The backbone is the part of the human body that provides strength and stability, holding the skeleton together. We often refer to people with reliable, resilient characters as “having backbone”. It’s the same in telecom networks, with the backbone providing shape, sturdiness and dependability to the overall system.
Without its backbone, connectivity would not be possible. And as Internet usage grows in volume and complexity, backbone networks must become more robust and capable of carrying the applications of tomorrow. However, there are significant challenges in scaling up backbone capabilities, primarily capacity, distance and energy usage.
The next phase of capacity expansion needed is 400G from metro to backbone networks. The term 400G refers to an optical transport network with capacity of 400 Gbps, a considerable jump in data-transfer speeds. This is driven by the demands of the digital and cloud era, in particular:
- Massive traffic volumes generated by 5G, 5G Advanced, fibre-based broadband and emerging service environments such as machine-type communications and ultralow-latency services. 5G needs edge computing architecture, which brings cloud resources — computing, storage and networking — closer to applications, devices and users. However, edge computing demands higher bandwidth.
- High-quality cloud interconnection and connectivity with the demands of service-level agreements, driven by standalone 5G and enterprise demand for services such as data centre interconnection. Research suggests that cloud-based data centres will take over 92% of data centre activity.
Fibre resources are relatively scarce in metropolitan area networks (MANs) and data centre interconnection environments, where short transmission distance and higher bandwidth are necessary. Typically, single-wavelength 400G is used. This combines the largest transmission bandwidth and the highest spectral efficiency with the simplest configuration, which effectively reduces transmission costs. Because of its smaller size and simple structure, single-carrier 400G can provide easy network management and low power usage.
However, in the backbone and some more complex MANs, requirements for transmission performance are more stringent because the transmission distance is longer with more network nodes. Here, 400G optical technology can extend the distance to several thousands of kilometres, helping operators deploy 400G backbone networks using as little bandwidth as possible.
400G optical capability has been evolving for the past six years for specific application environments, shown in the timeline below:
The first test of 400G for metropolitan use, typically short-distance and easier to achieve.
Metropolitan long-haul 400G is field-tested by China Mobile, and the first commercial 400G MAN is deployed by China Telecom
A more energy-efficient version of 400G MAN emerges, based on pluggable optical modules with 60% less power usage.
Creation of a prototype for 400G backbone and ultralong-haul 400G based on quadrature phase-shift keying (QPSK) — a digital modulation technique that enables the transmission of two bits per each zero or one, making it vital for bandwidth efficiency.
“Real” 400G, extending to all applications, is tested in lab and field networks.
400G becomes commercially available for use in all scenarios, from the MAN to the ultralong-haul backbone.
So, what are the criteria for building a 400G backbone? If we think of the backbone network as the highway of Internet connectivity, not only must it be ultrawide in terms of bandwidth, but also ultralong in terms of reach and more energy-efficient than previous iterations.
Optical fibre can be divided into several bands, with each being an independent channel to transmit a predetermined wavelength. According to ITU-T standards, single-mode fibre in the range over 1,260 nanometres is divided into six bands.
C-band, known as the conventional band, shows the lowest signal loss and offers advantages in long-distance transmission systems. As transmission distance increases, fibre optic amplifiers are used instead of optical-to-electronic-to-optical repeaters, so the C-band becomes more and more important. With the advent of dense wavelength division multiplexing, which allows multiple signals to share a single fibre, the use of C-band has been expanded. It’s typically deployed in urban areas, as well as long-distance and submarine optical transmission systems.
L-band, known as the long-wavelength band, has the second-lowest wavelength loss, so can be used when C-band doesn’t meet the bandwidth needs. To deliver ultrawide 400G capacity, the backbone of the future will require the combination of C-band and L-band. Network solution provider ZTE claims that it’s the first to complete a network test of this combination to accommodate 400G backbone capacity.
As a transmission technology, optical fibre has many benefits, but it’s prone to attenuation or loss as the light signal travels along the cabling. There are various factors that cause this, for example: intrinsic loss, owing to the characteristics of the fibre, such as material absorption; light dispersion loss or structural defects; and extrinsic loss, based on operating conditions like connector loss or cable bending, as light prefers to travel in a straight line.
With longer fibre, these losses can accumulate, so achieving the ultralong distance required by backbone networks — many thousands of kilometres — becomes highly challenging. This is especially true at higher capacity, as it’s also diminished by attenuation. And, as with capacity, new thinking is required so that backbone reach can continue expanding to meet growing Internet usage demands.
Various techniques can be deployed to yield moderate gains in achievable distance, but ZTE proposes that it’s only possible to increase the distance capability of 400G networks by up to 60% by combining three technology innovations:
- A new module in 400G QPSK capable of handling 1.2 terabits per second and is able to support up to 130G baud rates
- A 3D fusion package — a modelling tool that unifies design, engineering, electronics and manufacturing into a single software platform. This reduces internal connection distance and improves high-speed signal performance
- ZTE’s new Flex Shaping 2.0 algorithm.
This solution was tested by China Mobile in a backbone connection simulation, following a route from Zhejiang to Guizhou. According to the companies, a distance of 2,808 km was achieved, and in an extreme verification scenario the backbone achieved a reach of 5,616 km.
Backbone capacity and distance improvements must not be delivered at the cost of higher energy usage per bit. Here also, ZTE claims that some new capabilities are coming into play: new optical modules can reduce gigabit energy in the backbone by 35%, from 0.275 W to 0.175 W per gigabit — not a bad start.
Accommodating the rapidly rising traffic from Internet usage, and doing so with a phased-in approach to performance enhancement and cost, isn’t a trivial task for network operators. 400G backbone represents the next horizon to reach if they’re to stay ahead of demand. It won’t be the last development in the field, but it does mean that the backbone can continue to provide shape, sturdiness and dependability to the overall network for some time to come.
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