Similar to LTE core network, IMS has its own network.
LTE core network has various nodes such as MME, SGW, PGW/PCEF, PCRF, HSS.
IMS core network has various nodes such as P-CSCF, I-CSCF, S-CSCF , SLF , HSS.
Each node belongs either to LTE network or IMS network, has specific set of functions that it performs.