5g voice – network evolution aspects...2g/3g network, including support of srvcc ... cs voice...

5G voice – network evolution aspectsVoice services in a 5G System with 3GPP option 2 deployment

Paper – 2

2 Ericsson | 5G voice – network evolution aspects

Abstract

Disclaimer

The journey to 5G has several possible migration paths and steps, and voice services must be supported in all 5G evolution steps. This paper outlines the voice service aspects, migration aspects and recommendations for deployments using option 3 and option 2.

In initial 5G deployments using option 3- 5G EPS includes NR non-standalone in the RAN. 4G voice (VoLTE) and other IMS services continue to be used without the need for any core network upgrade. IMS is not aware of the updated RAN. All 4G VoLTE principles remain.

5G system (5GS) using opting 2 includes NR standalone in the RAN. At least initially, a 5G System (5GS) will not be deployed with full network coverage. Therefore, the 5GS needs to be tightly coupled to

an existing 4G VoLTE deployment, to provide seamless voice services across the whole 4G+5G network with good characteristics.

EPS fallback is used in early 5GS deployments; the UE falls back from NG-RAN to LTE during call establishment in such deployments. Hence all voice related capabilities are needed in NG-RAN, 5GC and IMS except those needed to completely establish the call over NG-RAN.

Voice over NR implies that voice calls can be made on NR. This requires NR in NG-RAN to support all voice related capabilities, being dimensioned for voice coverage.

The intended audience for this paper is familiar with 4G VoLTE and wants to know how to take the voice service into 5G.

The information provided in this document is based on ongoing 3GPP standardization and Ericsson’s current recommendations on network evolution. These aspects are subject to changes at any time, thus Ericsson assumes no liability of the info provided in this document.

Copyright

© Copyright 2018 Ericsson AB. All rights reserved.

This document is part of a series of papers explaining the network evolution aspects for 5G voice services. The intended audience for this paper is familiar with 4G VoLTE and wants to know how to take the voice service into 5G.

3Ericsson | 5G voice – network evolution aspects

Voice and EPS Fallback in 5GS - option 2 deployment

Architecture for EPS fallback and voice over NR At least initially, a 5G system (5GS) will not be deployed with full network coverage. Therefore, the 5GS needs to be tightly coupled to an existing 4G VoLTE deployment to provide seamless voice service across the network with good characteristics.

The architecture for this tightly-coupled system is based on: — A common gateway (Com. GW in the figure below),

which replaces the 4G P-GW for voice services in the combined EPS and 5GS for voice users, with 5GS enabled in subscription

— Interworking with N26 between MME and AMF in 5GC. This enables context transfer and network-controlled handover between EPC and 5GC

— Combined HSS/UDM network functions and PCF with Rx

Figure 7 Conceptual architecture for voice in 5GS tightly coupled to 4G VoLTE

The common gateway includes both control and user plane functions. IP flows use the same common GW, regardless of the UE camps on NR or LTE. The IP address of the UE can then be maintained. The common gateway supports the interfaces to SGw and MME. It ensures a mapping between 5GS and EPS QoS

parameters (5QI and QCI) and provides a single control point for a combined 4G/5G policy control function PCF. The common gateway is also used by outbound 5G roamers for operators supporting 5GC and/or EPC (see also section 4.1).

PCF

MME+ SGw

Com. Gw IMS

AMF

HSS/ UDM

E-UTRAN

NG-RAN

N26Inter system Handover

EPS Fallback

NNI


Figure 8 Common GW and interworking with N26 is required to meet voice KPIs for EPS fallback and inter system handover

As shown above, a pre-requisite for the 5GS architecture is that smartphones can register to 5G Core (5GC) and IMS over NR in NG-RAN. The selection of NR is enabled by an “IMS VoPS indicator” in the NAS signaling from the 5GC. The smartphone’s Radio Access Technology (RAT) preference setting must also be set to “NR preferred” so the smartphone stays in NR also when in overlapping LTE/NR coverage.

The combination of a common gateway and interworking with N26 between AMF and MME limits

the time, added by EPS fallback, during call setup. The architecture also ensures that inter-system handover can be performed, without exceeding the critical voice gap KPI of 300 milliseconds.

Ericsson does not recommend deploying 5G voice that rely on 4G coverage without tight interworking between MME and AMF (also called interworking without N26). The call setup would be longer, and network-controlled inter-system handover would not work at all.

PCF

MME

AMF

SMF/ PGw-C

E-UTRAN

NG-RAN

N26N11

N3

SSU

S5-C

N6/SGi

N10

N7 RxInter system Handover

EPS Fallback

NNI

UPF/ PGw-U

SGw

IMS

HSS/ UDM

Common Gw


Voice with EPS fallback

EPS fallback enables phones to use the 5GC with NR prior to all needed voice features in the phone and in NG-RAN, and before the NG-RAN is dimensioned and tuned for voice.

This is an intermediate step towards support of voice in a 5G system, as described in section 3.3 below.

The coverage and deployment scenarios for voice in early 5GS are as follows:

— NR coverage connected to a 5G Core, not supporting native voice services

— Wide-area LTE coverage controlled by EPC supporting VoLTE

— VoLTE network may interwork with an underlying 2G/3G network, including support of SRVCC

— Emergency services are handled by either the VoLTE or Circuit Switched (CS) network

Figure 9 Typical coverage scenario for voice in EPS and 5GS in early 5GS deployments using EPS fallback

Figure 10 Conceptual illustration of EPS fallback

A smartphone (UE), connected to NR, that tries to establish a voice connection, will perform an EPS fallback at call setup, triggered by the attempt to

establish the Quality-of-Service (QoS) bearer for the voice flow for voice in NR.

NR

LTE

AMF

NR in 5GSLTE in 5G EPS (voice)

CS voice coverage

4G SRVCC

NG-RAN

During call

Before call

E-UTRAN

EPS Fallback

NNI

MME+ SGw

IMSCom. Gw

HSS/ UDM

T


At the attempt to establish the QoS bearer for the voice flow over NR during call set-up, the NG-RAN rejects the QoS flow setup towards the common gateway with an indication that mobility is in progress. The NG-RAN initiates transfer of all PDU sessions from 5GS to EPS, using one of the two standardized procedures:

— Release with redirect — Inter-system handover

The EPS fallback procedure adds time to the call setup time which should be kept as short as possible. The release with redirect variant of EPS fallback only uses the UE context transfer part of N26.

Ericsson recommends EPS fallback with release with redirect in early deployments of 5GS. EPS fallback with

inter-system handover is recommended as soon as the required features are available in RAN, 5GC, and in phones, since it has superior performance.

If EPS Fallback is also required for emergency service, then Ericsson recommends using the same procedure as for voice, i.e. release with redirect in early deployments, and inter-system handover as soon as the required features are available. The UE is made aware of emergency support by the AMF indicating to the UE that emergency service is supported over NG-RAN. If there is no local regulatory requirement to indicate emergency support on NG-RAN, then at emergency attempt the UE will reselect to E-UTRAN or other access providing emergency support.


5GC 5G Core

5GS 5G System, consists of 5GC, NG-RAN and UE

5G Voice IMS voice in the 5G system

AMF Access and Mobility Management Function

Com. GW Common Gateway

EN-DC E-UTRA NR Dual Connectivity (in RAN connected to EPC)

eLTE LTE in NG-RAN connected to 5GC

EPC Evolved Packet Core, 4G Packet Core

EPS Evolved Packet System consist of EPC and LTE RAN

HSS Home Subscriber Server in 4G

IMS IP Multimedia Subsystem

KPI Key Performance Indicator

MME Mobility Management Entity in EPC

NG-RAN Next Generation RAN, RAN connected to 5G Core; can include NR standalone or eLTE standalone or both NR and eLTE

NR New Radio

PCF Policy Control Function in 5GC

PGW PDN Gateway in EPC

RAN Radio Access Network

RAT Radio Access Technology e.g. LTE, NR

SGW Serving Gateway in the evolved Packet Core

SMF Session Management Function in 5GC

UDM User Data Management in 5GC

UE User Equipment

UNI User Network Interface

UPF User Plane Function in 5GC

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The content of this document is subject to revision without notice due to continued progress in methodology, design and manufacturing. Ericsson shall have no liability for any error or damage of any kind resulting from the use of this document.

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5g voice – network evolution aspects...2g/3g network, including support of srvcc ... cs voice...

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