What is Circuit Switched Fallback (CSFB)? Our current generation of LTE handsets uses CSFB, which means they attach to the LTE network for data, but voice calls still use the 2G, 3G or HSPA+ networks.(1) When there’s an active voice call and data session, the device’s radio will use HSPA+ for both the phone call and the data session. In the future, handsets will support VoLTE, a technology which uses the LTE network for voice, messaging and data, thus eliminating the need for the device to fall back to 2G or HSPA+ networks to manage the voice call (VoLTE is dependent on VoLTE capable coverage). (1,2,3)
So what does CSFB do to your LTE phone? When a call comes in, the phone uses CSFB to fall back to the 2G, 3G or HSPA+ networks for voice and data. If there is data streaming during the phone call, the throughput drops to the speed of the next best available network. Once you are off the call, your phone can reattach to LTE if you’re still in an LTE coverage area (but does not have to). For your device to reattach to LTE, the device needs to go to an idle state for a few seconds. If your application is constantly streaming data, the phone will remain on the next best available network until the radio is freed up to make the move back to LTE.
(1) http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&sqi=2&ved=0CDMQFjAC&url=http%...
(2) http://3g4g.blogspot.com/2011/02/circuit-switched-fallback-csfb-quick.html
(3) http://www.youtube.com/watch?v=9gKbyJiZlF4
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