Femtocell Core Network Integration: IP RAN or SIP/IMS?

 

Dr. Asa Kalavade and Jonathan Morgan - Tatara Systems

 

 

Introduction
Femtocell access points (or 3G access points) are cellular base stations designed for use in residential environments that allow a user with an existing mobile phone to access cellular voice and data services over IP. Femtocells allow service providers to extend the reach of their services to users within a “home zone”, while leveraging the user's broadband connection. This not only allows operators to address coverage holes, but also gives them an opportunity to potentially shape end-user behavior by encouraging the use of 3G data services in the home femtocell where they can be faster and cheaper. In addition, mobile operators can leverage the user's IP backhaul to reduce their operating expenses and spectrum costs – all without requiring any changes to the user's handset. The business case for the operators typically involves reduced customer churn, ARPU increase through fixed-mobile substitution, reduced operations expenditures, increased uptake in 3G services, and an upside for additional new services that can be offered taking advantage of the mobile operator's new presence in the home. 

 

Users stand to benefit from femtocell based services. In fact, for vanilla voice and data services, a user does not even realize he is on a femtocell – he just gets the benefit of better coverage and faster access. In fact, many vendors are developing technologies that allow service parity between the macro network and the femtocell network. This includes voice, SMS, data, supplementary services, voicemail, and handoff. Further, new services can be made available through their service provider, such as IPTV on the mobile, remote access to home PC content, and softphone based services. Finally, the user also stands to benefit from better pricing due to the lower operator costs and bundling.

 

The architectural approach taken to integrate femtocells into the mobile operator's core network has a huge impact on the both the end-user's service experience and the mobile operator's business case.    This paper will compare and contrast the various approaches and will show that an “all IP” (or SIP/IMS) approach has clear advantages over the somewhat legacy IP radio access network (IP RAN) approaches (which includes UMA).

 

Femtocells: Why Now?
Indoor coverage has been an industry problem for year and vendors have unsuccessfully tried to develop relevant technology solutions for the home. Most services to date involved micro or pico base stations and did not really have the price points to support residential users. Alternative approaches tried most recently involve dual mode devices based on Wi-Fi. While technically compelling, these solutions depend on adoption of new (and expensive) handsets.

 

A number of factors are coming together to enable femtocell based solutions:

  • The high adoption of broadband connections allows the service providers to leverage the IP backhaul to reduce the backhaul costs for additional usage.
  • Advances in embedded technologies make it possible to offer a home base station at an acceptable price point approaching $100 over time
  • Users are now accustomed to the idea of having a home gateway – thanks to prevalent products like TiVo, Vonage, Slingbox, Wi-Fi AP, as well as DSL/cable modems.
  • As 3G adoption increases, indoor coverage becomes a challenge even in otherwise good 2G coverage regions such as Europe and Asia.
  • Operators who have trialed cell-site based home zone type services have seen the potential to leverage home zones for improved customer retention.
  • New low power GSM spectrum has enabled new players to participate in mobile offerings based on licensed band femtocells.

         

Core Network Integration Challenges

There are a number of technical challenges in developing femtocell Customer Premises Equipment (CPE). These primarily include solving radio interference and developing cost-effective CPE. However, there is now a critical mass of vendors, such as UbiquiSys and 3Way Networks, addressing these issues and this roadblock will be eliminated.

 

The next big challenge in making femtocells a reality is around network integration. In the past, this portion of the solution has been neglected because vendors have focused on developing femtocell CPE.  However, to make femtocell solutions viable, the challenges of network integration must be addressed, including: 

  • Scalability: There are expected to be millions of femtocells connected into an operator network, in contrast with thousands of macro cellular base stations deployed today. According to ABI Research, there will be 102 million users of femtocell products on 32 million access points worldwide by 2011.  Therefore, femtocell solutions must be scalable to support millions of end points efficiently.
  • Security: Femtocells are basically IP-connected devices hooking into a cellular network. Furthermore, femtocells will be deployed as CPE equipment.  It is extremely important that the security offered by these devices be equivalent to, or in some ways better than that of cellular networks.  Security is a topic onto itself and will be addressed in detail in another Tatara white paper.
  • Mobility: The overall femtocell experience should provide a way for users to transparently connect to a femtocell at home and to the macro network when outside. Voice calls and data sessions should be seamlessly switched between the two networks.
  • Service Parity: Since users continue to use their existing handsets within a femtocell, they will expect the same experience as they receive on a macro network. including supplementary voice services such as caller ID, 3 way calling, call forwarding, and call barring. In addition, other critical services such as SMS, Unstructured Supplementary Service Data (USSD) services, 3G data services, and regulatory services (e.g., emergency services and wire tapping), must be made available just as they are on the macro network.
  • Standards Compliance: In order for service providers to widely deploy services based on femtocells, it is important that the overall operation be compliant with existing network standards. Integration should not require any changes to the core network or impose any restriction on the deployment of these CPEs. Standards compliance also enables production of cost-effective CPEs.
  • Vendor Innovation and Choice: Femtocells provide mobile operators with an opportunity to break free from the vendor lock-in that today's mobile networks present and tap into the vendor innovation that has been proven again and again in the IP world.  An architecture that ties the femtocell to the same vendor that provides the mobile core is not a step in that direction.  Nor is an approach like UMA that in reality has only attracted a single independent vendor.
  • Support for Multi-vendor CPE: As service providers offer services to the general consumer population, it is important that the solution be plug and play across a range of CPE devices. This not only allows the operators to have multiple vendors in their network but also will eventually drive down the prices of the femtocells.
  • Reduce Cost: As with any new service, the overall capex and opex of the service must not be a barrier to the business case. This is especially true with femtocells since the initial business case is typically around reduced churn – the service provider cannot pass on service costs to users.
  • New Service Extensibility: A big driver femtocell rollout is the ability to offer new IP based services. These could be based on connecting into the home network (e.g. security and monitoring) or entertainment based services (e.g., IPTV) or remote access to home PCs (e.g., Orb type services). In addition, the service providers can also link the PC users with mobile services such as softphone based services tied to the mobile number.
  • Future-proof: In general, as service providers are evolving their core networks to SIP/IMS, it is important that the femtocell solutions be deployable today but also be future-proof to fit seamlessly into an IMS core.

 

Approaches for Core Network Integration

There are two broad femtocell integration approaches within a mobile service provider's network as illustrated in Figure 1:

  • Radio Access Network (IP RAN): The IP RAN based approach effectively considers a femtocell an extension into the operator RAN network and ties the femtocell into the circuit switch core at the edge of the network. This typically involves transporting “Iub” messages over IP into a Radio Network Controller (RNC) or a modified RNC/concentrator. (The Iub is the interface used by an RNC to control multiple Node B's in a UMTS network.) There are three different variants of this approach being pursued by different vendors.

    • Modified RNC: This approach uses existing or modified RNCs to connect to the circuit switched core network. The CPE connects to the RNC via Iub over IP.
    • Concentrator: This is similar to modified RAN in that it connects to the CS core, but it does it through a new ‘concentrator' device that interfaces with the CPE. The interface is again based on Iub over IP.
    • UMA: This approach incorporates a UMA client into the CPE and connects to the core network via a UMA UNC.

      All three of these variants require either the introduction of or major modification to network elements at the RNC layer of the mobile core network.

  • All-IP (SIP/IMS): The SIP/IMS based approach, on the other hand, integrates the femtocell through a SIP or IMS based network. This approach leverages a SIP based VoIP network for cost-effective delivery, while interworking with a cellular core to extend legacy circuit switched services. In this approach, the CPE converts cellular signals to SIP and interfaces to a SIP-MSC inter-working function (IWF) which connects to the SIP (or IMS) network as well as the circuit switched network.

 

 

Figure 1: Network Integration for Femtocells

 

IP RAN Based Approaches

 

Modified RNC: The modified RNC approach basically treats femotcell CPEs as extensions to the existing RAN network. CPEs transfer Iub messages over IP to an existing or modified RNC. The RNC then connects to the circuit switch and packet switch core as in any 3G network. This is a natural approach for incumbent RNC vendors to pursue. The key advantages of this approach are the ability to seamlessly enable mobility and supplementary services into the IP network. A key disadvantage is potential scalability issues since traditional RNCs were not designed to support millions of base stations. The approach is also an extension of legacy networks and does not migrate to or leverage developments in IP/IMS. As a result, this approach only captures the existing circuit switch/packet switch experience – it cannot expand to offer new IP based services. Further, the CPE is typically proprietary to specific RNC vendors since it often involves proprietary implementations of RNCs. 

 

RAN Concentrator: The RAN Concentrator approach is quite similar to the Modified RNC approach except that it typically involves new RNCs – called RAN concentrators. In this approach, the CPE communicates with RAN concentrators over a proprietary interface. The RAN concentrator typically might multiplex the data while feeding it into an existing CS core. This approach, at first glance, could eliminate the RNC overload issue in the modified RNC approach; however, it involves building new equivalent functionality in the concentrator. The CPE and the interface to the RAN concentrator are usually proprietary and vendor-specific. Like the Modified RNC approach, it also has similar limitations around service extensions. Most of the vendors developing RAN concentrator solutions will likely over time migrate to a SIP based architecture.

 

UMA:  The UMA approach leverages work done around UMA/GAN for dual-mode handsets and applies it to the femtocell market. The femtocell CPE encapsulates a UMA client that talks to a UMA Network Controller (UNC) in the core network. UMA vendors will position an advantage of this approach is that it leverages existing UMA based technology and mobility and supplementary services from the circuit switched network port directly into the femtocell environment. However, in reality, there is very little UMA network equipment that has been deployed to date due to the slow uptake of dual-mode handset FMC services.  Other disadvantages of the UMA approach include scalability (which is largely unproven) and load on the existing mobile core. UMA does not have the ability to offload IP based calls completely thus reducing the cost benefits provided by IP. UMA requires the deployment of the UNC – unlike the modified RNC where RNCs are already deployed. UMA is also not standardized for 3G.  The UMA vendor community is also somewhat constrained with a sole independent vendor and attracting very little in venture capital interest.  Finally, UMA suffers from other RAN approach limitations such as lack of IP service delivery. Some of the CPE vendors are planning on adding UMA support primarily for limited service trials.

 

SIP/IMS Based Approach:  In the SIP/IMS approach, the CPE converts 2G/3G signals from the mobile into appropriate SIP based messages over IP. The CPE interfaces with a SIP-MSC inter-working function (SIP-MSC IWF) that couples these SIP based messages with circuit switched messages as appropriate. Many femtocell vendors, such as UbiquiSys and 3Way Networks, are developing SIP/IMS based CPE. In addition, several of the RAN based vendors have plans to evolve to a SIP/IMS based solution because of its elegance and evolution to IMS.

 

Tatara Systems is developing the core infrastructure fixed-mobile convergence application server that enables the deployment of SIP-based femtocells.  The Tatara Convergence Server effectively provides the SIP-MSC Interworking Function (IWF) to enable integration of SIP based femtocells into the core network. The Tatara Convergence Server is based on an open platform that will work in conjunction with all SIP based femtocells. 

 

The SIP/IMS based approach has a number of clear advantages.

  • The solution is very scalable and does not load the existing RAN network or mobile core network. The solution is able to offload IP based calls completely to the IP or IMS network.
  • Because of the SIP based approach, it is also future-proof and migrates seamlessly to IMS networks, while still having the ability to be deployed today.
  • The use of IP in the core network significantly reduces the opex for carriers.
  • The solution has the potential to be the first ‘killer' application to leverage IMS deployments.
  • As a result of its IP and SIP based architecture, this approach can enable a number of new services into the femtocell environment. This will allow operators to leverage the femtocell to generate new revenue for advanced services, instead of just being a landline replacement service or cannibalizing their own pure voice revenues.



 As shown in Figure 2, the SIP/IMS solution can be deployed in both pre-IMS and IMS networks. For instance, in a pre-IMS network, the SIP-MSC IWF couples SIP messaging with the circuit switched MSC to offer security, mobility, and supplementary services functionality. In an IMS network, the same functionality is implemented as a SIP application that functions as a SIP Application Server in conjunction with an S-CSCF and HSS as well as VoIP elements like a MGW/MGCF.

 


Figure 2: SIP/IMS Convergence Solution

 

A limitation often mentioned in the context of the SIP/IMS approach is the lack of standards-based support for security, mobility, and supplementary services. Vendors such as Tatara Systems have developed mechanisms to address these challenges.  On the mobility side, the solutions enable handoff to/from femtocells. The solution also interworks circuit switched supplementary services (call barring, call forwarding, voicemail, USSD, SMS) between SIP from the CPE and the existing implementation of supplementary services in the circuit switched network. It also interworks other services such as call waiting, 3 way calling and conferencing calling using existing feature servers in deployed VoIP or IMS networks.

 

 

Enhanced IP Services Based on the SIP/IMS Architecture

Only the SIP/IMS approach has the ability to deliver new revenue generating IP services within the femtocell environment. This will help enable the service provider to win the home environment.  Some example services include:  

  • Softphone Offer: Since the SIP/IMS solution interworks SIP messages with a circuit switch core, it is possible to extend the overall service into any SIP based softphone environment. A service provider can offer, in addition to standard mobile phones, the ability to make/receive voice calls, messages, etc. on a PC through a softphone based application. This could easily be as a bundled offer or as a differentiating “home zone” feature.  Note that the platform used for femtocells extends to any SIP based client. The operator can offer the softphone as an extension to the service so users can be captive with the operator even on a PC based device in the home. Tatara Systems enables such a service with additional value-added capabilities such as the ability to ring the mobile as well as the PC or duplicate SMS to the mobile and PC. All the other supplementary services mentioned earlier can also be offered on the PC.  A derivative benefit of this to the mobile operator is that this mobile sotfphone application will then likely be used on laptops away from the home, extending the operators brand even further. 

  • Competitive VoIP Services: A SIP/IMS solution can be leveraged into today's network technology to enable mobile operators to compete, not only on price, but on offers like free home to home calls (where both homes have femtocells), creating a viral or network pull for femtocell adoption.  In addition, some femtocell CPEs have built-in RJ ports and SIMs allowing for the mobile operator to potentially provide a home VoIP number and compete directly and more elegantly with the likes of Vonage. 
  • Access to the