LTE network consists of multiple nodes such as MME, SGW, PGW/PCEF, PCRF, eNodeB etc. Almost every node involves in bearer establishment and has right to reject/cancel the establishment. MME acts an orchestra-tor for all the signaling which exchange over LTE Core and LTE radio network nodes. I would like to list down the call flow in the form of points.
1. After performing authentication and establishing security with UE, MME sends "Create Session Request" to SGW. If SGW can accept the request, it allocates the required resources.
2. After that SGW, forward the Create Session Request to PGW . Similar to SGW, PGW does the same operation and send message to PCRF.
3. If subscription and policy allows, PCRF sends successful response back to PGW. After receiving response, PGW prepares and sends Create Session Response to SGW. Then SGW sends Create Session Response to MME.
4. Once bearer is established as Core network, MME triggers Initial Context Setup Request/E-RAB Setup Request based on the NAS message received from the UE.
5. If eNodeB RRM/Application allows then context will be created at all the required layers and rrc connection reconfiguration will be sent to UE.
These are the things what I know in this context. Hope it will help you.
