Global Standardization Activities
Next-generation Metallic Access Technologies and Their Standardization Activities
This article introduces certain next-generation metallic access technologies and the standardization activities relating to them under study in the International Telecommunication Union’s Telecommunication Standardization Sector (ITU-T) Study Group 15 with a strong focus on G.fast (“G” stands for ITU-T G series of recommendations and “fast” stands for fast access to subscriber terminals) and MGfast (multi-gigabit fast access to subscriber terminals) that can provide optical-fiber-grade ultrahigh-speed transmission services using pre-installed metallic cables in the existing infrastructure. MGfast, which is expected to be a next-generation metallic access technology, targets a transmission rate of 5 to 10 Gbit/s (uplink and downlink combined) over twisted-pair cables and coaxial cables.
Keywords: MGfast, G.fast, access
1. Development of metallic access networks
Question 4, which is responsible for the standardization of metallic access technologies, in the International Telecommunication Union’s Telecommunication Standardization Sector (ITU-T) Study Group 15 (SG15) started to develop digital subscriber line (DSL)-related standards in 1998. Asymmetric DSL (ADSL) and very high-speed DSL (VDSL) technologies and standards have been developed by 2010 and provide capabilities supporting Internet access for home users. These technologies have supported the demand for high-speed Internet not only in Japan but also worldwide. Since then, network configurations that bring optical network termination closer to customer premises equipment (CPE), such as fiber to the cabinet (FTTC), fiber to the distribution point (FTTdp), fiber to the building (FTTB), and fiber to the home (FTTH), have been adopted due to the lower cost of optical fibers. One of these configurations connects optical fibers from the central office to the distribution point (DP) then to the CPE using G.fast (“G” stands for ITU-T G series of recommendations and “fast” stands for fast access to subscriber terminals). DPs are located in manholes, utility poles, and basements of apartment buildings, depending on the service provider. G.fast, included in Fig. 1, provides a transmission rate of 2 Gbit/s (uplink and downlink combined) over a 50-m twisted-pair cable. MGfast (multi-gigabit fast access to subscriber terminals), the next-generation metallic access technology described in detail in this article, targets a transmission rate of 5 Gbit/s (uplink and downlink combined) over a 30-m twisted-pair cable.
1.1 Ultrahigh-speed access technology G.fast
G.fast was originally standardized in 2014 (G.9700/G.9701) then revised in 2019 to achieve Gbit/s-level transmission rates with substantial expansion of frequency bands (adoption of 106- and 212-MHz profiles) and has the additional functionality of changing the transmission rates of uplink and downlink directions using the time division duplex (TDD) method. Unlike conventional ADSL and VDSL, which use the frequency division duplex (FDD) method, the TDD method allows easy control of the transmission rates in uplink and downlink directions, as shown in the frame structure in Fig. 2. There are also the following features of specific profiling, which are also listed in Table 1:
It should be mentioned that G.fast (106-MHz profile) is provided in Japan for commercial services for multi-dwelling units.
1.2 Multi-gigabit ultrahigh-speed access technology MGfast
While G.fast was being deployed commercially, it was agreed at the ITU-T SG15 meeting in June 2017 to start developing standards for next-generation ultrafast access technology to achieve further enhancements in functionality and performance. Requirements proposed by service providers in Europe and the U.S. brought about the study of MGfast targeting transmission rates of 5 to 10 Gbit/s. As was the case with G.fast, reliable ultrahigh-speed transmission services of optical-fiber grade are required for next-generation access networks. In transmission systems using metallic cables, signals from neighboring lines result in interference, and how to eliminate such noise (far-end crosstalk noise and near-end crosstalk noise) is a major issue. In DSL technology, which uses a multi-carrier modulation scheme to achieve higher speeds, the subcarrier spacing and number of subcarriers are important system parameters as well as the frequency band used to complete subcarrier-by-subcarrier processing. Although it is effective to increase the number of subcarriers to achieve higher speed, the computational complexity increases accordingly; thus, the subcarrier spacing is generally determined to limit the number of subcarriers. As shown in Table 2, the subcarrier spacing is 51.375 kHz and the number of subcarriers is 2048 for G.fast (106-MHz profile). MGfast (424-MHz profile), which is described later, has the same subcarrier spacing, but the number of subcarriers is significantly increased to 8192. In MGfast (848-MHz profile), which is expected to be standardized, the subcarrier spacing is anticipated to be double.
A full duplex (FDX) method is also effective to increase the transmission rate. However, to avoid interference from near-end crosstalk noise and echo signals, VDSL uses the FDD method, while G.fast uses the TDD method. In this context, MGfast, a newly developed next-generation technology, is aimed at doubling the transmission rate by enabling full-duplexing in the single-pair while eliminating the effects of near-end crosstalk noise and echo signals.
2. Next-generation metallic access technology: MGfast standardization activities
This section describes the technical specifications and standardization status of MGfast, which has been studied in ITU-T SG15. MGfast is a technology to achieve even higher transmission rates while maintaining compatibility with G.fast for easy migration from G.fast systems. MGfast is mainly applicable to apartment buildings.
2.1 Objective of MGfast standardization
Standardization have been undertaken since 2017 with the following objectives:
2.2 Features of MGfast
The following features have been specified to enable easy migration from G.fast systems:
The following new features are specified for the next-generation MGfast technology:
The MGfast related standards consist of the following:
3. Considerations for the future
System development based on the MGfast specifications approved at the ITU-T SG15 meeting held in April 2021 is expected to be accelerated. For service providers who do not have sufficient optical fiber assets, one of the options would be to provide high-performance services of optical-fiber grade using existing metallic cables. The standardization of the next-generation metallic access technologies described in this article is expected to continue to be studied to satisfy the demand for higher speed services by effectively using existing facilities. The following topics have been discussed in the standardization meetings as future themes for metallic access technology.
3.1 Specifications regarding MGfast (848-MHz profile)
The MGfast system for coaxial cables uses a frequency of up to 848 MHz. The doubling of the frequency band, combined with FDX technology, will make it possible to achieve transmission rates of up to 10 Gbit/s in one direction. It will be necessary to study the details of system parameters such as subcarrier spacing and the number of subcarriers, as explained earlier.
3.2 Bundling of multiple G.fast lines (G.fastback)
A project called G.fastback is under consideration as a technology to provide a transmission path between a cabinet and DPU as a single high-capacity line by combining multiple G.fast lines. It should be possible to use multiple metallic cables as an alternative to optical fiber cables with G.fastback technology. As application examples, this configuration would be connected in tandem in a number of stages to increase the distance, while in another example of G.fastback, multiple G.fast lines are bundled where G.fast signals are frequency multiplexed over the existing service lines such as ADSL and VDSL shown in Fig. 3. In both cases, the objective is to provide reliable and higher data-rate operations while effectively using metallic cables. Regarding G.fastback, there are several issues to be solved, which include how to eliminate near-end crosstalk noise in DPUs and signal synchronization between lines when multiple lines are connected.
In this article, the next-generation metallic access technologies and standardization activities being developed in ITU-T SG15 were described, which can provide ultrahigh-speed transmission operations of optical-fiber grade by using existing metallic cables specified in G.9700 series standards, as an extension of optical fiber in FTTdp and FTTB configurations. It will be necessary to contribute further to the standardization activities mentioned in this article and to cooperate with service providers in Europe and the U.S. to use existing network facilities effectively and economically.