In general, voice transmission problems in LTE networks are available in three ways: VoLTE (Voice Over LTE, LTE network direct transmission), CSFB (Circuit Switched Fallback), SVLTE (Simultaneous Voice and LTE, LTE and voice). Network synchronization support).
VoLTE is an IP Multimedia Subsystem (IMS)-based voice service, in conjunction with GSMA's voice service standards at LTE control and media level, developed in PRD IR.92. Using this scheme means that voice will be transmitted in the form of data streams over the LTE network, so there is no need to call a traditional circuit-switched network, and the old network will not need to be reserved. VoLTE recommends the use of AMR-WB voice coding, also known as HD Voice. This encoding supports a sampling rate of 16 KHz under the 3GPP standard family network.
At present, VoLTE has become the mainstream solution for voice and voice roaming of global operators in the LTE era. VoLTE provides a high-quality audio and video service experience, and adopts SRVCC/eSRVCC to support voice service continuity. As a key capability for operators to build next-generation converged communications, it can provide users with a richer service experience. With the coordinated development of FDD and TDD, more and more operators are investing in the deployment and construction of VoLTE networks, jointly promoting the construction of VoLTE international roaming network, with the goal of achieving true global seamless roaming.
The deployment of VoLTE is a system engineering, which involves related issues in 10 fields, such as IMS core network, EPC core network, CS core network, user data, signaling network, wireless network, and bearer network support system. It is an unprecedented complex in the telecommunications field. A network evolution is a major challenge to the transformation of carrier networks and communications services.
IMS core network deployment
IMS supports rich access types and multimedia services, and has been identified by 3GPP and GSMA as the standard architecture for mobile voice to All IP evolution. One of the key tasks in deploying a VoLTE network is to establish an IMS-centric target core network. The key network elements involved in the IMS core network include I/S-CSCF, DNS/ENUM, MRFC/MRFP, IBCF/BGCF, and MGCF/IM-. MGW, SBC (Session Border Controller), etc., mainly complete the access control, call routing, and service triggering functions of the VoLTE network.
The SBC is an important component of the VoLTE network. It is generally deployed in conjunction with the P-CSCF function and is the entry point for mobile users on the IMS network. Since the SBC generally carries VoLTE signaling and media at the same time, its deployment location has a significant impact on the user experience. Therefore, in the deployment strategy, it is necessary to comprehensively consider the equipment efficiency, engineering efficiency, and traffic switching of the SBC distributed deployment. effectiveness. As the proxy node and entry point of the VoLTE bearer, the SBC can support the ATCF/ATGW anchor node function at the same time, which can reduce the media reconstruction time when the user switches from 4G to 2G or 3G, and ensures smooth inter-system roaming call experience.
EPC core network deployment
The key functional nodes of the EPC core network include the MME and the S/P-GW, and need to support VoLTE related functions. For example, the IMS APN (Access Point Name) of the VoLTE user needs to be assigned an IPv4 or IPv6 address, the P-CSCF discovery and routing service request is deployed to the IMS core network, and the PCRF policy is established through the Gx interface to satisfy the service QoS. The required EPC and air interface bearer initiates a voice continuity request to the CS circuit domain through the Sv interface between the MME and the SRVCC-IWF.
APN configuration involves APN configuration on the terminal, MME, and P-GW, which is a key aspect to be considered for EPC core network deployment. The VoLTE service and the Internet service can share the APN or the independent APN. The two types of services use different IP addresses. The shared APN solution is simple to deploy, but telecom services such as voice are susceptible to Internet data services. In addition, the telecommunication voice service is generally based on the duration of the call, and the Internet data service is based on the flow rate. The shared APN solution needs to eliminate the related voice call traffic according to the ICID (IMS Charging IdenTIfier), and the charging support system adaptation process is more complicated. The independent APN solution does not have this problem. It can be charged according to the APN. Therefore, it is generally recommended to deploy the VoLTE service by using an independent APN.
CS core network transformation
The current CS network will still coexist with the VoLTE network for a long time, and the two networks need to cooperate to smooth the user experience. One is that CS needs to support the Sv interface, and the SRVCC/eSRVCC technology is deployed to ensure that voice calls are continuous when users roam from 4G to 2G or 3G networks. The second is to support the SRVCC/eSRVCC enhancement function to ensure that the supplementary calls can be supported after the voice call is switched to the CS, such as call waiting (CW), call hold (CH), and multi-party call (MulTI-Party). In the long run, the current international standards have also developed a solution for the CS network evolution access IMS. The CS network can be upgraded to support ICS, that is, the softswitch MSC evolves to enhance the MSC to support the I2 interface and other functions, and complete the service switching and call switching functions to the IMS. The migration will build a unified core network across the network to unify the user experience and improve operational efficiency.
In the initial stage of VoLTE deployment, the transformation of the CS network should be reduced as much as possible, and the deployment time of the CS network transformation should be shortened. For eSRVCC deployment, because the amount of inter-system handover is relatively small, and it is also a transitional network in the 4G migration process, it is suitable to centrally deploy one or a pair of enhanced MSCs as SRVCC-IWF network element bridging CS and EPC/IMS networks. , reducing the impact on existing CS networks.
4G PCC network deployment
The PCC (Policy and Charging Control) system is proposed in 3GPP R7 and acts on the end-to-end policy control architecture of the packet network. With the PCC architecture, the SBC/P-CSCF applies for the VoLTE audio and video bearer resources to the PCRF through the Rx interface. The PCRF maps the dynamic rules that meet the audio and video service requirements according to the configuration policy, and indicates the EPC and the wireless reservation to satisfy the service through the Gx interface. The proprietary bearer required by QoS ensures the quality of service.
The key to end-to-end QoS deployment lies in the configuration and mapping of QoS parameters of each relevant network element. The SAE-HSS issues key parameters such as QCI, ARP, APN-AMBR, and UE-AMBR based on the user subscription service type, and completes the QoS mechanism and parameters of the service layer network element, including the base station, MME, S/P-GW, and PCRF. Mapping, complete IP bearer layer network elements, such as routers based on QCI to DSCP differentiated control mapping. Since end-to-end QoS deployment involves many domains and network elements, this integration capability for operators or vendors is a big challenge.
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