IMS Based Architecture
With improving QoS delivery on IP transport, UMTS 3GPP has been migrating both signaling and bearer transport from traditional Signaling System 7 (SS7) to IP. Improved quality of voice calls over pure Voice over IP (VoIP) applications such as Skype has definitely made a case for this migration. Session Initiation Protocol (SIP) and Real-time Transport Protocol (RTP) have been the key pillars of this migration for signaling and bearer transport, respectively.

As shown Figure 5 below, the IMS based architecture for femtocell looks to leverage the IMS core and completely offload the mobile core network, which enables building future-proof femtocell networks where innovative application-level services can be deployed easily and delivered all the way to consumers. This architecture works best for vertically-integrated network operators such as Verizon, Orange, etc., that offer bundled wireless, wireline, and broadband services.

Figure 5: IMS based architecture

In this approach, the FAP interworks the UMTS signaling plane with the SIP signaling protocol over the public IP network. On the IMS core side, the FAP may directly interface with softswitches providing Call Session Control Function (CSCF) functionality using SIP and interface directly with the Home Subscriber Server (HSS) using the Diameter protocol for Authentication, Authorization, and Accounting (AAA) functionality.

Alternatively, the FAP may choose to interface with these devices through an aggregating packet data gateway. On the bearer plane, the FAP forwards voice traffic toward the IMS core as RTP packets. QoS depends on the connectivity offered by the public IP network, and as noted above IP delays and data/packet loss will impact service performance. TR-069 could again be used for zero-touch initial system configuration and service provisioning of the FAP.

Handovers in this approach are always Inter-CN (MSC/SGSN) in nature. The FAP would handle most resource management (bearer and control) functionality within the femtocell environment and would defer it to the CN during femtocell-to-macrocell handover, which is a key issue to tackle from a standardization perspective in this model. To ensure smooth femtocell-to-macrocell handover, the CN and IMS core would have to coordinate the MM and resource management control messages over disparate signaling transport while ensuring voice call continuity (VCC).

Figure 6 below illustrates one possible scenario of signaling and bearer transport switchover from IMS to CS and vice-versa during handover. The illustration assumes the macrocell is in a CS network and the femtocell is in an IMS-based network.

Figure 6: Signaling and bearer transport switchover for IMS to CS handover

Per the illustration in Figure 6, during hand-in the CN together with the UE identifies the appropriate FAP (using an approved neighbor list managed at the RNC). Once the FAP is identified, the MSC acts as the anchor for this call and initiates a signaling transport switchover through the IMS domain via the CSCF. In the IMS domain, the CSCF initiates SIP signaling to set up signaling and bearer (RTP) transport sessions and the RRC protocol at the FAP sets up the radio link with the UE.

Interworking between SIP and the RRC/MM function is done at the FAP. Once the radio link at the FAP is set up, the MSC transfers the call over to the IMS domain before terminating the radio link in the macrocell. During hand-out, the MSC continues to anchor the call " however it is also possible that the CSCF anchors the call and initiates signaling and bearer transport through the CS domain. To support VCC, a logical Domain Transfer Function (DTF) as defined in 3GPP TS23.206 is implemented in anchoring the CSCF or MSC; this function ensures seamless transfer of signaling and bearer transport across domains.

Table 2: Comparison of various femtocell network architectures

Conclusion
In femtocell, service providers and TEMs have tremendous opportunity to address coverage and capacity issues to offer innovative high-quality service and reduce customer churn. However, the future success of femtocell technology fulfilling this opportunity will depend largely on the progress of standardization by the Femto Forum and other industry bodies in the near future.

Each of the architectures described above has distinct advantages and specific issues to tackle. In all approaches, the FAP and FGW are common devices that need to be put in place.

The approach that is least intrusive to operators' existing network infrastructure, enables quick standardization (e.g., requiring minimal new protocols), is most cost effective, is easy for consumers to use, and provides reasonable quality of service will likely make the cut. Table 2 above attempts to compare these approaches on qualitative metrics.

Srinivasa Rao is network architect and and Ravi Raj Bhat is director of engineering at Continuous Computing.