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29 - Reducing the Cost per Bit with Coherent Technology

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July 17, 2024

Reducing the Cost per Bit with Coherent Technology

The cost per bit is a metric directly impacting network operators’ economic viability and competitive…

The cost per bit is a metric directly impacting network operators’ economic viability and competitive positioning. It represents the expense of transmitting a single bit of information across a network, encompassing infrastructure, operations, and maintenance costs. Lower costs per bit enable providers to offer more data-intensive services at competitive prices, attracting more customers and increasing revenue. Additionally, optimizing this metric helps achieve higher efficiency and sustainability in network operations.

Different segments of a telecommunication network—core, metro, and access—prioritize the cost per bit differently due to their distinct roles and technical requirements. The core network, which connects major cities and data centers, handles high data volumes over long distances. Economies of scale play a significant role here, as reducing the cost per bit is crucial for maintaining profitability on the vast amount of data transmitted. This segment typically invests in high-capacity, longer-haul technologies that, while expensive, reduce the cost per bit through enhanced efficiency and higher data throughput.

Conversely, edge networks face different challenges and priorities. They must prioritize flexibility and adaptability to handle varying traffic loads efficiently. Reducing the cost per bit involves deploying technologies that can scale quickly and cost-effectively. The access segment of edge networks, which brings connectivity directly to end-users, focuses on maximizing coverage and reliability. Here, the cost per bit needs to be managed against the need for extensive physical infrastructure that reaches individual customers.

Coherent technology is often seen as more expensive than direct detection (IM-DD), but in this article, we will explore some ways in which the initial investment in coherent technology can help networks reduce their cost per bit.

Coherent Increases Transmission Reach

The quality of the light signal degrades when traveling through an optical fiber by a process called dispersion. The same phenomenon happens when a prism splits white light into several colors. The fiber also adds other distortions due to nonlinear optical effects.

These effects get worse as the input power of the light signal increases, leading to a trade-off. You might want more power to transmit over longer distances, but the nonlinear distortions also become larger, which beats the point of using more power.

dispersion in optical fibers — Figure 1: Explaining dispersion compensation in optical fibers. A pulse of light will spread out and distort due to dispersion effects.

Coherent systems use sophisticated digital signal processing (DSP) technologies to automatically compensate for signal impairments, including chromatic and polarization mode dispersion. Coherent receivers are also highly sensitive, allowing them to detect signals over longer distances with higher fidelity than is possible with IM-DD systems. The dispersion compensation and increased sensitivity reduces the number of regenerative repeaters and other physical modules needed to boost the signal over longer distances.

Fewer repeaters mean lower energy consumption, reduced maintenance, and fewer operational disruptions, all contributing to a lower cost per bit. Additionally, the ability to transmit over extended distances without degradation in signal quality allows for more straightforward network architectures with longer point-to-point connections, simplifying the overall network design and further reducing costs

Coherent Increases Transmission Efficiency

Coherent systems use complex modulation formats that encode data on a light wave’s amplitude, phase, and polarization. By encoding multiple bits per symbol, these systems can transmit more data over a single wavelength than IM-DD, which primarily uses amplitude-only modulation, rather than just the intensity. This allows more bits to be transmitted per symbol, effectively increasing the data-carrying capacity of a single fiber.

Such efficiency can allow a single coherent channel to replace the work of several IM-DD channels. In our previous article, we provided an example of a single 100Gbps coherent channel replacing the link aggregation of four 10Gbps IM-DD channels found in some access and aggregation network architectures.

Figure 4x10G link aggregation — Figure 2: Comparison between 4x10G link aggregation and using a single coherent 100ZR link.

This substitution would replace eight SFP+ transceivers with just two coherent 100G transceivers, simplifying network configuration and operation. It more than doubles the capacity of 4x10Gbps link aggregation, allowing the network to handle more data traffic while reducing the required physical infrastructure, effectively reducing the cost per bit.

You can consult the recent Cignal AI report on 100ZR technologies to gain further insight into this link aggregation upgrade’s potential market and reach.

The Synergy with WDM Technology

Dense Wavelength Division Multiplexing (DWDM) is an optical technology that dramatically increases the amount of data transmitted over existing fiber networks. Data from various signals are separated, encoded on different wavelengths, and put together (multiplexed) in a single optical fiber.

The wavelengths are separated again and reconverted into the original digital signals at the receiving end. In other words, DWDM allows different data streams to be sent simultaneously over a single optical fiber without requiring the expensive installation of new fiber cables. In a way, it’s like adding more lanes to the information highway without building new roads!

Optical fiber with DWDM Channels — Figure 3: Example of a bidirectional DWDM system. At the central office, transceiver modules pick different wavelength channels for transmission, and these channels are combined into a single fiber through a device called a multiplexer. A demultiplexer splits the wavelength channels into separate fibers and the receiving modules on the remote end. Since this is a bidirectional system, transmission can go opposite too, from the remote end to the central office.

The tremendous expansion in data volume afforded with DWDM can be seen compared to other optical methods. A standard transceiver, often called a grey transceiver, is a single-channel device – each fiber has a single laser source. You can transmit 10 Gbps with grey optics. Coarse Wavelength Division Multiplexing (CWDM) has multiple channels, although far fewer than possible with DWDM. For example, with a 4-channel CWDM, you can transmit 40 Gbps. DWDM can accommodate up to 100 channels. You can transmit 1 Tbps or one trillion bps at that capacity – 100 times more data than grey optics and 25 times more than CWDM.

While the upgrade to DWDM requires some initial investment in new and more tunable transceivers, the use of this technology ultimately reduces the cost per bit transmitted to the network. Demand in access networks will continue to grow as we move toward IoT and 5G, and DWDM will be vital to scaling cost-effectively. Self-tuning modules have also helped further reduce the expenses associated with tunable transceivers.

Takeaways

Coherent systems minimize the need for frequent signal regeneration by compensating signal dispersion and enhancing signal reach. This simplifies network architecture and contributes to a lower cost per bit. Additionally, by employing complex modulation techniques, coherent technology maximizes the data capacity per wavelength, potentially replacing multiple IM-DD systems with a single coherent channel. This can further streamline network operations and reduce expenses.

The synergy of coherent technology with Dense Wavelength Division Multiplexing (DWDM) can multiply the data throughput of existing fiber infrastructures without requiring new infrastructure installations. Overall, while coherent technology involves a higher upfront investment compared to IM-DD systems, it can lower the cost per bit by enhancing the efficiency, reach, and capacity of data transmission.

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Tags: capacity, channels, Coherent technology, cost per bit, data, dispersion, DWDM, EFFECT Photonics, efficiency, fiber, IM-DD, infrastructure, modulation, multiplexing, network, operations, reach, signal, systems, technology, transmission

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November 15, 2023

The Highways of Light: How Optical Fiber Works

Optical fibers revolutionized how we transmit data, enabling faster long-distance connections. These slender strands of…

Optical fibers revolutionized how we transmit data, enabling faster long-distance connections. These slender strands of glass or plastic carry light pulses and serve as the backbone of modern telecommunication networks. Optical fibers have found applications beyond communications, including imaging, sensing, and medicine, further showcasing their versatility and impact in various fields.

Early optical fibers suffered significant losses during transmission, limiting their practicality for long-haul communication. In 1966, Charles Kao and George Hockham proposed that impurities in the glass were responsible for these losses, and they suggested that high-purity silica glass could achieve a much more reasonable attenuation of 20 dB per kilometer for telecommunications. This breakthrough, for which Kao received the Nobel Prize in Physics in 2009, kickstarted an era of explosive progress and growth for optical fiber.

In 1970, Corning scientists Robert Maurer, Donald Keck, and Peter Schultz successfully fabricated a glass fiber with an attenuation of 16 dB per kilometer, exceeding the performance benchmark set by Kao and Hockham. Two years later, Corning pushed the envelope further and achieved a loss of 4 dB/km, an order of magnitude improvement over their first effort. By 1979, Nippon Telegraph and Telephone (NTT) had reached a loss of 0.2 dB/km, meaning that only 5% of the light signal was lost over one kilometer. Optical fibers were ready for the world stage and deployed worldwide throughout the 1980s. The first transatlantic optical fiber link, spanning 6000 km, was established in 1988.

In this article, we will delve into the fascinating world of optical fibers, exploring how they work and what role optical transceivers play in fiber communications.

The Principle of Total Internal Reflection

Light bends when it passes from one material to another, such as from air to water. This bending occurs due to the change in the speed of light when it encounters a different material, causing the light rays to change direction. How much light changes direction depends on the angle at which it enters the new material and the factor by which the material slows down light. The latter factor is known as the refractive index of the material.

Figure 1 GREEN PENCIL IN GLASS OF WATER 1024 — Figure 1: Light refraction from a green-colored pencil inside a glass of water.

When light travels from a material of a higher refractive index to one of a lower index at a specific critical angle, the light will be entirely reflected into the high-index material. This phenomenon is called total internal reflection and is the fundamental principle behind the operation of optical fibers.

Optical Fibers and Total Internal Reflection

Optical fibers consist of a high-refractive-index core surrounded by a low-refractive-index cladding layer. Light entering the fiber core through one end at the correct critical angle will bounce back whenever it reaches the boundary between the core and the cladding. This behavior effectively traps light inside the core, allowing the light pulses to propagate through the fiber with minimal loss over long distances.

Figure 2: Diagram of light trapped inside an optical fiber by total internal reflection.

Optical fibers can achieve single-mode or multi-mode operation by carefully engineering the refractive indices of the core and cladding. Single-mode fibers have a small core diameter to transmit only a single optical mode or path. In contrast, multi-mode fibers have a larger core diameter, enabling the propagation of multiple modes simultaneously.

Figure 3: Comparing light propagation in multimode fiber and single-mode fiber. While the claddings are the same size, the single-mode fiber core is much smaller than the multimode one.

The quality of the light signal degrades when traveling through an optical fiber by a process called dispersion. The same phenomenon happens when a prism splits white light into several colors. The single-mode fiber design minimizes dispersion, making single-mode fibers ideal for long-distance communication. Since they are more susceptible to dispersion, multi-mode fibers are well-suited for short-distance applications such as local area networks (LANs).

The Role of Optical Transceivers in Fiber

Optical transceivers play a crucial role in fiber communications by converting electrical signals into optical signals for fiber transmission and vice versa when the optical signal is received. They act as the interface between fiber optical networks and electronic computing devices such as computers, routers, and switches.

As the name implies, an optical transceiver contains both a transmitter and a receiver within a single module. The transmitter has a laser or LED that generates light pulses representing the electronic data signal. On the receiving end, the receiver detects the optical signals and converts them back into electrical signals, which electronic devices can further process.

Figure 4: How transceivers convert electrical data signals into light pulses and vice-versa.

There are many different approaches to encode electrical data into light pulses. Two key approaches are intensity modulation/direct detection (IM-DD) and coherent transmission. IM-DD transmission only uses the amplitude of the light signal to encode data, while coherent transmission manipulates three different light properties to encode data: amplitude, phase, and polarization. These additional degrees of modulation allow for faster optical signals without compromising the transmission distance.

Direct Detection or Coherent? EFFECT Photonics explains

Takeaways

Optical fibers have transformed telecommunications in the last 50 years, enabling the rapid and efficient transmission of data over vast distances. By exploiting the principles of total internal reflection, these slender strands of glass or plastic carry pulses of light with minimal loss, ensuring high-speed communication worldwide. Optical transceivers act as the vital link between optical fiber and electronic networking devices, facilitating the conversion of electrical signals to optical signals and vice versa. Optical fibers and transceivers are at the forefront of our interconnected world, serving as the highways of light the digital age drives on.

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Tags: amplifiers, cladding, coherent, conversion, core, Corning, detector, Direct Detect, dispersion, EFFECT Photonics, electrical, fiber, laser, Networks, Nobel Prize, noise, NTT, optical, refraction, refractive index, Telecommunications, total internal reflections, total internal relection, Transceiver

Pluggables have becomes mainstream in datacom applications.

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February 8, 2023

A Story of Standards: From 400ZR and Open ROADM to OpenZR+

Coherent optical transmission has been crucial in addressing network operator problems for the last decade. In this time, coherent technology has expanded from a solution reserved only for premium long-distance links to one that impacts data center interconnects, metro, and access networks…

Coherent optical transmission has been crucial in addressing network operator problems for the last decade. In this time, coherent technology has expanded from a solution reserved only for premium long-distance links to one that impacts data center interconnects, metro, and access networks, as explained in the video below.

The development of the 400ZR standard by the Optical Internetworking Forum (OIF) proved to be a milestone in this regard. It was the result of several years of progress in electronic and photonic integration that enabled the miniaturization of 400G coherent systems into smaller pluggable form factors. With small enough modules to pack a router faceplate densely, the datacom sector could profit from an ideal solution for high-capacity data center interconnects. Telecom operators wanted to implement a similar solution in their metro links, so they combined the Open ROADM standardization initiative with the 400ZR initiative to develop the OpenZR+ agreement that better fits their use cases.

This article will elaborate on these standardization projects—400ZR, OpenROADM, and OpenZR+—and explain what use cases each was designed to tackle.

What Is 400G ZR?

To cope with growing bandwidth demands, providers wanted to implement the concept of IP over DWDM (IPoDWDM), in which tunable optics are integrated into the router. This integration eliminates the optical transponder shelf and the optics between the routers and DWDM systems, reducing the network capital expenditure (CAPEX). This is shown in the figure below.

Figure 1: Implementation of a traditional coherent transport architecture (left), with the router connected to a transponder line card and a Reconfigurable Optical Add/Drop Multiplexer (ROADM). In contrast, an IPoDWDM solution (right) has 400ZR pluggable transceivers that go directly into the router and use passive multiplexing filters. The IPoDWDM solution eliminates the line card and additional optics, reducing network CAPEX.

However, widely deploying IPoDWDM with coherent optics forced providers to face a router faceplate trade-off. Since DWDM modules have traditionally been much larger than the client optics, plugging DWDM modules into the router required sacrificing roughly half of the costly router faceplate capacity. This was unacceptable for datacom and telecom providers, who approached their suppliers and the Optical Internetworking Forum (OIF) about the need to develop a compact and interoperable coherent solution that addressed this trade-off.

These discussions and technology development led the OIF to publish the 400ZR implementation agreement in 2020. 400ZR specifies a (relatively) low-cost and interoperable 400 Gigabit coherent interface for a link with a single optical wavelength, using a double-polarization, 16-state quadrature amplitude modulation scheme (DP-16QAM). This modulation scheme uses sixteen different constellation points that arise from combining four different intensity levels and four phases. It doubles the usual 16-QAM transmission capacity by encoding information in two different polarizations of light.

The agreement specified a link reach of 80 kilometers without amplification and 120 kilometers with amplification. For forward error correction (FEC), the 400ZR standard supports a concatenated FEC (CFEC) method. CFEC combines inner and outer FEC codes to enhance the performance compared to a standard FEC code.

The 400ZR agreement does not specify a particular size or type of module, but its specifications targeted a footprint and power consumption that could fit in smaller modules such as the Quad Small Form-Factor Pluggable Double-Density (QSFP-DD) and Octal-Small Form-Factor Pluggable (OSFP). These form factors are small enough to provide the faceplate density that telecom and especially datacom operators need in their system architectures. So even if we often associate the 400ZR standard with QSFP-DD, other form factors, such as CFP2, can be used.

What Is Open ROADM?

In parallel with the 400ZR standardization efforts, telecom network operators had a different ongoing discussion.

Reconfigurable Optical Add-Drop Multiplexers (ROADMs) were a game-changer for optical communications when they entered the market in the 2000s. Before this technology, optical networks featured inefficient fixed routes and could not adapt to changes in traffic and demand. ROADMs allowed operators to remotely provision and manage their wavelength channels and bandwidth without redesigning the physical network infrastructure.

However, ROADMs were proprietary hardware with proprietary software. Changing the proprietary ROADM platform needed extensive testing and a lengthy integration process, so operators were usually reluctant to look for other platform alternatives. Besides, ROADMs still had several fixed, pre-defined elements that could have been configurable through open interfaces. This environment led to reduced competition and innovation in the ROADM space.

These trends drove the launch of the Open ROADM project in 2016 and the release of their first Multi-Source Agreement in 2017. The project aimed to disaggregate and open up these traditionally proprietary ROADM systems and make their provisioning and control more centralized through technologies such as software-defined networks (SDNs, explained in the diagram below).

Figure 2: In a traditional network paradigm (left), switches contain both the hardware that forwards traffic (the data plane), as well as the software that sets the rules of where to forward said traffic (the control plane). The SDN paradigm (right) separates the switching hardware from the software, effectively decoupling the data plane from the control plane. The language between this controller and the switches is an open protocol, with OpenFlow being the most commonly used one. The Open ROADM initiative proposed its own open protocol called NETCONF.

The Open ROADM project defined three disaggregated functions (pluggable optics, transponder, and ROADM), all controlled through an open standards-based API that could be accessed through an SDN controller. It defined 100G-400G interfaces for both Ethernet and Optical Transport Networking (OTN) protocols with a link reach of up to 500km. It also defined a stronger FEC algorithm called open FEC (oFEC) to support this reach. oFEC provides a greater enhancement than CFEC at the cost of more overhead and energy.

What Is OpenZR+?

The 400ZR agreement was primarily focused on addressing the needs of large-scale data center operators and their suppliers.

While it had some usefulness for telecom network operators, their transport network links usually span several hundreds of kilometers, so the interface and module power consumption defined in the 400ZR agreement could not handle such an extended reach. Besides, network operators needed extra flexibility when defining the transmission rate and the modulation type of their links.

Therefore, soon after the publication of the 400ZR agreement, the OpenZR+ Multi-Source Agreement (MSA) was published in September 2020. As the diagram below explains, this agreement can be seen as a combination of the 400ZR and Open ROADM standardization efforts.

Figure 3: Diagram of how different elements from the 400ZR and Open ROADM standardization projects were combined to form the OpenZR+ MSA. Diagram provided by the OpenZR+ website.

To better fit the telecom use cases of regional and long-haul transport links, OpenZR+ added a few changes to improve the link reach and flexibility over 400ZR:

Using the more powerful oFEC defined by the Open ROADM standard.
Multi-rate Ethernet that enables the multiplexing of 100G and 200G signals. This provides more options to optimize traffic in transport links.
Support for 100G, 200G, 300G, or 400G transport links using different modulation types (QPSK, 8QAM, or 16QAM). This enables further reach and capacity optimization for fiber links.
Higher dispersion compensation to make the fiber link more robust.

These changes allow QSFP-DD and OSFP modules to reach link lengths of up to 480 km (with optical amplifiers) at a 400G data rate. However, the FEC and dispersion compensation improvements that enable this extended reach come at the price of increased energy consumption. While the 400ZR standard targets a power consumption of 15 Watts, OpenZR+ standards aim for a power consumption of up to 20 Watts.

If operators need more performance, distances above 500 km, and support for OTN traffic (400ZR and OpenZR+ only support Ethernet), they must still use proprietary solutions, which are informally called 400ZR+. These 400ZR+ solutions feature larger module sizes (CFP2), higher performance proprietary FEC, and higher launch powers to achieve longer reach. These more powerful features come at the cost of even more power consumption, which can go up to 25 Watts.

Takeaways

The following table summarizes the use cases and characteristics of the approaches discussed in the article: 400ZR, Open ROADM, OpenZR+, and 400ZR+.

Technology	400ZR	Open ROADM	OpenZR+	400ZR+ Proprietary
Target Application	Edge Data Center Interconnects	Carrier ROADM Mesh Networks	Metro/Regional Carrier and Data Center Interconnects	Long-Haul Carrier
Target Reach @ 400G	120 km (amplified)	500 km (amplified)	480km (amplified)	1000 km (amplified)
Target Power Consumption	Up to 15 W	Up to 25 W	Up to 20 W	Up to 25W
Typical Module Option	QSFP-DD/OSFP	CFP2	QSFP-DD/OSFP	CFP2
Client Interface	400G Ethernet	100-400G Ethernet and OTN	100-400G Ethernet (Multi-rate)	100-400G Ethernet and OTN
Modulation Scheme	16QAM	QPSK, 8QAM, 16QAM	QPSK, 8QAM, 16QAM	QPSK, 8QAM, 16QAM
Forward Error Correction	CFEC	oFEC	oFEC	Proprietary
Standards / MSA	OIF	Open ROADM MSA	OpenZR+ MSA	Proprietary

400ZR is an agreement primarily focused on the needs of data center interconnects across distances of 80 – 120 kilometers. On the other hand, OpenROADM and OpenZR+ focused on addressing the needs of telecom carrier links, supporting link lengths of up to 500 km. These differences in reach are also reflected in the power consumption specs and the module form factors typically used. The 400ZR and OpenZR+ standards can only handle Ethernet traffic, while the Open ROADM and 400ZR+ solutions can handle both Ethernet and OTN traffic.

Tags: 400ZR, Data center, demand, disaggregation, extend, fiber, interoperable, Open ROADM, power, size, ZR+

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