An Introduction to LTE: LTE, LTE-Advanced, SAE, VoLTE and 4G Mobile Communications, 2nd Edition

Chapter 21Circuit Switched Fallback

As we explained in Chapter 1, LTE is designed as a data pipe: a system that delivers information to and from the user, but does not concern itself with the overlying application. For most data services, such as web browsing and emails, the applications are separate from the delivery system and are supplied by third parties, so this approach works well. However, for voice and text messages, the applications have previously been supplied by the operator's circuit switched network and have been tightly integrated into the delivery system. This is a very different principle from the one adopted for LTE.

The 3GPP specifications support two main approaches for the delivery of voice over LTE. The main short-term solution is circuit switched fallback, in which the mobile accesses the circuit switched domain of a legacy 2G or 3G network by moving to a 2G or 3G cell. The main long-term solution is the IP multimedia subsystem, an external network that handles the signalling functions needed for voice over IP. There are also three other approaches: the delivery of voice over IP services by a third party, the use of a dual radio device and another 2G/3G inter-working technique known as voice over LTE by generic access. Each approach can be adapted for the delivery of text messages, using either SMS or a proprietary messaging application.

In this chapter, we review the market for voice and text messaging and introduce each of the five approaches. We then ...