Basic Registration call flow is as shown in image below:
Below image shows the sequence of steps followed for Registration process:
First step involves in UE attaching to LTE Network.
Once UE is attached to LTE network, UE needs to register with SIP server to make VoLTE calls.
The above image looks like a lengthy process. But it is very simple to understand after reading below steps:
1. UA will send Register request to P-CSCF.
UA -> eNB -> SGW -> PGW -> P-CSCF
2. P-CSCF -> I-CSCF.
P-CSCF will forward Register request to I-CSCF.
3. I-CSCF -> HSS .
I-CSCF will ask User Authorization Request (UAR) to HSS.
4. HSS -> I-CSCF.
HSS will send User Authorization Answer to I-CSCF.
5. I-CSCF -> S-CSCF.
I-CSCF will forward Register request to S-CSCF.
6. S-CSCF -> HSS.
S-CSCF will send MAR (Multimedia Authentication Request) to HSS.
7. HSS -> S-CSCF
HSS will send MAA (Multimedia Authentication Answer) to S-CSCF.
8. 401 UnAuthorized S-CSCF -> I-CSCF.
www-authenticate: nonce=RAND-AUTN, ck, ik
9. 401 UnAuthorized I-CSCF -> P-CSCF.
10. 401 UnAuthorized P-CSCF -> PGW -> SGW -> eNB -> UA.
11. Register UA -> eNB -> SGW -> PGW -> P-CSCF
12. Register P-CSCF -> I-CSCF
13. UAR I-CSCF -> HSS
14. UAR HSS -> I-CSCF
15. REGISTER I-CSCF -> S-CSCF
16. SAR (Server Assignment Request) S-CSCF -> HSS
17. SAA (Server Assignment Answer) HSS -> S-CSCF
18. 200 OK S-CSCF -> I-CSCF
19. 200 OK I-CSCF -> P-CSCF
20. 200 OK P-CSCF -> UA