These are my lecture notes from IMT’s NET02x (4G Network Essentials) course, I thought I’d post them here as they may be useful to someone. You can find my complete notes here.
The LTE architecture compartmentalises the roles in the mobile network.
For example the eNB concentrates on radio connection management, while the MME focuses on security and mobility.
Non Access Stratum (NAS) messages are exchanged between the terminal and the MME.
Access Stratum (AS) messages are exchanged over the air between the UE and the eNB. It contains all the radio related information.
The eNB must map the NAS messages from an MME to a LCID and RNTI and transmit them over the air, and vice-versa. The eNB forwards this data without ever analyzing it.
Transport of S1 messages is carried over SCTP which I’ve spoken about in the past.
The above image overlaps with the radio version we talked about earlier.