The LTE radio access has been designed to be optimized for IP-based services. This means that LTE has no support for dedicated channels optimized for voice calls. It is a packet-only access with no connection to the circuit-switched mobile core network. This is different to GSM, WCDMA and CDMA, which support both circuit- and packet-switched services and it naturally impacts the technical solution for how to deliver voice to LTE users.
Depending on the network operator build-out plans and the frequency bands used for LTE, the radio coverage can be assumed to be non-continuous or even spotty, at least in the initial stages of LTE deployment. Voice as a service, however, relies on a continuous service coverage. In a mobile network, the support for continuous service coverage is realized through handovers between radio cells and between base stations.
For EPC, two basic approaches have been guiding the work in defining voice service support. Simply put, either voice services for LTE users are produced using the circuit-switched infrastructure that is used for voice calls in GSM, WCDMA and CDMA, or alternatively, IMS technology and the MMTel application are used.
Circuit-switched fallback (CSFB) is an alternative solution to using IMS and SRVCC (Single-radio voice call continuity) to provide voice services to users of LTE. The fundamental differences are that IMS is not part of the solution, and in fact, voice calls are never served over LTE at all. Instead, CSFB relies on a temporary inter-system that switches between LTE and a system where circuit-switched voice calls can be served.
The solution is based on the fact that LTE terminals ‘register’ in the circuit-switched domain when powered and attaching to LTE. This is handled through an interaction between the MME and the MSC-Server in the circuit-switched network domain. There are then two use cases to consider – voice calls initiated by the mobile user or voice calls received by the mobile user:
1. If the user is to make a voice call, the terminal switches from LTE (system A) to a system with circuit-switched voice support (system B). Any packet-based services that happened to be active on the end-user device at this time are either handed over and continue to run in system B but on lower data speeds or suspended until the voice call is terminated and the terminal switches back to LTE again and the packet services are resumed. Which of these cases that apply will depend on the capabilities of system B.
2. If there is an incoming voice call to an end-user that is currently attached to LTE, the MSC-Server will request a paging in LTE for the specific user. This is done via the interface between the MSC-Server and the MME. The terminal receives the page, and temporarily switches from LTE to system B where the voice call is received. Once the voice call is terminated, the terminal switches back to LTE.
The main idea behind CS fallback is to allow UEs to camp on LTE and utilize the LTE for data services but reuse the GSM, WCDMA or CDMA network for circuit-switched voice services. To support CS fallback, there is a set of modifications of existing procedures and also some additional CS fallback specific procedures added to EPS.
This article intends to illustrate the main principles of the CS fallback procedures by outlining an example of a CS fallback call.
There are special additions to the attach and TA update procedures which activates an interface, called SGs, between the MME and the MSC. This interface is used
by the MSC to send paging messages for CS calls to the UE camping on LTE. The procedure is shown in below and can be described in the steps that follow;
A.The MSC receives an incoming voice call and sends a CS page to the MME over a SGs interface. The MME uses the TMSI (or IMSI) received from the MSC to find the S-TMSI (which is used as the paging address on the LTE radio interface). The MME forwards the paging to the eNodeB in the TAs where the UE is registered. The eNodeBs perform the paging procedures in all the cells in the indicated TAs. The paging message includes a special CS indicator that informs the UE that the incoming paging is for a terminating CS call.
B.At the reception of the paging message, the UE performs a service request procedure which establishes the RRC connection and sends the Service Request to the MME. The Service Request message includes a special CS fallback indicator that informs the MME that the CS fallback is needed. This triggers the MME to activate the bearer context in the eNodeB with an indication to perform fallback to GERAN or UTRAN.
C.The eNodeB selects a suitable target cell, possibly by triggering the UE to send measurements on neighbour cells, and initiates a handover or cell change procedure. The selection between handover or cell change procedure is based on the target cell capabilities and is configured in the eNodeB.
D.After handover/cell change, the UE detects the new cell and establishes a radio connection and sends a page response to the MSC, via the target RAN.
E.When the page response arrives at the MSC, a normal mobile terminated call setup continues and the CS call is activated towards the UE.
The CS fallback specifications cover all necessary procedures to support fallback to GSM, WCDMA and 1xRTT for mobile originated and mobile terminating calls in both idle and active mode. The procedures for CS fallback are specified in 3GPP TS 23.272 [23.272].
Even though CS fallback was primarily introduced to support voice calls it also supports other CS services. In particular the support for SMS has been specified in such a way that it does not require the UE to switch to another radio interface. The UE can remain on LTE and still send and receive SMS. The SMS messages are tunnelled between the UE and the MSC through the MME NAS signalling and the SGs interface. 
