Standardization of Femtocells

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In this section:
Introduction
3GPP Standards for UMTS Femtocells
3GPP2 Standards for CDMA Femtocells
   - SIP/IMS-Based 1x Circuit-Services Architecture

The creation and adoption of standards is critical for the growth of the femtocell industry.  Femtocells must support millions of mobile devices already in use and they must connect to existing macrocell networks.  This means that femtocells need to be compliant with standards that define the interfaces to existing mobile devices and networks.  Further, mobile operators will want to purchase femtocell access points and network nodes from multiple vendors in order to drive market choice and competition.  Such multi-vendor deployments require standard interfaces between femtocells and various network nodes, such as the security gateway (SeG), the femtocell network gateway (FNG), femtocell convergence server (FCS) and femtocell device manager.

Standards for UMTS femtocells are developed in 3GPP and standards for CDMA femtocells are developed in 3GPP2.

While not a standards-publishing or standards-setting body, the Femto Forum has played a key role in driving early femtocell standardization activities, particularly through extensive collaborative exchanges among key vendors and system operators to reach industry consensus on the initial UMTS femtocell overall architecture (including Iuh), the IPsec/IKEv2-based security architecture, and the TR-069-based femtocell management architecture.  Discussion between femtocell service providers and key femtocell product vendors expedited the standardization activities in 3GPP and the Broadband Forum, permitting widely supported and effective standards to be produced on extremely tight schedules dictated by the needs of the femtocell service providers.

Operating, administering, and managing large numbers of Consumer Premises Equipment (CPE) femtocell devices in a cost-effective manner requires the use of a management architecture that is based on mature, proven CPE-device standards.  For this reason, the Femto Forum reached industry consensus that the Broadband Forum (BBF) TR-069 family of standards should serve as the basis for femtocell management.  This recommendation was subsequently agreed in 3GPP and 3GPP2, and both of these standards bodies have worked closely with the Broadband Forum to define device model extensions for femtocell devices (BBF TR-196).  The close and highly-successful collaboration between 3GPP, 3GPP2, and Broadband Forum to add new capabilities and support for additional radio technologies is continuing.

3GPP Standards for UMTS Femtocells
The 3rd Generation Project Partnership, or 3GPP, is the primary international standards organization dealing with mobile networks using the GSM/UMTS and LTE family of air interface technologies. Therefore 3GPP has been the nexus of standards for UMTS femtocells.  In 3GPP, most mobile operators and vendors preferred the legacy architecture with a direct interface to the circuit and packet core network based on the Iu interface (including Iu_cs for circuit services and Iu_ps for packet data services).  Early in 2008, the UMTS femtocell standardization topic gained tremendous interest from many mobile operators, and efforts to find a way to have the capability incorporated into UMTS Release 8 began with great intensity.  Areas of 3GPP femtocell standards included the following:

• Interface between the femtocell (Home Node B - HNB) and the femto network gateway (HNB Gateway, HNB-GW)
• Security protocols to authenticate femtocell (HNBs) and secure communications across the un-trusted Internet
• Management protocols for “touch free” Operations, Administration, and Management (OA&M) of femtocells (HNB devices)

The interface between the femtocell (HNB) and the HNB-GW is referred as Iuh interface (see Figure below).  The Iuh specification was published in near-record time by 3GPP2 in early 2009.

Following the initial Iuh specifications, 3GPP turned its attention to resolving other UMTS femtocell architecture issues, chiefly the critical security architecture and protocol stack for the tunnel between the HNBs and the security gateway.  3GPP has successfully reached consensus to use the IPsec/IKEv2 protocols along with a security architecture that is essentially common with that of 3GPP2, and the standardization for these aspects has been completed in the UMTS Release 9 (March, 2010).

Broadband Forum TR-069 has been selected as the framework for femtocell management and was widely supported by 3GPP2 and 3GPP vendors and carriers, as the Femtocell Management protocol.

Management architecture and TR-069 Auto-Configuration Server (ACS) are the backbone of the femtocell operational and provisioning call flows in 3GPP and 3GPP2. The femtocell management architecture including: Discovery/Registration, Redirection to alternate SeGW, ACS, or Femto Gateway, self-organizing network/auto-configuration, location verification, provisioning and radio parameter assignments.

 
Figure 1.  3GPP UMTS Femtocell (HNB) Architecture Overview

Femtocell Standard Specifications included in the UMTS Release 8:

• Iuh interface (and associated HNB-GW procedures)
• OA&M
   - Based on TR-069 family of standards that is widely used for DSL modems and other CPE devices
   - Base management object contents defined in 3GPP
   - HNB management object definitions developed and published by BBF (TR-196)

UMTS standards continue to add more capabilities to femtocells.  For example, UMTS Release 9 (March, 2010) completed the security specifications and added new features such as voice packet frame bundling (to support slow DSL uplinks), hand-in (macro to femto handoff), and basic enterprise femtocell features (e.g., femto-to-femto handover).  Release 9 specification also completes support for LTE femtocells.

Release 10 (targeted for completion by the first half of 2011) will add support for Local breakout (known as Local IP Access or LIPA) and additional support for enterprise femtocells, in particular enhanced femtocell-to-femtocell interference mitigation.

3GPP2 Standards for CDMA Femtocells
The CDMA2000 femtocell standard architecture includes the following aspects:

• SIP/IMS-based 1x circuit services architecture 
• Packet data architecture 
• Security framework 
• Enhancements to mobile devices to make them more femto-aware 
• Foundations of femtozone services (Local IP Access and Remote IP Access) 
• Femtocell management architecture 

SIP/IMS-based 1x circuit services architecture
For cdma2000 femtocells, 3GPP2 has selected the IMS model as the best architecture to deliver 1x legacy services to existing mobile devices.  These legacy services include voice, text messaging, emergency calling, location services, and other supplementary services.    IMS femtocell architecture applied to a basic IMS core network with an appropriately defined convergence server node connected to the legacy ANSI-41 core network offered the best balance between the numerous challenging real-world constraints (see Figure X).

Balance is the key to making femtocell architectures feasible for a mobile operator from a development, purchasing, deployment, and operating perspective.  Re-using standard SIP and RTP protocols and basic IMS elements such as CSCFs provides a huge scalability advantage in comparison to the legacy architecture (e.g., using A1/A2), while also providing an outstanding base for evolution to IP/SIP/IMS-based core networks and services in the future.  At the same time, carefully selected embedding of a small amount of legacy-related signaling information within the SIP messages from the femtocells to the core network allows for a Femtocell Convergence Server (FCS) to be built that serves as an efficient gateway into the wide range of legacy services – without requiring either a huge investment in IMS-based application servers or costly investment in legacy gear (e.g., MSCs or BSCs).  

Figure 2.  3GPP2 Femtocell Architecture Overview

The packet data service architecture includes a mode that is highly compatible with existing packet data service core network elements, re-using the A10/A11 interface to existing Packet Data Service Nodes (PDSNs), as well as a “flat” architecture alternative in which most of the PDSN functionality is incorporated into the femtocell (“Packet Data Service Termination – PDST” architecture).  This will allow system operators to evolve gracefully from existing packet data core network to highly scalable flat architectures as femtocell deployments grow.

A security architecture based on re-use of commercial off-the-shelf IPsec/IKEv2 security gateway products has been adopted in 3GPP2, as was the concept of using certificates for mutual authentication of femtocell access points (FAPs) and the security gateway (SeGW) in order to reduce system operators’ CAPEX and OPEX.

While femtocells must operate smoothly with all existing (legacy) mobile devices, new enhancements were added to future mobile devices to make them “femto-aware.”  Such devices could offer benefits such as reduced femtocell detection time, improved battery life in the presence of femtocells, better or more reliable handoff between femtocells and macro systems (or between femtocells), or additional femtozone services.   One example of a femto-aware feature is Enhanced System Selection (ESS) based on the Preferred User Zone List (PUZL), which is a database of information that can be downloaded into mobile devices or autonomously learned by the mobile devices to supplement the existing Preferred Roaming List (PRL) that all mobile devices have today when it comes to efficient system detection and selection. 

Standards for Femtozone Service Enablement
Two features that serve as enablers for femtozone services have long been championed in the Femto Forum, 3GPP2, and 3GPP – Local IP Access (LIPA, sometimes called local break-out) and Remote IP Access (RIPA, sometimes called local break-in).

LIPA allows for mobile devices being served by the femtocell to have direct access to IP-based services and devices in the home/local network, or to have direct access for some or all IP-based traffic to the public Internet, without going through the mobile operator’s network.  LIPA is a component that allows femtozone services to support applications that integrate mobile devices with networked devices in the end user’s household, but perhaps even more importantly, LIPA can provide the ability to offload all traffic destined for the public Internet without burdening the mobile operator’s packet data network, while at the same time improving the end-user experience.

RIPA allows mobile devices that are being served by the macrocell to access IP-based services and devices in the mobile device owner’s private home network.  This capability enhances femtozone services by making services and applications that depend on resources within the end user’s home network available from anywhere in a completely secure manner.  For example, the end user could have access to their media collection on a home media server no matter where their mobile device is getting service.

Standards for Device Management
A critical component in supporting a true multi-vendor femtocell ecosystem is the standardization of Femtocell Device Management.  Femtocell devices Management Objects (MOs) comprise the foundation of all of the operational, administrative, and management procedures and capabilities for femtocell access points.

The best standardized and open architecture and protocol family for the management of millions of CPE femtocell devices is the TR-069 family of standards defined in the Broadband Forum (BBF).  The first instance of that framework was developed by a collaborative effort catalyzed by the Femto Forum for UMTS femtocell devices, and that work was transitioned into both 3GPP and the Broadband Forum, ultimately resulting in the BBF TR-196 specification (containing both the framework as well as the UMTS-specific MOs).  TR-069 and TR-196 have also been selected by 3GPP2 as the management framework for CDMA2000-based femtocells.  TR-196, published by the BBF in mid-2009, defines the TR-069 device management extensions for the overall FF femtocell management object as well as the specific objects to support UMTS-based femtocells (HNBs).  Amendment 1 of TR-196 from the BBF (to be published by mid-2010) includes OA&M support for LTE-based femtocells (HeNBs) and CDMA2000 1x and EV-DO (HRPD)-based Femtocell Access Points (FAPs).

Below is a table of 3GPP2 Femtocell-related Specifications and their references.

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