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.

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.