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Wikipedia describes "femto" as an SI prefix in the International System of Units denoting a factor of 10-15, or one quadrillionth. In sharp contrast to a macro-cell, a femtocell is a very small wireless base station residing in a consumer's home. Femtocells are designed to provide excellent coverage in indoor environments and to work seamlessly with any existing 3G handset, thereby eliminating the need for power-hungry dual-mode handsets. Existing Internet Protocol (IP) broadband links (such as DSL or cable) are leveraged to backhaul the mobile voice, video, SMS, and data traffic from the home and integrate with an existing 3G Wireless Core Network.
Today's Wireless Network
Wireless network build-outs are created by strategically placing hundreds of wireless base stations (Node Bs) across a city and along freeways. These macro-cells provide wireless coverage enabling seamless mobility in the wireless networks. Carriers also strategically place micro-cells and pico-cells in dense urban population areas like Multi-Dwelling Units (MDUs) and airports to address both coverage and capacity challenges. Unlike femtocells, which leverage existing IP broadband links for backhaul, the macro-cells, micro-cells, and pico-cells are all connected to Radio Network Controllers (RNCs) over dedicated ATM links such as fractional E1/T1.
Figure 1. Simplified 3G Network Architecture
RNCs aggregate the traffic from Node Bs and deliver the aggregated traffic to the mobile core network. RNCs deliver Circuit Switched (CS) traffic --mostly voice --to the Mobile Switching Center (MSC) over the Iu-CS interface, while the Packet Switched (PS) traffic--mostly data --is sent to the Serving GPRS Support Node (SGSN) over the Iu-PS interface. Another key element of the core is the HLR/HSS, which holds customer provisioning data including subscriber profiles and location information for routing incoming calls.
Femtocell Architecture Options
Femtocells extend carriers' wireless networks directly into the homes. With this extension comes the challenge of integrating potentially millions of wireless base stations with the existing mobile infrastructure. There are many different ways of achieving this integration, and in the December 2007 Femto Forum meeting in Dallas, some 13 different network architectures were proposed.
For simplicity and ease of understanding we broadly classify these 13 architectures into the following three categories: UMTS-centric architectures, UMA-based architectures, and SIP/IMS-based architectures.
Figure 2. Different Femtocell Architectures
UMTS-Centric Architectures
In principle, Universal Mobile Telecommunications System (UMTS)-centric architectures leverage the existing UMTS core and integrate femtocells via an Iu-concentrator. In this case, RNC and Node B functions are built into the femtocell and the femtocell provides the standard 3G Iu interface over IP. An Iu concentrator aggregates the traffic from tens of thousands of femtocells and delivers the aggregated traffic to the MSC over an Iu-CS interface and to the SGSN over an Iu-PS interface. The Iu concentrator mirrors the Core Network (CN) functions toward femtocells and the RNC functions toward the CN, thereby making the integration seamless and not requiring any changes in the CN infrastructure. Because traffic from femtocells can potentially traverse public IP networks, for security reasons IPSec is used to encrypt the traffic. An Iu concentrator or a separate Security Gateway terminates the IPSec tunnels from the femtocells.
Figure 3. Tunneled Iu and corresponding Femtocell Signaling Interfaces
The UMTS-centric architecture essentially leverages the CN handoff functions as the mobile call is anchored at the MSC and/or SGSN and the handoff is supported by these network elements. The drawback of these architectures is that they do not offload the CN, as all the traffic from femtocells goes via the CN. Because each femtocell could potentially support 7.2Mbps HSDPA traffic, as volume rollouts start the aggregate mobile traffic could quickly multiply--potentially leading to the need for a CN infrastructure upgrade. The UMTS-centric architecture is the easiest and simplest way of integrating the femtocells with the existing mobile CN and enables carriers to leverage their existing CN assets. Nokia Siemens Networks in July 2007 announced 3G Femto Gateway which is essentially based on this architecture.
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