This approach reuses the same cellsite protocols as for 3G macrocells running over IP.
This is defined in the mobile standards by 3GPP Release 5 and connects the femtocell directly to the RNC using the Iub interface. It allows existing RNC vendors to reuse their products with minimum redevelopment to support femtocells.
There are some issues with this approach:
a) Timing and Synchronisation. The standard Iub interface is used for synchronisation and requires very high quality communication between the femtocell and the RNC. It is intolerant to delay, jitter and packet loss as typically experienced over the public internet.
b) Scalability of RNCs. These were designed to handle several hundred high capacity NodeBs connected via ATM, and may require substantial re-architecting to support thousands or tens of thousands of femtocells via IP.
c) Managability. Macrocellular UTRANs are not designed to handle thousands of cellsite moves and changes per day.
d) Security: There is no standard in place to secure the IP connection between the femtocell and RNC, although an IPsec tunnel could be used for this purpose.
e) Vendor Interoperability: Often the Iub interface implementation is not standard, and many vendors have added additional proprietary functions. This makes support of multi-vendor femtocell deployments more difficult.
f) Network operator bypass: Iub implementations cannot easily be architected to offload direct internet traffic at the femtocell itself, and instead would need to tunnel all data traffic to the mobile operators core network.