Mobility Management and Connection Management in LTE/LTE-Advanced (Reference 3GPP TS 23.401, Release 10)

Acronyms

GERAN (GPRS EDGE Radio Access Network)

UTRAN (UMTS Terrestrial Radio Access Network)

E-UTRAN (Evolved UMTS Terrestrial Radio Access Network)

EPC (Evolved Packet Core)

EPS (Evolved Packet System)

UE (User Equipment)

eNB (Evolved Node B)

MME (Mobility Management Entity)

SGSN (Serving GPRS Support Node)

GGSN (Gateway GPRS Support Node)

EMM (EPS Mobility Management)

ECM (EPS Connection Management)

RRC (Radio Resource Control)

RAT (Radio Access Technology)

TA (Tracking Area), same as Location Area in GSM/GPRS

TAU (Tracking Area Update), same as Location Area Update in GSM/GPRS

RAU (Routing Area Update)

PDN (Packet Data Network)

AKA (Authentication and Key Agreement)

S1 Interface (Interface Between eNB and MME/SGW)

S1-MME Interface (Interface Between eNB and MME)

S1-U Interface (Interface Between eNB and SGW)

EMM (EPS Mobility Management)

EMM states describe the mobility management states that result from the mobility management procedures e.g Attach and Tracking Area Update procedures.

Following are two types of EMM States

• EMM-Registered :

• The UE enters this state by a successful registration with an Attach procedure to either E-UTRAN or GERAN/UTRAN.

• The MME enters this state by a successful Tracking Area Update procedure for a UE selecting an E-UTRAN cell from GERAN/UTRAN or by an Attach procedure via E-UTRAN

• In this state, the UE can receive services that require registration in the EPS. The UE location is known in the MME to at least an accuracy of the tracking area list allocated to that UE

• In this state, the UE shall always have

  • at least one active PDN connection

  • setup the EPS security context

• EMM-Deregistered

  • EMM context in MME holds no valid location or routing information for the UE.

  • The UE is not reachable by a MME, as the UE location is not known

  • In this state, some UE context can still be stored in the UE and MME, e.g. to avoid running an

    AKA procedure during every Attach procedure

  • During the successful Inter-RAT TAU/RAU/handover procedure and ISR activated is not indicated to the UE, the old S4 SGSN/old MME changes the EMM state of the UE to GPRS-IDLE/PMM-DETACHED/EMM-DEREGISTERED.

EMM State Transitions in UE and MME

EMM State Model in UE

EMM State Model in MME

ECM (EPS Connection Management)

ECM states describe the signalling connectivity between the UE and EPC

Following are two types of EMM States

• ECM-IDLE

  • A UE is in this state when no NAS signalling connection between UE and network exists.

  • In this state, a UE performs cell selection/reselection according to TS 36.304 [34] and PLMN selection according to TS 23.122 [10].

  • There exists no UE context in E-UTRAN for the UE in this state.

  • There is no S1-MME and no S1-U connection for the UE in the ECM-IDLE state.

  • The UE and the MME shall enter the ECM-CONNECTED state when the signalling connection is established between the UE and the MME.

  • Initial NAS messages that initiate a transition from ECM-IDLE to ECM-CONNECTED state are Attach Request, Tracking Area Update Request, Service Request or Detach Request.

• ECM-Connected

  • The UE location is known in the MME with an accuracy of a serving eNodeB ID

  • The mobility of UE is handled by the handover procedure

  • For a UE in this state, there exists a signalling connection between the UE and the MME. The signalling connection is made up of an RRC connection (UE - eNB) and an S1_MME (eNB - MME) connection.

  • The UE shall enter the ECM-IDLE state when its signalling connection to the MME has been released or broken. This release or failure is explicitly indicated by the eNodeB to the UE or detected by the UE.

  • The S1 release procedure changes the state at both UE and MME from ECM-CONNECTED to ECM-IDLE. (NOTE: The UE may not receive the indication for the S1 release, e.g. due to radio link error or out of coverage. In this case, there can be temporal mismatch between the ECM-state in the UE and the ECM-state in the MME)

  • After a signalling procedure, the MME may decide to release the signalling connection to the UE, after which the state at both the UE and the MME is changed to ECM-IDLE.

  • When a UE changes to ECM-CONNECTED state and if a radio bearer cannot be established, or the UE cannot maintain a bearer in the ECM-CONNECTED state during handovers, the corresponding EPS bearer is deactivated.

ECM State Transitions in UE and MME

ECM State Model in UE

ECM State Model in MME

Functionality of E-UTRAN Logical Entities

Functionality of E-UTRAN

As shown in this post, E-UTRAN can be explained as E-UTRAN logical nodes and E-UTRAN interfaces

E-UTRAN Logical Nodes

1. eNB (evolved NodeB)
2. MME (Mobility Management Entity)
3. S-GW (Serving Gateway)

E-UTRAN Interfaces

1. X2 Interface (Interface between eNB's)

2. S1 Interface

   1. S1-MME Interface (Interface between eNB and MME)

   2. S1-U Interface (Interface between eNB and S-GW)

eNB Functionality

1. Functions for Radio Resource Management: Radio Bearer Control, Radio Admission Control, Connection

2. Mobility Control, Dynamic allocation of resources to UEs in both uplink and downlink (scheduling)

3. IP header compression and encryption of user data stream

4. Selection of an MME at UE attachment when no routing to an MME can be determined from the information

5. provided by the UE

6. Routing of User Plane data towards Serving Gateway

7. Scheduling and transmission of paging messages (originated from the MME)

8. Scheduling and transmission of broadcast information (originated from the MME or O&M)

9. Measurement and measurement reporting configuration for mobility and scheduling

10. Scheduling and transmission of PWS (which includes ETWS and CMAS) messages (originated from the MME)

11. CSG (Closed Subscriber Group) handling

12. Transport level packet marking in the uplink

MME Functionality

1. NAS signalling

2. NAS signalling security

3. AS Security control

4. Inter CN node signalling for mobility between 3GPP access networks

5. Idle mode UE Reachability (including control and execution of paging retransmission)

6. Tracking Area list management (for UE in idle and active mode)

7. PDN GW and Serving GW selection

8. MME selection for handovers with MME change

9. SGSN selection for handovers to 2G or 3G 3GPP access networks

10. Roaming

11. Authentication

12. Bearer management functions including dedicated bearer establishment

13. Support for PWS (which includes ETWS and CMAS) message transmission

14. Optionally performing paging optimization

S-GW Functionality

1. The local Mobility Anchor point for inter-eNB handover

2. Mobility anchoring for inter-3GPP mobility

3. E-UTRAN idle mode downlink packet buffering and initiation of network triggered service request procedure

4. Lawful Interception

5. Packet routing and forwarding

6. Transport level packet marking in the uplink and the downlink

7. Accounting on user and QCI granularity for inter-operator charging

8. UL and DL charging per UE, PDN, and QCI

Functionality and other details of the Interfaces will be explained in next post

LTE E-UTRAN Atchtecture

E-UTRAN Architecture (Evolved-UMTS Terrestrial Radio Access Network)

eNB - Evolved NodeB
S-GW - Serving Gateway
MME - Mobility Management Entity