Femtocell Network Architecture

WHITE PAPER Femtocell Network Architecture Author: Woojune Kim, Vice President, Technology, Network Architecture May 2010

© 2010 Airvana Corp.

© 2010 Airvana Corp. 2

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EXECUTIVE SUMMARY

Connecting femtocells to operator networks requires unique architectures that address

the security needs of operators and mobile users and support the deployment of

scalable femtocell networks that can serve millions of subscribers. The femtocell

network architecture is also designed to allow ordinary consumers to install them with

plug-and-play simplicity. Significant adaptive and self-organizing capabilities built into

this architecture support zero-touch service activation by the user. Network architecture

is also critical in supporting emergency calling, which can now be delivered to mobile

devices inside buildings with the same reliability and accuracy as fixed-line emergency

calling. This paper addresses these critical topics, presents alternative approaches that

are used in femtocell products, and highlights some of the unique solutions used in

Airvana femtocell products.


Connecting femtocells to existing operator networks requires a network architecture that addresses the security needs of operators and mobile users, while supporting the scalable deployment of millions of femtocells. In addition, it must allow ordinary consumers to install them with plug-and-play simplicity and ensure that critical services such as emergency calling are also supported with the same reliability and accuracy as fixed-line emergency calling. The femtocell network architecture describes the major nodes and connections in a femtocell network, and how they achieve the objectives of mobile subscribers and operators. The femtocell network architecture supports the following key requirements:

• Service Parity: Femtocells support the same voice and broadband data services that mobile users are currently receiving on the macrocell network. This includes circuit-switched services such as text messaging and various voice features, such as call forwarding, caller ID, voicemail and emergency calling.

• Call Continuity: Femtocell networks are well-integrated with the macrocell network so that calls originating on either macrocell or femtocell networks can continue when the user moves into or out of femtocell coverage. Femtocell network architecture needs to include the necessary connectivity between the femtocell and macrocell networks to support such call continuity.

• Security: Femtocells use the same over-the-air security mechanisms that are used in macrocell radio networks. But additional security capabilities need to be supported to protect against threats that originate from the Internet or through tampering with the femtocell itself. Femtocell network architecture provides network access security, and includes subscriber and femtocell authentication and authorization procedures to protect against fraud.

• Self-Installation & Simple Operational Management: Femtocells are installed by end-users. Therefore, the femtocell network architecture must support an extremely simple installation procedure with automatic configuration of the femtocell and automated operational management with “zero-touch” by the end-user.

• Scalability: Femtocell networks can have millions of access points. Therefore the femtocell network architecture must be scalable to grow into such large networks, while at the same time maintaining reliability and manageability.

Common Elements of the Femtocell Network Architecture As shown in Figure 1 there are three network elements that are common to any femtocell network architecture. These are:

• Femtocell Access Point (FAP) • Security Gateway (SeGW) • Femtocell Device Management System (FMS)

Two other elements that are in all femtocell network architectures are entities that enable connectivity to the mobile operator core. Depending on the specific architecture used for circuit switched calls, there can be either a Femtocell Convergence Server (FCS) or a


Femtocell Network Gateway (FNG). This is also shown in Figure 1. For packet calls, depending on the airlink technology, there can be either a PDSN or xGSN (GGSN/SGSN) in the core. In most cases, the PDSN / xGSN are the same as those used for macro networks.

Figure 1 : Common Components of Femtocell Network Architecture

Femtocell Access Point (FAP) Femtocell Access Point is the primary node in a femtocell network that resides in the user premises (e.g., home or office). The FAP implements the functions of the base station and base station controller and connects to the operator network over a secure tunnel via the Internet. A FAP can be introduced into a home in multiple ways. A standalone FAP can be directly connected to the home router. In some applications, the FAP may also include a built-in router, which is useful in prioritizing FAP voice traffic over other Internet traffic in the home network. More advanced FAP’s include an Analog Terminal Adapter (ATA) to connect a fixed-line phone. In some cases, FAP’s are full-blown residential gateways with built-in Wi-Fi and a broadband modem (xDSL, cable). Security Gateway The security gateway is a network node that secures the Internet connection between femtocell users and the mobile operator core network. It uses standard Internet security protocols such as IPSec and IKEv2 to authenticate and authorize femtocells and provide encryption support for all signaling and user traffic.


The security gateway supports a large number of femtocells connecting to the operator’s network. While similar to traditional VPN gateways used in enterprises, femtocell security gateways are designed for use in carrier networks and meet carrier-grade requirements such as scalability, high availability, and network management.

Femtocell Device Management System The femtocell management system, also located in the operator network, plays a critical role in the provisioning, activation and operational management of femtocells using industry standards such as TR-069. The management system is perhaps the most critical node in ensuring the scalability of a femtocell network to millions of devices. To ensure low-cost deployment and easy setup for subscribers, the activation and provisioning of the femtocell must be plug-and-play with no on-site assistance (sometimes called a “truck roll”) required from the mobile operator. Various standards bodies specify the use of the TR-069 family of standards as the base device management framework for femtocells. This protocol is widely used in DSL modem and residential gateway deployments, and uses a proven web-based architecture that can scale to support millions of devices. Airvana’s Femtocell Service Manager (FSM) is a comprehensive TR-069-based femtocell management system that supports efficient management of large numbers of femtocells using a clustering and load balancing architecture. The FSM is composed of two primary elements, the Device Manager application and the Automatic Network Planner application. The Device Manager implements functions such as remote configuration, remote diagnostics, fault management, software upgrade, performance data collection and device authentication. The Automatic Network Planner adds RF planning algorithms, RF configuration and a northbound interface to Operational Support Systems (OSS).


Figure 2 : Femtocell Service Manager

FCS or FNG The FCS or FNG enables femtocells to connect to the operator core network. This is important for the operation of femtocells as this is what allows femtocells to communicate with the core elements in the operator’s networks and allow seamless service for the mobile. For example, basic call setup requires communicating with the MSC and PSTN of the operator core. The FCS or FNG allows this to happen. As will be shown below, depending on the specific architectural model used to support Circuit-Switched Services the FCS /FNG can be used.

PDSN /xGSN The PDSN / xGSN enable femtocell users to receive packet data services over the mobile operator’s core. In most cases, these will be the same as those used by the mobile operator’s macro network.

Architectural Models to Support Circuit-Switched Services Macrocell networks consist of a radio network and a core network. The radio network is made up of base stations, base station controllers (BSC’s) and a radio network management system. The core network includes the Mobile Switching Centers (MSC’s), mobile data nodes such as Packet Data Serving Node (PDSN) in CDMA and Serving/Gateway GPRS Serving Nodes (SGSN/GGSN) in UMTS, subscriber data bases, such as the Home Location Register (HLR), and various billing systems.


Femtocells connect to a mobile operator core network to deliver both circuit-switched and packet data services. Broadly speaking, there are two distinct architectural models for supporting circuit-switched services in femtocells. In the “SIP/IMS model”, femtocells connect into an overlay SIP/IMS core network. In the alternate so-called “legacy” model, femtocells connect directly into the mobile operator’s legacy core network (MSC’s). In the remainder of this section, we will review these models in detail.

Figure 3 : Femtocell Network Architectures for Supporting Voice (Circuit Switched Services)

SIP/IMS Network Model for Femtocells In this model, the femtocell connects to a “new” core network of the mobile operator that is based on the SIP/IMS architecture. This is achieved by having the femtocells behave towards the SIP/IMS network like a SIP/IMS client by converting the circuit-switched 3G signaling to SIP/IMS signaling, and by transporting the voice traffic over RTP as defined in the IETF standards. To support femtocells, a new network node called a “Femtocell Convergence Server (FCS)” is added to the SIP/IMS network. The FCS also attaches to the legacy mobile core network – by acting like an MSC towards the legacy core network, the FCS helps support handoffs between femtocell and macrocell networks, accesses subscriber databases such as HLR’s and provides the supplementary services required for feature parity. 3GPP2 standards for CDMA femtocells use the SIP/IMS Model, and the


description that follows mirrors the use of this architecture in Airvana’s CDMA femtocells. The following figure shows the detailed diagram for the SIP/IMS femtocell network for CDMA femtocells. As can be seen, the CDMA femtocell connects to a SIP/IMS core network which in turn has a signaling connection to the legacy CDMA core network, whose interfaces are defined in the TIA IS-41 standard. The SIP/IMS network is used for the call control and media routing while the legacy core network is used to retrieve subscriber data stored on the legacy network and to support handoffs to/from the macrocell network, which exclusively uses the legacy core network.

Figure 4 : SIP/IMS Network Model

The main components in the SIP/IMS architecture are as follows:

• Femtocell Access Point with SIP/IMS Client • SIP/IMS Core Network • Femtocell Convergence Server (FCS)

The SIP/IMS core network can be a full-blown IMS network or it can be a basic SIP-based VoIP network. An IMS core network would have the IMS nodes, such as the Call Signaling Control Function (CSCF) nodes for call control, Home Subscriber System (HSS) to manage subscriber data, and Media Gateway (MGW)/Media Gateway Control Function (MGCF) nodes to connect to the Public Telephone Network (PSTN). In a more basic SIP network, the functionality of the CSCF and MGCF functions may be integrated into a SIP soft switch, and the HSS function may be handled by a Radius server. FCS is the key component in this architecture. It fits into an IMS core network as an Application Server (AS) that connects to the CSCF’s using the standard ISC interface. FCS connects to the legacy core network like an MSC using standard IS-41 network interfaces.


The SIP/IMS model is a forward-looking approach for delivering services over femtocells. It offers not only a scalable approach for delivering services, but can also be used to offer converged fixed-mobile services to both mobile devices and fixed-line phones. In the SIP/IMS model, support of active handoff is done through the FCS. As the FCS essentially acts as a peer MSC in the mobile core network, the handoff uses the well known inter-MSC active handoff mechanisms using existing core network interfaces, as defined in IS-41. When a user moves from femtocell coverage to macro network coverage, the femtocell sends and receives messages that correspond to the handoff request messages sent in the macro network in handoff scenarios.

Legacy Network Model for Femtocells In the legacy network model, the femtocell connects directly to the existing mobile operator core network. A network node called a femto network gateway sits between the femtocell and the legacy core network and performs the necessary translations to ensure the femtocells appear as a radio network controller to existing MSC’s. 3GPP standards for UMTS femtocells use the Legacy Core Network Model, so the description that follows largely mirrors the use of this architecture in Airvana’s UMTS femtocells. The main components of the legacy network model are:

• Femtocell Access Point • Femtocell Network Gateway (FNG) • Security Gateway

The FNG is responsible for connecting the femtocell to the MSC’s in the legacy core network. Using the standardized UMTS interface known as Iu, the FNG behaves like a macrocell Radio Network Controller (RNC) and as such requires no modifications to existing MSC’s to support femtocells. Towards femtocells, the FNG uses a variant of the Iu interface, known as Iuh, which has been standardized in 3GPP. The legacy network architecture is easier to deploy when a SIP/IMS network is not already in place, since it allows the operator to reuse the existing mobile core network. Using the existing core network implies 100% compatibility with many service features and avoids the need to replicate these features on a new SIP/IMS network. In this model support of active handoff is done through the legacy MSC. As the FNG essentially acts as an RNC in the core network, the handoff is implemented using the well known Iu interface between the RNC and MSC/SGSN. When a user moves from femtocell coverage to macro network coverage, the femtocell sends and receives messages that correspond to the handoff request messages sent in the macro network in handoff scenarios.


Figure 5 : Legacy Network Model for UMTS

Although the legacy network model is currently not supported in CDMA femtocell standards, it is supported by Airvana’s CDMA femtocell. As shown in Figure 6, in this architecture the FNG uses the 3GPP2 standard A1p/A2p interfaces to connect to the MSC in the legacy network and a SIP-based interface to connect femtocells to the FNG. Using the SIP-based interface between the femtocell and the FNG makes the FNG more scalable and also creates a natural migration path from the legacy model to the SIP/IMS model.

Figure 6 : Legacy Network Model for CDMA

Interworking with Packet Data Networks Femtocells connect to the mobile core packet data network through existing packet data network nodes, such as SGSN/GGSN’s in UMTS and PDSN’s in CDMA. 3GPP standards for UMTS femtocells and 3GPP2 standards for CDMA femtocells both use this method. In CDMA networks, femtocells can connect to the PDSN directly via the security gateway. This is possible because existing PDSN’s can generally handle a very


large number of femtocells without any modifications. In UMTS networks on the other hand, the femto network gateway acts as an intermediary between the SGSN/GGSN and the femtocell makes the data traffic from these devices appear as if it is coming from an RNC.

Local Breakout Femtocells also support a feature known as local breakout, which allows a femtocell user to connect their mobile devices to the local home or office network without traversing the mobile operator’s core network. For traffic destined to the global Internet, local breakout also bypasses the operator core network, thus reducing the network load. This is shown conceptually in figure 7.

Figure 7 : Local Breakout

From an end user perspective the potential benefits include

• Access to in-home multimedia devices and content • Improved data performance with direct access to the Internet

From an operator perspective the potential benefits include

• Avoiding costly packet data core capacity upgrades by offloading Internet web/streaming and corporate VPN traffic

Emergency Services Support One of the most important requirements for a femtocell is the support for emergency calling services, which is known in North America as e911. When the mobile user makes



an emergency call, an emergency service is dispatched to user’s current location as soon as possible.

Whether the call is made from a mobile handset or a fixed-line phone, the call is directed to a Public Safety Answering Point (PSAP) that handles the geographic region in which the call was initiated. For a fixed-line call, in most cases, the street address of the fixed-line phone is known, so an emergency dispatch (e.g. police) is sent immediately to that address. When a call originates from a mobile phone, the location of the user is inmost cases less certain. In North America, it has become mandatory for mobile phones to support location identification for e911 calls, so they can report the user’s location to the PSAP with a high degree of accuracy. Other countries are adopting similar requirements for emergency calling from mobile devices. To support emergency services, femtocells provide critical information to the mobile operator core network, such as the location of the caller to identify the nearest PSAP and a callback number to call the user in case of a disconnection. Femtocells support emergency services even for those users who are not authorized to use the femtocell.

Summary As shown above, femtocells are not simple standalone devices. They must be integrated into the mobile operator’s network to enable seamless service and to ensure optimal performance across both femtocell and macrocell networks. The architectures for the UMTS and CDMA solutions have been defined by their respective standards bodies (3GPP and 3GPP2). Both architectures enable a better experience, with service parity for users, while ensuring security and scalable solutions for operators.