Fixed Mobile Convergent Access in the 5G Era

 

Background of FMC Development 

Fixed mobile convergence (FMC) allows fixed and mobile networks to interwork for convergent and full service operation. The FMC technology provides users with diverse quality services including communications, information and entertainment, independent of the terminal, network, application and location.
Mobile broadband and fixed broadband both compete and rely on each other. In areas such as internet access and data service, mobile broadband replaces some of the fixed network functions. For example, smartphones have gradually taken the place of fixed terminals including computers as the primary mode of accessing the internet. Mobile and fixed networks are also interdependent. A heterogeneous network based on cellular and cloud-RAN (C-RAN) technologies requires a large amount of mobile backhaul networking. In the access segment, in particular, mobile and fixed networks both need a great deal of optical infrastructure.
As competition intensifies in an increasingly mature telecom market, operators are compelled to find ways to prevent customer churn and attract new users. Since 2012, some operators in Europe have gained more mobile or fixed users by engaging in mergers and acquisitions (M&A) and tapping the potential of existing users. Notable cases include the acquisition of British mobile operator EE by incumbent fixed-network operator British Telecom (BT) and the success of Deutsche Telekom (DT) in signing up 70% of its mobile users to its fixed broadband services. FMC has become an effective means used by operators in their battle for users.
In the LTE era, an IP multimedia subsystem (IMS)-based FMC solution can provide fixed and mobile users with unified voice, data and video services to improve quality of experience (QoE). By adopting the new operating model of binding fixed and mobile users, operators can retain customers, boost average revenue per user (ARPU) and improve profitability.
In the 5G era, in-depth convergence of cloud network architecture, virtual network functions (VNFs), and service functions brings new opportunities and challenges for FMC.

Analysis of Application Scenarios and Technical Architecture

FMC services need to have the following characteristics: 
● Seamless connection is set up between the equipment and network layers, allowing diverse applications to be transported between different network platforms. Handover between different networks does not interrupt or degrade quality of service (QoS). 
● Various user access modes are supported. Converged services and devices can select different access modes such as Wi-Fi or cellular networks according to the user location, required application, QoS and voice traffic. 
● Users can use one terminal rather than previous multiple terminals to implement all applications.
● Personalized services are supported. End users can set services and interfaces as desired. Fixed networks also support diverse settings like smartphones.

User equipment (UE) in the 5G era falls into two types: 5G RG and NG UE (Fig. 1). 5G RG is an upgraded version of the traditional RG and is connected as a UE to the 5G core network through a fixed or mobile network. 5G RG can be connected to the core network via fixed wireless access (FWA) or hybrid links. NG UE is an upgraded version of the original UE and is connected to the 5G core network through WLAN or cellular networks. NG UE can be connected via Wi-Fi or cellular networks to offload some traffic, or via both WLAN and cellular networks. In both cases, RG serves as a trusted node to provide Wi-Fi access.
From the perspective of fixed access node or access network for user terminals, 5G FMC involves architecture convergence and function convergence in the access network. 

Resource Sharing and Architecture Convergence

In the 5G era, scenarios such as dense urban or hotspot areas need to adopt C-RAN architecture where DUs are centrally deployed. For fixed network access, the distance from the DU pool to the user is basically the same as that from the access office to the user. In areas where the access office has ample resources, fixed and mobile equipment can be co-located so that the resources can be shared to reduce Capex. The existing fixed network resources including the access office, power supplies, air-conditioners and transmission equipment can be shared, which facilitates the centralized equipment management (Fig. 2).

 

Network Simplification and Function Convergence

Network architecture, operation and maintenance can be simplified to reduce Opex. The overlapping part of authentication, control and management functions of the fixed and mobile networks are converged to provide users with consistent service experience and boost network operation and maintenance. After convergence, the fixed and mobile networks can provide:
● Standard interfaces: The 5G core network becomes more functionally independent. It is decoupled from access modes and gradually transforms into an agnostic architecture. By reducing AN-CN interface coupling, seamless and agnostic access can be achieved.
● Unified authentication: Previously fixed and mobile users were authenticated by different operating systems. The FMC architecture allows the users to be authenticated by one operator. This saves network resources and simplifies the management.
● Unified resource management: With virtualization and programmable technologies, some overlapping functions of the fixed and mobile networks can be abstracted and converged, and the operator’s resources can be orchestrated in a unified manner. 

There are three typical 5G FMC access modes currently being discussed by the standards bodies 3GPP and BBF, aiming to converge functions and simplify the network (Fig. 1).
● Integration mode: The access gateway function (AGF) is provided at the access node and can be directly connected to the 5G CN. Key modules include 5G RG and AGF.
● Interworking mode: In the existing access mode, the transmission equipment (such as BNG) that interconnects with CN supports fixed-mobile interworking function (FMIF) interface. Key modules include FN-RG and FMIF. The FN-RG is a traditional RG that can provide Wi-Fi access.
● Hybrid mode: Fixed access and cellular access co-exist in this mode.
Of the three modes, integration is the most recommended mode because its network has the simplest architecture. As L3 is moved down to the access node, the control and user plane interfaces to 5G CN are more direct, which reduces the amount of intermediary transmission equipment.
Re-architected access office (AO) makes it easier to deploy future SDN and NFV services such as 5G AGF. In terms of functionality and location, 5G AGF will best run on the NFVI of the access office. That enables the NFV platform in the re-architected AO to be fully used to implement in-depth architecture and function convergence required by FMC.

 

In the integration mode, all mobile users are connected via wired access, and are provided with converged 5G CN functions. The N1’/N2’/N3’/N4’ interfaces in Fig. 3 are interfaces between wired AN and core network. All mobile devices are connected to NG-RG (also called 5G RG), where user access to the core network is authenticated. The core network only needs to authenticate and charge NG-RG, without authenticating and charging individual mobile devices connected to NG-RG such as UEs, PADs, and PCs.

Industry Development and Network Evolution

FMC has attracted widespread attention in the industry. The operators that have engaged in discussions on 5G FMC standards at 3GPP and BBF are AT&T, BT, China Mobile, DT, KT, NTT, Orange, Telecom Italia, Telefonica, Telenor and Vodafone. They all have both fixed and mobile network resources. 
The research paper on 5G FMC standardization at BBF is SD-407: 5G Fixed Mobile Convergence Study, while the research at 3GPP is included in Release 15 SA2 TS23.706. The research and cooperation at 3GPP corresponding to the Release 16 timetable will soon kick off. 
The evolution target of FMC networks in the 5G era involves implementing agnostic access with a non-3GPP access network based on the 5G core network, making 5G networks more flexible with modular software and software-driven function development, supporting new 5G use cases in the industrial, public and user networks with the future-proof architecture and design, and converging more access network services with a converged cloud-based network platform.
The 5G FMC architecture is still being discussed. Many issues need to be resolved with input from operators and vendors, which will promote the continuous improvement of the 5G FMC solution.