Mobility Management in LTE/EPS

Mobility Management is the function of controlling the mobility (and security) of the UE when using the RAN while the Session Management is the function of providing IP connectivity (and resources) to the UE. Connection Management is a subfunction of Mobility Management controlling the connection of the UE to the network. These functions are implemented by protocols belonging to the NAS (Non-Access Stratum).



Mobility Management in LTE/EPS known as EPS Mobility Management (EMM) is similar to the UMTS one with some changes stemming mainly from the LTE requirement for “Always On” IP connectivity. This requirement is achieved by providing an EPS Bearer (Default EPS Bearer) to the UE once it attaches to the network and by keeping it until it switches off. Thus the Mobility and Session Management mechanisms in LTE are interconnected. For more info on the EPS Bearer look here.

Mobility Management (EMM)

The main EMM procedures are as follows:

• Attach
  
• Detach
  
• Tracking Area update
  
• Paging
  
• Identification
  
• Security Mode Control

The EMM defines two main states for the UE, while other intermediate states are also defined:

EMM-Deregistered
When the UE is not attached to the network and thus the network doesn’t know its location and cannot reach it.

EMM-Registered
When the UE is attached to the network and an EPS Bearer is established for it.

Independently of the EMM states there are two EPS Connection Management (ECM) states the UE can be in:

ECM-Idle
When there is no NAS connection established for the UE and no physical resources are allocated to it (i.e. Radio and S1 Bearers). It can still exchange data as its EPS Bearer still exists but a NAS connection needs to be established first in order to get physical resources allocated.

ECM-Connected
When there is a NAS connection established for the UE and physical resources are allocated to it. In order to get to the EMM-Registered state the UE needs to get to the ECM-Connected mode first.

The NAS connection which changes the ECM-Idle to the ECM-Connected mode is established when the UE needs to send an initial NAS message such as Attach/Detach Request, Tracking Area Update, etc. Then an RRC connection needs to be established between the UE and the eNodeB and in sequence an S1_MME connection. After that the NAS connection between the UE and the MME is established. The initial NAS message can then be sent by the UE. In the same manner the NAS connection release which brings the UE back to ECM-Idle mode takes place after the RRC connection and the S1_MME connections are released.

Session Management (ESM)

The main EPS Session Management (ESM) procedures which realize the bearer handling and IP connectivity are as follows:

• Default EPS Bearer Context Activation
  
• EPS Bearer Context Deactivation
  
• Dedicated EPS Bearer Context Activation
  
• EPS Bearer Context Modification
  
• UE Requested PDN Connectivity
  
• UE Requested Disconnect
  
• UE Requested Bearer Resource Allocation
  
• UE Requested Bearer Resource Modification

All these procedures can be performed only after a NAS connection is established (ECM-Connected). The Default EPS Bearer setup is performed during the EMM Attach procedure.

ESM defines two main states for the UE:

Bearer Context Inactive
When there is no EPS Bearer for the UE.

Bearer Context Active
When there is at least EPS Bearer for the UE. An EPS Bearer Context can exist even if no physical resources are allocated to the UE (i.e. Radio and S1 Bearers).

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