during QSIG call establishment.
  • When the session is terminated, gateways send a SIP BYE message. If the session is terminated by notice of a QSIG RELEASE COMPLETE message, that message can be encapsulated into the SIP BYE message.

This section lists QSIG message elements and their associated SIP message elements when QSIG messages are tunneled over a SIP trunk.

To create a tunnel for QSIG messages across a SIP trunk using the same signaling forward setting for all interfaces on a gateway, configure the signaling forward settings in voice service voip configuration mode.

To create a tunnel for QSIG messages across a SIP trunk on a specific interface on a gateway, configure the signaling forward settings in dial peer configuration mode.