1. LTE UE Initial Attach
This is a huge call flow and there is a lot of processing involved. I would like give a brief out
line of messages.
S1 Setup: This is where eNB is attached to the network. As long the eNB is functioning the
S1 setup remains.
RRC Connections: Once UE comes up a RRC connection is established for communication
with the network.
NAS: After RRC is established then begins the NAS signaling. UE sends Attach request along
with PDN connectivity request to network. Attach is for attaching to the network and other
message message is for establishing the bearers.
HSS: This is Home Subscriber System and it understands diameter protocol. Once MME
receives Attach Request, it queries HSS for authentication details. HSS sends the
authentication vectors to MME in Authentication Info Answer.

2. Authentication/Security: Networks requests UE for Auth vectors. Once UE provides the same
MME compares the same with what HSS has sent. If they match UE is authenticated. Next is
security. After the security all the NAS messages are encrypted using the security
algorithms that were exchanged.
Bearers: After security network creates the EPS bearers first. (GTP messages). Then the
radio bearers are created and RRC connections are modified accordingly. Once the radio
bearers are created eNB downlink addresses are sent to SGW in GTP messages.
LTE UE Initiated Detach
UE decides to detach from the network. Its sends a detach request message to network.
Network deletes the EPS bearers then the radio bearers are torn down. Finally RRC
connection is released.