Assessing Femtocell Network Architecture and Signaling Protocol alternatives

As a technology, a femtocell is a fairly simple concept of extending the 3G/2G wireless "last mile" into the indoor arena (Figure 1, below), through which current wireless infrastructure " base-stations and/or Node Bs " finds it difficult to penetrate.

This coverage extension is achieved by putting a new device called a Femto Access Point (FAP) within consumers' proximity and leveraging their public internet connectivity (cable, DSL, or fiber) for backhaul. FAPs will be indoor, low-cost, and aesthetically-designed brethrens of the big, expensive, and ugly outdoor base-stations and/or Node Bs " but the devil lies in the details of how FAPs are engineered and managed.

Figure 1: High-level femtocell network architecture

Early pioneers like Ubiquisys, RadioFrame Networks, and ip.access have led the charge in femtocell technology, spurring interest from bigger Telecom Equipment Manufacturers (TEMs) to jockey for position in this nascent market. As expected, all of these players have approached femtocells from their respective positions of strength, resulting in fragmented and disparate approaches to the challenge.

Continuing on this path would result in over-hyped technology solutions that would make it difficult to interoperate, thereby locking in consumers and service providers with specific vendors and in turn keeping the cost of FAPs at a level where the industry would only see a few takers.

To avoid such a fate, the industry is rallying together under the Femto Forum  banner to work with various standardization bodies and industry forums such as the International Telecommunication Union (ITU), European Telecommunication Standards Institute (ETSI), Internet Engineering Task Force (IETF), 3rd Generation Partnership Project (3GPP), and Digital Subscriber Line (DSL) Forum to standardize the architecture for femtocell networks and the protocols used by the network devices to communicate with each other.

Table 1 below lists the focus areas and status of the various organizations influencing femtocell standardization activities.

At a broad level, various approaches being proposed at recent Femto Forum meetings can be classified into three categories: Universal Mobile Telecommunications System (UMTS) based architecture, Unlicensed Mobile Access (UMA) or Generic Access Network (GAN) based architecture, and IP Multimedia Subsystem (IMS) based architecture.

The purpose in this article is to take a closer look at each of these approaches from the perspective of signaling protocols that need to run on each device and the what work will be need to be done by the Femto Forum and the various standardization bodies.

Table 1: Organizations influencing femtocell standardization activity

UMTS Based Architecture
The UMTS based architecture focuses on leveraging the existing core network (CN) behind the Radio Network Controller (RNC) and/or Mobile Service Controller (MSC), Serving GPRS Support Node (SGSN) and tunneling the traffic between CN and Node B over the subscriber's broadband IP network. At a high level, this approach is also referred to as Iu-over-IP. Here again, recommendations diverge on whether we need to tunnel traffic over IP between the RNC and Node-B (i.e., Iub-over-IP) or between the MSC/SGSN and RNC (i.e., Iu-over- IP).

In either case, a new device loosely called an Iu Concentrator (also known as Femto Gateway and hereafter referred to as FGW) is introduced to scale the limited capacity CN to connect to millions of femtocells expected to spring up. One of the significant drawbacks of the Iub-over-IP approach, however, is the fact that the Iub interface is left largely undefined by 3GPP and therefore proprietary vendor implementations of the Iub solutions for femtocells using Iub-over-IP would require significant Iub interface standardization effort.