Abstract: With the in-depth development of 5G network construction in my country, 5G inter-network interconnection has become the focus of attention of operators and telecom authorities. 5G inter-network services include voice/video, rich media messages, inter-network roaming, etc. Both the program and the regulation put forward new requirements. This paper introduces the inter-network interconnection scheme of 5G services, and analyzes the issues that need to be paid attention to in interconnection supervision.

Key words: 5G; interconnection; IMS; RCS; inter-network roaming

1 Introduction

As a new type of infrastructure, 5G can provide key support for the digital, networked and intelligent transformation of society, and has become an important link in the strategic layout of my country’s information modernization construction. Currently in the critical period of centralized construction of 5G network, it is necessary to coordinate the layout, increase policy support and implementation, and further give full play to the role of telecom operators as the main body of construction and operation. Entering 2020, the 5G construction process has accelerated significantly. As an important part of network construction, inter-network interconnection has received attention from all parties. On the one hand, 5G service interconnection puts forward new requirements for the network construction of telecommunications companies; on the other hand, the service quality, settlement plan, and security guarantee of 5G service interconnection have also become important issues that telecommunications authorities pay attention to.

2 5G voice/video interworking

At present, the voice network of each operator is gradually evolving to the full IMS target network. VoLTE has been activated on 4G networks. Each operator is deploying the core network of 5G independent networking architecture, and will provide VoNR voice and video services in the future. At this stage, the calls between users within each operator’s network can use the high-definition voice/video and various audio and video CRBT services provided by the IMS, but the inter-network calls still use the circuit domain gateway office to communicate. When calls between users of different operators pass through the gateway office in the inter-network circuit domain, the signaling is converted into ISUP, and the media encoding is converted into PCM, so the inter-network calls cannot provide high-definition voice and video services.

In order to meet the needs of 4G/5G business development, the Ministry of Industry and Information Technology organized China Telecom, China Mobile and China Unicom to carry out IMS network interconnection pilot projects in Hunan and Sichuan provinces in 2019, and verified IMS services on 4G networks. Interoperability between networks. With the deployment of 5G independent networking architecture and the implementation of VoNR, 5G network audio and video services will also be interconnected through IP interworking gateway offices, providing high-definition voice/video, video color ringtones and other services between networks.

2.1 IMS Inter-Network Interworking Scheme

2.1.1 IMS Inter-Network Interworking Architecture

The interworking architecture of IMS networks is shown in Figure 1, following YD/T 3369-2018 “Technical Requirements for Interworking Between IMS Networks of Different Operators”[1]The IP bearer network is used for interconnection between the networks, and the intercommunication is carried out through the IBCF/TrGW. The Ici interface between the IBCFs is responsible for the forwarding of SIP signaling between the networks, and the TrGW completes the media channel establishment, media streaming, and media encoding and decoding under the control of the IBCF. Conversion, etc., the Izi interface between TrGWs is responsible for media forwarding between networks.

Figure 1 Interworking architecture between IMS networks of different operators

2.1.2 IMS Inter-Network Interworking Point Settings

Interconnection points between IMS networks are constructed on a provincial basis. IBCF is deployed in provincial capital cities, and TrGW is deployed in provincial capital cities or sinks to various cities according to the volume of interworking media traffic.

2.1.3 IMS inter-network routing

When a user dials the user number of another network, if the calling user and the called user belong to the same province, they will communicate within the province. .

2.2 Analysis of IMS Interworking Supervision Issues

Compared with the traditional circuit domain interconnection, the interconnection between IMS networks has changed in terms of interconnection protocols, interconnection point settings, service types, and billing and settlement. Corresponding adjustments need to be made in interconnection and interconnection supervision strategies.

2.2.1 Quality of Service

The monitoring of the quality of service of inter-network calls is the key content of interconnection supervision. As early as 2003, the telecommunications authority built an inter-network settlement and interconnection order monitoring system based on No.7 signaling. With the migration of the inter-network gateway office from the circuit domain to the IP network, the technical means of interconnection supervision have also changed accordingly. First, the collected signaling becomes IP data, and the monitoring points are changed from scattered collection in various cities to centralized collection at a single point in the province. Secondly, since the inter-network communication command is changed from ISUP to SIP, the signaling monitoring system needs to be transformed accordingly to realize the functions of SIP protocol analysis and analysis and processing. In addition, due to the best-effort feature of the IP network, attention should also be paid to the operation quality of the IP bearer network, such as packet loss, delay, and jitter.

2.2.2 Inter-network settlement

The original circuit domain gateway offices are deployed in various cities, and the inter-network call settlement distinguishes between local calls and long-distance calls, among which long-distance calls are further divided into inter-provincial long-distance and intra-provincial long-distance. IMS intercommunication gateways are deployed in provincial capital cities, and single-point intercommunication is adopted within the province, so the concept of intra-provincial long-distance disappears. In order to adapt to the changes in the IMS network interoperability architecture, the inter-network settlement rules can be simplified, and a unified settlement rate is set within the province, and no distinction is made between local calls within the province and long-distance calls within the province.

2.2.3 Management of fraudulent calls and harassing calls

In recent years, the governance of fraudulent calls and harassing calls has been deepened, and bad information on the Internet has also become one of the contents of the governance work. Each operator shall transmit calling numbers between networks in accordance with industry standards and relevant management regulations, and manage and control illegal calls initiated within the network. In terms of identifying and intercepting fraudulent calls and harassing calls on the Internet, operators should unify rules to avoid arbitrarily intercepting calls from the Internet.

3 5G message exchange

On April 8, 2020, China Telecom, China Mobile, and China Unicom jointly released the “5G Message White Paper”[2], the white paper proposes the concept of 5G messaging, and introduces the requirements of 5G messaging in terms of services, terminals, and networks. The 5G message service not only supports multimedia message interaction between individual users, but also enables industry customers to provide their users with new digital interactive services based on rich media. In terms of standardization, 5G messages are based on the Rich Communication Suite (RCS) mature standard (GSMA Universal Profile 2.4). After more than ten years of development, there has been extensive standard accumulation and practice accumulation, and it has a good technical foundation. my country’s 5G message standardization work is in progress, and the formulation of communication industry standards such as the overall technical requirements for 5G messages and terminal technical requirements has been basically completed.

Similar to short messages, each operator builds a 5G message center to provide services for users of its own network, and users of different operators need to use the inter-network interworking function to realize the interaction of 5G messages.

3.1 5G message network interworking solution

The industry standard for 5G message interworking is in the process of being formulated, which mainly stipulates the network architecture, routing organization, interworking function, interworking protocol, security and inter-network settlement requirements for 5G message interworking between users of different operators. The operator sets up a 5G message interworking gateway, which is responsible for the interworking of the 5G message signaling plane and media plane. The gateways are connected by dedicated lines or VPNs (see Figure 2).

Figure 2 Interoperability architecture between different operators’ 5G message networks

The following differences exist between 5G message interworking and short message interworking.

(1) Multimedia message transmission: Traditional short message gateways carry text information through signaling. 5G message interworking requires signaling and media layers to complete the transmission of multimedia messages. The protocols used include SIP, MSRP, and HTTP. Therefore, it is necessary to define the interworking interface to achieve message interaction.

(2) The focus of group chat messages is shifted: Different from traditional short messages, 5G messages provide group chat functions. When establishing a group, the 5G message center of one operator needs to establish a group focus, which is responsible for maintaining the user list and historical message content in the group. Users who create a group can transfer the group focus to other operators’ 5G messages Center, which involves the process of inter-network interaction.

3.2 Analysis of 5G Message Interworking Regulatory Issues

5G messaging is a new type of messaging service. Due to the introduction of rich media functions, it faces new challenges in terms of message transmission service quality and content security.

3.2.1 Quality of Service

5G messages are mainly multimedia data, including voice, image, video and other formats. The Internet transmission of various data should meet specific performance indicators. The competent telecommunications department can establish a monitoring system to monitor the transmission delay, delivery rate and other indicators of inter-network messages to ensure that the business meets the corresponding service quality requirements.

3.2.2 Inter-network settlement

Traditional short messages are billed and settled by piece. 5G messages introduce multimedia content. When interconnecting, they should have the ability to bill messages between networks based on traffic, number of pieces, and duration. The 5G message charging field should include charging type, calling and called party identification, media type, message content length, etc. The specific settlement rules and rates can be determined by the operator through negotiation.

3.2.3 Spam governance

The 5G message format has been upgraded from text to multimedia, posing new challenges to the governance of spam messages such as harassment and marketing. While carrying out 5G messaging services, operators will also provide technical means to prevent spam messages. Internet messages may become a blind spot for spam management, and telecommunications authorities need to coordinate the deployment of operators in advance to prevent the spread of spam and purify the cyberspace.

4 5G core network inter-network roaming

my country’s 5G networks are mostly constructed in high frequency bands, the network sites are dense, and the construction investment is huge; 5G network bandwidth is large, maintaining the same power spectrum density, 5G base stations need higher transmit power, resulting in increased base station power consumption; 5G active antenna units and The 2G/3G/4G passive antennas on the existing network need to be deployed independently of each other, which increases the rent of base stations and puts pressure on operating costs.To this end, the Ministry of Industry and Information Technology released the 5G development strategy[3]guide basic telecommunications companies to strengthen coordination and cooperation, give full play to market mechanisms, integrate advantageous resources, carry out 5G network sharing and inter-network roaming, accelerate the formation of a network pattern with multiple networks coexisting in hotspot areas and one network in remote areas, and create resource-intensive and operational Efficient 5G network.

4.1 5G Core Network Inter-Network Roaming Solution

5G core network inter-network roaming refers to the construction of a complete 5G network by an operator, and users of other operators access the network through roaming and use mobile communication services. In remote areas where the economy is underdeveloped, the number of users is small, and the demand for services is small, building a 5G underpinning network, and users of other operators can access the 5G network through core network roaming, which can greatly save construction investment and improve network efficiency. The specific technical requirements for inter-network roaming in the 5G core network are as follows.

(1) Technical system: Priority is given to realizing 5G inter-network roaming under the independent networking architecture, and it can also provide inter-network roaming with non-independent networking architecture.

(2) Equipment capabilities: network equipment and terminals should support 5G inter-network roaming with independent networking architecture, terminals should support automatic network selection mode, support 5G network priority network selection strategy, and support the simultaneous Display of two operator logos.

(3) Operator identification: In the roaming state, the terminal simultaneously displays the identification of the home network and the visited network to remind the user that the user is in a roaming state of different networks.

(4) Service realization: The voice, data and short message services provided by the operator to the users of this network and roaming users shall ensure the same communication quality. With VoNR as the 5G voice target solution, you can fall back/switch to the 4G network for calls if necessary.

(5) Routing mode: The home routing mode is preferred, that is, the roaming data is sent to the home network, and the user service is handled by the home network; the local routing mode can also be selected, that is, the user service is handled by the visited network.

4.2 Analysis of 5G Core Network Inter-Network Roaming Supervision Issues

Inter-network roaming in the 5G core network involves coordination between different operators at the technical and management levels. The business process is complex, and there are many issues to be paid attention to in terms of supervision.

4.2.1 Quality of Service

Operators providing roaming services should ensure that roaming users have the same quality of service as users on their own networks, so that roaming users can have a good 5G service experience. The competent telecommunication department may establish a monitoring system to monitor the service quality of inter-network roaming.

4.2.2 Inter-network settlement

Both parties opening roaming services should sign a settlement agreement, and the settlement rate should adhere to the principle of market dominance, and should be negotiated by each operator; if an agreement cannot be reached through negotiation, the telecommunications authority should intervene in coordination, refer to the comprehensive tariff level of roaming operators, and put forward coordination opinions or make an administrative decision. According to the market development, the competent telecommunications department shall regularly evaluate and adjust the settlement policies and settlement standards for different network roaming in a timely manner.

4.2.3 Network Security

Core network inter-network roaming needs to be connected to core networks of different operators, and there is a lot of data exchanged between the two parties, including user authentication information and service data. When establishing a roaming interworking link, the interface for the division of network security responsibilities should be clarified, a network security assurance plan should be proposed simultaneously, and a technical plan for preventing fraudulent calls and harassing calls should be made on the Internet.

5 Conclusion

my country has carried out 5G network construction in an all-round way. In order to cope with the new situation of 5G service development and enable users to obtain better service experience, operators have begun to deploy 5G service-oriented inter-network interconnection infrastructure. The competent telecommunications department needs to strengthen the supervision and coordination of interconnection between networks, and formulate interconnection policies that conform to the current situation of my country’s network, so as to effectively promote the construction of 5G networks and industrial development.