The UE modes of operation for LTE are defined in 3GPP TS 24.301 (e.g.
section 4.3 in v10.3.0):
* PS mode 1: EPS only, 'voice centric'
* PS mode 2: EPS only, 'data centric'
* CS/PS mode 1: EPS and non-EPS, 'voice centric'
* CS/PS mode 2: EPS and non-EPS, 'data centric'
The mode specifies, among other things, how the UE should behave w.r.t
CS fallback depending on the capabilities reported by the network.
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