What is circuit switched fallback?

What is circuit switched fallback?

LTE is an all-IP technology and therefore cannot transport switched services such as voice or SMS. However, voice is still an important part of any telco’s business due to consumer expectation and the huge contribution it makes to revenue. Circuit switched fallback (CSFB) is a protocol devised to allow voice to travel over LTE networks, which cannot normally handle circuit switched services.


The Global System for Mobile Communications Association (GSMA) has launched voice over LTE (VoLTE) as an initiative to try and support the move to voice and SMS over LTE.

According to a report by Rohde and Schwarz, the organisation that formally announced the initiative in February last year, the transition should be based on existing IP multimedia systems (IMS) telephony concepts. This contradicts the initial expectation that IMS networks would be widely deployed as LTE became more available, which so far has not been the case.

The third-generation partnership project (3GPP) decided that, in order to tackle this problem, operators should use CSFB when using circuit switched-based services.

How does this work during a phone call?

When a call is made, it reaches a mobile switching centre (MSC), which communicates to a mobility management entity (MME) to identify network compatibility. If it is an LTE network attempting to connect to a legacy network, the MME recognises this and subsequently routes the call through a 2G or 3G network. It is important to note that this process only works when an LTE network’s coverage area is also covered by GSM, UMTS or CDMA networks.

Where is the technology implemented?

Robin Kent, director of operations at Adax Europe, explains that the standard was defined by the 3GPP: “Essentially you have an architecture, which is part of that standard, showing you the route the call has to take.”

The physical technology required is a multimode handset that can receive LTE calls but can also take calls from existing 2G and 3G networks. For the switch to be implemented, the technology needs to be available next to the MSC, which has traditionally existed in the media gateway that detects the call and routes it accordingly. The existing CSFB standard states that there does not need to be any change to the user plane transport service.

However, a few companies have been trying to combine CFSB with an IP core, which has resulted in interworking the user plane at the network edge. Companies such as Adax, Surf Solutions and RadiSys have recently launched similar solutions for GSM edge radio access network (GERAN) and universal terrestrial radio access network (UTRAN).

How does this benefit operators?

By allowing voice traffic to transfer from LTE to 2G and 3G networks, it provides a solution for operators which have deployed LTE but where VoIP is still unavailable. The other option would be to make heavy investment into IMS. Adax claims the investment in CFSB is not only a lot smaller but will take less time to put into effect.

“While voice revenues are declining, it is still a major part of an operator’s revenue and it is a service that they have to deliver,” says Kent. “Things are moving towards being completely transferred in packets and therefore everything is data, [but] there is still a need to deliver voice.”

If this technology only serves to switch between LTE and 2G and 3G networks, doesn’t it have a limited life span?

CFSB is seen as a temporary measure but it is expected to be used for some time yet as operators need more money and time to fulfill the idea of all-IP, LTE and IMS. The deployments of LTE thus far have been data based to accommodate the rising demand for high-speed data.

The widespread deployment of IMS is still some way off as many operators will not be able to handle the cost. The 2G and 3G networks are capable of supporting voice service requirements and consequently operators are predicted to keep using the existing systems while they can.

What are the possible complications?

Issues have been raised over possible timing and delays in the network that could affect the reliability of the calls. “Some of the operators have said that you will have to accept that, while others will look at their networks and tune them to make sure that’s not an issue,” adds Kent.

“In the trials we have done so far we haven’t come across this problem but it is the potential downfall.” Another possible problem could arise over roaming and interconnection of networks, but CFSB has been implemented in some areas to make sure roaming agreements are met.

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