Three questions about it:
(a) ISDN/SIP interworking spec [ETSI TS 183 036] proposes that in the
Q.931/SIP interworking, some EIs Q.931 are codec to SDP and/or inserted
in the SIP body as a PSTN XML element.
Under what criteria, an Q.931 EI should be?:
(1) Coding to SDP
AND / OR (this is also important)
(2) Inserted in the SIP body as a PSTN XML element
(3) Coding to the URI SIP or and SIP header
(4) None (only interpreted by the AGW, without being carried over SIP)
(b) Why does [ETSI TS 183 036]define the mapping of the IE "User to
user" to the SIP header User-to-User [draft-johnston-sipping-cc-uui-09],
instead of transporting it over an PSTN XML element? (the IE "User to
user" must be transported transparently by the nerwork)
(c) Although as optional, [ETSI TS 183 036]defines the coding to SDP of
the "High Layer Characteristics Identification" field of the IE
HLC[Q.931].According to Q.931, this EI HLC should be used by the
end-to-end terminals, not by the LEs (or AGWs in NGN). Then:
(1) Why this SDP coding?
(2) And why is it optional?
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