Cell phones are everywhere, and femtocells may follow. Designed for homes and small businesses, these desktop cell-phone basestations connect to the cell-phone network via an existing high-speed Internet connection using a DSL or cable TV modem.
The femtocell is a home version of the micro and pico cells used in buildings and other densely populated environments. They enable the network to handle more subscribers, and they improve indoor handset performance. And not only will they greatly improve home cell-phone connectivity, they’ll also help unburden the carriers’ already heavily loaded backhaul channels.
Femtocells promise unparalleled in-building coverage (i.e., five bars guaranteed in your home or office), dedicated highspeed 3G data services, and aggressive bundled tariff packages that allow flat-fee rates for unlimited voice and data calls while using your femtocell, for example. Beyond these three major benefits, femtocells are seen as a platform for service innovation, primarily by exploiting the fact that the device will be deployed in the customer’s residence or office.
Multiple architecture options are already on the market and under consideration for standardization (see the table). These include the Universal Mobile Telecommunications System (UMTS), Iub-over-IP, Iu-over-IP, Unlicensed Mobile Access/ Generic Access Network (UMA/GAN), and the IP Multimedia Subsystem (IMS).
UMTS-BASED FEMTOS
Proponents of the UMTS-based architecture cite its ability to leverage existing mobile core network equipment, standards, and capabilities to deliver femtocell service faster with minimal added equipment investment. UMTS-based approaches provide an accelerated path to macro-network equivalent services, so operators can trial the femtocell value proposition with customers earlier.
The first UMTS-based approach is the Iub-over-IP architecture (Fig. 1). The femtocell access point (FAP), also known as the Home NodeB (HNB), takes on the role of the Node B (i.e., 3G basestation). The femtocell gateway (FGW), also known as the Home NodeB Gateway (HNB-GW), lies between the FAP and the radio network controller (RNC).
This approach specifically suits small-office/home-office (SOHO) or single-home environments where only a few subscribers would access services through the FAP. Depending on the number of FAPs connected to the FGW, the RNC and FGW could even be collapsed into one device.
The FAP communicates with the FGW using a 16-bit cell ID that uniquely identifies a femtocell. The FGW multiplexes the traffic coming from different FAPs and forwards it to the RNC using the Framing Protocol (FP). The FGW doesn’t modify any of the FP packets, especially the FAP identifier. At startup, the FAP establishes a security association with the FGW to avoid compromising subscriber information over the public IP network. The FAP could use TR-069 or some other similar mechanism to discover and obtain IP addresses from an auto-configuration server (ACS).
The RNC would handle the resource management (bearer and control) functionality. The RNC and FGW would take care of delay jitter for the bearer traffic and control signaling (specifically forced/hard handover mobility management).
To communicate with the pre-R4 Core Network (CN), which doesn’t support IP transport, the RNC will have to accomplish IP-to-ATM and vice-versa transport conversion. In this architecture, the FAP looks like a normal basestation to the CN. Also, all of the mobility management (MM) and call control (CC), including handover, remains in the CN equipment domain.
The second UMTS-based approach is Iu-over-IP (Fig. 2). The FAP takes on the role of both the NodeB and the RNC. A collapsed architecture makes it possible to offload existing RNCs, freeing up macro-network capacity while still leveraging existing protocols and cellular MM and CC procedures.
The configuration suits small and medium business (SMB) and multiple dwelling unit (MDU) environments, where multiple customers can access services through the FAP. The Iu-over- IP configuration creates a greater demand on the FAP to handle frequent MM and resource management (RM) procedures and places an even greater premium on advanced quality-of-service (QoS) capability in the FAP itself. The FAP communicates with the FGW using a 12-bit RNC-ID. The FGW tunnels Radio Access Network Application Part (RANAP) signaling from the FAP to the CN. It may also convert the IP transport from/to ATM transport using SIGTRAN functionality if the CN doesn’t support IP transport.
At startup, the FAP establishes a security association with the FGW to avoid compromising subscriber information over the public IP network. The FAP could use TR-069, or some other similar mechanism could be employed to discover and obtain IP addresses from an ACS for the FGW. In addition, the FAP uses the ACS to obtain RM parameters and algorithms to be exercised in the femtocell environment.
In this approach, the FAP handles most of the RM (i.e., bearer and control) functionality within the femtocell environment. It defers to the CN for these procedures during femtocell-tomacrocell handover.
UMA/GAN-BASED FEMTOS
The UMA/GAN-based architecture takes advantage of a proven and standards-based approach for providing cellular-grade services over the best-effort Internet (Fig. 3). UMA is an approach to fixed-mobile convergence that bridges cellular radio and unlicensed 802.11 Wi-Fi technologies.
The standard UMA architecture requires a dual-mode phone with specialized client software for managing handover and call control when moving from cellular to Wi-Fi networks and vice versa. The UMA/GAN-based femtocell architecture moves the specialized client, or UMA client, from the handset to the FAP, eliminating the requirement for a dual-mode phone while leveraging UMA’s signaling and QoS innovations.
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