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.

• Home-routed roaming enables subscribers to access the visited network through the home PDN gateway (H-PGW) and obtain services provided by their home networks.
• LocalBreakOut roaming enables subscribers to access the visited network through the visited PGW (V-PGW) and obtain services, which can be provided by the home or visited network. The visited policy and charging rules function (V-PCRF) must obtain PCC policies of the home network from the home PCRF (H-PCRF) over the S9 interface.

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:

• For intra-network roaming, subscribers who do not have special services should use LocalBreakOut, and subscribers who have special services (such as enterprise VPNs) use Home-routed.
• For inter-network roaming, Home-routed is recommended at the early and middle phases of LTE deployment. Once the Internet Packet Exchange (IPX) network has matured and local services (such as voice) have largely rolled out, operators should use LocalBreakOut roaming.

3.Roaming Networking Mode

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.

3.1 Direct Interconnection

This solution is simple and fast and facilitates interworking. It can be implemented in either of the following ways:

• Establish tunnels on the public IP network, for example, by using the Internet Protocol Security (IPsec). This method does not meet carrier-class mobile communication requirements and is not recommended.
• Use private lines such as FR and ATM, or establish VPNs on private networks. Although this network easily solves the QoS and security issues, it will greatly increase costs, especially for direct connections with many international point-to-point (P2P) private lines. In addition, private lines must be established when an operator is added. This is time consuming and costly. This connection is usually used between friendly operators or between multiple subnets in an operator.

3.2 GRX/IPX-based Interconnection

3.2.1 GRX Networking Mode

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:

• Low-cost network services: Redundant network connections and international transmission services are provided at low costs. GRX service providers also provide extendable service agent functions, such as the multimedia messaging service (MMS) server agent.
• High security: A GRX network is an isolated private network, which has strict security policies.
• Better scalability: Connections between operator networks and GRX networks will not be affected by changes to services and operators who have signed roaming agreements. Therefore, GRX connections can be used for a long time after being established.
• Easily provide roaming services: Server manufacturers provide comprehensive service support so that GPRS operators can focus on network construction.

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:

• Multiple networks need to be used to transmit different roaming data.
• Multiple roaming agreements must be signed among multiple networks.
• Networks cannot extend to support new IP services.
• Network complexity hinders the rollout of new services. 

3.2.2 IPX Networking Mode

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:

• Security: An IPX network is an isolated, transparent network. Its business agreement complies with security regulations and protects the interests of all partners. Data of a single operator is isolated, and the IPX network is completely invisible to end users.
• Flexible interconnections: A single protocol can be used for bilateral and multilateral connection services. In addition to simple and secure channels, an IPX network can provide complex and hub-based connection services.
• Cascading payment: Data volume management lets operators who have signed roaming agreements to settle their accounts, benefiting all links in the value chain.
• Efficient service guarantee: By complying with the service level agreement (SLA), IPX network services are guaranteed.

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:

• One IPX network can bear all service data.
• A single network protocol can connect all networks.
• Open all-IP networks can provide new IP services.
• Networks are simplified so that new services are rolled out efficiently.

3.2.3 GRX Evolution to IPX

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

Networking Mode	GRX	IPX
Business scope	Applies only to MNOs	Applies to MNOs and FNOs as well as content and app providers
Data borne	Most roaming data	Most international P2P data, such as MMS, voice packets, and instant messages
Service provided	Simple SLA	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:

• It is not imperative to change circuit switching to packet switching for legacy voice services, since circuit switching services, such as voice, SS7 signaling, and SMS, run stably on the existing platforms.
• MNOs do not have a deep understanding of IPX networks, although they offer transparent E2E costs and operate efficiently.

Due to the following factors, LTE network deployment will consolidate the position of IPX networks:

• Roaming data under explosive growth

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.

• LTE roaming security guarantee: Diameter

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.

4 Signaling Network Roaming Deployment for Roaming

4.1 SS7 Signaling Networking for Roaming

Figure 4-1 SS7 signaling networking for roaming

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.

4.2 Diameter Signaling Networking for Roaming

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.

5 Roaming Service

5.1 Charging

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:

• Method 1: Enable the interface between the GGSN/PGW in the visited network and the billing center in the home network.

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.

• Method 2: Create local access accounts for subscribers in the visited network. These accounts are used only for LocalBreakOut charging.

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.

• Method 3: Based on method 1 and method 2, enable an intermediate operator to coordinate and provide bill settlements. Specifically, when the visited GGSN/PGW reports service information to the visited billing center, it also sends one copy to the intermediate operator billing center, such as a GRX/IPX network. Then, the billing center in the GRX/IPX network charges and provides bill settlements between operators who have signed roaming agreements.

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.

5.2 Preferred Roaming

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.

5.3 Roaming Security

6 Huawei Signaling Roaming Solution and Value

6.1 Overview

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:

• Communication with the S6a, S6d, and S9 interfaces

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.

• Signaling mediation

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.

• Charging

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.

• KPI statistics

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.

• Diameter signaling firewall

Operators use the DEA for signaling, which enhances network security. Operators can use flexible security policies on the DEA to protect network security, such as controlling IP address access, restricting device access, and deploying firewalls.

6.2 Value

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:

• Improved efficiency

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.

• Secure roaming

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.

• Increased revenue

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.