During IMS registration, which node issues the 401 Unauthorized challenge?

Interview
1

Q. During IMS registration, which node issues the 401 Unauthorized challenge?

a) HSS
b) P-CSCF
c) I-CSCF
d) S-CSCF

Check 🔎

:arrow_right: Correct answer: Letter d.

During IMS registration, the S-CSCF (Serving Call Session Control Function) issues the 401 Unauthorized challenge as part of the SIP authentication procedure. The S-CSCF generates the challenge after receiving authentication vectors from the HSS.

Q. During IMS registration, which node issues the 401 Unauthorized challenge?

a) :x: “HSS”
→ Incorrect. HSS provides authentication vectors but does not issue SIP challenges.

b) :x: “P-CSCF”
→ Incorrect. P-CSCF forwards messages but does not handle SIP challenges.

c) :x: “I-CSCF”
→ Incorrect. I-CSCF routes SIP requests but does not generate the 401 challenge.

d) :white_check_mark: “S-CSCF”
→ Correct. The S-CSCF generates the 401 Unauthorized challenge during registration. The Serving CSCF sends the 401 challenge with AKA parameters to authenticate the UE.