What is the purpose of the Diameter 2nd UAR message in IMS Registration Call flow?

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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:arrow_right: 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) :x: Authenticate the user
→ Incorrect. Authentication is triggered via MAR/MAA between S-CSCF and HSS.

b) :x: Deliver voice packets
→ Incorrect. Voice packet delivery is handled by bearers, not UAR messages.

c) :x: Allocate IP address
→ Incorrect. IP address assignment is done via PDN connectivity, not Diameter UAR.

d) :white_check_mark: 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.