Q. What is the purpose of the Diameter 2nd UAR message in IMS Registration Call flow?
a) Authenticate the user
b) Deliver voice packets
c) Allocate IP address
d) Determine the S-CSCF
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Correct answer: Letter d.
During IMS registration, the I-CSCF sends a User-Authorization-Request (UAR) to the HSS to determine the appropriate S-CSCF for the user.
- The 2nd UAR typically occurs after the UE sends a REGISTER message.
- The HSS replies with a User-Authorization-Answer (UAA) that includes the S-CSCF assignment.
- This is crucial for routing the REGISTER message to the correct S-CSCF.
Q. What is the purpose of the Diameter 2nd UAR message in IMS Registration Call flow?
a) Authenticate the user
→ Incorrect. Authentication is triggered via MAR/MAA between S-CSCF and HSS.
b) Deliver voice packets
→ Incorrect. Voice packet delivery is handled by bearers, not UAR messages.
c) Allocate IP address
→ Incorrect. IP address assignment is done via PDN connectivity, not Diameter UAR.
d) Determine the S-CSCF
→ Correct. The 2nd UAR is used by the I-CSCF to query the HSS and receive the assigned S-CSCF for the user. The I-CSCF sends a UAR to the HSS to find out which S-CSCF address.
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