Integrating Femtocells with Existing Wireless InfrastructureDocument Transcript
Integrating Femtocells with Existing Wireless Infrastructure
By Manish Singh, Vice President of Product Line Management, Continuous Computing
Unlike UMA, femtocells operate in strictly regulated licensed spectrum sparking the challenge of radio
frequency (RF) interference with the existing macro-cellular network. How can interference be minimized?
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
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
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.
Fixed-Mobile Convergence (FMC) led to the development of the Unlicensed Mobile Access (UMA)
architecture, which was later standardized by 3GPP for Generic Access Networks (GAN). To integrate the
mobile traffic via a Generic IP network (such as 802.11) to the mobile core network, a Generic Access Network
Controller (GANC) was defined. The GANC provides the Up interface toward mobile handsets and a standard
A/Gb interface toward the mobile core network.
In principle, UMA-based architectures extend the 3GPP GAN architecture to integrate femtocells in a similar
way as UMA mobile handsets. The GANC, also commonly known as a UMA Network Controller (UNC),
aggregates the traffic from femtocells and delivers it to the existing mobile core over a standard Iu-interface.
The UNC also provides support for security functions and terminates IPSec tunnels from femtocells.
Figure 4. UMA Based Architecture and corresponding Femtocell Signaling Interfaces
Similar to UMTS-centric architectures, UMA-based architectures leverage the mobile CN to anchor mobile
calls and support handoffs. Traffic generated from femtocells traverses through the CN rather than being
offloaded. UMA-based architectures bring strategic advantage to operators who already have commercialized
UMA services and can leverage their existing UNC assets to quickly and efficiently integrate femtocells into
their CN. For example, T-Mobile currently offers HotSpot@Home services in the United States and recently
announced femtocell trials in the UK, Germany, and the Netherlands .
SIP / IMS-Based Architectures
Session Initiation Protocol/IP Multimedia Subsystem-based architectures integrate femtocells directly with the
IP Multimedia Subsystem (IMS) core. Alternative architectures under this category include softswitch-based
implementations where the femtocell is integrated to a softswitch via a SIP interface.
Integrating femtocells directly with the IMS core offers distinct advantages. First, the mobile core network is
completely offloaded, and as femtocell traffic scales the operator avoids the need for CN infrastructure
upgrades. Second, traffic latency challenges are mitigated as the number of hops a packet must traverse is
greatly reduced. Third, this architecture is forward-looking as it simultaneously solves the near-term 3G
coverage challenge while also providing long-term options for delivering rich innovative services via an IMS
core that leverages femtocells.
Figure 5. SIP/IMS-Based Architecture and corresponding Femtocell Signaling Interfaces
The biggest challenge that needs to be addressed with SIP/IMS-based architectures is supporting seamless
handoff between the femtocell and the macro-cellular network. As the mobile CN is completely bypassed,
there is the need for a Voice-Call-Continuity (VCC)-like application to support handoff functions. This
architecture has the strongest appeal to carriers who have a strong IMS core and have both fixed and wireless
assets. Carriers who are deploying FTTx (fiber to the home/curb/etc.) for delivering IPTV services can leverage
this architecture to deliver “triple-screen play” services by efficiently place-shifting content across the TV, PC,
and mobile handset via the femtocell.
Conclusions on Architecture Options
As discussed above, it is clear that there are many different ways of integrating millions of femtocells with
existing wireless core network infrastructure. Each of these architectures provides distinct advantages and has
strong appeal to a subset of operators – but there is no single choice of architecture that is best for all. It
should also be noted that the underlying femtocell hardware does not necessarily change with each variation
of network architecture; what essentially changes is the software and the signaling interfaces that the femtocell
must support. As a result, ensuring remote software upgradeability within femtocells is critical to ensuring that
the widest range of near-term and long-term opportunities is addressable.
Too many architecture choices can also lead to challenges pertaining to interoperability and market
fragmentation. For femtocells to be widely deployed, it is essential to achieve low per-unit price and
simultaneously reduce the CAPEX and OPEX savings for carriers. Architecture harmonization and phased
evolution is essential for the industry to achieve the economies of scale which will drive overall market
success, and the good news is that the Femto Forum has recently decided to make this a priority.
While this article primarily focused on the core network integration options and challenges, it must be noted
that there are other significant integration challenges that need to be resolved.
Unlike UMA, femtocells operate in strictly regulated licensed spectrum. While this provides distinct advantages
to deliver high quality voice, it also brings the challenge of radio frequency (RF) interference with the existing
macro-cellular network. Femto-to-macro signals, macro-to-femto signals, and femto-to-femto signals all can
potentially interfere, creating “dead-spots” in the network. Solutions to these challenges will require dynamic
scanning of the RF environment and rapidly configuring femtocells with appropriate operating frequencies and
code to minimize interference.
Quality of service (QoS) and traffic prioritization are other issues to consider as well. While for data the
bottleneck might still be the radio as the maximum HSDPA downlink rate supported is 7.2Mbps, carrying real-
time voice on a shared IP link is altogether a different matter. As real-time voice competes with Internet data
and peer-to-peer (P2P) traffic on a shared IP link to a consumer’s home, for QoS it is essential that real-time
traffic be prioritized right from the home network all the way into the ISP’s network. Because mobile operators
do not necessarily control the ISP networks, service level agreements and business arrangements need to be
addressed. Femtocells are thus a disruptive force that could even lead to further industry consolidation,
thereby accelerating FMC.
As with any new disruptive technology, challenges abound for femtocells – such as customer premise
equipment (CPE) device provisioning and management; potential consumer concerns around health, security,
and access control; regulatory hurdles; and so on. (Note: Many of these challenges and their potential
solutions will be addressed in a future installment of this series). However, none of these challenges is
insurmountable. With the amount of investment that is pouring into the femtocell market, successfully
addressing these challenges is just a matter of when – not if.
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