A quarter century of increasing fixed-broadband speeds
By ITU News
If you were born before the millennium, you may just remember the sound of a dial-up modem establishing a connection to the Internet.
We have come a very long way since then. We now stream Ultra-HD ‘4K’ and ‘8K’ video, play online games massively multi-player, and look to virtual reality for ever more immersive experiences.
Digital subscriber line (DSL) for broadband access over traditional telephone wiring, which brought many of us our first taste of the Internet, has evolved to reach unprecedented speeds. Over time, fibre to the home (FTTH) will come to support all fixed broadband.
This year marks the 25th anniversary of two pioneering working groups at the International Telecommunication Union (ITU) that made these technologies possible.
The ITU standardization (ITU-T) working groups on broadband access over metallic conductors (Q4/15) and optical systems for fibre access networks (Q2/15), both established in 1997, laid the foundations for fixed broadband and have since then facilitated the meteoric rise in access speeds.
Both groups are part of ITU-T Study Group 15, which looks at networks, technologies and infrastructures for transport, access and home.
Q4/15 was formed to make DSL globally scalable.
“What followed was a 25-year journey of dedicated engineers fighting physics for ever-higher broadband speeds, through several generations of ITU-standardized DSL technology,” says Q4/15 Rapporteur Frank Van der Putten.
Building on prior work by Alliance for Telecommunications Industry Solutions (ATIS) committee T1E1.4 and European Telecommunications Standards Institute (ETSI) working group TM6, the DSL technologies standardized by ITU now connect over 600 million homes and businesses to the Internet.
“DSL changed the world by enabling mass-market broadband,” Van der Putten says.
DSL enabled rapid broadband deployment at low cost because it used the existing telephone wires to the home.
“Championed at first by an impassioned few, continually provoking debates among Q4/15 experts, it has been an intellectual catalyst for the advancement of communications technology,” he adds. “We are proud to have played a part in that.”
While ADSL (asymmetric DSL), as defined by ITU in 1999, could deliver 8 megabits per second (Mbit/s), it was followed by ADSL2plus in 2003 at 24 Mbit/s and the very high speed VDSL2 at 70 Mbit/s. With the introduction of vectoring, VDSL2 reached 100 Mbit/s by 2010 and 300 Mbit/s by 2014.
In 2014, G.fast raised the bar to 1 Gbit/s, doubling this to 2 Gbit/s in 2016. Its successor standard, MGfast, achieves an aggregate bit rate up to 8 gigabits per second (Gbit/s) in Full Duplex mode and 4 Gbit/s in Time Division Duplexing mode.
The architecture standards for DSL, G.fast and MGfast were defined by the Broadband Forum (once known as the ASDL forum), which also plays a key part in promoting interoperability.
Van der Putten explains: “Both technologies intend to meet service providers’ need for a complement to the fibre-to-the-home technologies in scenarios where G.fast or MGfast prove the more cost-efficient strategy.”
The continual upgrading of ITU’s standards has also sparked huge upward revisions in forecasts for the life left in traditional telephone wiring.
Future directions for Q4/15 work include G.fast-based backhaul, MGfast at aggregate data rates of 10 Gbit/s, and ultra-low latency transmission optimized for 5G wireless back/mid-haul, he says.
Cost-efficient fibre access
Q2/15 paved the way for passive optical network (PON) technologies as a highly cost-efficient means of enabling FTTH. Optical access networks now serve over a billion users worldwide, mostly based on PON .
Q2/15 works closely with Full Service Access Network (FSAN), which collects system requirements from operators to determine common requirements for ITU standards.
“The result has been systems ideally suited for a large group of networks and applications,” says Q2/15 Rapporteur Frank Effenberger.
“The first widely deployed system, G-PON [Gigabit PON], is found almost everywhere now,” he adds.
Q2/15 has developed seven generations of PON systems. The first, pi-PON, operated at 50 Mbit/s. This was followed by A-PON (155 Mbit/s), B-PON (622 Mbit/s), G-PON (2.5 Gbit/s), XG(S)-PON (10 Gbit/s), and NG-PON2 (4 x 10 Gbit/s).
To provide the basis for interoperability, ITU standards specify the control system for PON systems. Q2/15 has also developed a range of implementer’s guides and works closely with FSAN, ATIS, and the Broadband Forum to foster common designs and interoperability.
From 10 to 50 Gbit/s
Demand for higher capacity keeps growing fast. Optical access solutions also support 5G wireless communications and innovation for smart cities and factories.
“What we are seeing is a gradual evolution from G-PON to XG-PON [a 10G, or 10 Gbit/s, network] and XGS-PON [a 10G symmetric network], which is now being deployed at scale in many countries,” says Effenberger.
The latest generation of ITU-standardized PON, known as “Higher Speed PON”, provides for speeds of 50 Gbit/s per wavelength, up from the 10 Gbit/s of its predecessors. Market demand for Higher Speed PON is expected to begin in 2024.
“Given the large size and cost of the fixed access network, upgrades generally come once per decade,” says Effenberger.
Higher Speed PON includes both single-channel 50 Gbit/s systems to succeed XG(S)-PON and multi-channel 50 Gbit/s systems to succeed today’s NG-PON2 – a 40G PON that operates at 10Gbit/s per wavelength.
Although Higher Speed PON offers a five-fold capacity increase over its predecessors, it has been designed to work with the same fibre plant as G-PON, XG(S)-PON and NG-PON2.
“A successful technology requires a coincidence of both technical feasibility and strong global market demand,” notes Effenberger. “We strongly believe that 50G PON will provide the right capacity, at the right price, and at the right time.”
Q2/15 aims to continue delivering higher-capacity PON solutions, such as a multi-wavelength version of Higher Speed PON, and speeds even higher than 50 Gbit/s on a single wavelength. But passive networks cannot handle all foreseen demand.
“Certain applications will require more dedicated and higher-capacity solutions than PON,” says Effenberger, highlighting the motivations behind Q2/15’s development of various point-to-point bidirectional optics with speeds of 1 Gbit/s, 10 Gbit/s, 25 Gbit/s, and 50 Gbit/s.
Q2/15 continues to study 100 Gbit/s transmission and point-to-point wavelength connections over a shared optical distribution network based on wavelength division multiplexing. “These are likely to find use in wireless fronthaul applications, given their exacting latency requirements,” says Effenberger.
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