Video source: 51 Learning Communication "5G Core Network Foundation, Protocol and Signaling Process"
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https://zhuanlan.zhihu.com/p/484677694
main content
Introduce the functions of BSF network elements, that is, why BSF is introduced
Introduce the services and service operations provided by BSF
Briefly introduce the session binding process related to BSF
Background generated by BSF: - 4G Gx/Rx session binding
In 4G, DRA is needed to bind Gx and Rx sessions, so as to ensure that Diameter signaling is always routed to the same PCRF for processing. Usually, the DRA and PCRF in the existing network are deployed in pairs.
Consequences and solutions if Gx and Rx session binding is not done:
PDN connection establishment process of IMS APN:
- PGW-->DRA1-->PCRF1 takes the rules of Gx port PCC for the default bearer of IMS APN.
- The DRA1 needs to record the correspondence between the user's IMSI, IP and PCRF. If all subsequent Gx and Rx traffic goes to DRA1, it will be fine. But this is not guaranteed. Therefore, DRA1 and DRA2 are also required to synchronize data.
VoLTE call flow:
- P-CSCF-->DRA2 [DRA2 may be selected based on the load sharing principle].
- DRA2 has a considerable probability to forward the Rx message to PCRF2. [Unless the previous data synchronization with DRA1 will be transferred to PCRF1]
- However, there is no session to the PGW on PCRF2, and the PCC rules generated before cannot be obtained. Causes inconsistencies in PCC rules.
An overview of the basic functions of BSF
VoNR top-down assurance trilogy:
- AF asks for PCF (features of SDF that need to be guaranteed, how much QoS needs to be given to me)
- PCF creates PCC rules as needed (eg QoS flow, GFBR)
- PCF issues PCC rules to PCEF (SMF, UPF) for execution
[It can be seen that the PCF is a link between the previous and the next, and it is necessary to ensure that the correct PCC rules are issued to the SDF. Therefore, if there are multiple PCFs, it is necessary to ensure that the PCF in the signaling process is the same.]
BFS (Binding Support Function) is the binding function. It is mainly used for session binding of N7 (Gx) and N5 (Rx) ports to support VoNR services, so that users can choose the same PCF. In 4G, the manufacturer's private solution is used (such as the way the active and standby DRAs synchronize data), while 5G achieves standardization by defining BSF network elements and related services.
Note: Since the current AF on the existing network is mainly the P-CSCF of the IMS, it does not support the SBI interface, so the N5 interface is not commercially available, and the Diameter protocol, that is, the Rx interface, is still used between the P-CSCF and the PCF.
Overview of BSF's Services
BSF implements the function of session binding by exposing the services of the SBI interface.
The services of BSF are defined in TS29.521. It mainly includes 1 service Nbsf_Management and 4 service operations:
N7/Rx Session Binding Process (Macro) - Implementation from a Pure Specification Perspective
