How is location update processed in the network? That is the question we are going to answer in this video. Up till now, we’ve seen how it works from the point of view of the terminal. Now we’re going to see the network's side. We’ll take a very simple case, with an eNodeB that is part of the Tracking Area TA1, a terminal that has the TA0-TA1 list in memory, a second cell corresponding to a second eNodeB belonging to TA2. The various base stations here are linked to the same Serving Gateway and depend on the same MME. We’re going to assume that the terminal is correctly connected to the network. That means, first, it is in EMM-Registered state. Second, it has no radio activity, I mean the user has no application running and generating exchanges. The user is moving. If there is no radio activity, that means it is not in ECM-Connected state but in ECM-Idle state. If it is ECM-Connected state, moving will trigger a handover, which will be dealt with in the following videos. The terminal is moving and enters the coverage of base station 2. It notes at that moment that the tracking area identity broadcast is not on its list, so, it must make a location update. The first thing to do, is to establish a radio connection. Just after, an S1-AP connection is set up as we have seen last week. The MME knows the state of the terminal, so it knows for this terminal that it broadcast the list TA0, TA1 and that the terminal was located in a cell belonging to this list. The terminal, once all the connections have been established, sends an NAS, Non Access Stratum, type message, which is EMM Tracking Area Update Request. The MME will then erase from its memory the former location of the terminal and update it by sending a new list. I’ve assumed that it sends a list starting with at least TA2, TA3 which isn’t represented here. The message sent back is an EMM Tracking Area Update Accept message with TA2, TA3 and so on. Thus, the terminal updates the list in its memory,. Small detail, this list is stored in the SIM card. The update is made correctly, the terminal can return to ECM-Idle state. Let’s see how that happens, with this message sequence chart. The terminal detects that it is in a cell that does not belong to the list. It establishes the radio connection, it sends the Tracking Area Update Request message, indicating its GUTI and where it was before. There can be authentication and key agreement, a procedure commonly called AKA, and then cyphering activation. In general, the MME will take advantage of this to allocate a new TMSI, which enables it to construct a new GUTI, Globally Unique Temporary Identity. In my example, the MME chooses a list with TA2, TA3,… and it sends this list to the terminal. The terminal updates its GUTI, and the list, and it indicates that it has taken this list into account. From this moment on, the MME knows that, until it receives a new message, the UE is located in a cell on the list. It can then release the radio and S1‑AP connections. In a few words, location update is made in ECM-idle state. It starts with the establishment of a radio connection and an S1-AP connection. Then, NAS signaling messages are exchanged between the UE and the MME. The MME may renew the GUTI and updates the Tracking Area list.
