We can compare how Voice of LTE (VoLTE) is better than Circuit Switch Fall back (CSFB) in several ways. Performance-wise, capacity, quality, etc. However, in general terms, VoLTE is better than CSFB in 6 steps as follows.
- VoLTE provides a first-line telephony service with high voice quality and short call setup.
- In the case of VoLTE, Voice, and video use Quality of Service (QoS) bearers with guaranteed bit rate to secure the service characteristic.
- VoLTE allows swift call establishment ( ~ 1 sec ) versus CSFB towards 3G (~ 5 sec) and even more in case of CSFB towards the GSM (~ 8 sec)
- VoLTE avoids 4G data service interruptions and preserves the LTE data experience during speech communications. In contrast, the throughput of concurrent data sessions is typically reduced in the case of CSFB to 3G and even suspended in the case of CSFB to 2G.
- 3GPP standardizes VoLTE, and it's considered to be the target network infrastructure from a long term perspective.
- VoLTE phone can work in a variety of modes, attached to the different networks (2G, 3G, and LTE), even also connected to two networks by dual-standby terminal and therefore when UE originate/terminate the call; terminal/network needs to select which network to be accessed. This process is called domain selection according to the network registration information.
Source: LTE Optimization Engineering Handbook by X. Zhang