QSIG Tunneling

The device supports QSIG tunneling over SIP, according to IETF Internet-Draft draft-elwell-sipping-qsig-tunnel-03 ("Tunnelling of QSIG over SIP") and ECMA-355/ISO/IEC 22535. This is applicable to all ISDN variants. QSIG tunneling can be applied to all calls or to specific calls using IP Profiles.

QSIG tunneling sends all QSIG messages as raw data in corresponding SIP messages using a dedicated message body. This is used, for example, to enable two QSIG subscribers connected to the same or different QSIG PBX to communicate with each other over an IP network. Tunneling is supported in both directions (Tel-to-IP and IP-to-Tel).

The term tunneling means that messages are transferred ‘as is’ to the remote side without being converted (QSIG > SIP > QSIG). The advantage of tunneling over QSIG-to-SIP interworking is that by using interworking, QSIG functionality can only be partially achieved. When tunneling is used, all QSIG capabilities are supported and the tunneling medium (the SIP network) doesn't need to process these messages.

QSIG messages are transferred in SIP messages in a separate Multipurpose Internet Mail Extensions (MIME) body. Therefore, if a message contains more than one body (e.g., SDP and QSIG), multipart MIME must be used. The Content-Type of the QSIG tunneled message is ‘application/QSIG’. The device also adds a Content-Disposition header in the following format:

Content-Disposition: signal; handling=required.

QSIG tunneling is done as follows:

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Call setup (originating device): The QSIG Setup request is encapsulated in the SIP INVITE message without being altered. After the SIP INVITE request is sent, the device doesn't encapsulate the subsequent QSIG message until a SIP 200 OK response is received. If the originating device receives a 4xx, 5xx, or 6xx response, it disconnects the QSIG call with a ‘no route to destination’ cause.

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Call setup (terminating device): After the terminating device receives a SIP INVITE request with a 'Content-Type: application/QSIG', it sends the encapsulated QSIG Setup message to the Tel side and sends a 200 OK response (no 1xx response is sent) to IP. The 200 OK response includes an encapsulated QSIG Call Proceeding message (without waiting for a Call Proceeding message from the Tel side). If tunneling is disabled and the incoming INVITE includes a QSIG body, a 415 response is sent.

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Mid-call communication: After the SIP connection is established, all QSIG messages are encapsulated in SIP INFO messages.

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Call tear-down: The SIP connection is terminated once the QSIG call is complete. The Release Complete message is encapsulated in the SIP BYE message that terminates the session.

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To enable QSIG tunneling:

1.

Open the Digital Gateway Settings page (Setup menu > Signaling & Media tab > Gateway folder > Digital Gateway > Digital Gateway Settings), and then from the 'Enable QSIG Tunneling' drop-down list (EnableQSIGTunneling), select Enable on the originating and terminating devices.

2.

Configure the [QSIGTunnelingMode] parameter for defining the format of encapsulated QSIG message data in the SIP message MIME body (0 for ASCII presentation; 1 for binary encoding).

3.

Configure the [ISDNDuplicateQ931BuffMode] parameter to 128 to duplicate all messages.

4.

Configure the [ISDNInCallsBehavior] parameter to 4096.

5.

Configure the [ISDNRxOverlap] parameter to 0 for tunneling of QSIG overlap-dialed digits (see below for description).

The [ISDNInCallsBehavior] and [ISDNRxOverlap] parameters enable tunneling of QSIG overlap-dialed digits (Tel to IP). In this configuration, the device delays the sending of the QSIG Setup Ack message upon receipt of the QSIG Setup message. Instead, the device sends the Setup Ack message to QSIG only when it receives the SIP INFO message with Setup Ack encapsulated in its MIME body. The PBX sends QSIG Information messages (to complete the Called Party Number) only after it receives the Setup Ack. The device relays these Information messages encapsulated in SIP INFO messages to the remote party.