LIR/LIA pair is primarily used when?

Interview
1

Q. LIR/LIA pair is primarily used when?

a) Handling incoming (terminating) calls
b) For PCC decisions
c) During deregistration
d) During registration

Check 🔎

:arrow_right: Correct answer: Letter a.

In IMS, the LIR (Location Info Request) / LIA (Location Info Answer) pair is primarily used during incoming (terminating) call handling.

The I-CSCF sends an LIR to the HSS to determine which S-CSCF is currently serving the called user.
The HSS replies with an LIA, containing the S-CSCF address.
This process is essential for routing SIP INVITE messages to the correct serving S-CSCF during call delivery.

Q. LIR/LIA pair is primarily used when?

a) :white_check_mark: Handling incoming (terminating) calls
→ Correct. LIR/LIA is triggered when an incoming request arrives for a registered user, allowing I-CSCF to query the HSS and route the call to the correct S-CSCF.

b) :x: For PCC decisions
→ Incorrect. PCC (Policy and Charging Control) decisions involve the PCRF and the Gx interface, not LIR/LIA.

c) :x: During deregistration
→ Incorrect. Deregistration uses SAR/STA (Server Assignment Request/Answer) messages, not LIR/LIA.

d) :x: During registration
→ Incorrect. Registration uses MAR/MAA (Multimedia-Auth-Request/Answer) and SAR/SAA messages. LIR/LIA is not part of the primary registration flow.