Combining Enterprise VoIP with
2G/3G Mobility
Wouter Franx, Bob Smeets and Jacopo Pianigiani
Enterprise customers are looking for more
cost-efficient and more integrated solutions for their fixed
and mobile communications needs. This paper
evalu-ates the different options to combine ‘the best of
both worlds’ of enterprise VoIP solutions on the one
hand and full mobile networks on the other hand.
Also it will describe the opportunity to support
roaming and hand-over services between Wi-Fi and
2G/3G cellular network, while ensuring seamless
service behaviour to the end-user.
Introduction
Today, enterprise customers often need to rely on both a fixed network service as well as a mobile network service for their communications needs. These fixed and mobile services are usually provided in parallel and in relative isolation of each other. For example, certain employees of a company have both a fixed phone (with fixed number) and a mobile phone (with mobile number), but the integration between the two respective services is often minimal. In a drive to increase reachability and employee effectiveness, while lowering total cost of ownership, enterprises are looking for more integrated solutions.
The respective fixed and mobile infra-structures that these enterprises are using for their communications needs – that is, increasingly enterprise voice over IP (VoIP) on the fixed side and 2G/3G on the mobile
side – also have very differing business and technology backgrounds. On the supply side different players are active (mobile operators versus enterprise VoIP solution providers), technologies are different (TDM voice for mobile versus VoIP for enterprise), and network domains are different (wide area mobility versus enterprise).
With the further increasing of conver-gence between fixed and mobile networks, this poses a challenge for solution providers to integrate these domains, and to develop more integrated service solutions for the enterprise segment (refer to Figure 1).
Mobile operators have developed ‘mobile VPN solutions’ where enterprise PBXs can be connected to the mobile network infrastructure, hereby creating a virtual private network (VPN) numbering plan and associated VPN billing plan. However, these solutions are based on traditional circuit technology and cannot easily take advantage of new service capabilities enabled by next-generation network (NGN) and VoIP technology.
PBX vendors are adding ‘mobile extensions’ to their IP-PBX solutions, thus trying to attain a first level of integration between fixed and mobile service. However, these PBX solutions lack specific value and efficiency that can uniquely be provided from the service provider network.
Another ‘leaf on the branch’ of fixed mobile convergence is the support of dual-mode Wi-Fi/GSM handsets with roaming and handover. Through converged Wi-Fi/ GSM solutions, enterprises hope to lower the costs and improve the coverage of in-building calls to mobiles.
At the moment, various solution approaches are being considered by the industry, each of them with its own stage of maturity. These solution approaches range from ‘give everybody a mobile phone and throw away the PBX’ to ‘deploy Wi-Fi-enabled IP-PBXs’. Probably as a result of this multitude of solution approaches, a recent commentary from Ovum† (based on a round table conference with service providers, solution providers and repre-sentatives of enterprise telecom managers) concluded that there is quite some ‘confu-sion’ in the market. Also, the view was
† See http://www.ovum.com/go/content/ c,55761
Authors
Wouter Franx and Bob Smeets Lucent Technologies
Larenseweg 50 P.O. Box 1168 1200 BD Hilversum The Netherlands
Emails: [email protected]; [email protected]
Jacopo Pianigiani Lucent Technologies Via Cesare Giulio Viola 65 00148 Rome
Italy
Email: [email protected]
Figure 1
Challenge of integrating two different domains?
IP PBX IP Centrex LAN SIP Control Access IP IP VPN+ Services GSM Network INAP CAP Control Access TDM TDM
Enterprise VoIP 2G/3G Mobility
MSC
?
expressed that the enterprise solutions offered by the mobile industry ‘are weak and lack the maturity of fixed solutions’. This paper examines the market and technology developments, and identifies a number (not all!) of solution approaches for service providers – being either fixed, mobile or combined – and analyses the pros and cons respectively.
Market Drivers and
Segmentation
The market requirements for converged enterprise solutions are quite dependent on the target segment; that is, large enterprise versus SME. Large enterprises often have their own fixed voice network (TDM PBX, IP PBX) and high investments in local infra-structure. Moreover, the IT managers usually have strong control over the network. As a result, in general, large enterprises are more ‘slow’ to migrate to new converged solutions.
The SME market however is less sophisticated in terms of infrastructure and employees are more open to a ‘mobile-centric’ solution. So here the opportunity to introduce fixed-mobile converged solutions is more apparent.
For both market segments, we can identify several common market drivers and requirements:
• reduce cost by moving from dual (non-integrated) network infrastructure (fixed PBX + mobile) to single converged network ➔ single subscription + single handsets + integrated feature set; • avoid/control increasing costs as a result
of increasing mobile usage, especially international roaming costs, fixed-to-mobile access charges, etc;
• allow reachability of the enterprise via both fixed and mobile numbers; • lower the cost and improve the coverage
of in-building calls to mobiles (for example, by using Wi-Fi as an alternative technology to 2G/3G);
• provide convenience of PBX features on or in combination with mobile phones; and • support single number and single
voicemail solution from call originator and call recipient point of view.
As enterprise customers provide a strong revenue source for service providers, these service providers will look at a combination of new revenue generation, improved customer retention and new cost-efficient solutions: • derive new revenue streams via new
service offerings (for example, substitut-ing revenues that are normally gained by fixed network operator or vice versa); • improve spectral efficiency – get more out
of the spectrum (for example, using Wi-Fi for hot spots and in-building coverage);
• combat subscriber drain to alternative communications technologies; and • own the end-user, increase revenue per
subscriber.
The following sections describe the solution architectures that address the above requirements, today and in the future.
TDM-Based Mobile (and
Fixed) VPN Solutions
In legacy 2G networks, voice VPN services have been offered for quite a long time. The majority of the voice VPN service offers try to address the need of a corporation to emulate the behaviour of a PBX between GSM company mobile phones and the PBX extensions, but, due to some intrinsic architectural limitations, the majority of features that a legacy voice VPN solution can offer falls into the categories below: • private numbering plan (ability to dial
short codes/extensions to make phone calls across phones – either PBX exten-sions or mobile phones);
• charging control (ability to provide attractive pricing for calls and features usage between mobile phones and PBX extensions, ability to discriminate between personal and professional calls); • call screening (originating screening,
terminating calls screening); and • a limited set of PBX-like features to GSM
phones, mostly provided by the mobile switching core network nodes (MSCs) themselves, some of them provided by the intelligent network node (this is mainly because intelligent network services are not usually designed to provide PBX features).
Several solution layouts are possible to implement this service offer: one approach consists of connecting directly the corporate PBX to the MSC via E1 lines (for example, by ETSI PRA interfaces as shown in Figure 2, other derived protocols – for example, DPNSS, Q.SIG – or via other legacy signalling protocols – for example, various CAS signalling variants). Another consists of
‘virtually’ connecting the PBX to the operator’s network via carrier selection, provided the operator is allowed to use this service for this purpose by local regulations.
The first approach causes significantly higher costs of the service infrastructure due to the leased line needed to connect the PBX, and limits the general scalability of the solution.
The second approach does not require additional operating expenditure to imple-ment the service infrastructure but requires either modifications to the legacy PBX (which is not necessarily achievable) or additional components that implement carrier selection dialling on a call basis on PBX-outgoing calls.
Concluding, these mobile VPN solutions only allow limited convergence and – because of their circuit-switched back-ground – do not take advantage of new service and network capabilities enabled by new VoIP technology.
Integrating Enterprise
VoIP
Where VoIP is expanding its deployment and maturity in the enterprise segment, the question is valid on how service providers will anticipate on integrating and converging enterprise VoIP solutions with their mobility solutions. Strategies of service providers are likely to be different, dependent on whether they are for example a well-established mobile-only provider, a fixed-only provider or a combination of both.
On the solution side, well-established GSM operators will likely opt for a so-called ‘GSM-centric’ approach where the large investments in the applications and service control layer of the mobile network can be leveraged, and VoIP is added as a new access technology (evolutionary model). However, smaller and new entrepreneurial operators might choose for a VoIP-centric approach where GSM is merely used as a (TDM-based) wireless access technology and a new VoIP control infrastructure is
Figure 2
TDM-based Mobile VPN solutions
Fixed Network PBX 2G/3G Network INAP CAP TDM (ETSI PRA) MSC VPN TDM Connect (leased line)
introduced to provide converged services across both GSM and VoIP access (overlay model). This is illustrated in Figure 3.
Although it is quite common to highlight the reusability of certain approaches for almost every type of requirement, we acknowledge that it is not seamless to have a common solution architecture for every applicable purpose. The bottom-line of this is that the solution to be adopted has to be tailored to the market requirements, the type and characteristics of subscribers that a specific operator is targeting (mobile or wired, legacy TDM or VoIP, large corpora-tions or small and medium enterprises) and the network/infrastructure assets in which an operator has already invested.
Under these constraints, four solution approaches are further described in the sections below.
• The Mobile VPN with Enterprise VoIP Connect solution is a VoIP-based version of the TDM-based predecessor, and does not provide truly converged control. • The IP Centrex with Mobile Extensions
solution is a typical example of a VoIP centric approach.
• The Mobile VPN with VoIP Virtual MSC solution is typical example of GSM-centric approach.
• The Hybrid IP Centrex/Mobile VPN solution is an example of hybrid control for both VoIP and the GSM domain.
Mobile VPN with Enterprise VoIP
Connect
In essence, the Mobile VPN with Enterprise VoIP Connect as shown in Figure 4 is the VoIP-based variant of the current based solution (Figure 2). Here the TDM-based PBX is replaced by a IP-PBX or hosted IP-Centrex platform. The IP-Centrex approach allows service providers to offer a fully hosted VoIP/mobile VPN service from dedicated or shared platforms in the network, such that the enterprise customer does not need to invest in and operate its own infrastructure. The solution also provides the opportunity for mobile operators to fully take over both the ‘fixed’ and the ‘mobile’ traffic from the enterprise
and bundle this into an integrated offering, although the fact that the two different domains (wireless and wireline/VoIP) are managed by different service layers limits somehow the cross-domain functional integration. Furthermore, as the solution is IP-based on the enterprise side, there is no need anymore for E1 leased lines into the mobile VPN network. Instead, the traffic can be offered via VoIP into a centralised media gateway (MG) within the mobile network, converting the VoIP traffic to circuit-based voice for the MSCs.
In this architecture, the end-users get the service from the network (IP network or mobile network) they are attached to. This implies there is no seamless service between the two domains, as the end-user feature set will be different across the two domains. Therefore, similar to its TDM-based pred-ecessor, this solution architecture still implies a coexistence of mobile VPN and enterprise VoIP rather than convergence.
IP Centrex with Mobile Extensions
A VoIP-centric approach can be realised with the IP Centrex with Mobile Extensions architecture (Figure 5). All calls to/from GSM handsets are routed via the IP Centrex application. In this way, the service and application control is centralised in a single platform, such that uniform service behav-iour can be provided across both VoIP terminals and GSM handsets. This also allows support for all services (for example, attendant/receptionist, boss/secretary – and all in combinations between fixed SIP end points and mobile phones) where real-time monitoring of the status of the end point is essential. Furthermore, the solution allows
Figure 4
Mobile VPN with Enterprise VoIP Connect
Fixed Network 2G/3G Network VPN INAP CAP MG IP PBX IP Centrex MG TDM VoIP Trunking IP Network SIP MSC SIP
Figure 5
IP Centrex with Mobile Extensions
Fixed Network MSC MG IP PBX IP Centrex MG TDM VoIP IP Network SIP SIP 2G/3G Network
Figure 3
Strategies for enterprise VoIP-mobility integration
GSM Centric VoIP Centric Hybrid
Control TDM VoIP Hybrid
Access TDM + VoIP VoIP + TDM VoIP + TDM
Solution Example Virtual VoIP MSC IP Centrex with Mobile Extensions Hybrid IP Centrex Mobile VPN
for one-number reachability (via either fixed number or mobile number or combination) where the end-user can fully control whether incoming calls are routed to the VoIP terminal, the GSM handset or both (simultaneous ringing).
A disadvantage of this architecture is that existing investments in mobile VPN functionality in the mobile network cannot be fully leveraged. Furthermore, the solution may raise scalability concerns as all calls to/ from GSM handsets need to be routed to the VoIP domain. This will impact the trunking and port capacity on existing MSC switches in the mobile network.
Mobile VPN with VoIP Virtual MSC
A GSM-centric approach can be taken through the deployment of a VoIP virtual MSC as shown in Figure 6. Here a ‘virtual’ MSC is provided in the IP domain. To the mobile VPN application, the VoIP MSC will appear as a regular MSC with integrated VLR functions. To the VoIP terminals, however, it will appear as a SIP server. In this way, the VoIP terminals are just ‘another type’ of GSM terminals from a VPN applications point of view. This also implies that the mobile VPN application can provide single point of control for both network domains.
A disadvantage of the architecture is that the end-user feature functionality remains limited to the features that existing GSM and mobile VPN applications can deliver in today’s networks. This implies that new features (like attendant/reception-ist) may not be easily introduced, without lengthy and costly upgrading of existing platforms. Also, the architecture is not really geared to support the one-number service as described with the IP Centrex with Mobile Extensions solution. The VoIP terminal will get a new (=second) mobile number in addition to the mobile number of the GSM handset.
Hybrid IP Centrex/Mobile VPN
Another variant allows a hybrid solution approach that combines some of the attributes of the previous variants. The Hybrid IP Centrex/Mobile VPN architecture (Figure 7) contains a logically centralised application platform that can control both the VoIP domain and the mobile domain. This implies that also specific control information (for example, line status of a line, user profile logic) can be shared across both domains. For example, for supporting the attendant feature, the application has access to both the VoIP and the mobile
domain, as to how to distribute incoming calls to available colleagues.
From an implementation point of view, the hybrid application may also be realised through separated IP Centrex and mobile VPN platforms that allow dynamic interac-tion via either SIP or PINT and SPIRITS interface (see alternative configuration in Figure 7).
The architecture intends to minimise hair-pinning of calls between the VoIP and mobile domain, which provides an advan-tage compared to the IP Centrex with Mobile Extensions architecture. A disadvantage however remains that users connected to the mobile network may still have limited features (through limitations of the legacy mobile network).
However, an evolutionary approach toward this target can be achieved by gradually evolving a legacy VPN solution to – by taking a step-by-step approach – include incrementally both the support of SIP end points accessing legacy features and then, as a subsequent phase, enhancing the legacy VPN including more advanced features; for example, all those services that are based on line status monitoring. Although we acknowledge a certain level of complexity in implementing those services across legacy and new domains (SIP and GSM), nevertheless this approach, if appropriately designed as an evolution plan, can result in capital expenditure reuse for next-generation applications.
Integrating Wi-Fi/GSM
Roaming and Hand-over
The previous sections addressed the integra-tion and convergence of VoIP domain with the existing mobile domains, especially at the
control layer. It did not specifically address
the possible access technologies like DSL, cable, Wi-Fi, and private LAN that can carry
Figure 6
Mobile VPN with VoIP Virtual MSC
Fixed Network 2G/3G Network MSC MG VPN MG TDM IP Network VoIP SIP Query INAP CAP INAP CAP Query VoIP MSC
Figure 7
Hybrid IP Centrex/Mobile VPN
Fixed Network 2G/3G Network MSC MG IP Centrex and VPN+ Services MG TDM IP Network VoIP SIP Query INAP CAP INAP CAP Query 2G/3G Network IP Network IP Centrex MG SIP, PINT, SPIRITS
SIP IN GW MSC INAP CAP VoIP Alternative Configuration
the VoIP-based services. Clearly, Wi-Fi is receiving major interest from the industry, as it provides a potentially lower-cost alternative for the licensed 2G/3G spectrum. So with the introduction of Wi-Fi for in-building coverage and hot spots, further convergence can also be realised at the access layer. From the end-user perspective this implies that users can roam from Wi-Fi domain to GSM domain, and ideally also support roaming across the two domains. The sections below provide architectural alternatives to support Wi-Fi/ GSM roaming. In a sense they provide an ‘extra dimension’ to the solution architectures presented in the previous sections.
Unlicensed Mobile Access (UMA)
The Unlicensed Mobile Access (UMA) Forum is a well-established industry activity with memberships including equipment vendors and service providers. The solution defined by UMA provides the ability for GSM operators to integrate GSM and Wi-Fi/ Bluetooth networks, supporting both GSM and GPRS-based services. The solution architecture is shown in Figure 8. A dual-mode handset for UMA allows access via either Wi-Fi or the regular GSM air interface. In both cases, all traffic is handled by the existing MSCs in the GSM network. A new UMA network controller is added to control and handle the Wi-Fi-based traffic and interface this into the MSC. Therefore, from the MSC point of view the arrangement is quite transparent, and the solution also provides roaming and hand-over.
The UMA approach has definite appeal to mobile operators, as they can leverage existing GSM infrastructure to deliver
seamless GSM services over Wi-Fi. Cur-rently, multiple UMA trials are being conducted in Europe.
On the other hand, UMA is also believed to have some weaknesses. With UMA the services are limited to the current 2G-based circuit-switched services sup-ported from the MSCs. Therefore it may not provide an evolutionary approach to new all-IP IMS-based services (blended serv-ices). Combined with the limited availabil-ity of dual-mode handsets, some analysts believe that the window of UMA is closing. In addition there is another major disad-vantage in UMA, which resides in binding an ‘access component on a non-typically licensed spectrum’ to a standardised (3GPP, 3GPP2) core network. In order to maintain a seamless end-used experience while core network technologies evolve over time, UMA will need to consider embracing new access technologies both from a standardisation and implementation point of view.
IP Centrex with Wi-Fi/GSM
Roaming
Building upon some of the principles of the IP Centrex with Mobile Extensions architec-ture as presented above, it is possible to extend this architecture to also support Wi-Fi/GSM roaming. The high-level architecture is shown in Figure 9. The centralised IP Centrex control is complemented with a mobility manager function that intelligently communicates with the dual-mode handset on whether it is reachable via Wi-Fi or GSM. The IP Centrex application uses the mobility manager to route calls to the dual-mode handset accordingly (via Wi-Fi or GSM). In principle, this architecture can also support hand-over when a user roams from Wi-Fi to GSM and vice versa, as long as the IP Centrex has centralised control of the call. Such hand-over can be supported through an ‘underwater’ call transfer from for example Wi-Fi to GSM, without the end-user
noticing. This solution however requires dedicated dual-mode handsets with special software to support the above.
In this solution, the GSM network is mainly used as a transparent access network. For mobile operators with large GSM investments this may be less attrac-tive, as they have less opportunity to leverage their service and application platform. The solution might be attractive for fixed-only operators who want to complement their IP Centrex service offering with GSM access.
IMS-based Wi-Fi/GSM Roaming
Last but not least, an IMS-based solution for Wi-Fi/GSM roaming is shown in Figure 10. As the IP multimedia subsystem (IMS) is generally viewed as the target architecture for NGN and fixed-mobile convergence, it provides a good foundation to also support Wi-Fi/GSM roaming.
The IMS session control (CSCF func-tions) allows the hosting of multiple applications, including the blending of these applications. The mobility manage-ment function is similar as in the previous section and basically provides a ‘virtual MSC/VLR’ function within the IMS. Through this arrangement, it will be possible to roam with other 2G/3G mobile networks.
The major advantages of this architec-ture are its ability to provide a seamless service experience independently of the type of radio access technology – and potentially terminal type – being used. In fact in IMS, the IMS core can:
• include handling of calls originated from Abis interface on the GSM side; and • include VoIP calls handling from either 3G
access or 4G access (Wi-Fi, WiMAX, etc.) and still, by leveraging a mobility manage-ment function, provide seamless MAP-based roaming across different technologies.
Through the IMS architecture where HSS, HLR and mobility management
Figure 8
Unlicensed Mobile
Access (UMA)
2G/3G Network INAP CAP VPN Base Station Controller UMA Station Controller DSL Cable Wi-Fi AP Dual-Mode Handset MSCFigure 9
IP Centrex with Wi-Fi/GSM Roaming
MSC IP Centrex TDM IP Network VoIP SIP Mobility Manager Fixed Network MG MG SIP 2G/3G Network Wi-Fi AP Dual-Mode Handset
Biographies
functions can be supported on a single platform, it will allow all dual-mode handset subscriber information to be maintained in one place, thereby reducing provisioning costs and allowing the exchange of feature activation information between GSM and IMS systems.
Given the typical IMS service layer architecture, that foresees the access to next-generation applications (for example, IP Centrex services) the handling of interaction toward a single service layer is managed by for example an IM-SSF function as foreseen in IMS standards that ‘adapts’ the client domain signalling (SIP or INAP/CAMEL) to the application server or through an application server that can support both legacy and SIP end points, as seen for the hybrid IN/Centrex approach.
Conclusions
Today’s communications services are offered to enterprises in relative separation. Enterprise users are driving the demand for communications convergence for its lower costs, more convenience and potentially better indoor coverage. Network operators serving these enterprises are faced with challenges when supporting the conver-gence due to the different network technolo-gies currently in place, both in the operator as well as in the enterprise domain.
In this paper, four different options are described that facilitate enterprise VoIP and mobility solution integration on the network control side. Each solution is qualified and, depending on the context of the situation, a particular solution may be beneficial for the operator. In addition, three directions are described that increase mobility for the enterprise end-user. Due to readily available
broadband IP-access infrastructure within the enterprise domain, mobile communica-tions services based on IP-based access technologies are becoming increasingly
market-ready, thus bypassing the licensed 2G or 3G public land mobile network when indoors.
A reference architecture for Wi-Fi/GSM mobility is proposed based on the IP multimedia subsystem architecture. IMS provides the service provider maximum flexibility to address the wide variety of enterprise configurations and service needs that exist in the market place.
With the advent of new converged services for enterprise users that provide further benefits to the end-user (beyond basic voice and messaging), traditional network elements like the PBX, or 2G MSCs, are likely to become a bottleneck preventing a smooth migration of services to the next-generation era. Solutions are commercially available today that provide converged end-user experience from networks that are still relatively apart.
VoIP and IP Centrex allow mobile operators to take a larger share of enterprise voice revenues in different networking models, while VoIP and IP Centrex easily allow VoIP operators to provide mobile extensions as part of their offering as well.
Wouter Franx joined AT&T in 1987 and held several functions in sales and product management. He worked on various customer projects in the
area of intelligent networking, switching and NGN. During the last four years he has gained experience with various VoIP and NGN projects for major service providers in the EMEA region. Currently, he is a network architect in the Network Architecture & Technology Strategy group within Lucent EMEA. He holds a degree in Electrical Engineering from Delft University of Technology.
W. G. Franx
Lucent Technologies
Bob Smeets holds a degree in Applied Physics from Delft University of Technology. He joined AT&T (later Lucent Technologies) in 1996.
His initial assignment was with 5ESS product development to design and implement a framework for regulatory services. Then he joined the specification department where he
R. A. M. Smeets
Lucent Technologies
Jacopo Pianigiani has more than 12 years of experience in the telecommunications industry. He currently works in experience to Lucent’s Global Account Team for Vodafone. His
group is responsible for Lucent’s technology strategy and technical support for Vodafone. He joined Lucent in 1999. Before that, Jacopo has worked for Vodafone UK in 1992, then in 1993 as network architect in a consortium bidding for the second GSM licence. Subsequently he has lead the Design and Implementation team in Telexis, the telecommunications branch of FIAT Group in charge of providing all services (voice, data, LAN, call centres) to all FIAT companies. From 1998, he moved to Wind, a service provider, to lead the Business Market Technical Presales team. Jacopo graduated in Electronic Engineering in 1993.
Jacopo Pianigiani
Lucent Technologies
became responsible for software production process improvements in the switching portfolio of Lucent Technologies. Since 2000 he has been working in technical pre-sales in the area of voice over packet and signalling toward major network operators in the North-West of Europe.
Figure 10
IMS-based Wi-Fi/GSM Roaming
MSC TDM IP Network VoIP SIP Fixed Network MG MG 2G/3G Network Wi-Fi AP Dual-Mode Handset I-CSCF P-CSCF S-CSCF Mobility Management Handoff VLR Services Services Services IMS Services HSS/ HLR IMS VLR Services MAP
