5G Core Solution
20.2.0
Mobility Management
Issue Date
2020-01-23
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Mobility Management
Contents
1 Mobility Management
1
1.1 Involved NFs
1
1.2 Mobility Management Objects
1
1.2.1 UE Identifiers
1
1.2.2 UE Status
2
1.3 Mobility Management Procedure
3
1.4 Mobility Restrictions
4
1.5 Mobility Management Example
5
1.6 Conclusion
5
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Mobility Management
1 Mobility Management
Mobile communication is evolving from 4G to 5G. The network architecture and service types have changed greatly. 5G
mobility management must adapt to changes in the network architecture and meet diversified service requirements. This
chapter introduces 5G mobility management based on the comparison and analysis of mobility management principles on
4G and 5G networks.
1.1 Involved NFs
We have learned that both the Access and Mobility Management function (AMF) in the 5G service based architecture
(SBA) and mobility management entity (MME) on a 4G network can provide the access and mobility management
functions. The following figure shows a comparison of 4G and 5G network architecture.
The 5G system (5GS) architecture provides
fixed mobile convergence (FMC) to allow the AMF to provide services for both 3GPP and non-3GPP access networks,
ensuring subscriber service experience. On a 4G network, when a UE accesses a core network through an untrusted non3GPP network, the UE is directly connected to the gateway through the ePDG, and the UE is not connected to the MME for
access and mobility management. However, when the 3GPP and non-3GPP access networks used by a 5G UE belong to the
same PLMN, the same AMF can perform mobility management for the UE, providing seamless services of the UE. The
following figure shows the differences of 4G and 5G architectures over non-3GPP access.
To provide FMC and improve subscriber experience, the 5G network allocates and uses the same temporary identity for a
UE on 3GPP and non-3GPP access networks, introduces UE connection management status, and flexibly defines service
request procedures. The following describes differences between 5G and 4G networks.
1.2 Mobility Management Objects
1.2.1 UE Identifiers
UE identifiers are involved when a network performs mobility management on a UE. The UE identifiers used on a 5G
network are as follows:
• Subscription permanent identifier (SUPI): An SUPI is globally unique and identifies a subscriber on the entire
network. An IMSI is used for 3GPP access, and a network access identifier (NAI) is used for non-3GPP access.
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• Permanent equipment identifier (PEI): A PEI is globally unique and identifies a device on a 5G network.
• 5G-GUTI: A temporary identifier is allocated by the core network. It is used to prevent the transmission of a
permanent identifier on a network and prevent attackers from tracking the location and activity status of the UE.
The following table compares UE identifiers between 4G and 5G networks.
Table 1 5G and 4G UE identifiers
Category
UE permanent identifi
5G Identifier
4G Identifier
3GPP Access
Non-3GPP Access
3GPP Access
Non-3GPP Access
IMSI
NAI
IMSI
NAI
N/A
IMEI
N/A
N/A
er
UE permanent equipm PEI
ent identifier
UE temporary identifi
5G-GUTI
EPS-GUTI
er
<5G-GUTI> :=
<GUTI>:= <GUMME
<GUAMI> <5G-TMSI>
I><M-TMSI>
5G protocols optimize security of UE identity. A network does not directly transmit a SUPI, but uses an encrypted SUPI,
that is, subscription concealed identifier (SUCI). The AMF can obtain an SUPI after a security procedure is complete.
After 5G introduces network slicing, the format of the temporary identifier of a UE changes. When the UE moves between
the 5GS and E-UTRAN, the 5G-GUTI needs to be mapped to the EPS-GUTI or the EPS-GUTI needs to be mapped to the
5G-GUTI according to the mapping shown in the following figure, and the mapped EPS-GUTI or 5G-GUTI is sent to the
AMF or MME in the corresponding message.
The following figure compares 5G-GUTI and EPS-GUTI.
The concept of AMF set is introduced into the 5G UE temporary identifier. An AMF set consists of some AMFs that serve a
given region and a network slice. An AMF region consists of one or more AMF sets. After 5G introduces the concept of
network slicing, the AMFs in a region are divided into different sets according to the capability to support network slicing.
The AMFs in a set have the same capability (equivalent to a 4G MME pool).
1.2.2 UE Status
The mobility management performed by a network for UEs varies with the UE status. On a 4G network, UE status is
classified into EPS mobility management (EMM) and EPS connection management (ECM). On a 5G network, UE status is
classified into registration management (RM) and connection management (CM). The following figure shows the
differences between the management status of 4G and 5G.
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The RRC Inactive state is added to 5G CM states so that a network can quickly recover a link when data needs to be
transmitted, and to meet power saving requirements for terminals. In this case, the RAN and CN are still in connected
mode, and the signaling between the UE and RAN is released, as shown in the following figure.
1.3 Mobility Management Procedure
In a mobile communications system, a UE or a mobile device is always in a constantly moving state. The core network
needs to change the state of the UE according to the service condition of the UE, and ensure that the UE location
information can be obtained and data transmission continuity can be ensured when the UE moves. These functions need to
be supported using a mobility management procedure.
Table 1 Differences between 4G and 5G mobility management procedures.
5G Mobility Management Procedure of 5G
4G Mobility Management
Function
Procedure
Registration
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Initial registration
Attach
The UE accesses a network.
Mobility registration update
TAU
The network updates the TA
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5G Mobility Management Procedure of 5G
4G Mobility Management
Function
Procedure
and related parameters of the
UE.
Periodic registration update
Periodic TAU
The UE periodically updates
the location.
AN Release
S1 release
The UE enters the idle state.
Service request
Service request
The UE enters the connected
mode.
Handover
Handover
The UE in the connected stat
e is successfully handed ove
r to a new cell.
De-registration
Detach
The UE location information
is deleted from the network.
The mobility management procedure of 5G is similar to that of 4G. Some 5G procedures, such as the AN release procedure
and deregistration procedure, are similar to those of 4G. The only difference with these procedures is the names. The other
differences between mobility management procedures are as follows:
• The attach, TAU, and periodic TAU procedures on a 4G network are all implemented using a registration procedure
on a 5G network. The registration types are different, namely, initial registration, mobility registration update, and
periodic registration update.
• Only some PDU sessions can be activated during a service request procedure of 5G. When the UE is in the connected
state, the service request procedure of 5G can also be used to activate certain PDU sessions and establish user plane
connections for data transmission.
1.4 Mobility Restrictions
Network mobility management must ensure that the UE is reachable and the data transmission is continuous. In addition,
the UE must be prevented from accessing certain forbidden areas or requesting services that are not authorized. Mobility
Restrictions include the access restrictions and mobility restrictions. Mobility restrictions only apply to 3GPP access, they
do not apply to non-3GPP access.
4G mobility restrictions are enforced using some optional features. 5G protocols define multiple mobility restrictions:
forbidden area, RAT restriction, service area restriction, and core network type restriction.
• Forbidden area: In a forbidden area, a UE is not permitted to send messages over the network.
• RAT restriction: It defines the RAT that a UE is not allowed to access.
• Service area restriction: It defines the allowed areas or non-allowed areas. The UE can access a network in the allowed
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area. The UE can initiate periodic updates and registration requests but cannot initiate SR or any session-related
signaling procedures in the non-allowed area.
• A service area restriction contains one or more complete tracking areas (TAs).
1.5 Mobility Management Example
The following figure shows the main procedures involved in the mobility management of UEs during the daily use of UEs.
1.6 Conclusion
Compared with 4G mobility management, 5G mobility management has changed in terms of UE identifiers, UE status, and
management procedures to adapt to changes in the network architecture and meet diversified service requirements.
A 5G network supports both 3GPP access and non-3GPP access, and the AMF provides access and mobility management
services. The UE uses a same temporary identifier in both 3GPP access and non-3GPP access, and it contains AMF set
information for network slicing. 5G mobility management inherits the 4G CM status: idle and connected states, and also
introduces the RRC Inactive state to the UEs to meet high power saving requirements. 5G mobility management simplifies
the management procedures. The attach, TAU, and periodic TAU procedures are completed by a registration procedure on a
5G network, and the registration types are different.
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