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
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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) 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) 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) 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) 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.
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