N.3 En-bloc conversion function

24.2293GPPIP multimedia call control protocol based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP)Release 18Stage 3TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

N.3.1 General

The en-bloc conversion function may be performed in an entity acting as B2BUA. The en-bloc conversion function may be implemented in different network nodes depending on the operator’s deployment strategy (e.g. AS, IBCF).

If the initial INVITE request is to be forwarded towards a network, or towards a network entity, that does not support overlap signalling, the en-bloc conversion function shall determine the end of address signalling.

The following methods can be used to determine the end of the address signalling:

1) the maximum number of digits used in a national numbering plan has been received;

2) number analysis, e.g. using a provisioned dial plan, is used to determinte that the complete number of digits has been received; or

3) an inter digit timer expires, and the minimum number of digits required for routeing the call have been received. The timer is started when the initial INVITE request is received, and re-started every time new digit(s) are received.

NOTE: The inter digit timer is similar to the protection timer used in PSTN/ISDN. The timer value range is between 5 and 15 seconds, and the default value is 10 seconds.

The procedures for collecting additional digits are described in subclauses N.3.2 and N.3.3. When end of address signalling has been determined, the en-bloc conversion function shall generate an INVITE request, add all digits to the request and forwards the request towards its destination.

N.3.2 Multiple-INVITE method

Upon reception of an INVITE request, the en-bloc conversion function shall:

1) if an inter digit timer is running for a previously received INVITE request with the same Call-ID and From header, and

a) if the number of digits within that previous INVITE request is below the number of digits received in the new INVITE request (or as an equivalent test if the CSeqID of the previous INVITE request is below the CSeqID of the new INVITE request), stop the inter-digit timer for that previous INVITE request and send a 484 (Address Incomplete) response for it; and

b) if the number of digits with the previous INVITE request is above or equal to the number of digits received in the new INVITE request (or as an equivalent test if the CSeqID of the previous INVITE request is below the CSeqID of the new INVITE request), send a 484 (Address Incomplete) response for the new INVITE request; and

2) if the en-bloc conversion function determines that the number received in the INVITE request is complete, forward the INVITE request; and

3) if the en-bloc conversion function determines that it will not be able to forward the request even if additional digits are received, send a 404 (Not Found) response; and

4) if the en-bloc conversion function chooses to collect additional digits, store the INVITE request and start an inter-digit timer to wait for possible INVITE requests with the same Call ID and From header.

When the inter-digit timer expires the en-bloc conversion function shall:

1) if it determines that the number received in the stored INVITE request is incomplete (e. g. by number analysis), terminate the call setup by sending a sending a 484 (Address Incomplete) response towards the sender of the INVITE request; and

2) if it does not determines that the number received in the last INVITE request is incomplete, forward the corresponding stored INVITE request to the next hop.

After forwarding an INVITE request, the en-bloc conversion function shall apply SIP proxy procedures for all subsequent SIP messages within the corresponding dialogue, unless other functionality not related to en-bloc conversion allocated in the same physical norde requires a different behaviour.

N.3.3 In-dialog method

Upon receiving an initial INVITE request carrying an SDP offer, the en-bloc conversion function shall:

1) if it determines that the request contains a complete number, forward the request towards it’s destination;

2) if the en-bloc conversion function chooses to collect additional digits in INFO requests before forwarding the request, and the sender of the INVITE request has indicated support of reliable responses, store the received digits, send a reliable 183 (Session Progress) provisional response without an SDP answer in order to establish an early dialog with the sender of the INVITE request, and start an inter digit timer; or

3) if the en-bloc conversion function determines that it will not be able to forward the request even if additional digits are received, send a 404 (Not Found) response.

Upon receiving an initial INVITE request without an SDP offer, the en-bloc conversion function shall:

1) if it is determined that the request contains a complete number, forward the request towards it’s destination; or

2) send a 404 (Not Found) response.

NOTE: If the initial INVITE request does not contain an SDP offer, a reliable 18x provisional response generated by the en-bloc conversion function would have to contain an SDP offer. The en-bloc conversion function for the in-dialog method specified here does not support en-bloc conversion for calls with an initial INVITE request that does not contain an SDP offer in the present release. However, en-bloc conversion for an initial INVITE request that does not contain an SDP offer can be performed by the originating SIP entity (e.g. MGCF).

Upon receiving an INFO request carrying additional digits, and an early dialog towards the destination of the initial SIP INVITE request for the call has not been created, the en-bloc conversion function shall:

1) send a 200 (OK) response to the INFO request;

2) add the received digits to the previously stored digits for the call;

3) re-start the inter digit timer; and

4) check if it can determine that a complete number for the call has been received.

When the en-bloc conversion function determines that a complete number for the call has been received, it shall add all stored digits to the initial INVITE request and forward the request towards its destination and stop the inter digit timer.

When the inter-digit timer expires the en-bloc conversion function shall

1) if it determines that the number so far is incomplete (e. g. by number analysis), terminate the call setup by sending a sending a 484 (Address Incomplete) response towards the sender of the INVITE request; or

2) if it does not determines that the number received in the last INVITE request is incomplete, forward the INVITE request to the next hop, including all received digits.

Upon receiving an INFO request carrying additional digits, if the en-bloc conversion function has forwarded the initial INVITE request towards its destination, the en-bloc conversion function shall send a 200 (OK) response to the INFO request and not forward the INFO request.

Apart from INFO requests carrying additional digits received after the initial INVITE request has been forwarded, the en-bloc conversion function shall forward all SIP messages.

The en bloc conversion function will receive a "tag" To header field parameter value from the receiver of the initial INVITE request, which is different from the "tag" To header field parameter value that the digit collection function inserted in the 183 (Session Progress) response that it sent when it received the initial INVITE request. The digit collection function shall modify the "tag" To header field parameter value accordingly when forwarding in-dialog SIP messages.

Annex O (normative):
IP-Connectivity Access Network specific concepts when using the EPC via cdma2000^® HRPD to access IM CN subsystem