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I have been reading core architecture topics on LTE. In a session, I was introduced to CSFB, as a service handover, instead of the coverage handover associated with the SRVCC.

While I am able to understand SRVCC quite well, I have a query on CSFB. I have an example I use to understand the concept. Let me know if my understanding is correct.

Suppose that I want to connect with my friend, who has a user equipment equipped with only 2G/3G support. Since 2G/3G-only user equipment can't run any handshake sequence to sign in onto an LTE network, I have to call using my 2G/3G SIM. When I call my friend, a request goes to the core LTE network, which then instructs my user equipment to leave 4G and access the 2G/3G to make the call. This fallback from 4G to 2G/3G is what is CSFB, right?

ocrdu
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Kaswechiha
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1 Answers1

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Not quite, but close.

If your phone supports VoLTE (voice over LTE) and your provider supports it for you phone (make and model), then there will be no CSFB. You can place your call through LTE, and the network will send it to your friend through 2G or 3G.

But if your phone doesn't support VoLTE, or if your provider hasn't put it on its list of supported models, then any voice call you place will trigger the core LTE network to instruct your phone to move to 3G or, failing that, to 2G.

Paul
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  • I get the point. So, there is conceptually a difference between a VoLTE supported LTE subscriber trying to connect with a 2G/3G-only supported user and a subscriber with non-VoLTE support, on an LTE network, that is trying to connect with a 2G/3G-only supported user. – Kaswechiha Aug 12 '22 at 09:52
  • 'You can place your call through LTE, and the network will send it to your friend through 2G or 3G'- this means that the LTE network will first contact the core 2G or 3G network and put a request for establishing the call, through the 2G or 3G, by allocating the resources, right? – Kaswechiha Aug 12 '22 at 09:57
  • Correct! The friend could also be on a different network, or on a landline, and the mechanism will be the same. – Paul Aug 12 '22 at 10:09