ISDN/SIP interworking [ETSI TS 183 036]

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

or

(3) Coding to the URI SIP or and SIP header

or

(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?

Regards,

Javi

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