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Top 10 E2E VoLTE deploymentconsiderationsBy Wang Yachen

VoLTE is the mainstream solutionfor voice and voice-roamingservices in the LTE era. VoLTEprovides high-quality voice and

video services and supports single-radio voice-call continuity (SRVCC) and enhanced SRVCC(eSRVCC). Through its provision of rich serviceexperiences to end users, VoLTE is crucial tonext-generation convergent communications.

With the coordinated development of frequency

division duplexing (FDD) and time division

VoLTE deployment is unprecedented in its complexity and a great challenge to

carrier networks and telco business transformation. This article details the ten most

important deployment considerations.

duplexing (TDD), more and more carriersare investing in VoLTE. They are also jointlypromoting the construction of VoLTE roamingnetworks that will offer seamless roaming on aglobal scale.

IMS core networkdeployment

IP multimedia subsystem (IMS) supports a

Wang Yachen

Director of

Multimedia

Network Lab, China

Mobile Research

Institute, and

Reporter of SG13

Q10 Work Team,

ITU.

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variety of access types and multimedia services.It has been acknowledged by the 3GPP andGSMA as the standard architecture for all-IPmobile voice services. Deployment of an IMScore network is a key to VoLTE deployment.It contains many network elements (NEs),such as the interrogating-call session controlfunction (I-CSCF), serving-call session controlfunction (S-CSCF), domain name server (DNS),E.164 number to URI mapping (ENUM)server, multimedia resource function controller(MRFC), multimedia resource functionprocessor (MRFP), interconnection bordercontrol function (I-BCF), breakout gatewaycontrol function (BGCF), media gatewaycontrol function (MGCF), IP multimediamedia gateway (IM-MGW), and session bordercontroller (SBC). The IMS core network isresponsible for access control, call routing, andservice triggering.

The SBC is a key NE on a VoLTE network,usually co-located with the proxy-call sessioncontrol function (P-CSCF), which acts as thefirst contact point for mobile users within IMSnetworks. Generally, the SBC transmits both

VoLTE signals and media streams. Its locationon the VoLTE network has a great impact on

user experience, so deployment efficiency andengineering efficiency must be considered duringcentralized SBC deployment, with the trafficswitching efficiency warranting considerationduring distributed SBC deployment. As theproxy node and access point for VoLTEmedia streams, the SBC also acts as both theaccess transfer control function (ATCF) andaccess transfer gateway (ATGW), with thesefunctionalities reducing the media channelreestablishment time for handover from LTE to2G/3G, allowing for smooth inter-system call

roaming.

EPC core networkdeployment

The evolved packet core (EPC) corenetwork includes the mobility managemententity (MME), serving gateway (S-GW),and PDN gateway (P-GW). It supports thefollowing VoLTE-related functions; assignmentof IPv4 or IPv6 addresses to IMS access point

names (APNs); P-CSCF discovery deployment;service request routing to the IMS core network;enforcement of policy and charging rulesfunction (PCRF) policies over the Gx interfaceto establish EPC and air interface bearers thatmeet QoS requirements; and handover requestinitiation for CS networks over the Sv interfacebetween the MME and single-radio voice callcontinuity interworking function (SRVCCIWF).

APN configuration, which must beperformed on the user equipment (UE), MME,and P-GW, is a key element of EPC corenetwork deployment. Call and data services canshare an APN or use independent APNs, anddifferent IP addresses are required for different

APNs. If they share an APN, deployment iseasy, but voice could be impacted by dataservices. When the sharing APN solutionis used, voice call-related traffic must beseparated from data traffic by the IMS chargingidentifier (ICID) as voice calls are charged bycall duration while data services are chargedby traffic volume, and this separation involvescomplex adaptation of the billing system. Thisproblem will not occur, however, when anindependent APN solution is in use.

CS core networkreconstruction

The circuit-switched (CS) and VoLTEnetworks will coexist for a long time to come,and coordination between the two is crucialto seamless user experience. This requiresCS network support of the Sv interface forSRVCC/eSRVCC handover, which ensures

voice cal l continuity when users roam from

LTE to 2G/3G. A CS network must alsosupport enhanced SRVCC/eSRVCC forcommon supplementary services after handover,such as call waiting (CW), call hold (CH), andmultiparty (MPTY). An international standardsolution is prescribed for evolution from CSto IMS networking CS network upgradeto support IMS centralized services (ICS).This solution calls for the mobile switchingcenter (MSC) server to be transformed into anenhanced MSC (eMSC) in order to supportthe I2 interface, migrating the service switching

Top 10 E2E VoLTE deployment considerations

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eSRVCC deployment is an interimsolution for evolution to LTE.

LTE PCC deployment

Policy and charging control (PCC)architecture, proposed in 3GPP R7,provides end-to-end (E2E) policy controlon packet-switched (PS) networks. InPCC architecture, the SBC/P-CSCFapplies for VoLTE voice and video

bearer resources from the PCRF overthe Rx interface. The PCRF generatesdynamic rules for the voice and videoservices, based on the configured policies;then, over the Gx interface, the PCRFinstructs the EPC and wireless networkto reserve dedicated service resources inorder to ensure QoS.

E2E QoS depends on QoS parameterconfiguration and mapping on eachNE. The configuration and mappinginclude provisioning of key parameters,

such as QCI, ARP, APN-AMBR, andUE-AMBR, in the system architectureevolution home subscriber server (SAE-HSS) based on subscribed services; theQoS mechanism and parameter mappingof service-layer NEs (e.g. base station,MME, S-GW, P-GW, and PCRF), anddifferentiated control mapping betweenthe QCI and differentiated services codepoint (DSCP) for IP bearer-layer NEs(e.g. router). E2E QoS deploymentinvolves multiple fields and numerous

An international standard solution is prescribed for

network evolution from CS to IMS CS network upgrade

to support ICS so that the service switching and call

switching functions can be migrated to the IMS network.

A unied core network can then be built to improve user

experience and operational efciency.

IMS core network

deployment

Deployment of an

IMS core network

is a key to VoLTE

deployment. It

contains many NEs,such as I-CSCF,

S-CSCF, etc. The

IMS core network

is responsible for

access control, callrouting, and servicetriggering.

EPC core network

deployment

The EPC core

network includes

the MME, S-GW,

and P-GW,

supports manyVoLTE-related

functions. APN

conguration,

which must be

performed onthe UE, MME,and P-GW, is a

key element of

EPC core network

deployment.

CS core network

reconstruction

The CS and VoLTE

networks will

coexist for a long

time to come.

In the future,the CS network

will be upgraded

to support IMS

centralized

services, thena unied corenetwork can be

built to improve

user experience

and operational

efciency.

LTE PCC

deployment

PCC architecture

provides E2E

policy control on

PS networks. In

PCC architecture,the SBC/P-CSCF

applies for VoLTE

voice and video

bearer resources

from the PCRF, thePCRF instructs theEPC and wireless

network to reserve

dedicated service

resources in order

to ensure QoS.

Service platform and

IN reconstruction

In terms of service

deployment, there

are two challenges.

The rst is service

experienceconsistency

for voice and

supplementary

services on 2G/3G

and LTE networks.The second isservice migration

from 2G/3G to LTE.

and call switching functions to the IMSnetwork. A unified core network canthen be built to improve user experienceand operational efficiency.

However, CS network reconstructionmust be minimal during the initialphase of VoLTE deployment. Toimplement eSRVCC, carriers need onlydeploy one or two eMSCs to functionas the SRVCC IWF to bridge the CSand EPC/IMS networks, as inter-system handovers are relatively few and

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in CS networks. This solution is notmature and requires tremendous CSnetwork reconstruction.

Solution 2: Users subscribe tooriginating CAMEL subscriptioninformation (O-CSI) and terminatingCAMEL subscription information(T-CSI). When they make or receivecalls on a CS network, it triggers theO-CSI or T-CSI to route the callrequests to the anchor applicationserver (AS), which then provides an

IMS routing number (IMRN). Basedon the IMRN, the CS network anchorsthe calls in the IMS network for serviceprocessing. This process is known asanchoring. It is easy to deploy butreduces service processing efficiency,so most carriers currently use mobileterminated (MT) anchoring only.

Solution 3: Parallel CS/IMS serviceprocessing: neither mobile originated(MO) calls nor MT calls are anchoredin IMS networks. This solution requires

Convergent HLR/

HSS deployment

Convergent HLR/

HSS (co-located

HLR, SAE-HSS,

and IMS-HSS)

avoids large-scalesynchronization

of authentication

or eSRVCC data

between the HLR/

SAE-HSS andIMS-HSS. It alsosimplies BSS/OSS

reconstruction.

Diameter signaling

network deployment

Diameter is the

basic protocol

for EPC, PCC,

and IMS network

communication.If the STP on

a live network

can be directly

upgraded to

support Diameter,it can be reused,with convergent

Diameter/SS7

relay equipment

deployed

otherwise.

BSS/OSS and

NMS deployment

VoLTE service

provisioning

involves many

NEs. The BSS/OSS

must be rebuilt tosupport various

service provisioning

interfaces, such

as VoLTE service

provisioning andbilling. VoLTE's all-IP architecture and

complex service

processes pose

great challenges

to networkmanagement.

Wireless network

reconstruction

The LTE network

must be upgraded

to support VoLTE.

Real-time telecom

services, such asvoice and video, are

sensitive to delay,

imposing strict

requirements on

E2E QoS, includingQoS on the wirelessside.

IP bearer network

reconstruction

IPv6 address

deployment is an

important task

for large carriers;

the UE, IP bearernetwork, EPC core

network, IMS core

network, and SBC

are all involved.

QoS improvementis important to thereconstruction of

bearer networks.

NEs, posing a great challenge to carrierand vendor integration capabilities.

Service platform andintelligent networkreconstruction

In terms of service deployment,there are two challenges. The firstis service experience consistency for

voice and supplementary services on2G/3G and LTE networks. The secondis intelligent network (IN) servicemigration from 2G/3G to LTE sothat VoLTE users can use value-addedservices such as virtual private mobilenetwork (VPMN) and 400.

There are three ways to resolve thefirst challenge.

Solution 1:Deploy an ICS architecture-based target network so that users canuse IMS services even when roaming

service data synchronization betweenthe home location register (HLR)/SAE-HSS and IMS-HSS.

Considering the availability ofsolutions and time to market, carriersshould select solution 2 or 3, based ontheir network conditions.

There are several ways to helpcarriers migrate IN services. Carrierscan deploy the IP multimedia serviceswitching function (IM-SSF) which,similar to the service switching point

(SSP) on the CS network, implementsIN service control by routing CAMELor INAP signaling to the service controlpoint (SCP) and online charging system(OCS). In this case, it is recommendedthat the IM-SSF be co-located with themultimedia telephony application server(MMTel AS). Carriers could also opt totransform an IN service platform thatalready supports or can be upgraded tosupport SIP as a SIP AS. This wouldallow the S-CSCF to contact the SIP AS


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through initial filter criteria (iFC) to implementservice control. Another solution would be toconfigure the MGCF to route interactive voiceservices to the MSC/SSP, on a CS network, tocomplete intelligent interaction. Carriers canchoose a suitable solution based on their networkconditions and specific services.

Convergent HLR/HSSdeployment

Convergent HLR/HSS (co-located HLR,SAE-HSS, and IMS-HSS) avoids large-scale

synchronization of authentication or eSRVCCdata between the HLR/SAE-HSS and IMS-HSS. It also simplifies BSS/OSS reconstructionbecause it doesn't require VoLTE subscriptiondata provisioning to two sets of equipment.The unified service subscription data ensuressupplementary service consistency on CS and

VoLTE networks, enabling the same serviceexperience whether users are on CS or VoLTEnetworks.

The optimal solution for convergentHLR/HSS deployment is HLR upgrade on the

live network so that it functions as both theSAE-HSS and IMS-HSS. If the HLR on a livenetwork does not support such an upgrade,a convergent HLR/HSS must be deployed,

with the legacy HLR either coexisting withthe convergent HLR/HSS indefinitely orbeing replaced directly. Coexistence allows forsmooth deployment, but it requires flexiblenumber routing (FNR), as most users aremigrated to LTE without a change to theirphone numbers. FNR distinguishes routesto the existing HLR and the newly deployed

convergent HLR/HSS. HLR replacement doesnot require FNR deployment, but it is stillrisky and requires comprehensive verificationof every serv ice scenar io . Carr ier s candetermine a database deployment solution andprocess based on their network conditions.

Diameter signaling networkdeployment

Diameter is the basic protocol for EPC,PCC, and IMS network communication andis used by interfaces connecting to the PCRF,OCS, and HSS. As stipulated in 3GPP, theDiameter routing agent (DRA) is used forDiameter signal routing aggregation and appliesto core network routing scenarios. As definedby the GSMA, the DRA is used for securityisolation in 3G/LTE inter-network roamingand interworking with legacy SS7 signalingnetworks; it applies to inter-network roaminginterwork scenarios. The DRA is deployedbetween the EPC, PCC, and IMS networksand binds sessions from the same user overdifferent NE interfaces to the same PCRF. DRAdeployment helps simplify deployment and

maintenance of Diameter signaling networksand accelerates service rollout.

As user s migrate to LTE, the requiredcapacity of Diameter signaling networks willincrease while the capacity of SS7 signalingnetworks will decrease. If the STP on a livenetwork can be directly upgraded to supportDiameter, it can be reused, with convergentDiameter/SS7 relay equipment deployedotherwise. With user migration and IP-basedreconstruction of SS7, convergent Diameter/SS7 networks will gradually replace legacy SS7.

BSS/OSS and NMSdeployment

VoLTE service provisioning involves manyNEs, including the convergent HLR/HSS,MMTel AS, IM-SSF, DNS, ENUM server,SCP, and other AS. The BSS/OSS must berebuilt to support various service provisioninginterfaces, with the call detail record (CDR)processing and online charging interfaces of

As users migrate from 2G/3G to 4G LTE, the

required capacity of Diameter signaling networks

will increase while the capacity of SS7 signaling

networks will decrease. Carriers are advised

to deploy Diameter/SS7 convergent signaling

networks. The SS7 networks will be completely

replaced as their lifecycles end.

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the MMTel AS and P-GW rebuilt to supportVoLTE service provisioning and billing.

VoLTE's all-IP architecture and complexservice processes pose great challenges to networkmanagement. Less than five NEs exist on theCS core network, while VoLTE will have morethan twelve core NEs. With VoLTE, far moreNEs and interfaces are involved in basic serviceprocesses, increasing their complexity. Carriersmust consider improving E2E troubleshootingand service provisioning efficiencies, buildinga service quality KPI system for LTE, andmanaging E2E voice quality in the all-IParchitecture.

Wireless networkreconstruction

The LTE network must be upgraded tosupport VoLTE. Real-time telecom services,such as voice and video, are sensitive to delay,imposing strict requirements on E2E QoS,including QoS on the wireless side. Accordingto test results, wireless-related features, suchas robust header compression (ROHC),transmission time interval (TTI) bundling, and

semi-persistent scheduling (SPS), can improvethe VoLTE coverage and system capacity. TheE-UTRAN NodeB (eNodeB) must be correctlyconfigured with neighboring GSM or UMTSlocation areas and eSRVCC measurementcontrol parameters. The configurations mustbe adjusted correspondingly as the networkchanges. In addition, the base station controller(BSC) or radio network controller (RNC) mustbe configured to support reselection from a2G/3G network back to a 4G network when the4G network is available.

IP bearer networkreconstruction

Generally, virtual private networks (VPNs)are used to distinguish signaling, media, andmaintenance management data from telco networksto improve service security. IMS signaling VPNsand IMS media VPNs are used to distinguish IMSsignaling from media, for example. Unlike 2G/3GPS users, whose IP addresses are automatically

recycled when they do not consume data traffic fora period of time, VoLTE users are always online.Therefore, IPv6 address deployment is an importanttask for large carriers; the UE, IP bearer network,EPC core network, IMS core network, and SBC areall involved.

QoS improvement is important to thereconstruction of bearer networks, as they mustbe enabled with DiffServ queue scheduling in

order to differentiate QoS. Priority parametermapping between the QCI and DSCP must beconfigured to prioritize VoLTE services overother packet services so that QoS for VoLTEsignaling, voice, and video services is assured.

VoLTE deployment involves many issuesrelated to operation and network optimization;all require persistent effort and rich experienceto overcome. We hope that carriers, networkequipment vendors, and device and chipmanufacturers all cooperate to promote thedevelopment of the VoLTE industry.

VoLTE deployment involves many issues related to

operation and network optimization; all require

persistent effort, and rich experience to overcome.

We hope that carriers, network equipment

vendors, and device and chip manufacturers all

cooperate to promote the development of the

VoLTE industry.


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