This is one of the basic, but very interesting flavors of the LTE architecture. This sequential signal flow ensures the mobility as well as security of each individual subscriber.  At the time the user turns on the device (UE), it starts attaching to the network using the unique IMSI. This attachment procedure may vary depending upon the scenario but let us now discuss the very fundamental steps.

Initial UE Message:

UE initially sets up an RRC connection with the eNB and sends the ATTACH REQUEST message as a part of the RRC CONNECTION SETUP COMPLETE message. As there is no encryption initially, the information within this request is very minimal. Within ATTACH REQUEST UE specifies the identifier (IMSI for initial attachment, or old GUTI for subsequent attachment), UE network capabilities, and specific IP service request (if any).

Upon receiving ATTACH REQUEST eNB embed this message within INITIAL UE MESSAGE. Messages between eNB and the NAS layer of MME are sent over the S1-MME interface as an S1AP message.

Auth Info Request/Response and Mutual Authentication: 

After collecting the IMSI and network capabilities from ATTACH REQUEST MME starts performing authentication. MME sends the AUTHENTICATION INFORMATION REQUEST (Diameter Protocol) to HSS via the s6a interface and receives the authentication vectors(key K1 and response R1) within the AUTHENTICATION INFORMATION ANSWER.

MME challenges the UE by sending the key within AUTHENTICATION REQUEST and checks the response (say R2) that is received within AUTHENTICATION RESPONSE with previously received R1.  Equality of R1 and R2 indicates that authentication is complete.

Update Location Request/Answer: 

MME now issues UPDATE LOCATION REQUEST message to HSS including the following information mentioned in the image:

If there is an old MME entry in the HSS, a CANCEL LOCATION REQUEST with MME_UPDATE_PROCEDURE is sent to notify the MME about this change via the s10 interface so that context can be transferred to the new MME via the s10 interface. Old MME acknowledges with CANCEL LOCATION ANSWER and then deletes the previous context data.           

HSS now sends updated location Answers to new MME specifying the following items:

• Subscribed QoS profile.
• Subscribed APN

APN also carries the IP address of the PDN gateway. This is used as the route for the traffic towards the internet and the IP address that is assigned to UE.

Author: Arindam Ghosh