Table of Contents
UE attempts a handover or cell reselection when the received signal strength from its serving cell becomes weak as it moves. If there is no neighbor cell at all near the travelling UE, the signal strength will get gradually weaker until finally the UE is detached from the network. Then, once within an LTE coverage again, it will make initial attach to the network again through cell selection.
This document discusses EMM cases where UE detaches from the network as it moves out of one LTE coverage area, and moves into another one, attaching the network again, as in EMM Case 10, “Move to Another City”, and EMM Case 11, “Initial Attach in Another City” .
 Netmanias Technical Document, “Eleven EMM Cases in an EMM Scenario”
Figure 1 illustrates the two cases.
Figure 1. EMM Cases 10 and 11
For the purpose of the two EMM cases, this document assumes a mobile operator that:
Figure 1 shows City 1 and City 2 only, and the user is moving from City 1 to City 2 in a car.
In EMM Case 10, UE detaches from the network as it leaves City 1, thereby moving out of the LTE coverage. The UE, either while using services in Connected state (EMM-Registered, ECM-Connected, RRC-Connected) or while camping on its serving cell in Idle state (EMM-Registered, ECM-Idle, RRC-Idle), transits to Detach state (EMM-Deregistered, ECM-Idle, RRC-Idle) as it leaves City 1, and hence is detached from the network by MME.1
In EMM Case 11, UE attaches the network again as it moves into an LTE coverage after arriving in City 2. Unlike the case in the previous document , the network (Old MME) has already had the UE context when the UE attaches the network (New MME). The UE makes initial attach to the network (New MME) using the UE ID (Old GUTI) that was assigned by the network (Old MME). By that, it transits from Detach state (EMM-Deregistered, ECM-Idle, RRC-Idle) to Connected state (EMM-Registered, ECM-Connected, RRC-Connected).
This document is organized as follows:
Chapter 2 and 3 will describe the EMM Case 10 and 11 procedures, respectively. And
Chapter 4 will explain how information elements in EPS entities are different before and after each procedure.
Thanks for your share. It's very helpful for me.
But I have question : When UE perform move from MME1 to MME2, UE need "initial attach in Another City" => UE will no service (short time), right? If true, how many time UE no service?
I like this helpful article, but I have one quetion related to the implicit detach timer.
I know when the UE moves out of the shadow area while the implicit detach timer is running, the UE should establish a radio link to the MME by operations such as a tracking area update or a service request. here is the question, if it's a TAU, then this is TAU type should be a normal TAU or a periodic TAU? thank you, and respond to me, please~