Because the current performance indicators of VoIP services fail to meet the quality of existing circuit-domain voice services, and IMS needs to be deployed across the entire network, three different LTE voice solutions have been formed on the basis of existing networks: based on dual standby terminals Solution, CSFB and VoLTE. Both CSFB and VoLTE are LTE voice solutions defined by 3GPP. Both solutions are clearly defined in the 3GPP specifications. VoLTE requires full support and optimization of terminals, wireless and core networks, and from the current perspective, the implementation complexity is relatively large. CSFB is a relatively simple voice solution proposed when VoLTE is not implemented in the industry.
Dual standby: simple technology implementation
The dual standby terminal can stand by in the LTE network and the 3G / 2G network at the same time, and can simultaneously receive and send signals from the LTE and 3G / 2G networks. The essence of its voice solution is to use the traditional 3G / 2G network, which has nothing to do with LTE.
The voice solution based on dual standby terminals is a relatively simple solution. The terminal chip can be implemented with two chips (1 3G / 2G chip and 1 LTE chip) or a multimode chip. There is no interoperability between LTE and 3G / 2G modes. The terminal does not need to implement different system measurements. The implementation is simple.
CSFB: gradually improve
The main idea of ​​the CSFB scheme is to drop back from the LTE network to the circuit domain of 3G / 2G when the user needs to perform voice services, and initiate or answer voice services according to the business process of the circuit domain.
To achieve CSFB, it is necessary to establish an SGs interface between the MME and the MSC equipment of the 3G / 2G network (shown by the purple line in Figure 1). The SGs association plays a bridge role in the CSFB technology, and can connect two different systems to realize the continuity of users' voice services between different systems. The so-called SGs association established for a user means that the address of the user's MME is stored in the VLR, and at the same time, the address of the user's VLR is also stored in the MME.
As can be seen from Figure 1, the CSFB technology will affect the existing 3G / 2G network. The MSC of the original network needs to add an SGs interface with the MME, and the SGSN adds an S3 interface with the MME.
Terminals that support CSFB must have multi-mode capabilities, and can access EPC through E-UTRAN or CS domain through GERAN / UTRAN. CSFB capable terminals must be able to perform joint RAU / LAU attach, location update, and detach procedures.
In order to support the CSFB function, the MSC in the original 3G / 2G network needs to be added to the SGs interface of the MME, supports the SGs protocol stack, and maintains the SGs association; The joint mobility management and other functions.
For the wireless subsystem (base station, base station controller) of the original 3G / 2G network, the neighbor cell configuration of LTE needs to be increased. The wireless subsystem of the original network needs to support the Fast Return function; in order to optimize the delay of the terminal falling back from LTE to 3G / 2G, the wireless subsystem of the original network needs to support the RIM function, etc.
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