EPC Operations

4.iii

CONTENTS CONTENTS

Dedicated B earer C reation . . . .4.26 Connected Mode Procedures. . . .4.27 Direct and Indirect Forwarding. . . .4.28 Intra E-UTRAN Handover (X2-based) . . . .4.29

Intra-EUTRAN Handover (S1-based, no forwarding). . . .4.30 Intra-EUTRAN Handover (S1-based, with indirect forwarding) . . . .4.31 Intra-EUTRAN Handover (S1-based, with S-GW Change) . . . .4.32 Intra-EUTRAN Handover (S1-based, with MME Change) . . . .4.33 Intra-EUTRAN Handover (S1-based, MME change, S-GW change, forwarding) . . . .4.34

Inter-RAT Handover . . . .4.35 Inter-RAT Handover (E-UTRAN-UTRAN). . . .4.36 EPC Detach .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .. .4.37 UE-initiated Detach. . . .4.38

LTE Evolved Packet Core Network

4.iv

At the end of this section you will be able to:

OBJECTIVES OBJECTIVES

■ describe the set of ‘state machines’ employed within the EPS

■ outline the roles of the EPS Mobility Management and EPS Connection Management state machines

■ discuss the concept of the EPS Bearer Context and the EPS Session Management state machine

■ describe the EPS network selection and Attach processes

■ outline the set of activities required to establish a default EPS bearer and discuss the reasoning behind the decision to make this a stage in the attach process

■ discuss the roles played by various devices involved in EPS device selection including the role played by DNS

■ outline the set of functions a UE will perform when in idle mode

■ describe the functions performed by the EPC in support of UEs in Idle Mode, including ISR

■ outline the set of activities related to the TAU process

■ discuss the activities performed to allow UEs to be paged

■ describe the activities related to S1 Release and Downlink Data Notification

■ describe the actions performed by the MME and HSS in support of UE Reachability

■ discuss the use of the Service Request process and its relationship to the modify and create bearer functions

■ outline the processes used to establish CS Fallback services for EPS attached subscribers

■ outline the EPC’s support for charging

■ describe the functions that enable connected mode mobility management to operate

■ outline the processes employed to support various handover scenarios including intra-E-UTRAN and inter-RAT handover

■ describe the procedures employed to detach a UE from the EPS

LTE Evolved Packet Core Network

4.vi

LT3604/v4.0 © Wray Castle Limited 4.1

EPC Operations

NAS Protocols and UE States NAS Protocols and UE States

The RRC protocol controls radio communications between the UE and the eNB. A UE’s current connection capabilities are associated with its RRC state, which can take two values from the point of view of the eNB and the access stratum, namely RRC_Idle and RRC_Connected, which describe whether the mobile is on standby or actively communicating with the eNB.

Two NAS protocols control higher-level communications between the UE and the EPC. The ESM protocol manages EPS Bearers and Bearer Contexts. Each individual EPS Bearer Context can be in one of two states: active if it has been set up, and inactive otherwise.

The EMM protocol handles other issues, such as location management and security. It is associated with two state diagrams. The ECM (EPS Connection Management) state diagram mirrors the RRC state diagram, but from the viewpoint of the MME and the non access stratum. The EMM state diagram describes whether or not the UE is registered with an MME.

In order to offer effective service to UEs, the EPS needs to be able to define and keep track of the availability and reachability of each terminal. It achieves this by maintaining two sets of ‘contexts’ for each UE – an EMM context and an ECM context – each of which is handled by ‘state machines’ located in the UE and the MME.

The EMM state machine tracks the UE’s network registration status (registered or unregistered) and therefore provides overall details on the UE’s reachability. The ECM state machine tracks the status of the NAS connection that exists between a UE and its serving MME (idle or connected) and indicates whether it is currently possible or not possible to exchange NAS messages, forwarded by the NAS EMM and ESM protocols, with the UE.

A further state machine operates in the UE and serving eNB to track the terminal’s RRC state, which can be either RRC-IDLE (which relates to a UE in idle mode) or RRC-CONNECTED (which relates to a UE with an active traffic bearer).

Further Reading: 3GPP TS323.401:4.6, 24.301:4.2; TS 23.401:4.6 NAS

Radio Bearer GTP Tunnel GTP Tunnel

E-RAB

4.2

LTE Evolved Packet Core Network

EMM States EMM States

EMM is analogous to the MM (Mobility Management) processes undertaken in legacy networks and seeks to ensure that the MME maintains enough location data to be able to offer service to each UE when required. EMM Messages are created by the NAS EMM protocol and include Attach/Detach messages and those that manage GUTI allocation, Authentication and Security, Tracking Area Updates and general MME-UE communication.

The two EMM states maintained by the MME are EMM-DEREGISTERED and EMM-REGISTERED.

A UE in the EMM-DEREGISTERED state has no valid context stored in an MME, so its current location is unknown and paging and traffic routing cannot take place. This is generally consistent with a UE that is either powered off or is out of EPC-connected network coverage.

The EMM-REGISTERED state relates to UEs that have performed an EPS Attach, a Combined IMSI/

EPS Attach or a TAU and for which the MME maintains a valid context. In this state the UE will have been assigned an M-TMSI and will be performing TAU functions when necessary. This means that the MME knows the UE’s location (at least to the current TA level) and can page and route traffic for it.

A UE in the EMM-REGISTERED state will have at least one active EPS bearer (the ‘always-on’ initial or default bearer) and as a result will have at least one IP address.

In order for a UE in ECM-Idle state to perform an Explicit Detach and move from EMM-Registered to EMM-DEREGISTERED, it must first move to ECM-Connected state to ensure that a signalling bearer is available to carry the Detach message.

Further Reading: 3GPP TS 23.401:4.6.2 MME

EMM-Registered

EMM-Registered EMM-Deregistered

EMM-Deregistered

Detach, Attach Reject

TAU Reject All Bearers deactivated

Attach accept, TAU accept

Detach, Attach Reject

TAU Reject

E-UTRAN interface switched off due to Non-3GPP handover All Bearers deactivated

Attach accept, TAU accept

LT3604/v4.0 © Wray Castle Limited 4.3

EPC Operations

ECM States ECM States

The ECM states describe a UE’s current NAS connectivity status with the EPC, i.e. whether a NAS signalling connection exists between a UE and an MME.

There are two ECM states, ECM-IDLE and ECM-CONNECTED.

A UE in ECM-IDLE has no S1-MME active and no active NAS connection to an MME, although UE and Bearer Contexts will be stored in the serving MME and other network elements. No NAS EMM or ESM signalling will be passing between the UE and the MME.

A UE in this state will perform network and cell selection/reselection and will send TAU messages, but has no RRC or S1 traffic bearers established. In ECM-IDLE the location of the UE is known by the MME only to the level of the current TA or TA List.

In the ECM-CONNECTED state a UE has established a NAS signalling relationship with an MME, which will know the UE’s location to the eNB level, not the current cell level. The UE’s Bearer Contexts will be activated and RRC and S1 transport resources will have been assigned to it.

A UE moves from ECM-IDLE to ECM-CONNECTED by sending a Service Request to the MME and also during functions such as Attach, TAU and Detach.

Further Reading: 3GPP TS 23.401:4.6.3 MME

ECM-Connected

ECM-Connected ECM-Idle

ECM-Idle

S1 connection released

S1 connection established

RRC connection released

RRC connection established

4.4

LTE Evolved Packet Core Network

Combining EPS States Combining EPS States

Although the EMM and ECM states are independent of each other they are related and any discussion of a UE’s reachability is best served by viewing these states in a combined fashion.

There are three main phases of UE activity, each of which can be described by a combination of EMM and ECM states.

In the UE Powered Off/Unreachable phase a UE is not contactable via the EPS and cannot use the EPS network’s services. This may be because the UE is powered off, has no signal, or is connected to a non-3GPP access network. This could be described as EMM-DEREGISTERED/ECM-IDLE.

RRC will also be idle.

In the UE Powered On but Idle phase a UE is powered on and has attached to the EPS network, but is idle. This could be described as EMM-REGISTERED/ECM-IDLE. RRC will also be idle.

In the UE with Active Traffic Connection phase the UE has an established EPS bearer over which traffic is flowing. This could be described as EMM-REGISTERED/ECM-CONNECTED. RRC will also be connected.

LTE Evolved Packet Core Network

EPS Bearers and

EPS Bearers and Bearer ContextsBearer Contexts

An EPS Bearer Context will be in the Inactive state if no resources have been assigned to it. Details of the UE’s subscribed PDN Connections (which will implicitly provide details of the PDN Connection’s Default EPS Bearer) will be stored in the user’s subscriber profile (held in the HSS, MME and SIM) but the EPS Bearer itself has not been activated.

An EPS Bearer Context will be in the Active state if the associated EPS Bearer has been established and resources have been assigned to it in the network.

An EPS Bearer Context will still be considered to be in the Active state even if the UE has moved to the ECM-Idle state and S1 Release has occurred. In this scenario, even though physical resources on the air and S1 interfaces have been released, details relating to the activated EPS Bearers are retained by the UE and the MME and the S5/S8 portion of the bearers remain active.

Further Reading: 3GPP TS24.301:6.1 No Bearer Context details held

PDN

EPC Operations

Attach and Registration Requirements Attach and Registration Requirements

As with legacy 3GPP systems, a UE can only access network services after it has performed an Attach and can only access IMS services once it has Registered.

An Attach is usually required when a UE is powered on or after it returns from a period outside of network coverage. Prior to the attach being initiated, the UE must perform either ‘stored information’ or ‘initial’ cell selection functions to allow it to determine the best available cell resource via which to connect.

The EPS specifications include the ‘stored information cell selection’ process that allows details of the

‘last used’ cell to be retained in the USIM so that the UE can attempt to reconnect quickly to that resource on power on. Stored last cell details must include the EARFCN (E-UTRAN Absolute Radio Frequency Channel Number) and may include other cell parameters such as PCI (Physical Cell ID).

If there are no stored cell details, or if the stored cell is unavailable, the UE must scan for available cells and perform an ‘initial cell selection’. Again, data stored on the USIM can aid this process allowing the UE to be instructed to search for preferred EARFCNs, preferred networks and preferred radio access technologies.

Cell selection is based on similar criteria to those employed in legacy systems – after searching at least a minimum number of carriers the UE will have compiled a list of ‘acceptable’ cells from which it will select a ‘suitable’ cell, which is the one it regards as offering the best service, and will attempt to attach.

Before attempting to attach the UE must check to ensure that the selected cell is not barred and that it meets the USIM access priority level indicated on the cell’s BCCH.

Further Reading: 3GPP TS 23.011; 36.300:10

Initial Cell selection:

Scan frequency bands, compile list of strongest allowed cells

Stored Information:

Last used EARFCN

4.8

LTE Evolved Packet Core Network

Device Selection Device Selection

Each EPS Bearer will be carried between or controlled by a specific set of devices. For an EPS Bearer established between the UE’s home E-UTRAN and home EPC, this set of devices will be an eNB, an MME, an S-GW and a PDN-GW. As each network can be expected to have a number of each type of device deployed to it, the methods by which the devices involved in serving a bearer are chosen must be clearly stated. Device selection for EPS connections operates as follows:

The UE selects the eNB to use based on air interface selection and reselection actions.

The eNB selects the MME to use from the MME Pool available based on load balancing principles and any current overload notifications. Load balancing is managed using the MME ‘weighting factor’, which is related to the MME’s capacity and is signalled to eNBs using the MME Relative Capacity information element in the MME Configuration message during S1 set-up. An MME with a capacity of 0 is not accepting connections, whilst an MME with a capacity of 255 has the highest capacity relative to other MMEs in its pool. The eNB does not select the MME in the case of MME Relocation, when a target MME is selected by the source MME. The SGSN is responsible for MME selection in the case of inter-RAT handover from GERAN/UTRAN to E-UTRAN.

The MME selects the S-GW to use from the set associated with the UE’s current TA and takes any current overload notifications into account. The MME may also take the UE’s current TA List into account, by selecting an S-GW that serves more than one of the TAs included on the list.

The PDN-GW is selected by the MME based on APN details stored in the user’s HSS subscription data or on the USIM. If no APN is specified the MME selects one based on its own configured data. It is common for network operators to ensure service resilience by deploying multiple instances of the same APN to different PDN-GWs; this ensures that if one PDN-GW fails the APN service can still continue.

Each UE will only be served by one MME, one S-GW and one PCRF at any one time (except for the brief but inevitable overlap that occurs during a relocation), each UE may be served by more than one PDN-GW if multiple PDN connections have been established. These restrictions continue to apply even if LIPA, SIPTO or Multi-Access PDN services have been invoked.

EPS-Attached UEs are able to roam onto GERAN or UTRAN cells whilst still maintaining connections to PDN-GW Access Points. It is not possible to simultaneously mix core network connectivity, however. If a UE is EPS Attached and has S5 or S8 connections (to one or more PDN-GWs) established, it is not possible for that UE to also be separately 2G/3G Attached and have Gn or Gp connections (to one or more GGSNs) established – the two core network services are mutually exclusive.

Further Reading: 3GPP TS 23.401:4.3.8 (Device Selection), 36.413:9.2.3 (MME Relative Capacity) UE

eNB

UE selects eNB based on air interface S and R criteria

MME

eNB selects MME from associated Pool based on load-balancing parameters

S-GW

MME selects S-GW from the set associated with UE’s current TA

PDN-GW

PDN-GW selected from the set that supports the indicated APN

DNS plays a key role in facilitating the selection of a PDN-GW during PDN Connection establishment and can also play a role in S-GW selection.

During PDN Connection establishment the MME determines the APN to use for the requested connection – this can be provided by the UE, or it can be contained in the UE’s subscription data or could be

determined by the MME based on local policy. The MME will use the network’s private DNS to resolve the selected APN into an IP address to be used during session creation.

In scenarios where a network has configured the same APN to be available on multiple PDN-GWs, for resilience and load balancing purposes, the DNS record for the APN can include weighting factors which will determine the relative frequency with which each IP address should be used. The DNS server will supply the full list of applicable IP addresses to the MME, which will then select one based on the weighting factor; the higher the weighting the higher, proportionally, that entry will be selected. In the example, for the selected APN ‘internet’, out of a choice of four, PDN-GW 4 was selected as it had the

In document LTE Evolved Packet Core Network (Page 127-169)