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

Interview
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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.