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Mobility Management Entity Overview

Features and Functionality - Base Software ▀

Cisco ASR 5000 Series Product Overview ▄

OL-22938-02

If the HSS provides an APN of a PDN and the subscription allows for allocation of a PDN GW from the visited PLMN
for this APN, the PDN GW selection function derives a PDN GW address from the visited PLMN. If a visited PDN GW
address cannot be derived, or if the subscription does not allow for allocation of a PDN GW from the visited PLMN,
then the APN is used to derive a PDN GW address from the HPLMN.

Serving Gateway (S-GW) Selection

The Serving GW selection function selects an available Serving GW to serve a UE. This feature reduces the probability
of changing the Serving Gateway and a load balancing between Serving Gateways The MME uses DNS procedure to
for S-GW selection.

S-GW selection chooses an available S-GW to serve a UE. The selection is based on network topology, i.e. the selected
S-GW serves the UE‘s location and in the case of overlapping S-GW service areas, the selection may prefer S-GWs
with service areas that reduce the probability of changing the Serving GW. If a subscriber of a GTP only network roams
into a P-MIP network, the PDN GWs selected for local breakout supports the P-MIP protocol, while P-GWs for home
routed traffic use GTP. This means the S-GW selected for such subscribers may need to support both GTP and PMIP, so
that it is possible to set up both local breakout and home routed sessions for these subscribers.

3GPP R8 Identity Support

Provides the identity allocation of following type:

EPS Bearer Identity

Globally Unique Temporary UE Identity (GUTI)

Tracking Area Identity (TAI)

MME S1-AP UE Identity (MME S1-AP UE ID)

EPS Bearer Identity

: An EPS bearer identity uniquely identifies EPS bearers within a user session for

attachment to the E-UTRAN access and EPC core networks. The EPS Bearer Identity is allocated by the MME.
There is a one to one mapping between EPS Radio Bearers via the E-UTRAN radio access network and EPS
Bearers via the S1-MME interface between the eNodeB and MME. There is also a one-to-one mapping
between EPS Radio Bearer Identity via the S1 and X2 interfaces and the EPS Bearer Identity assigned by the
MME.

Globally Unique Temporary UE Identity (GUTI)

: The MME allocates a Globally Unique Temporary Identity

(GUTI) to the UE. A GUTI has; 1) unique identity for MME which allocated the GUTI; and 2) the unique
identity of the UE within the MME that allocated the GUTI.

Within the MME, the mobile is identified by the M-TMSI.

The Globally Unique MME Identifier (GUMMEI) is constructed from MCC, MNC and MME Identifier (MMEI). In
turn the MMEI is constructed from an MME Group ID (MMEGI) and an MME Code (MMEC).

The GUTI is constructed from the GUMMEI and the M-TMSI.

For paging, the mobile is paged with the S-TMSI. The S-TMSI is constructed from the MMEC and the M-TMSI.

The operator needs to ensure that the MMEC is unique within the MME pool area and, if overlapping pool areas are in
use, unique within the area of overlapping MME pools.

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