VoLTE Basic Self-Learning Series | Which scenarios will trigger CSFB on VoLTE terminals?

Video source: 51 Learning Communication "VoLTE Basics"

Organize the teacher's course content and test notes while studying, and share it with everyone. The infringement will be deleted immediately. Thank you for your support!

Attached a summary post: VoLTE basic self-study series | Summary_COCOgsta's blog-CSDN blog

VoLTE terminal using CSFB preset conditions

Premise:

1) The VoLTE terminal is successfully attached under LTE and completes the capability declaration shown in the figure below.

2) The VoLTE terminal completes the establishment of a PDN connection to the IMS under LTE and registers with the IMS, and is assigned an IPv6 address.

How should VoLTE terminals use voice services?

Note:

This table may only be an implementation at this stage, and it does not mean that it will be the same in the future.

Scenario when VoLTE terminal triggers CSFB

When the terminal resides in the VoLTE coverage area and the terminal cannot initiate a VoLTE call successfully, the terminal works in CSFB voice mode and uses CSFB to provide voice;

When the terminal resides in a non-VoLTE LTE coverage area, the terminal should work in CSFB voice mode and use CSFB to provide voice;

When the terminal resides in the VoLTE coverage area and needs to make emergency calls, CSFB falls back to 2/3G CS to make emergency calls.

What is Silent Redial?

is a function of VoLTE terminal.

Applicable to VoLTE calling terminal.

When the VoLTE calling terminal calls the called party (Tcall timer) times out, this function is called and the CSFB process is triggered to help the calling VoLTE terminal fall back to the 2/3G circuit domain and try to call the called party again.

Silent Redial signaling process overview

Note:

1) In this process, the calling party is required to be a VoLTE subscriber and is currently registered to the IMS through LTE.

2) The called UE is not required, it can be a VoLTE or non-VoLTE user, it can be in LTE or it can also be in 2/3G.

Summary: What is the relationship between Silent Redial and CSFB?

is a causal relationship.CSFB because of Silent Redial.

The logic is that there is a problem with the IMS network where the calling UE is located at this time (the calling P-CSCF does not return 100 trying, which causes the Tcall timeout), so a mechanism is needed to trigger the CSFB, so that the calling party can go to 2/3G to find the receiver again.Call.This mechanism is Silent Redial.

Failure to establish a dedicated load causes CSFB to fall back

Note:

1) In this process, the calling party is required to be a VoLTE subscriber and is currently registered to the IMS through LTE.

2) The called UE is not required, it can be a VoLTE or non-VoLTE user, it can be in LTE or it can also be in 2/3G.

The core network reply does not support IMS to guide the UE to initiate CSFB fallback

Note:

The core network mentioned in this example does not support IMS, and it is not necessarily that the core network does not deploy IMS, and there may be other reasons such as the data definition for eSRVCC is not defined in the HSS.

VoLTE emergency call: the core network replies 380 to guide the UE to fall back to CS

Other cases:

• I have also seen on the Internet that the called T-ADS is inaccurate, resulting in the called CSFB

Author: Correspondent who loves programming
Link: https://juejin.cn/post/7128664316097593351/
Source: Rare Earth Nuggets
The copyright belongs to the author.For commercial reprints, please contact the author for authorization, and for non-commercial reprints, please indicate the source.