Deploying LTE networks is faster than ever. Therefore, it is urgent for operators to better implement worldwide seamless roaming to provide subscribers with various services, including voice, data, and multimedia.
To address the preceding issue, operators are rolling out international roaming services. These services can expand their scope and help enhance their competitiveness, by improving subscriber satisfaction and loyalty, and attracting new subscribers. In addition, operators can increase revenue by providing services to subscribers served by other international operators.
However, due to different standards, systems, and inventory markets, applying international roaming on LTE networks have the following difficulties:
In summary, to implement international roaming, operators are facing a technical issue and also required to determine a solution.
Roaming services enable mobile subscribers to use services in countries or areas outside of home networks. Roaming is allowed among networks that have the same network standards and only usable in areas or countries where operators have signed a roaming agreement.
For operators, roaming services can be classified into inbound and outbound roaming. The inbound roaming service allows subscribers from other operators to access the local network and services. The outbound roaming service allows subscribers from the local network to access another operator's network and services.Roaming can be classified into national and international roaming based on the physical locations accessed by subscribers. National roaming indicates that mobile subscribers can access other operator networks and services in the home country. International roaming indicates that mobile subscribers can access operator networks and services abroad.
Figure 2-1 LTE Roaming Architecture
Based on the roaming service access policies used by mobile terminals, two roaming types are supported: Home-routed and LocalBreakOut.
Roaming subscribers can choose Home-routed, LocalBreakOut, or both to access a visited network based on service access policies.
Home-routed roaming is widely used in 2G/3G networks and has had all bugs fixed. At the early phase of LTE roaming deployment, this roaming type is recommended. The Diameter edge agent (DEA) is deployed over the S6a interface.
LocalBreakOut can relatively reduce user plane loops and transmission resources required, thereby reducing the roaming service delay and providing better user experience. However, service control, policy control, and charging are complex.
To tackle the preceding problems, the following roaming access policies are recommended:
To ensure service continuity while roaming, connections must be established and roaming agreements must be signed between operator networks. Roaming agreements allow operators to set policies to control network access for roaming subscribers and manage roaming services.Operator networks can connect directly or connect through a GPRS Roaming Exchange (GRX) or IPX network.
This solution is simple and fast and facilitates interworking. It can be implemented in either of the following ways:
GRX/IPX networks are operated and managed by third parties. A GRX/IPX connection endpoint can be connected to multiple operator networks with a roaming agreement. Private lines do not need to be individually established on the network by a single operator, greatly reducing roaming costs. In addition, GRX/IPX-based interconnections provide better service scalability. This connection is the preferred solution.
A GRX acts as a hub to connect different mobile networks through GPRS roaming networks established by GRX service providers.
GRX-based interconnections have the following advantages:
However, GRX networks only support transmission of GPRS, EDGE, 3G, and HSPA roaming data and MMS service data. Other service data is borne over dedicated networks. To be specific, when roaming, signaling is transmitted through SS7 signaling networks, voice data is transmitted through PSTN networks, and SMS data is transmitted through SMS-IG networks.
GRX-based interconnections are used only between specific mobile operator networks. As these connections do not provide QoS guarantee, they cannot support LTE network or IMS service deployment.
According to the preceding analysis, GRX networking has the following disadvantages:
IPX has evolved from the GRX framework and provides a more open and flexible environment to interconnect fixed-lines and mobile service providers.
An IPX network has the following features:
IPX networks not only support voice, MMS, and data roaming services but also support potential services and apps, such as video sharing, instant communication, enhanced messaging, and rich communications suite.
By using IPX networks to transmit international voice data, mobile network operators (MNOs) and fixed network operators (FNOs) can expand their business scope wider, to include the public switched telephone network (PSTN).
According to the preceding analysis, IPX networking has the following advantages:
The more All-IP is transformed and LTE networks are deployed, IPX networks will gradually replace GRX, SS7, and TDM networks. This means existing services, such as voice, signaling, and SMS transmissions, will be migrated to IPX networks.Compared with GRX networks, IPX networks provide end-to-end (E2E) QoS and security guarantees, making VoIP possible.
Table 3-1 GRX and IPX comparison
Applies only to MNOs
Applies to MNOs and FNOs as well as content and app providers
Most roaming data
Most international P2P data, such as MMS, voice packets, and instant messages
Comprehensive E2E SLA
GRX and IPX networks will continue to coexist for the time being. To fully replace GRX networks, IPX networks face the following challenges:
Due to the following factors, LTE network deployment will consolidate the position of IPX networks:
On legacy 2G and 3G mobile networks, roaming data refers to all data services transmitted from the visited network to the home network through the GRX network. Different from traditional mobile technologies, LTE roaming must support LocalBreakOut roaming, in which roaming subscribers are allowed to access the Internet by using a visited network.
The deployment of LTE networks will require more bandwidth and will have more requirements on GRX networks while offering more services.
LTE operators need to find GRX services that are able to offer E2E SLA for future service growth. Only GRX services provided by IPX networks can offer E2E SLA.
Information used for interaction between the visited and home networks, subscription and authentication data for visited network access, and policy and charging control information are transmitted when roaming. Information is exchanged by using the Authentication, Authorization, and Accounting (AAA) functionality.The AAA functionality is provided by using the Diameter protocol, an enhancement to the RADIUS protocol. Diameter is based on IP. As defined by 3GPP and GSMA, IPX should be the network that provides Diameter connections between MNOs.
To support roaming for 2G/3G subscribers, signaling network connections must be configured between the 2G/3G networks.
National and international SS7 signaling networks are independent, and the format of their signaling point codes (SPCs) also differs. Therefore, networks from different carriers must be connected using the signaling transfer point (STP), and an international gateway must be deployed to connect the national and international signaling networks.
Each international gateway must be allocated two SPCs: an international SPC allocated by the international signaling network, and a national SPC allocated by the national signaling network.
The international gateway forwards messages between the national and international signaling networks.
Diameter has been widely used in the S6a, S6d, S9, S13, Gx, Gxc, and Rx interfaces of the 3GPP evolved packet core (EPC) architecture.
Figure 4-2 Diameter signaling networking for roaming
An edge agent is a Diameter routing agent (DRA) deployed at the network edge and functions as the DEA. On an IPX network, the DEA must be deployed to support route addressing for Diameter signaling between the visited and home networks.The DEA is mainly used for route addressing and forwarding of Diameter signaling, including mobility management, charging policy, and charging information about internationally roaming subscribers, transmitted over the S6a, S6d, and S9 interfaces. When the DEA is deployed as a unified Diameter signaling gateway, all signaling messages are transmitted through the DEA. This allows an operator network to only send or receive signaling messages to or from DEAs deployed on other operator networks. It no longer needs to learn the internal structure of other networks. In addition, its network architecture is not provided to others.
Figure 5-1 Home-routed roaming
In Home-routed roaming, the access gateway GGSN/PGW resides in the home network and can directly communicate with the charging account in the home network of a roaming subscriber. This allows for services to be controlled in real time. However, if network elements (NEs) in the visited network need to implement real-time service control, the interface must be available for interworking between these NEs and the charging account.
In most cases, the billing center in the home network is not available to NEs in the visited network. Therefore, the NEs in the visited network can implement only offline charging.
In summary, service control is performed by the GGSN/PGW in the home network, and the charging control is performed by the billing center in the home network.
In LocalBreakOut roaming, all roaming service NEs reside in the visited network, and service control is performed by the GGSN/PGW in the visited network.
For charging, roaming information must be associated with charging accounts. However, the visited network does not have subscriber charging information and the home network does not have subscriber roaming information.
Therefore, the following three methods can be used to solve the preceding problem:
When the GGSN/PGW reports billing information to the billing center in the visited network, it also reports one copy to the home-billing center. The home-billing center serves as the charging control point.
This method requires operators to open up billing center interfaces to operators with whom roaming agreements have been signed.
The GGSN/PGW only reports billing information to the billing center in the visited network. The billing center in the visited network serves as the charging control point.
Service and billing information is stored only in the visited network, whereas the home network does not have any information. Therefore, roaming charges cannot be processed, and the home operator does not receive payment.
If an account is created in the home network, the billing center in the home network opens up interfaces to the intermediate operator billing center. This allows the GRX/IPX network to synchronize charging.If there are many operators who have signed roaming agreements, the home operator needs to open up billing center interfaces with all other operators. Method 3 saves time and efforts by establishing settlement relationships with all relevant operators.
Multiple MNOs in the visited country may have signed roaming agreements with the home operator. However, charging rates may differ. The preferred roaming service enables the home operator to select a mobile operator in the visited county to provide preferential rates for a subscriber who roams abroad.
Third-party IPX/GRX networks are untrusted for the DEA. Security threats from third-party IPX/GRX networks include IP-layer attacks, Diameter-layer network path sniffing or information leaks, malicious packet attacks, signaling fraud, and Denial of Service (DoS)/Distributed Denial of Service (DDoS) attacks. Protective measures against these security threats must be taken, including network isolation, firewall deployment, topology hiding, and signaling screening.
The Huawei service processing system (SPS) can be deployed as a DEA on the network edge to function as a Diameter agent or in the IPX network to support Diameter signaling route addressing between the visited and home network. When the SPS is deployed as a DEA, it provides the following functions:
All signaling messages are sent and received by the SPS. This provides a unified Diameter signaling interface for other operators. The local operator only needs to receive signaling messages from other operators through gateways, and messages sent by the local operator only need to be sent through signaling gateways of other operators.
Diameter signaling between different networks and devices of various vendors is time consuming. To solve this problem, the Huawei SPS provides flexible Diameter signaling mediation capabilities, thereby improving network efficiency and success rate.
If the SPS is deployed on the network edge, it functions as a unified interface on the network and accurate charges and provides usage statistics. If the SPS is deployed on an IPX network, it performs charging data record (CDR) settlement with the DEA deployed on the LTE network and provides the international roaming rental service.
The Huawei signaling roaming solution implements KPI statistics about the roaming signaling and provides information about the usage status and prediction of the roaming signaling.
The Huawei signaling roaming solution provides a deployment plan and suggestions for international roaming services using mainstream industrial viewpoints and current progress of an LTE deployment.
The value of this solution is demonstrated in the following aspects:
The Huawei roaming solution provides flexible routing and diverse mediation capabilities, and interoperability test (IOT) experience. The solution is flexible, fast, and it affordably interconnects different networks and devices from different vendors to accelerate the development of roaming services.
Various signaling firewall functions, such as message screening, IPsec, and DoS/DDoS attack defense, prevent operator networks from external attacks. In addition, topology hiding prevents networks from being exposed.
The Huawei roaming solution uses digital charging and refined management to guarantee efficiency. It combines roaming KPI and preferred roaming analysis, improves roaming quality, and increases revenue.By deploying roaming services, operators can cover a wider market, competition is enhanced, and subscriber satisfaction is improved. This competitive edge generates new subscribers and maintains subscriber loyalty.