Voice Over LTE

Network Research Department

China Mobile Research Institute

Voice over LTE

2Content

2

1 Alternatives of LTE voice solution

3 SRVCC enhancement

Voice over LTE solution

4 Industry status on Voice of LTE

3Different strategies for LTE voice service provision

Voice service can be provided both by CS network or LTE network No impact to legacy CS network if voice service remains in CS LTE system is an all-IP network without CS domain VoIP service can be provided on LTE system with IMS based voice call control

functionality

SAE

RAN/GERAN

IMSMMEeNodeB

CS

Vo

ice c

on

tinu

ity

LTE covertage

CS coverage Voice service keeps in legacy CS network

LTE system provides only high speed data service

Strategy 1: Voice over CS

LTE system provides VoIP service and high speed data service simultaneously

IMS is used for voice call control Voice should be provided in CS

network when out of LTE coverage

Strategy 2 : Voice over LTE

4Alternatives of LTE voice solution

3GPP

Standard Solution

3GPP

Standard Solution

Network

Independent

Solution

Voice over LTEVoIMS+SRVCC: Providing IMS-based voice over LTE, and supporting PS/CS

interworking between LTE and 2/3G

CS Fallback (LTE First)If voice is needed, reselecting from LTE to 2/3G

to provide.

CS Fallback (2/3G First)First select 2/3G, if data service is needed,

reselecting from 2/3G to LTE based on

some condition

Multi-mode Dual-standby

Single-USIM TerminalThe UE access 2G/3G and LTE

simultaneously, voice over 2G or 3G,

data service over LTE or 2G/3G

Multi-mode Dual-standby Single-USIM

Voice over CS

Multi-mode Single-standby

Voice over CS

5Content

2


3 SRVCC enhancement



6General Principle of Voice over LTE

PS

nework

2G/3G

CS

network

2G/3G terminal

Internet

2G/3G

IMS

network

LTE

Internet

Legacy

Legacy networks provide different

services via CS and PS domains

CS domain provides voice and SMS

services

PS domain provides data service

VoIP service and high speed data service are simultaneously offered over LTE

Due to lack of call control function, EPC network needs IMS network for call control and service provision

IMS network provides voice continuity from LTE to CS by anchoring voice call

LTE terminal uses the same MSISDN in LTE and CS networks

LTE terminal

Voice & SMS

services

Voice &

multimedia

servicesData service

Data service

EPC provides beaers for IP services

IMS based call control

After LTE and IMS are introduced

PS

nework

CS

network

LTE terminal

7VoIMS+SRVCC: Target Solution for Voice over LTE

SRVCC: VoLTE Target Solution

The consensus has been reached by the industry that the target solution for VoLTE should be based on IMS.

SRVCC provides the voice continuity from LTE coverage to 2/3G CS, necessarily ensuring the pleasant

experience for VoLTE users at early stage of commercial deployment.

VoLTE UE under LTE coverage initiates voice sessions via IMS network, which will anchor the

session and provide SIP-based session control

and applications.

When the UE moves from LTE to 2/3G CS coverage with the ongoing IMS session, IMS

switches the session to the CS domain, keeping

both parties of the session unaware of the

handover.

SRVCC Principle

The first version of SRVCC solution has been specified in 3GPP Release 8, but the handover

performance of this version need to be enhanced.

The Work Item of SRVCC enhancement was created and completed in 3GPP Release 10,

aiming at the comparable handover performance

with legacy CS network.

SRVCC Enhancements

SAE-GW

EPC

HSS

RAN/GE

RAN

MGC

F

CSCF

SCC AS

IMS

MMEeNodeB

MGW

SR-IWF

MSC Server

SV

Vo

ice c

on

tinu

ity

PCRF

Internet

CS

LTE

SGW/PGW

SRVCC UEeMSC

MME

MSC Server

IMS

2G/3G

2G/3G

Remote UE

P-CSCFATCF

ATGW

MGW

I/S-

CSCF

SCC AS

8Single Radio Voice Call Continuity

SAE-GW

EPC

HSS

RAN/GE

RAN

MGCF

CSCF

SCC AS

IMS

MME

eNodeB

MGW

IWF

MSC Server

SV

Vo

ice c

on

tinu

ity

PCRF

Internet

CS

Within LTE coverage voice call is initiated from LTE/EPC via IMS, which controls and anchors

the voice call

When the UE leaves the coverage of LTE, IMS will transfer the ongoing call to CS network

SRVCC fundamental

SRVCC applies to the scenario that LTE coverage is not continual

SRVCC applies to multi-mode single radio UE SRVCC is essentially a handover procedure,

including transport level handover and session

level handover

Considering the impacts to the networks introduced, SRVCC is not recommended until

LTE becomes middle level coverage

SRVCC requires deployment of IMS network Currently SRVCC only supports voice continuity

from LTE to CS

SRVCC features

Sv interface between MSC server and MME

is defined to handle SRVCC procedure

9SR-VCC general procedure

UE E-UTRAN MME MSC ServerTarget

UTRAN/GERAN

Measurement

Reports

Handover to UTRAN/GERAN

required

3GPP IMS

Initiates SRVCC for voice component

CS handover preparation

IMS Service Continuity Procedure

Handles PS-PS HO for

non-voice if needed

CS HO response to MMETo eUTRAN

Coordinates SRVCC

and PS HO response Handover CMD

Handover

execution

1. EUTRAN indicates MME of SRVCC handover after measurement;

2. MME initiates SRVCC procedure via Sv interface to MSC server;

3. MME also handles PS handover procedure for non-voice sessions

4. MSC Server reserves bearers in CS network, then initiates IMS session transfer procedure;

5. After successful bearer reservation, MSC Server indicates MME, who will then notify UE to reselect to

UTRAN/GERAN;

6. UE continues the ongoing call when successfully reselecting to UTRAN/GERAN

10

Content

2


3 SRVCC enhancement



11

EPC

2G/TD

SRVCC eMSC

MGW

MME SAE-PGW

HSS

CS Sv

MGCF

SCC AS

IMSCSCF

MGW

MSCS

LTE

SRVCC performance issues Session transfer and remote UE update may need a long time No sync between session transfer and RAT retuning procedures

The performance issues have notable negative impact on user experience

Since SRVCC is an IMS based inter-RAT handover procedure from LTE to the 2G/3G CS, it is much more

complex than that of the legacy 2G/3G networks, the question is how to keep the comparable handover

performance?

Remote IMS UE

1. SRVCC UE initiates voice call to another IMS UE via LTE.

2. The call is successfully established, with voice media

between UEs

3. SRVCC happens when SRVCC UE leaves LTE coverage.

SAE network informs SRVCC MSC to prepare for

handover. MSC completes the resource reservation

4. The UE is informed to switch to 2G/3G. The voice is

interrupted during the switching procedure, for about

200ms in T1

5. SRVCC MSC informs the remote UE to receive voice

media from SRVCC MSC, not SRVCC UE

6. The remote UE switch media connection to SRVCC MSC

7. voice media will be interrupted again during the media

switching procedure above, for about 800ms(if remote UE

is roaming, the interruption could be longer)interrupt time

T1

interrupt time

T2

The whole interruption time 800msThe longest voice interrupt could be more than1s

signal

media

The problem of handover performance in existing

SRVCC solution

SRVCC UESRVCC UE

12

SRVCC optimization alternatives

LTE Wireless access network SGW/PG

W

UE

IWF

MME

MSC Server

IMS

2G/3G Wireless access network

2G/3G

UE

SCC AS

User plane during SRVCC

User plane before SRVCC

User plane after SRVCC

SGW/PGW

UE

IWF

MME

MSC Server

IMS2G/3G UE

SCC AS

MRFP



SGW/PGW

UE

IWF

MME

MSC Server

IMS2G/3G UE

SCC AS



The media is anchored in visited network during call setup The anchor point redirects media flow to CS when SRVCC Pros

good performanceCons

Impacts to visited network, requiring visited network support

SRVCC optimization targetvoice interruption less than 300ms in roaming and the non-roaming; no impact to UE; the impact to the network is minimized.

The media is anchored in home network during call setup The anchor point bi-cast media flow to LTE and CS during

SRVCC

ProsLittle impact on network, easy to deploy

ConsLocal breakout is impossibleAll session need to be anchored back to home network

Synchronize IMS session transfer procedure with RAT retuning procedure to overlap the interruptions

ProsLittle impact on network, easy to deploy

ConsDifficult to accurate synchronizationPerformance not optimized, the best result is no better than the longer one in the two interruptions

LTE Wireless access network


LTE Wireless access network


13

eSRVCC standardizations

synchronization

Media anchored in home network

Media anchored in visited network

Sol 12media anchored in SGW

Sol 10media anchored in PGW

Sol 4 media anchored in MGW

Sol 11 media anchored in ALG/TrGW

Sol 1synchronization based on measurement

Sol 2serial handover

Sol 5ST optimization based on sol 2

Sol 9 synchronization based on packet check

Sol 8media surface anchors on MRFC/MRFP of home network

performance not

fulfiled

After transfer

Du

ring

tran

sfer

UE Before transferRemote

end

CS-MGWUE

Before and during transferS/P-GW

UL voice media

DL voice media

After transfer After transfer

UE

I/S-CSCF,

SCC ASATCF

Before transfer

Remote

Leg

SN HN

MSC

ServerUE

Before transfer

Gm

Mw

Serving Leg Home Leg

After transfer Remote

End

Access LegIMS based solution

EPC based solution

In order to enhance the handover performance of SRVCC, a WI named eSRVCC (SRVCC enhancement) is established in 3GPP SA2 in Release 10, with China Mobile as the rapporteur

on July 1, 2010, in the conference call of eSRVCC work group, the solution of eSRVCC to be standardized was determined out of 12 alternatives

IMS based solution, i.e. anchoring the media in visited IMS network, is selected

All 3GPP standardization work has been completed in March, 2011

14

eSRVCC solution

LTE wireless access network SGW/PGW

SRVCC UE

eMSC

MME

MSC Server

IMS

2G/3Gwireless access network

CS network

Remote UE

P-CSCFATCF

ATGW

MGW

SCC AS

The solution is based on IMS anchoring, which introduces new entities ATCF (Access Transfer Control Function) and ATGW (Access Transfer Gateway)

As functional entities, ATCF and ATGW can be collocated with other entities, e.g. P-CSCF, IBCF, or MSC ServerATCF, anchor point for SIP signaling, stays in the SIP signaling path during registration and session setup, controling ATGW to anchor the voice media when required

when SRVCC happens, ATCF indicates ATGW to redirect the media flow to CS, keeping other part of media path unchanged

Sv

I/S-

CSCF

Voice media is anchored in visited network, therefore the media redirection is executed locally, which is much faster, normally less than 100ms

IMS session transfer and remote update procedures are avoided Interruption by RAT change can not be avoided, which is normally 200ms The total interruption time will be less than 300 ms

EPC network

15

Control and user planes during SRVCC handover

After transfer After transfer

UE

I/S-CSCF,

SCC ASATCF

Before transfer

Remote

Leg

SN HN

MSC

ServerUE

Before transfer

Gm

Mw

Serving Leg Home Leg

After transfer Remote

End

Access Leg

P-CSCF

After transfer

After transfer

UE PDN GWBefore transferRemote

endRemote leg

CS-MGWUE

ATGWBefore transferServing GW Before transfer

Media Path of Serving Leg

SN

Media Path of Remote Leg

The change on control

plane during SRVCC

handover

Both the control plane and user plane are anchored in the network which the SRVCC UE

is attached, the handover is executed locally

The change on control

plane during SRVCC

handover

16

Content

2


3 SRVCC enhancement



17

VoLTE standardization status

R5-R7 Rel-8 Rel-9 Rel-10 Rel-11

(ongoing)

IMS introduced

LTE introducedSRVCCdefined

SRVCCsupporting mid-callVoLTE emergency call

SRVCC enhancement

rSRVCCvSRVCCVoLTE roaming architecture

IR.92 defines VoLTE profile

Requirements for VoLTE roaming

architecture

VoLTE roaming architecture

IR.88 defines LTE roaming

architecture for

VoLTE

IR.64 IMS Service

Centralization

and Continuity

Guidelines

VoLTE basic

requirementsVoLTE international roaming & interworking

VoLTE futher

requirements

IR.65 defines principles for

IMS roaming &

interworking

18

14Q111Q3 11Q411Q1 11Q2 12Q3 12Q412Q1 12Q2 13Q3 13Q413Q1 13Q2

Vendor 1

SRVCC roadmap

of chip vendor

eSRVCC roadmap

of nw vendors

Vendor 2

Vendor 3

Vendor 4

Vendor 5

VoLTE Roadmap

of operators

Operator 2

Operator 3

Operator 4

Chip vendor 1

VoLTE roadmap of the industry

Operator 1

Chip vendor 2

Documents

Voice Over LTE