IMS for Enterprise Applications

Li Mo

（G lobal Marketing and Research Center, ZTE USA, 2425 North Central Expressway, Suite 323,Richardson, TX 75080, US）

Abstract:In response to the change of communication needs,IP Multim ed ia Subsystem（IMS）was envisioned to p rovide a sound session contro l for the m ultim ed ia conversional services.IMS technology has been slow ly adop ted by operators for variety of services.In fact,the IMS used in the enterp rise has d ifferent features and capab ilities than the one used by the genera l pub lic.This paper p rovides a very b rief overview of IMS and then investigates the IMS which is used in the enterp rise and focuses on the challenges in p rovid ing IMS to the enterp rise and ZTE's solutions to those issues.

I PMultimed ia Subsystem（IMS）［1］technology has been slow ly adop ted by operators for variety ofservices.The p rom ise ofdelivering and controlling themultimed ia conversional services for subscribers holds g reat appealand potential fora seam less integration of IT technology and telephony technology.

Traditionally,there are two d istinct g roups of customers in the telephony world.The Pub lic Sw itched Telephone Network（PSTN）［2］is created to serve the generalpub lic and many businesses.The PSTN is operated by a common carrierora telephone com pany.

Anothergroup ofcustomers is the business customer,which is normally served by Private Branch Exchange（PBX）located on p rem ises.Fora large corporation,a numberofPBXsmay be used and interconnec ted by a common carrier.The PBXw illalso be connected to PSTN for communications outside the corporation.The PBX is normally purchased,managed,and operated by the corporation.

There aremany unique characteristics ofPBX,such as short numberd ialing,executive p re-emp tion,numberd ialing policies,and numbering p lans［3］.Those features are very different from the ones offered to the general pub lic.In general,the ownership of the PBX,the interconnection of them in the multinationalenterp rise,management aspects,and operationalaspects of PBX c reated a lucrativemarketing segment forequipmentvendors and service p roviders,in the case ofmanaged services.

In today's enterp rise,the communication needs of the enterp rise are no longer telephony centric.For exam p le,the emailsystem in the enterp rise w illbemuchmore important than the PBX.Furthermore,the communication ismore peop le centered instead ofextension centered（i.e.PBX is only capab le ofcommunicating to a specific extension,nota specific person）.Today,peop le should communicate w ith peop le inmultimedia,notan im personalextension.Themerge ofcorporate IT infrastructure and communication infrastructure seems inevitab le,p roductive,and econom ical.

In response to the change of communication needs,IMSwas envisioned to p rovide a sound session control for themultimed ia conversional services.In fact,the IMS technology created the possibility ofmerging IT infrastructure w ith the communication infrastructure.Just like its telephony counterpart,the IMS used in the enterp rise w illhave different features and capabilities from the one used by the generalpub lic.

In this paper,the IMSused in the enterp rise w illbe investigated.These discussions w ill focus on the challenges in p rovid ing IMS to the enterp rise and ZTE's solutions to these issues.Before the detailed d iscussion on p rovid ing IMS to the enterp rise,a very briefoverview of IMS is p rovided in the nextsection to setup the context for furtherd iscussions in the subsequentsections.

1 IMS Overview

The IMSwas originally designed for mobile consumermarketby 3GPP.Its main function is to p rovide session control for the packet-based mobile network.With the advances ofmobile access technologies,the packet-based w ireless access network demands IMS technology to p rovide needed control.This factmakes IMS technology an integ ralpartof the futuremobile network.

The IMS architecture has also been slightlymodified for the fixed network by Telecommunications and Internet converged Services and Protocols for Advanced Networking（TISPAN）ofETSI.By encompasses both the fixed and mobile networks,the IMS became the only standard means ofsession control for the packet-based network.

IMS is both network architec ture and a setofstandards which were p roposed and standard ized by the IETF.Instead of p roposing new standards by IMS,those IETF standards have been＂p rofiled＂（i.e.IMSarticulates how those standards should be used）to build an end-to-end solution for the service p roviders.

In the p rotocolaspects of IMS,the basic p rotocols are RTP（Real-Time TransportProtocol）for themed ia flow,Session Initiation Protocol（SIP）for session control,and Diameter for authentication and accounting related matters.In IMS technology,those p rotocols are to be im p lemented overa p redefined architecture for conversationalservices over the packet network using IP technology.

▲Figure 1. IMS architecture in mobile network.

In the architecture aspects of IMS,the service controlw illalways be p rovided by the home network,which the subsc riberand the service p roviderhave business relationship.In fact,the services w illbe p rovided by the app lication servers in the home network where the session information is p rovided by an architecture com ponentcalled Serving CallSession ControlFunction（S-CSCF）.The S-CSCF in the IMS network has to hand le subsc riber registration and session control（session setup,maintenance,and tear-down）.

In order to supportmobility,twomore CallSession ControlFunctions have been defined in the IMS architecture.One is the Proxy CallSession Control Function（P-CSCF）whichmay be located in the visited network or the home network,depend ing on subscriber's location.The subsc ribermay nothave business relationship w ith the service p roviderwhose P-CSCF is used（i.e.P-CSCF located in the visited network）.But the home network service p roviderand the service p roviderwhose P-CSCF is used w illneed to have roam ing ag reement.

The otherCSCF to supportmobility is Interrogate CallSession ControlFunc tion（I-CSCF）whose purpose is to locate the S-CSCF in the home network.In general,I-CSCF is the entry pointof the home network from any roam ing partners.The IMS architecture is shown in Figure 1 for easy reference in the subsequent sections.

The IMS technology also add resses severalissues which are important for service p roviders,inc lud ing roam ing control,QoS negotiation,app lication invocation and sequencing,charging,and interoperation w ith circuitsw itched networks.

Developed forpub licmobile users,the IMShas also been dep loyed in the enterp rises.There is certainly a lotof sim ilarities and differences between session controlmechanisms forboth generalpub lic and enterp rises.In the nextsection,these sim ilarities and d ifferences w illbe articulated.

2 Enterprise IMS

At the session control level,there is little d ifference between an enterp rise focused IMS system and a general pub lic focused IMS system,in both mobile and fixed environment.In technicalterm s,there is no difference at the call flow levelbetween those two differentenvironments（i.e.enterp rise environmentand pub lic environment）.

The d ifferences w illbe in the app lications,and themanagementof those app lications.In the IMSworld,the app lications are p rovided by the app lication servers.Hence,allthe enterp rise specific features w illbe p rovided by the app lication servers.There are three d ifferentscenarios on how those app lications are p rovided.

2.1 Managed App lications

In this case,the service p roviderw ill p rovide app lications w ith enterp rise specific features,just like themanaged Centrex services p rovided by carriers.

The entire network and its app lications w ill reside inside the network.The em p loyees w illbe ab le to use the network services viamobile devices,and fixed devices.The fixed access w illbe connected to a setof P-CSCF inside the carrierwhile the mobile accessmay use one of the carrier's P-CSCFora roam ing partner's P-CSCF.

Geog raphic coverage w illbe p rovided by the service p roviderand its roam ing partners,even for the fixed access,due to the geog raphic locations ofvarious b ranch offices.Considering that the home service p rovider p rovides the app lications,the user interface w illbe consistent regard less ofgeog raphic locations.

Themanaged app lication scenario w illbemore suitab le forsmallbusiness for its lack of technicalexpertise,small working force,and less sophisticated app lication requirements.Figure 2 depicts this particularscenario.

This app lication scenario is sim p le for enterp rise butw ith lim itations.First,the enterp rise features w illbe lim ited to whatever the service p rovider is offering.Second,the change ofanything,from

emp loyee enrollment to security policy,has to be coordinated w ith the service p rovider.Third,because the geog raphic coverage is p rovided by the roam ing partners,the costwould also be an issue.

▲Figure 2. Service provider centric IMS.

ZTE's IMS portfolio w ill fully support this app lication scenario.

2.2 Enterprise-Based Applications

In this app lication scenario,the enterp rise w illuse the p rem ises-based app lication server.The S-CSCF in the carrierw illinteractw ith the p rem ises based app lication server,across the standard ISC interface,to perform the sessionmanagement related activities.

In the enterp rise based app lication app roach,it is possib le for the service p rovider to p rovide value-added app lications in the network in conjunction w ith the enterp rise-based app lication servers.In this case,the service p rovider maymaxim ize the add ressab lemarket for certain app lications while refraining from getting involved into enterp rise specific app lications and its related management.This app lication scenario is shown in Figure 3.

Ifam ixture ofnetwork based app lication serverand p rem ises based app lication server is used,the feature interaction and sequencing between those app lications need to be carefully arranged.Anymod ification ofenterp rise app licationmay involve datamod ification in HSS,whichmay notalways be feasib le.

The advantage of this app roach is the flexibility ofapp licationmanagement whichmay be tailored towards the specific enterp rise and its working model.In the enterp rise based app lication scenario,the geog raphic coverage w illalso need to be covered by the roam ing partnerswhich would be a cost issue.On the otherhand,because it is the home service p rovider that p rovides the needed app lications,interface consistence can be achieved.

Considering that ZTE's IMS portfolio is com p letely standard based,thismode ofoperation can be accom p lished.

2.3 Enterprise-Based IMS

In this case,the enterp rise takes the comp lexity ofowning and managing the IMS network.In particular,the enterp rise may own various CSCFs,the HSS,and the app lication servers.In p ractice,only largemultinationals w illhave the budget and the technicalexpertise tomanage a com p lete IMSnetwork.

The advantage ofsuch app roach is userexperience consistence across d ifferent regions,management flexibility,and security.There are twomeans to p rovide geog raphic coverage:via the enterp rise based IMS directly and via a roam ing partner,whose P-CSCFw ill interactw ith the I-CSCF in the enterp rise.

A dep loymentscenario which could be of interestis that the enterp rise based IMSw illbe responsib le for the fixed access and Wi-Fiaccesswhich is also p rem ises based.The cellularaccess w ill be p rovided by roam ing mobile operators due tomobile licenses issues and geographic coverage issues.The enterp rise based IMSw illbe the home IMS for the enterp rise and allcellular access w illbe considered to be roam ing access.Figure 4 illustrates this scenario.

In this app roach,in particular,various b ranch officesmay be connected to a centralized P-CSCF formanagement and cost reasons.ZTE has dep loyment experience for such network configuration（an enterp rise based IMS network w ith centralized P-CSCF）.

While this app roach hasmany freedoms and flexibilities,the costof such app roach,especially the maintenance and operation ofsuch network,would be p rohibitive for small-ormed ium-sized businesses.ZTE's IMS solution has been p rovid ing this type ofaccessmechanism from the very beginning and is well poised to take challenges p resented in the enterp rise-based IMS,which is be discussed in the nextsection.

3 Challenges of IMS in Enterprise Environment

There aremany challenges in dep loying the IMS in the enterp rise.In this paper,two ofsuch challenges,togetherw ith ZTE's solutions,w illbe d iscussed.The firstchallenge is emergency support for centralized P-CSCF,and the otherwould be the Network Add ress Translation（NAT）and IP version（IPv4 v.s.IPv6）m ismatch.

▲Figure 3. Promises-based application.

◀Figure 4.Enterprise-based IMS.

3.1 Emergency Call Support

The emergency callsupport for dep loyment in Figure 2 does notp rovide any challenges due to the fac t that the P-CSCFw illbe local,and the standard emergency hand ling mechanisms can be used.

The emergency callsupport for dep loymentin Figure 3may notp resent any issue if the enterp rise based app lication does notperform special p rocessing of the emergency call.If the enterp rise based app lication p rovides specialp rocessing for the emergency call,how to supportsuch callis an open question.In p rac tice,if the enterp rise based app lication server p rocesses the emergency callin a specific way,the enterp rise should considerdep loying the com p lete IMS inside the enterp rise,such as the one dep icted in Figure 4.

Due to the sharing nature of the P-CSCFamong many differentbranches for the centralized P-CSCFsetup,the emergency callp rocessing by the P-CSCFneeds to d irec t the call to the p roperpub lic safety site.This can be achieved via carefularrangementof the source IP add resses of the d ifferent branch offices.

Ifspecialp rocessing of the emergency callis required in the app lication,the P-CSCFcan be configured to hand le the emergency call as a normalcalland the p roper app lication can be a trigger to further p rocess the emergency support.

3.2 NAT and IP version

In the p revious d iscussions,the branch office is dep icted as if there is only one fixed access point.In p ractice,this is muchmore com p licated.In fact,the b ranch office w illhave the comp lexity of a comp lete IT infrastructure of the enterp rise

In particular,there w illbe NAT［4］involved to solve the IPv4 add ress shortage p rob lem and p rovide certain security features.There could be IP versionm ismatch among communication partners（IPv4 and IPv6［5］）.

If the service p rovider（in the case of scenarios depicted in Figures 2 and 3）does nothave controlof the NAT in the b ranch offices,the IMS c lients inside the b ranch office need to ob tain a pub lic add ress before the firstSIPmessage can be sentout.There aremanymeans to obtain a pub lic add ress,such as Interactive Connectivity Estab lishment（ICE）and TraversalUsing Relay NAT（TURN）,which has to be used in any SIP messages.

For the scenarios in Figure 4（or the service p rovider canmanage the NAT in the enterp rise IP gateway in Figures 2 and 3）,the P-CSCF,togetherw ith its SIP App lication LayerGateway（ALG）function,can be used to supportNAT traversaland IP versionm ismatch issues.In particular,ZTE has developed technology which w illsatisfy the follow ing criteria:

（1）The solution w illensure that Network Add ress and PortTranslation ProtocolTranslation（NAPT-PT）w illonly be invoked once inside the network for themed ia path.

（2）NAPToperations w illbe combined with the NAPT-PToperations wheneverpossib le.

（3）The solution w illcause detourof themed ia traffic due to NAPT-PTusage only when there is no NAPT-PT capab le network elements along the normal med ia path.

（4）The solution w illbe independent ofnetwork IPv4 add ressing p lans in the access network.

（5）The solution w illavoid unnecessary invocation ofservice logic in S-CSCFbecause of recursion of the signaling path.

ZTE's solution,which satisfies the abovementioned criteria,w illp rovide the mostcost-effective solution to the NAT and IP versionm ismatch p rob lem.

4 Conclusions

Providing IMS in the enterp rise w illhave a lotof issues and challenges.In this paper,various possib le dep loyment scenarios,along w ith issues w ith those scenarios,have been d iscussed.The enterp rise IMS forestab lishing the multimed ia conversationalservices can be analogue to the PBX in the telephony for voice services,where flexibility,security,and manageability are very d ifferent from the IMS system defined for generalpub lic.