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Content for  TS 23.172  Word version:  18.0.0

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4.3.2 Originating side - initiation of call setup  4.3.2.1 Originating MSC Handling  4.3.2.2 VMSC Handling at Originating Side 
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4.3.2  Originating side - initiation of call setup  p. 23

4.3.2.1  Originating MSC Handling |R6|  p. 23

The originating MSC has a list of supported codec types, listed in order of preference.
If the SETUP message received from the UE contains a Repeat Indicator with a value of "service change and fallback", in addition to a multimedia BC-IE and a speech BC-IE, the MSC shall include a 3G-324.M codec in the list of supported codec types according to the following rule:
  • If the multimedia BC-IE is listed first, then the 3G-324.M codec shall be the first codec in the list (see Figure 4.15) If the originating MSC supports a network-initiated service change from speech to multimedia, it shall include the 3G-324.M2 codec as second codec in the list.
  • If the speech BC-IE is listed first, then the 3G-324.M codec shall be the last codec in the list (see Figure 4.16) sent by a originating MSC not supporting a network-initiated service change from speech to multimedia. If the originating MSC supports a network-initiated service change from speech to multimedia, the 3G-324.M codec shall be the last but one codec in the list and the 3G-324.M2 codec shall be the last codec in the list.. In the case that the maximum number of codecs is already reached before insertion of the 3G-324M and possibly 3G-324.M2 codec(s), the optional speech codec(s) with the least preference shall be replaced by those codec(s).
The list shall then be sent according to the Out-of-Band Transcoder Control codec negotiation procedures. The TMR field, although mandatory BICC/ISUP parameter, has no meaning when using OoBTC/BICC codec negotiation (the link characteristics and QoS are determined from the codec type and signalled to any intermediate switches via the bearer control protocol) and thus shall be set arbitrarily to "speech". A transit node that requires interaction with the user plane will remove from the list the codecs it does not support. If the 3G-324.M codec is not supported, and thus removed, the call shall be turned into a normal speech call (fallback to speech) and follow the normal call setup procedures.
Copy of original 3GPP image for 3GPP TS 23.172, Fig. 4.15: Multimedia BC as first BC
Figure 4.15: Multimedia BC as first BC
(⇒ copy of original 3GPP image)
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Copy of original 3GPP image for 3GPP TS 23.172, Fig. 4.16: Speech BC as first BC
Figure 4.16: Speech BC as first BC
(⇒ copy of original 3GPP image)
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4.3.2.2  VMSC Handling at Originating Side |R6|  p. 24

Depending on operator policy, the VMSC may remove the 3G-324.M2 codec from the list of supported codec types if the call is routed to selected other PLMNs.

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