LTE Voice Solution

LTE VOICE SOLUTION v1.0

Technical Solution Description

1. Executive Overview...............................................................31.1 Solution highlights.......................................................................................31.2 Solution benefits for user............................................................................31.3 Solution benefits to operator.......................................................................31.4 End user services........................................................................................31.5 Usability requirements.................................................................................31.6 Security.......................................................................................................41.7 Reference architecture................................................................................41.8 Technical Solution.......................................................................................9

1.8.1 MSC Server (MSC-s)..............................................................................91.8.1.1 NVS.............................................................................................................91.8.1.2 SRVCC......................................................................................................101.8.1.3 DTF...........................................................................................................10

1.8.2 MGW....................................................................................................10

2. End user use-cases and network functionalities.................112.1 Registration to LTE....................................................................................112.2 Mobile Originated Call...............................................................................132.3 Emergency Call.........................................................................................142.4 Mobile Terminated Call..............................................................................142.5 Short Message Service.............................................................................172.6 Single Radio Voice Call Continuity (SRVCC)............................................202.7 Service evolution.......................................................................................222.8 Roaming evolution....................................................................................24

2.8.1 Phase 1................................................................................................242.8.2 Phase 2................................................................................................252.8.3 Phase 3................................................................................................26

1. Executive Overview

1.1 Solution highlights How to introduce the voice service for the LTE deployment Utilization of existing assets and customer loyalty Provide same end user voice service and therefore transparency for

the end used Solution works fully with IMS deployment and has also an optimized

configuration (fast track)

1.2 Solution benefits for user Possibility to utilize LTE access data services simultaneously with

voice service Preserves the current user interface for voice service Enables service continuation between the LTE and the 2G/3G access

networks

1.3 Solution benefits to operator Can use GSM voice core functionality Service machinery e.g IN and CAMEL can be used without change Support systems like charging and subscriber management can work

as in current GSM networks Solution is based on standard 3GPP release 8 components thus

enabling easy upgrade possibility toward full IMS solution while operator prefers the change

NSN can plan, deliver and maintain the e2e solution

1.4 End user servicesFollowing capabilities exist for the end user voice services

Voice call SMS Interworking between the LTE access and the 2/3G accessess

1.5 Usability requirements Dialing format, CLI display and SMS sending works same way as

calling between any GSM- and other terminal The use of LTE access technology is hidden by the terminal from the

end user

1.6 Security User plane security is based on the LTE access security. On the control plane (SIP signaling) USIM based IMS AKA is applied for the authentication and the IMS signaling security capabilities based on the IPsec are applied.

1.7 Reference architectureThe solution is from the terminal perspective exactly as the release 8 specified voice solution for the LTE environment. This means that the terminal uses SIP as specified for the Gm interface, while it is using LTE access for the voice delivery and the CS-access while it is accessing network through 2G/3G access technologies.

On the core network the Gm-interface is terminated by the A-SBC (Access-Session Border Control) and provides the Mw –interface toward the IMS core.

The MSC-s (Mobily Switching Center-server) is enhanced with the NVS (NSN Voice application Server) capability. NVS terminates the SIP signaling and adapts to the CS-Core functionality. MMtel is defined in release 8 24.173 specification. NVS is a functionality that maps the MMTel service to the CS-core functionality, therefore the functionality set is defined by the CS-supplementary services and by the CS-networks addressing capabilities. With this approach the end user will get seamless user experience, which is independent from the used access technology and independent on the used call control protocol, be it either 24.008, or SIP (24.229). Basic principle of the NVS in the core network architecture from voice and SMS perspective is shown in the Figure 1.

LTEUEUE

eNbeNb SAE-GW

MME

HLR

CSCF

PCRF

MSS/NVS

MGW

S6a

Rx

Gx

McGm

SIP/ISC

Mb

NcSv

MAP

MGWNb

Mc

NGVM Voice solution

Standard interfaces to “new” world and 3GPP Rel-8 terminalsStandard interfaces to “new” world and 3GPP Rel-8 terminals

slide added MSC-S

OAM, LI, IN

Charging

NVSVoice solution

Unchanged interfaces to “old”world, reuse CS coreUnchanged interfaces to “old”world, reuse CS core

Figure 1 NVS functionality on a high level

MSC-s is enhanced with two inter system handover related components. The first one being the SRVCC(Single Radio Voice Call Continuity), which is the functionality defined in the rel-8 for enabling the inter system handover from the LTE-access toward the CS-core. According to the specification this is an additional capability in the MSC-s. The other additional component for the system is the DTF (Domain Transfer Function). DTF has been defined in the rel 7 Dual Radio VCC concept (23.206) and is used for the anchoring of the calls. In the SRVCC context the DTF is also used fordoing the anchoring. Release 8 SRVCC defines only the domain transfer from the LTE access to the CS network and therefore in this architecture the anchoring is done only for the calls that are initially using the LTE access.

Domain selection is based on the terminals access information in the HLR, therefore there is no separate terminating access network selection entity in the architecture.

Below is two possible architectures for implementing the solution for the core network side. First option is the voice service deployment with IMS-core, in this case the NVS uses the ISC interface toward the IMS core. IMS option is shown in Figure 2. If the CS-access networks services are to be moved to the IMS side the CS-network may be upgraded by having a I2 interface toward the IMS, or the calls may be routed through CAMEL routing capabilities to the IMS as indicated in the Figure 3.

UE

E-UTRAN

GERAN

UTRAN

MSC-s

GW

MME

Um

Uu

Gm

Iu-CS

A

S1-MME

S1-USGi

MwSR-VCC

PCRF

A-SBC MMtel

GW

D

• Gm interface as specified in 24.229

SRVCC

MGW

SV

DTF

Gx

Rx

S11

CORE-IMS

I-S-CSCF

HSS

Cx/Dx

ISC

Figure 2 Network reference architecture with IMS

UE

E-UTRAN

GERAN

UTRAN

GW

MME

Um

Uu

Gm

Iu-CS

A

S1-MME

S1-USGi

MwSR-VCC

PCRF

A-SBC NVS

HLRHSS

D


SRVCC

MGW

SV

DTF

Gx

Rx

S11

CORE-IMS

I-S-CSCF

HSS

Cx/Dx

ISC

Sh

MSS/MGCF

Mg

Figure 3 Network reference architecture with IMS enhanced to include also IMS centralized services

For a MSS based solution there exists also a possibility to deploy the NVS only for the voice and SMS, for some part of the subscribers, then there is also an option for deploying the NVS through the Mw´ interface toward the A-SBC (Figure 4).

NVS is implemented as part of the MSC-s. In both solutions the SRVCC is the same as in the release 8 specification. The NVS and the SRVCC may be in the same MSC-s network element or may be in different network elements. The DTF functionality is in close relation with the NVS and is addressable with the STN-SR specified for the subscriber as specified in the SRVCC specification.

UE

E-UTRAN

GERAN

UTRAN

MSC-s

GW

MME

Um

Uu

Gm

Iu-CS

A

S1-MME

S1-USGi

Mw´SR-VCC

PCRF

A-SBC MMtel

GW

HLRHSS

D


SRVCC

MGW

SV

DTF

Gx

Rx

S11

Figure 4 Network reference architecture with fast track option

1.8 Technical Solution The solution is based on following subsystems and components

1.8.1 MSC Server (MSC-s)

MSC-s is the physical network element that implements firstly the basic MSC-s and VLR functions for the CS-access, but in addition has the capability to support the NVS, SRVCC and DTF software components.

1.8.1.1 NVS

Nokia Siemens Networks Mobile Voip Server (NVS) is collocated with the 3GPP R4 MSC Server (MSS). The solution is mainly targeted at operators with existing 3GPP R4 infrastructure that want to introduce VoIP service by re-using existing investments. Reuse of MSC Server has the advantage of having one common MGCF both in the 3GPP R4 network and in the IMS (when applicable), and a common Media Gateway (MGW) as well. The NVS being part of MSS server platform can also be reused to provide services both for fixed and mobile subscribers.

The MSS can host the three product roles:

1. VoIP Server (NVS)2. Media Gateway Control Function (MGCF)3. MSC Server Functionality (MSS)

The MSS-based VoIP Server provides voice telephony services to the SIP/VoIP clients including the following services:

ability to use facility codes for the services supported by NVS CFU, CFNRy, CFNRc, Barrings, ACR

services provided together with NVS and terminal : Call Hold and resume, CLIP & CLIR, CFB, Call Transfer,

Missed call log 100% terminal provided services :

Call Waiting, Call Switching, Do not disturb, Direct dialing in, 3-party call

NVS also provides SMS delivery over SIP

In addition to main features mentioned above, NVS also introduces Digest AKA authentication method. This is essential functionality of NVS from the solution perspective.

1.8.1.2 SRVCC

SRVCC functionality is an new component for the MSC-s network element, which implements LTE domain transfer capabilities in the CS-core network side. The reference point related to the SRVCC functionality is the SV and the related protocol is GTP.

SRVCC initiation is coming from the MME and the SRVCC capability is converting the Sv functionalities to the CS-access network A/Iu-interface signaling in the case of intra MSC handover, or to the E-interface signaling in case of inter MSC handover. During SRVCC initiation MME delivers the related STN-SR relating to the subscriber. SRVCC functionality is responsible for initiating the call toward the anchoring point (indicated by STN-SR).

1.8.1.3 DTF

Domain Transfer Functionality (DTF) is responsible for doing the anchoring of the calls. In LTE case only the VoIP calls are anchored. If call originates in CS domain the anchoring is not applied, while the SRVCC from CS to LTE domain is not supported at a moment. With this approach also networks resource usage is optimized.

1.8.2 MGW

MSS MGCF / Multimedia Gateway (MGW U4.1) ensures connectivity to the CS/PSTN domain. The MGW consists of several functional elements configured under the gateway architecture. The operator can use a number of different configurations of the Multimedia Gateway depending on the services the operator requires.

The main functions of the MGW are as follows:

Adapting the conventional signaling (control plane) between the MSC Server (Media Gateway Control Function) and different network interfaces

Connecting the user data (user plane) from the ATM or the IP backbone into the radio access network or the circuit-switched networks. Media resources are under the control of the MSC Server (MGCF) via the H.248 (MEGACO) protocol.

Supporting G.711 (A-law / µ-law), G.723.1, G.729 A/B, AMR-NB, iLBC Providing tones and announcements to the end users. Performing the transcoding and signal processing for the user plane,

when needed. The MGW transmits and converts the user plane traffic in both circuit-

switched core networks and IP Multimedia Subsystem networks, and also as a border element between the different kinds of networks.

2. End user use-cases and network functionalitiesThe following use cases describe only the LTE network related functionalities. The assumption is that on the 2G/3G access networks the CS-access network is used exactly the same way as currently.

2.1 Registration to LTEVoice over CS for 2G/3G Access

Voice over IP for LTE Access

LTE

BSC

UEUE

UEUE

eNbeNb SAE-GW

MGW

MSC-S

MME

2G/3

G

S11

MAP

S6a

NVS

PSTN/PLMNPSTN/PLMN

RNC

IP NetworkIP Network

Gx

Operators IPServicesOperators IPServices

HLR

A-SBC/IMS

1. SIP registration

1. SIP registration

2. MAP locationupdate & data from HLR to

VLR

2. MAP locationupdate & data from HLR to

VLR

3. HLR cancel’s

location fromCS domain,

NVS updatedto VLR address

Figure 5 Terminal registration to LTE access

Figure 5 describes terminals registration procedure to the LTE access. While the access network selection criterias for LTE are met and the LTE EPS attachment is succesfull, then the terminal initiates the SIP registration toward the A-BCF. A-BCF forwards the registration toward the NVS and the NVS initiates the MAP location updating toward the HLR. IMSI in the location updating is derived from the IMPI from terminals REGISTER message. In basic case the terminal is authenticated during the registration based on the IMS AKA. After a succesfull authentication the HLR initiates Cancel Location MAP procedure toward the old VLR. After the completion of this procedure calls will be routed to LTE access.

Figure 6 Registration signalling flow

Figure 6 the terminals registration signalling in detail. Figure 7 shows the terminals de-registration signalling.

Figure 7 De-Registration signalling flow

2.2 Mobile Originated Call

Voice over CS for 2G/3G Access


LTE

BSC

UEUE

UEUE

eNbeNb SAE-GW

MGW

MSC-S

MME

2G/3

G

S11

MAP

S6a

NVS

PSTN/PLMNPSTN/PLMN

RNC


Gx


HLR

A-SBC/IMS 1. Callestablishment(Voice/Video)

1. Callestablishment(Voice/Video)

4. Call is routed

onwardsintended

destination..

4. Call is routed

onwardsintended

destination..

3. Originatingside serviceexecution

2. Callanchoring

Figure 8 Mobile originated call in LTE access

Terminal sends INVITE in the LTE access. During the call establishment signaling A-BCF links the SIP session setups media negotiation to the LTE access resource reservation. Media reservation is done from A-BCF over the Rx interface toward PCRF, which is responsible for policy control functionalities. PCRF forwards the resource request indication to the SAE-GW, which then handles the media reservation further toward the LTE access. Policy control functionality ensures controlled use of LTE access resources.

Call is anchored for enabling the potential domain transfer to the CS-domain. NVS performs the originating service execution, including IN and other services. At the end the call is routed toward the terminating network.

Figure 9 Mobile originated call setup toward PSTN

Figure 9 shows the mobile originated call setup toward the PSTN. In LTE access the bearer setup is done form network toward the terminal and the A-SBC contacts the police control (PCRF) for the sesource control.

2.3 Emergency Call Emergency call can be handled either in the LTE domain or it the terminal may make a fallback to the CS-domain. LTE emergency call has not been fully standardized in release 8 and therefore all the functionalities are not in the same functional level on LTE-domain than in the CS-domain. E.g. the emergency call without (U)SIM is not supported in the NVS. The other functionality, which is not supported in the location services, so the location can not be derived in more accurate way than to the cell level.

NVS supports the emergency call delivery on a per cell level routing for registered subscribers. If that is sufficient for the regulatory requirements the emergency calls can be delivered in LTE-domain. If this is not sufficient the terminal needs to be configured such way that it performs terminal initiated fall back to CS network in the emergency call case.

2.4 Mobile Terminated Call



LTE

BSC

UEUE

UEUE

eNbeNb SAE-GW

MGW

MSC-S

MME

2G/3

G

S11

MAP

S6a

NVS

PSTN/PLMNPSTN/PLMN

RNC


Gx


HLR

A-SBC/IMS



1. Call is routed to NVS as normal mobile

terminated call (e.g. HLR enquiry has been donealready prior this point)

1. Call is routed to NVS as normal mobile

terminated call (e.g. HLR enquiry has been donealready prior this point)

2. Terminatingside serviceexecution

3. Callanchoring

Figure 10 Mobile terminated call in LTE access

While GMSC receives terminating call for a subscriber, it makes the HLR enquiry (SRI=Send Routing Information). If the terminal is registered to the LTE access the roaming number is allocated from the NVS. GMSC routes the call toward the NVS. NVS performs the subscribers terminating services makes the call anchoring and forwards the INVITE toward the called party.

During the call establishment signaling A-SBC links the SIP session setups media negotiation to the LTE access resource reservation. Media reservation is done from A-SBC over the Rx interface toward PCRF, which is responsible for policy control functionalities. PCRF forwards the resource request indication to the SAE-GW, which then handles the media reservation further toward the LTE access. Policy control functionality ensures controlled use of LTE access resources.

Figure 11 Mobile terminated call from PSTN

Figure 11 shows the signalling flow for the mobile terminating call from PSTN. Call setup from GMSC toward the NVS is using standard MAP based routing mechanisms. Call setup from the NVS toward the terminal (SIP-UA) is done with release 8 IMS SIP signalling mechanisms.

2.5 Short Message Service



LTE

BSC

UEUE

UEUE

eNbeNb SAE-GW

MGW

MSC-S

MME

2G/3

G

S11

MAPNVS

PSTN/PLMNPSTN/PLMN

RNC


Gx


HLR

A-SBC/IMS

SMS is routedto NVS as

normal mobile terminated SMS

(e.g. HLR enquiry returns

NVS-VLR address

SMSC

SMS

GMSC

SIP MESSAGE containing SMS

payload == sameas GSM service





S6a

Figure 12 Mobile terminated SMS in LTE access

SMS service functions on the network side exactly the same way as in basic CS-access network, using the same MAP procedures and related interfaces. From the NVS toward the LTE access NVS uses SMS-payload type in the SIP-MESSAGE method as defined by 3GPP, so the terminal interface is the same as in SMS over IP specification. With this approach the standard HLR functionality can be applied without any additional requirements.

Figure 12 describes how the Mobile Terminated SMS is delivered toward the terminal in the LTE access.

Mobile originating SMS delivery follows the same principles, so that over the LTE access the delivery is done with SMS over SIP and NVS converts the SMS delivery to the CS-core network methods. Figure 13 describes the MO SMS signalling interoperation

Figure 13 Mobile originated SMS delivery

1. MESSAGE is received on the Normal Access interface.

3. MESSAGE is converted to MO-SM and the originating services (like barrings, analyses, IN services, PNP, etc.) are executed.

4. The MO-SM is submitted to SMSC via sending MOForwardSM MAP operation.

5. SMSC sends back acknowledgement for the received MO-SM in MAP MOForwardSM-Ack.

6. NVS makes CDR generation and statistics reporting.

7. Because the MESSAGE was not delivered yet to the recipient and it was just successfully submitted to SMSC for delivery not 200 OK but 202 Accepted is sent back to the UA-A.

The terminating SMS delivery signalling is shown in the Figure 14.

Figure 14 Mobile terminated SMS delivery

1. SMS-GMSC functionality in SMSC makes the HLR inquiry based on MSISDN-B.

2. HLR-B executes part of the terminating services (e.g.: barring checking, SMS CFU, etc.) and returns the VMSC address of the B-subscriber.

3. SMSC delivers the MT-SM via routing the MAP MTForwardSM operation to the NVS-B based on the received VMSC address.

4. NVS-B executes the terminating services (like analyses, IN services, etc.) after it receives the MT-SM. From the radio access information it is determined that the user has SIP access. Then NVS makes SPD query to retrieve the subscriber’s Contact address (IP address and port where the MESSAGE has to be sent) and the registration source (in case of the subscriber is registered via normal registration from the home domain then the MESSAGE has to be sent out to the subscriber on the Normal Access interface). The MT-SM is converted to SIP MESSAGE.

5. The MESSAGE is sent out to the UA-B on SIP Normal Access interface.

6. In case of successful delivery UA-B responds with 200 OK.

7. NVS sends back acknowledgement to SMSC via MAP MTForwardSM-Ack.

8. NVS makes CDR generation and statistics reporting.

2.6 Single Radio Voice Call Continuity (SRVCC)Voice over CS for 2G/3G Access


LTE

BSS

UEUE

eNbeNb SAE-GW

MGW

MME

S11

MAP

PSTN/PLMNPSTN/PLMN

BSC


Gx


HLR

1.Call establishment(Voice/Video)

NVSNVS

A-SBC/IMS

SvSv

2.Call anchoring

Anchor MSC-s MSC-s

Figure 15 SRVCC call initiation

SRVCC enables the end users voice calls continuity while moving from LTE VoIP coverage to the 2G/3G CS-voice domain. Figure 15 describes the functionality during the call setup. UE initiates the call and the call gets anchored in the DTF residing in the NVS. While only the LTE to 2G/3G direction has been specified and also for network resource usage optimization resons the call is anchored only in the case when the call isi initiated in the LTE access. For 2G/3G originated calls the call is routed normally and there is no need for extra resources.



LTE

BSS

eNbeNb SAE-GW

MGW

Anchor MSC-s

MME

S11

MAP

PSTN/PLMNPSTN/PLMN

BSC


Gx


HLR

MSC-s

NVSNVS

A-SBC/IMS

SvSv

2.MME initiatesSR-VCC toward

MSC-S

UEUE

1.Terminal movesfrom LTE to 2G/3G

coverage

3.SRVCC MSC-S initiates intra

MSS handovertoward the BSS

4. Anchor MSC-smakes the

anchoring call to SR-VCC anchor

point

Figure 16 SRVCC domain change from LTE toward CS

Figure 16 describes the functionality when the UE moves from the LTE coverage area to the 2G/3G area. UE sends measurement information to the eNodeB, which initiates handover toward MME. While MME receives a intersystem handover request for certain QoS value (QCI=1) it triggers an SRVCC toward the MSS. LTE subscriber has in the subscription information STN-SR information, which is used in the MSS for VCC anchoring call setup. When MSS receives the SRVCC indication from the MME, it initiates the resource setup toward the target BSS/RNC and also initiates the call toward the VCC anchoring point by using the STN-SR.

Figure 17 Signalling connection after the domain change

Figure 17 shows the signaling what is the signaling connection after the SRVCC. In the LTE side the resources are released. And the call continues on the CS-access. In parallel with the voice calls SRVCC functionality there may also be a parallel packet connections inter system handover functionality. This functionality is of course dependent, whether the UE has parallel packet session ongoing, whether the target access network has parallel voice and packet access capability (3G, or DTM) and whether the packet core is supporting the PS-PS inter system handover capability.

2.7 Service evolutionFast track LTE voice solution is having the same services supported as in the CS-core currently, namely voice and SMS. For enabling the smooth service evolution fast track solution smoothly supports introduction of the IMS services like RCS to the network. Still there may be different customer segmets for having a basic voice type of service set and some other customer segment can have richer set of services.

The selection of the service environment is done during initial registration and if the subscriber has the subscription in the IMS/HSS the subscriber will be served by the IMS and if not then the registration will be forwarded to NVS and the fast track voice system is utilized.



LTE

BSS

eNbeNb SAE-GW

MGW

MME

S11

MAP

PSTN/PLMNPSTN/PLMN

BSC


Gx


HLRNVSNVS

A-SBC/IMS

SvSv

UEUE

Signallingconnection afterthe inter system

SR-VCC

Anchor MSC-s MSC-s

Figure 18 Fast track and IMS coexistence

2.8 Roaming evolution

2.8.1 Phase 1

Figure 19 Phase 1 LTE roaming

While the LTE is introduced with the 3GPP release 8 functionalities are existing in the network. Release 8 does not yet specify the emergency call handling in the LTE environment, so the assumption here is that the emergency calls would be using CS networks in both nome network and visited network cases.

Fast track VoIP solution is used only when own LTE network is used. While the end user moves to some other network then the CS fallback and/or CS network is used. With this approach the LTE introduction can be done such way that the existing roaming agreements can still be used without changes.

2.8.2 Phase 2


Figure 20 describes one potential enhancement scenario in addition to the phase 1 functionality. In this scenario fast track voice solution solution is used for normal calls also when roaming..A-SBC is assumed to be in the home network. Emergency calls are handled through the CS-network while roaming. For providing seamless service in networks without full LTE coverage the SRVCC functionality needs to be supported in the roamed network. This functionality requires sufficient IMS and/or data roaming agreements between the operators.

2.8.3 Phase 3


Phase 3 solution assumes already wide IMS deployment also from visited networks. In this phase all the calls from LTE access can be handled in IMS domain and the CS-network is used only when using 2G or 3G-access.

In the network evolution the phase 1 will probably exist quite long time, but with the LTE fast track voice approach proposed in this document a good basis for future evolution is created also for the roaming functionalities.

Documents

LTE Voice Solution