MME: Functions and Interfaces

Summary:

This article deals with one of the important element of the EPC architecture, MME (Mobility Management Entity). It also describes some of the interfaces of the MME with the other network elements.

How ever this article can not describe the full functionalities performed by the MME during the different EPS procedures.

MME (Mobility Management Entity):

The MME is the key control-node for the LTE access-network. It is responsible for idle mode UE (User Equipment) tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a UE at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. 

It is responsible for authenticating the user (by interacting with the HSS). The Non Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to UEs. It checks the authorization of the UE to camp on the service provider’s Public Land Mobile Network (PLMN) and enforces UE roaming restrictions. 

The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. 

The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming UEs.

MME is the control plane entity within EPS supporting functions as listed below.

Mobility Management,

- NAS signalling and security;

- Inter CN node signalling for mobility between 3GPP access networks;

- Tracking Area list management;

- PDN GW and Serving GW selection;

- SGSN selection for handovers to 2G or 3G 3GPP access networks;

- Roaming ;

- Authentication;

- Bearer management functions including dedicated bearer establishment.

- Lawful Interception of signalling traffic.

In order to support 3GPP2 access, MME supports:

- HRPD access node selection and maintenance for handovers to HRPD;

- Transparent transfer of HRPD signaling messages and transfer of status information between E-UTRAN and HRPD access.

The procedures for information transfer between the SGSN, the MME and the HSS are defined in TS 23.401 and TS 23.060.

The CS fallback enabled MME supports the following additional functions according to TS 23.272:

- Deriving a VLR number and LAI out of the TAI

- Maintaining of SGs association towards MSC/VLR for EPS/IMSI attached UE

- Initiating IMSI detach at EPS detach

- Initiating paging procedure towards eNodeB when MSC pages the UE for CS services

- Supporting SMS procedures for CS Fallback.

- Support CS Fallback interface and related functions for 1xRTT CDMA access.

When the MME supports the interworking to 3GPP CS, the MME supports the following functions as specified in TS 23.216 [83]:

- Performing the PS bearer splitting function by separating the voice PS bearer from the non-voice PS bearers.

- Handling the non-voice PS bearers handover with the target cell as according to Inter RAT handover procedure as defined in TS 23.401 [76].

- Initiating the SRVCC handover procedure for handover of the voice component to the target cell.

- Coordinating PS handover and SRVCC handover procedures when both procedures are performed,

- support interworking and SRVCC related functions for 1xRTT CDMA access.

The Basic Interfaces of MME:

1: Interface between MSC/VLR and MME (SGs-interface) for EPC

The SGs interface is the reference point between the MME and MSC/VLR which is used for the mobility management and paging procedures between EPS and CS domain, and it is based on the Gs interface procedures. The SGs interface is used to provide Fallback for Location Services (LCS) and Call Independent Supplementary Services (SS). The SGs interface is also used for the delivery of both mobile originating and mobile terminating SMS over EUTRAN in case SMSIP is not used.

CS Fallback function and SMS delivery via the CS core network is realized by reusing Gs interface mechanisms as defined in 3GPP TS 29.018 on the interface between the MME in the EPS and the VLR. This interface is called SGs interface. The SGs interface connects the databases in the VLR and the MME.

The basis for the interworking between a VLR and an MME is the existence of a SGs association between those entities per UE. The SGs association is applicable to UEs with CS Fallback capability activated and to UEs configured for SMS delivery via the CS core network.

SGs interface is a logical interface between the MME and the VLR. All the SGsAP messages described require an SCTP association between the MME and the VLR.

The signaling on this interface uses SCTP. The SGs interface is defined in TS 23.272 and TS 29.118.

2: Interface between 3GPP MSC Server and MME or SGSN (Sv-interface)

The Sv reference point provides SRVCC support for functions like sending Forward Relocation Message between MME or SGSN and 3GPP MSC Server. This interface is specified in TS 29.280.The Sv messages are based on GTP. The Sv application defines a set of messages between the MME/SGSN and MSC Server to provide SRVCC as defined in 3GPP TS 23.216 . The Sv message header is defined in 3GPP TS 29.274. The message format is coded as per GTP in 3GPP TS 29.274.

3: Interface between MME and E-UTRAN (S1-MME-interface)

Reference point for the control plane protocol between E-UTRAN and MME. This interface is defined in the TS 36.41x-series of Technical Specifications and in TS 24.301.

The non-access stratum (NAS) protocols form the highest stratum of the control plane between the user equipment (UE) and MME. NAS protocols support the mobility of the UE and the session management procedures to establish and maintain IP connectivity between the UE and a PDN GW. They define the rules for a mapping between parameters during inter-system mobility with 3G networks. They also provide the NAS security by integrity protection and ciphering of NAS signaling messages. EPS provides the subscriber with a "ready-to-use" IP connectivity and an "always-on" experience by linking between mobility

management and session management procedures during the UE attach procedure.

Complete NAS transactions consist of specific sequences of elementary procedures with EPS Mobility Management (EMM) and EPS Session Management (ESM) protocols.

EMM (EPS Mobility Management): The EPS Mobility Management (EMM) protocol provides procedures for the control of mobility when the User Equipment (UE) uses the Evolved UMTS Terrestrial Radio Access Network (E-UTRAN). It also provides control of security for the NAS protocols.

EMM involves different types of procedures such as:

1:= EMM common procedures -- can always be initiated whilst a NAS signalling connection exists. The procedures belonging to this type are initiated by the network. They include GUTI reallocation, Authentication, Security mode control, Identification, EMM information.

2:= EMM specific procedures -- specific to the UE only. At any time only one UE-initiated EMM specific procedure can run. The procedures belonging to this type are Attach and combined attach, Detach or combined detach, Normal tracking area update and combined tracking area update (S1 mode only), Periodic tracking area update (S1 mode only).

3:= EMM connection management procedures -- manage the connection of UE with network:

ESM (EPS Session Management): The EPS Session Management (ESM) protocol provides procedures for the handling of EPS bearer contexts. Together with the bearer control provided by the access stratum, it provides the control of user plane bearers. The transmission of ESM messages is suspended during EMM procedures except for the attach procedure.

1:= EPS bearer contexts procedures -- initiated by the network and are used for the manipulation of EPS bearer contexts, including Default EPS bearer context activation, Dedicated EPS bearer context activation, EPS bearer context modification, EPS bearer context deactivation.

2:= Transaction related procedures -- initiated by the UE to request for resources, i.e. a new PDN connection or dedicated bearer resources, or to release these resources. They include PDN connectivity procedure, PDN disconnect procedure, Bearer resource allocation procedure, Bearer resource modification procedure.

4:= Interface between MME and HSS (S6a-interface)

This interface is used to exchange the data related to the location of the mobile station and to the management of the subscriber. The main service provided to the mobile subscriber is the capability to transfer packet data within the whole service area. The MME informs the HSS of the location of a mobile station managed by the latter. The HSS sends to the MME all the data needed to support the service to the mobile subscriber. Exchanges of data may occur when the mobile subscriber requires a particular service, when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means.

The S6a interface enables the transfer of subscriber related data between the MME and the HSS.

Signaling on this interface uses Diameter S6a/S6d Application as specified in TS 29.272.

5:= Interface between MME and S-GW (S11-interface)

This interface is used to support mobility and bearer management between the MME and S-GW. This interface is specified in TS 29.274.

6:= Interface between MME and MME (S10-interface)

This interface is used to support user information transfer and MME relocation support between the MMEs. This interface is specified in TS 29.274.

7:= Interface between MME and EIR (S13-interface)

This interface is used between MME and EIR to exchange data, in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station.

The S13 interface shall enable the ME Identity check procedure between the MME and the EIR as described in the 3GPP TS 23.401.

This Mobile Equipment Identity Check Procedure shall be used between the MME and the EIR and between the SGSN and the EIR to check the Mobile Equipment's identity status (e.g. to check that it has not been stolen, or, to verify that it does not have faults).

Signaling on this interface uses the Diameter S13 Application in TS 29.272.

8:= Interface between MME and SGSN (S3-interface)

This interface enables user and bearer information exchange for inter 3GPP access network mobility in idle and/or active state. This interface is specified in TS 29.274.

4:= Interface between MME and HSS (S6a-interface)

This interface is used to exchange the data related to the location of the mobile station and to the management of the subscriber. The main service provided to the mobile subscriber is the capability to transfer packet data within the whole service area. The MME informs the HSS of the location of a mobile station managed by the latter. The HSS sends to the MME all the data needed to support the service to the mobile subscriber. Exchanges of data may occur when the mobile subscriber requires a particular service, when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means.

The S6a interface enables the transfer of subscriber related data between the MME and the HSS.

Signaling on this interface uses Diameter S6a/S6d Application as specified in TS 29.272.

5:= Interface between MME and S-GW (S11-interface)

This interface is used to support mobility and bearer management between the MME and S-GW. This interface is specified in TS 29.274.

6:= Interface between MME and MME (S10-interface)

This interface is used to support user information transfer and MME relocation support between the MMEs. This interface is specified in TS 29.274.

7:= Interface between MME and EIR (S13-interface)

This interface is used between MME and EIR to exchange data, in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station.

The S13 interface shall enable the ME Identity check procedure between the MME and the EIR as described in the 3GPP TS 23.401.

This Mobile Equipment Identity Check Procedure shall be used between the MME and the EIR and between the SGSN and the EIR to check the Mobile Equipment's identity status (e.g. to check that it has not been stolen, or, to verify that it does not have faults).

Signaling on this interface uses the Diameter S13 Application in TS 29.272.

8:= Interface between MME and SGSN (S3-interface)

This interface enables user and bearer information exchange for inter 3GPP access network mobility in idle and/or active state. This interface is specified in TS 29.274.

9:= Reference point between HRPD AN and MME (S101-reference point)

The S101 interface supports procedures for Pre-Registration, Session Maintenance and Active handovers between E UTRAN and HRPD networks. This is based on tunneling over S101 signaling of one technology while the UE is in the other technology as specified in TS 23.402. The HRPD air interface messages tunneled over S101 in E UTRAN to HRPD mobility are defined in 3GPP2 C.S0087 0.

10:= Reference point between 3GPP2 1xCS IWS and MME (S102-reference point)

The S102 reference point provides a tunnel between MME and 3GPP2 1xCS IWS to relay 3GPP2 1xCS signaling messages in order to support SRVCC as specified in TS 23.216 and CS Fallback as specified in TS 23.272. 1x CS signaling messages are those messages that are defined for A21 interface as described in 3GPP2 A.S0008-C.

In case of CS Fallback to 1x CS network as specified in TS 23.272, S102 Tunnel Redirection Procedure is used when the UE perform Tracking Area Update with MME change (as specified in TS 23.402 in case of S101 tunnel redirection) while the UE is registered with the 1xRTT CS domain and the S102 session exists between the MME and the 1xCS IWS.

11:= Interface between GMLC and MME (SLg-interface)

The SLg interface is used by the GMLC to communicate with MMEs. The SLg interface is defined in TS 29.172.

12:= Interface between MME and E-SMLC (SLs-interface)

The SLs interface is used to convey LCS-AP messages and parameters between the MME to the E-SMLC. It is also used for tunneling LTE Positioning Protocols (LPP between the E-SMLC and the target UE, LPPa between the E-SMLC and the eNB), which are transparent to the MME as described in 3GPP TS 36.305.

The SLs interface is defined in TS 29.171.

SCTP (see IETF RFC 4960) shall be supported as the transport layer of LCS-AP messages. The MME shall establish the SCTP association.

The port number (9082) and the payload protocol identifier (29) for LCS-AP is assigned by IANA.

(http://www.t1shopper.com/tools/port-number/9082/ and http://www.iana.org/assignments/sctp-parameters)

13:= Interface between the CBC and MME (SBc Interface)

Reference point between CBC and MME for warning message delivery and control functions. This interface is specified in TS 29.168.

SBc-AP interface is a logical interface between the MME and the CBC. All the SBc-AP messages require an SCTP association between the MME and the CBC.

Multiple local SCTP endpoints may be supported. Multiple remote SCTP endpoints shall be supported. When multiple local or remote SCTP endpoints are configured, several simultaneous SCTP associations shall be supported between MME and CBC.

The CBC shall establish the SCTP association. The registered port number for SBc-AP is 29168.The registered payload protocol identifier for SBc-AP is 24.

This is a short description of the  interfaces that the MME handles.
 

References:

TS 23.401: 3rd Generation Partnership Project;
Technical Specification Group Services and System Aspects;
General Packet Radio Service (GPRS) enhancements for
Evolved Universal Terrestrial Radio Access Network
(E-UTRAN) access
, 

TS 23.402 : 3rd Generation Partnership Project;
Technical Specification Group Services and System Aspects;
Architecture enhancements for non-3GPP accesses

TS 36.300: 3rd Generation Partnership Project;
Technical Specification Group Radio Access Network;
Evolved Universal Terrestrial Radio Access (E-UTRA)
and Evolved Universal Terrestrial Radio Access Network
(E-UTRAN);
Overall description;
Stage 2

TS 23.002: 3rd Generation Partnership Project;
Technical Specification Group Services and Systems Aspects;
Network architecture
 

TS 29.274: 3rd Generation Partnership Project;
Technical Specification Group Core Network and Terminals;
3GPP Evolved Packet System (EPS);
Evolved General Packet Radio Service (GPRS)
Tunnelling Protocol for Control plane (GTPv2-C);
Stage 3

TS 29.276:3rd Generation Partnership Project;
Technical Specification Group Core Network and Terminals;
3GPP Evolved Packet System (EPS);
Optimized Handover Procedures and Protocols between
E-UTRAN access and cdma2000 HRPD Access;
Stage 3

TS 29.171: 3rd Generation Partnership Project;
Technical Specification Group Core Network and Terminals;
Location Services (LCS);
LCS Application Protocol (LCS-AP) between the
Mobile Management Entity (MME) and
Evolved Serving Mobile Location Centre (E-SMLC);
SLs interface