Which of the below nodes have IMS realm and which have the EPC realm?

Interview
1

Q. Which of the below nodes have IMS realm and which have the EPC realm?

I-CSCF, S-CSCF, P-CSCF, HSS, AS, PCRF, MME, PGW

a) I-CSCF, P-CSCF, S-CSCF, PGW and AS belong to IMS realm and HSS, PCRF, MME belong EPC realm
b) I-CSCF, P-CSCF, S-CSCF and AS belong to IMS realm and remaining nodes belong to EPC realm
c) I-CSCF, P-CSCF and S-CSCF belong to IMS realm and remaining nodes belong to EPC realm
d) All nodes belong to IMS realm except MME and PGW which belong to EPC realm

Check 🔎

:arrow_right: Correct answer: Letter b.

In IMS architecture, I-CSCF, P-CSCF, S-CSCF, and Application Servers (AS) operate in the IMS realm, handling SIP-based signaling and service logic. The remaining nodes — HSS, PCRF, MME, PGW — belong to the EPC realm, which handles mobility, transport, and policy control.

Q. Which of the below nodes have IMS realm and which have the EPC realm?

a) :x: I-CSCF, P-CSCF, S-CSCF, PGW and AS belong to IMS realm and HSS, PCRF, MME belong EPC realm
→ Incorrect. PGW is an EPC gateway responsible for IP connectivity and bearer services — it is not part of IMS.

b) :white_check_mark: I-CSCF, P-CSCF, S-CSCF and AS belong to IMS realm and remaining nodes belong to EPC realm
→ Correct. These are the core IMS signaling and application entities. The other nodes support data plane, mobility, and policy control in the EPC.

c) :x: I-CSCF, P-CSCF and S-CSCF belong to IMS realm and remaining nodes belong to EPC realm
→ Incorrect. While the IMS nodes listed are correct, AS is missing, which also belongs to the IMS realm.

d) :x: All nodes belong to IMS realm except MME and PGW which belong to EPC realm
→ Incorrect. HSS and PCRF are core EPC components as well. The HSS interfaces with IMS, but its functional domain is within EPC.