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The femtocell appears to the standard 3G phone as just another cellsite from the host mobile operator, and can thus be used by almost any phone including roamers visiting from other countries. The mobile operators telephone switch (MSC) and data switch (SGSN) also communicate to the femtocell controller in the same way as other mobile calls. Therefore, all services including phone numbers, call diversion, voicemail etc. all operate in exactly the same way and appear the same to the end user. The connection between the femtocell and the femtocell controller uses a secure IP encryption (IPsec), which avoids interception and there is also authentication of the femtocell itself to ensure it is a valid access point. Femtocell controller protocols and interfaces. There are several competing approaches to how femtocells are controlled within the mobile operator’s network, with different vendors promoting each solution. Operators are insisting that the industry standardise, to allow them to buy from multiple vendors and to reduce interoperability problems. The main focal point of standardisation is the Iu interface, which separates the 3G core network (MSC voice switches, GSN packet service controllers etc) from the radio access network (base stations and base station controllers). The main contenders for a 3G femtocell standard are UMA and various forms of (proprietary) Iub over IP. 1. Iub over IP. Use the similar cellsite protocols as for macrocells and run them over IP, although more functionality of the RNC is incorporated into the femtocell. A femtocell gateway is used to aggregate many thousands (potentially hundreds of thousands) of femtocells into a single standard Iu interface. Different vendors have implemented different (and not necessarily interworking) solutions, especially where the same vendor supplies both the femtocell and gateway controller. 2. UMA. Reuses the standard developed for WiFi dual-mode phones between the femtocell and the mobile network. This also reuses the UNC products developed for WiFi. The 2G interfaces into the core network were formally standardised a few years ago and are in commercial use, whilst work on the 3G Iub interface was expected to be approved during Q1 2008. There are several alternatives which we believe are much less likely to succeed. 3. Automonous SIP. Incorporate the mobile network MSC and SGSN functionality into the femtocell, so it becomes a mini operator roaming network. Unlikely to be adopted in the short term, because the operator will wish to retain control and ensure continued revenue stream. 4. Emulation using SIP. Tatara networks proposes a system which converts all the standardised signalling into SIP based protocols, then unwinds them to present a standard interface to the existing network. We can't see the value in this, the additional complexity is intended to be ready for future evolution towards IMS. 5. SIP core network with Iu interface. Sonera have developed a gateway product which converts Iu into their common SIP-based core network. Whilst IMS is supported, other SIP based standards are typically used to provide the same features and services for both wireline and wireless devices. Examples given include being able to divert to mobile (or fixed) when the system knows you are present. These options are described in more detail, together with some advantages and disadvantages. Vendors appear to be consolidating in two camps: - Those who want to reuse their existing RNC product investments, and so are promoting a standard Iub interface. For example Nokia-Siemens offers a proprietary IP interface on their RNC which they wish other femtocell vendors to develop and interwork with. - Those who have developed a specific UNC product designed for very large scale, small capacity cells. For example, Kineto wireless offers a UNC which already interworks with many WiFi and Femtocell access points. The mobile operators will have to decide whether the operational costs of managing a different network node from a different vendor (ie the UNC) provide more benefits than reusing the same vendor and operational systems to run what is quite a different type of mobile network. If the predicted volume of femtocells are deployed in the next few years, it seems likely that significant additional service management, operational and billing investments will be required to provide a high quality customer experience.
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