CSFB (Circuit Switched Fallback)

CSFB stands for Circuit Switched Fallback. It is a mechanism that allows a mobile device to continue a voice call while being handed over from an LTE network to a 2G or 3G network. LTE Network is using CS and PS call.

When the mobile device moves out of range of the LTE network, it will be handed over to a 2G or 3G network. This is because 2G and 3G networks use circuit-switched (CS) connections to carry voice calls. CS connections are more reliable than PS connections, and they are less likely to be dropped during a handover.

During a CSFB handover, the mobile device will temporarily disconnect from the LTE network and connect to the 2G or 3G network. The voice call will be transferred to the 2G or 3G network. Once the mobile device has connected to the 2G or 3G network, the voice call will be resumed.

During CSFB, the LTE device temporarily leaves the LTE network and accesses the legacy circuit-switched network (2G or 3G) to establish the voice call. The device switches its radio connection to the legacy network for the duration of the call.

After the voice call ends, the device returns to the LTE network for data services, as LTE provides faster data speeds and better network capacity.

Volte stands for voice over Lte. It’s a mechanism that allows a mobile device is making a voice call over LTE, it is connected to the LTE network using a packet-switched (PS) connection. A PS connection is a connection that is used to carry data, such as voice calls, text messages, and data transfers.

In LTE networks, voice calls are typically carried over the IP-based LTE network using VoLTE (Voice over LTE) technology. However, if an LTE device needs to make or receive a voice call, and there is no VoLTE coverage or the device doesn't support VoLTE, CSFB is triggered.

Triggers of CSFB (When to perform CSFB)

Followings are the list of situation where UE initiate CSFB. These are not the full list of those situations, but can be most common situations (triggers) for CSCF.

 

1.                   UE is not capable of VoLTE and is registered (or in communication) with LTE network. User make voice call (MO Call)

2.                   UE is not capable of VoLTE and is registered (or in communication) with LTE network. UE recieves a Paging message for voice call (MT Call)

3.                   UE is capable of VoLTE, but ims PDN is not established (UE is not registered in CSCF). User make voice call (MO Call)

4.                   UE is capable of VoLTE, but ims PDN is not established (UE is not registered in CSCF). UE recieves a Paging message for voice call (MT Call)

5.                   UE is capable of VoLTE, ims PDN is established and UE is registered in CSCF, but Network notifies 'VoLTE not supported' in NAS message.

6.                   UE is capable of VoLTE, ims PDN is established but UE failed to register in CSCF. User make voice call (MO Call)

7.                   UE is capable of VoLTE, ims PDN is established but UE failed to register in CSCF. UE recieves a Paging message for voice call (MT Call)

8.                   UE is capable of VoLTE, ims PDN is established and UE is registered in CSCF but SIP INVITE for VoLTE failed to go through (e.g, getting some error or reject code right after SIP INVITE)