Glenn Wellbrock’s engineering roots
After four decades shaping optical networking, Glenn Wellbrock has retired. He shares his career highlights, industry insights, and his plans to embrace a quieter life of farming and hands-on projects in rural Kansas.
Glenn Wellbrock’s (pictured) fascination with telecommunications began at an early age. “I didn’t understand how it worked, and I wanted to know,” he recalls.
Wellbrock’s uncle had a small, rural telephone company where he worked while studying, setting the stage for his first full-time job at telecom operator, MCI. Wellbrock entered a world of microwave and satellite systems; MCI was originally named Microwave Communications Incorporated. “They were all ex-military guys, and I’m the rookie coming out of school trying to do my best and learn,” says Wellbrock.
The challenge that dominated the first decade of this century.
The arrival of fibre optics in the late 1980s marked a pivotal shift. As colleagues hesitated to embrace the new “glass” technology, Wellbrock seized the opportunity. “I became the fibre guy,” he says. “My boss said, ‘Anything breaks over there, it’s your problem. You go fix it.’”
This hands-on role propelled him into the early days of optical networking, where he worked on asynchronous systems with bit rates ranging from hundreds of kilobits to over a megabit, before SONET/SDH standards took over.
By the 1990s, with a young family, Wellbrock moved to Texas, contributing to MCI’s development of OC-48 (2.5 gigabit-per-second or Gbps) systems, a precursor to the high-capacity networks that would define his career.
Hitting a speed wall
One of Wellbrock’s proudest achievements was overcoming the barrier to get to speeds faster than 10Gbps, a challenge that dominated the first decade of this century.
Polarisation mode dispersion (PMD) in an optical fibre was a significant hurdle, limiting the distance and reliability of high-speed links. By then, he was working at a start-up and did not doubt that using phase modulation was the answer.
Wellbrock recalls conversations he had with venture capitalists at the time: “I said: ‘Okay, I get we are a company of 40 guys and I don’t even know if they can build it, but somebody’s going to do it, and they’re going to own this place.’”
Wellbrock admits he didn’t know the answer would be coherent optics, but he knew intensity modulation direct detection had reached its limits.
For a short period, Wellbrock was part of Marconi before joining Verizon in 2006. In 2007, he was involved in a Verizon field trial between Miami and Tampa, 300 miles apart, which demonstrated a 100Gbps direct-detection system. “It was so manual,” he admits. “It took three of us working through the night to keep it working so we could show it to the executives in the morning.”
While the trial passed video, it was clear that direct detection wouldn’t scale. The solution lay in coherent detection, which Wellbrock’s team, working with Nortel (acquired by Ciena), finally brought to market by 2009.
“Coherent was like seeing a door,” he says. “PMD was killing you, but you open the door, and it’s a vast room. We had breathing room for almost two decades.”
Verizon’s lab in Texas had multiple strands of production fibre that looped back to the lab every 80km. “We could use real-world glass with all the impairments, but keep equipment in one location,” says Wellbrock.
This setup enabled rigorous testing and led to numerous post-deadline papers at OFC, cementing Verizon’s reputation for optical networking innovation.
Rise of the hyperscalers
Wellbrock’s career spanned a transformative era in telecom, from telco-driven innovation to the rise of hyperscalers like Google and Microsoft.
He acknowledges the hyperscalers’ influence as inevitable due to their scale. “If you buy a million devices, you’re going to get attention,” he says. “We’re buying 100 of the same thing.”
Hyperscalers’ massive orders for pluggable modules and tunable lasers—technologies telcos like Verizon helped pioneer—have driven costs down, benefiting the industry.
However, Wellbrock notes that telcos remain vital for universal connectivity. “Every person, every device is connected,” he says. “Telcos aren’t going anywhere.”
Reliability remains a core challenge, particularly as networks grow. Wellbrock emphasises dual homing—redundant network paths—as telecom’s time-tested solution. “You can’t have zero failures,” he says. “Everything’s got a failure rate associated with it.”
He sees hyperscalers grappling with similar issues, as evidenced by a Google keynote at the Executive Forum at OFC 2025, which sought solutions for network failures linking thousands of AI accelerators in a data centre.
Wellbrock’s approach to such challenges is rooted in collaboration. “You’ve got to work with the ecosystem,” he insists. “Nobody solves every problem alone.”
Hollow-core fibre
Looking forward, what excites Wellbrock is hollow-core fibre, which he believes could be as transformative as SONET, optical amplifiers, and coherent detection.
Unlike traditional fibre, hollow-core fibre uses air-filled waveguides, offering near-zero loss, low latency, and vast bandwidth potential. “If we could get hollow-core fibre with near-zero loss and as much bandwidth as you needed, it would give us another ride at 20 years’ worth of growth,” he says. “It’s like opening another door.”
While companies like Microsoft are experimenting with hollow-core fibre, Wellbrock cautions that widespread adoption is years away. “They’re probably putting in [a high fibre glass] 864 [strand]-count standard glass and a few hollow core [strands],” he notes.
For long-haul routes, the technology promises lower latency and freedom from nonlinear effects, but challenges remain in developing compatible transmitters, receivers, and amplifiers. “All we’ve got to do is build those,” he says, laughing, acknowledging the complexity.
Wellbrock also highlights fibre sensing as a practical innovation, enabling real-time detection of cable damage. “If we can detect an excavator getting closer, we can stop it before it breaks a fibre link,” he explains. This technology, developed in collaboration with partners like NEC and Ciena, integrates optical time-domain reflectometry (OTDR) into transmission systems, thereby enhancing network reliability.
Learnings
Wellbrock’s approach to innovation centres on clearly defining problems to engage the broader ecosystem. “Defining the problem is two-thirds of solving it,” he says, crediting a Verizon colleague, Tiejun J. Xia, for the insight. “If you articulate it well, lots of smart people can help you fix it.”
This philosophy drove his success at OFC, where he used the conference to share challenges, such as fibre sensing, and rally vendor support. “You’ve got to explain the value of solving it,” he adds. “Then you’ll get 10 companies and 1,000 engineers working on it.”
He advises against preconceived solutions or excluding potential partners. “Never say never,” he says. “Be open to ideas and work with anybody willing to address the problem.”
This collaborative mindset, paired with a willingness to explore multiple solutions, defined his work with Xia, a PhD associate fellow at Verizon. “Our favourite Friday afternoon was picking the next thing to explore,” he recalls. “We’d write down 10 possible things and pull on the string that had legs.”
Fibre to Farming
As Wellbrock steps into retirement, he is teaming up with his brother.
The two own 400 acres in Kansas, where wheat farming, hunting, and fishing will define their days. “I won’t miss 100 emails a day or meetings all day long,” he admits. “But I’ll miss the interaction and building stuff.”
Farming offers a chance to work with one’s hands, doing welding and creating things from metal. “I love to build things,” he says. “It’s fun to go, ‘Why hasn’t somebody built this before?’
Farming projects can be completed in a day or over a weekend. “Networks take a long time to build,” he notes. “I’m looking forward to starting a project and finishing it quickly.”
He plans to cultivate half their land to fund their hobbies, using “old equipment” that requires hands-on maintenance—a nod to his engineering roots.
OFC farewell
Wellbrock retired just before the OFC show in March 2025. His attendance was less about work and more about transition, where he spent the conference introducing his successor to vendors and industry peers, ensuring a smooth handoff.
“I didn’t work as hard as I normally do at OFC,” he says. “It’s about meeting with vendors, doing a proper handoff, and saying goodbye to folks, especially international ones.” He also took part in this year’s OFC Rump Session.
Wellbrock admits to some sadness. Yet, he remains optimistic about his future, with plans to possibly return to OFC as a visitor. “Maybe I’ll come just to visit with people,” he muses.
Timeline
- 1984: MCI
- 1987: Started working on fibre
- 2000: Joined start-ups and, for a short period, was part of Marconi
- 2004: Joined Worldcom, which had bought MCI
- 2006: Joined Verizon
- 2025: Retired from Verizon
A tribute
Prof. Andrew Lord, Senior Manager, optical and quantum research, BT
I have had the privilege of knowing Glenn since the 1990s, when BT had a temporary alliance with MCI. We shared a vendor trip to Japan, where I first learnt of his appetite for breakfasting at McDonald’s!
Glenn has been a pivotal figure in our industry since then. A highlight would be the series of ambitious Requests For Information (RFIs) issued by Verizon, which would send vendor account managers scurrying to their R&D departments for cover.
Another highlight would be the annual world-breaking Post-Deadline Paper results at OFC: those thrilling sessions won’t be the same without a Wellbrock paper and neither will the OFC rump sessions, which have benefited from his often brutal pragmatism, always delivered with grace (which somehow made it even worse when defeating me in an argument!).
But it’s grace that defines the man who always has time for people and is always generous enough to share his views and experiences. Glenn will be sorely missed, but he deserves a fulfilling and happy retirement.
OIF adds a short-reach design to its 1600ZR/ ZR+ portfolio
The OIF (Optical Internetworking Forum) has broadened its 1600-gigabit coherent optics specification work to include a third project, complementing the 1600ZR and 1600ZR+ initiatives.
The latest project will add a short-reach ‘coherent-lite’ digital design to deliver a reach of 2km to 20km and possibly 40km with a low latency below 300ns
The low latency will suit workloads and computing resources distributed across data centres.
“The coherent-lite is more than just the LR (long reach) work that we have done [at 400 gigabits and 800 gigabits],” says Karl Gass, optical vice chair of the OIF’s physical link layer (PLL) working group, adding that the 1600-gigabit coherent-lite will be a distinct digital design.
Doubling the data rate from 800 gigabits to 1600 gigabits is the latest battle line between direct-detect and coherent pluggable optics for reaches of 2km to 40km.
At 800 gigabits, the OIF members debated whether the same coherent digital signal processor would implement 800ZR and 800-gigabit LR. Certain OIF members argued that unless a distinct, coherent DSP is developed, a coherent optics design will never be able to compete with direct-detect LR optics.
“We have that same acknowledgement that unless it’s a specific design for [1600 gigabit] coherent-lite, then it’s not going to compete with the direct detect,” says Gass.
OIF’s 1600-gigabit specification work
The OIF’s 1600-gigabit roadmap has evolved rapidly in the last year.
In September 2023, the OIF announced the 1600ZR project to develop 1.6-terabit coherent optics with a reach of 80km to 120km. In January 2024, the OIF announced it would undertake a 1600ZR+ specification, an enhanced version of 1600ZR with a reach of 1,000km.
The OIF’s taking the lead in ZR+ specification work is a significant shift in the industry, promising industry-wide interoperability compared to the previous 400ZR+ and 800ZR+ developments.
Now, the OIF has started a third 1600-gigabit coherent-lite design.
1600ZR development status
Work remains to complete the 1600ZR Implementation Agreement, the OIF’s specification document. However, member companies have agreed upon the main elements, such as the framing schemes for the client side and the digital signal processing and using oFEC as the forward error correction scheme.
oFEC is a robust forward error correction scheme but adds to the link’s latency. It has also been chosen as the forward error correction scheme for 1600ZR. The OIF members want the ‘coherent-lite’ version to use a less powerful forward error correction to achieve lower latency.
The 1600ZR symbol rate chosen is around 235 gigabaud (GBd), while the modulation scheme is 16-ary quadrature amplitude modulation (16-QAM). The specified reach will be 80km to 120km. (See table below.)
The members will likely agree on the digital issues this quarter before starting the optical specification work. Before completing the Implementation Agreement, members must also spell out interoperability testing.
1600ZR+ development status
The 1600ZR+ work still has some open questions.
One is whether members choose a single carrier, two sub-carriers, or four to achieve the 1,000km reach. The issue is equalisation-enhanced phase noise (EEPN), which imposes tighter constraints on the received laser. Using sub-carriers, the laser constraints can be relaxed, enabling more suppliers. The single-carrier camp argues that sub-carriers complicate the design of the coherent digital signal processor (DSP).
The workgroup members have also to choose the probabilistic constellation shaping to use. Probabilistic constellation shaping gain can extend the reach, but it can also reduce the symbol rate and, hence, the bandwidth specification of the coherent modem’s components.
The symbol rate of the 1600ZR+ is targeted in the range of 247GBd to 263GBd.
Power consumption
The 1600ZR design’s power consumption was hoped to be 26W, but it is now expected to be 30W or more. The 1600ZR+ is expected to be even higher.
The coherent pluggable’s power consumption will depend on the CMOS process that the coherent DSP developers choose for their 1600ZR and 1600ZR+ ASIC designs. Will they choose the state-of-the-art 3nm CMOS process or wait for 2nm or even 1.8nm to become available to gain a design advantage?
Timescales
The target remains to complete the 1600ZR Implementation Agreement document quickly. Gass says the 1600ZR and 1600ZR+ Implementation Agreements could be completed this year, paving the way for the first 1600ZR/ZR+ products in 2026.
“We are being pushed by customers, which isn’t a bad thing,” says Gass.
The coherent-lite design will be completed later given that it has only just started. At present, the OIF will specify the digital design and not the associated optics, but this may change, says Gass.
How scaling optical networks is soon to change
Carrier division multiplexing and spatial division multiplexing (CSDM) are both needed, argues Lumentum’s Brian Smith.
The era of coherent-based optical transmission as is implemented today is coming to an end, argues Lumentum in a White Paper.
Brian Smith
The author of the paper, Brian Smith, product and technology strategy, CTO Office at Lumentum, says two factors account for the looming change.
One is Shannon’s limit that defines how much information can be sent across a communications channel, in this case an optical fibre.
The second, less discussed regarding coherent-based optical transport, is how Moore’s law is slowing down.
”Both are happening coincidentally,” says Smith. “We believe what that means is that we, as an industry, are going to have to change how we scale capacity.”
Accommodating traffic growth
A common view in telecoms, based on years of reporting, is that internet traffic is growing 30 per cent annually. The CEO of AT&T mentioned over 30 per cent traffic growth in its network for the last three years during the company’s last quarterly report of 2023.
Smith says that data on the rate of traffic growth is limited. He points to a 2023 study by market research firm TeleGeography that shows traffic growth is dependent on region, ranging from 25 to 45 per cent CAGR.
Since the deployment of the first optical networking systems using coherent transmission in 2010, almost all networking capacity growth has been achieved in the C-band of a fibre, which comprises approximately 5 terahertz (THz) of spectrum.
Cramming more data into the C-band has come about by increasing the symbol rate used to transmit data and the modulation scheme used by the coherent transceivers, says Smith.
The current coherent era – labelled the 5th on the chart – is coming to an end. Source: Lumentum.
Pushing up baud rate
Because of the Shannon limit being approached, marginal gains exist to squeeze more data within the C-band. It means that more spectrum is required. In turn, the channel bandwidth occupied by an optical wavelength now goes up with baud rate such that while each wavelength carries more data, the capacity limit within the C-band has largely been reached.
Current systems use a symbol rate of 130-150 gigabaud (GBd). Later this year Ciena will introduce its 200GBd WaveLogic 6e coherent modem, while the industry has started work on developing the next generation 240-280GBd systems.
Reconfigurable optical add-drop multiplexers (ROADMs) have had to become ‘flexible’ in the last decade to accommodate changing channel widths. For example, a 400-gigabit wavelength fits in a 75GHz channel while an 800-gigabit wavelength fits within a 150GHz channel.
Another consequence of Shannon’s limit is that the transmission distance limit for a certain modulation scheme has been reached. Using 16-ary quadrature amplitude modulation (16-QAM), the distance ranges from 800-1200km. Doubling the baud rate doubles the data rate per wavelength but the link span remains fixed.
“There is a fundamentally limit to the maximum reach that you can achieve with that modulation scheme because of the Shannon limit,” says Smith.
At the recent OFC show held in March in San Diego, a workshop discussed whether a capacity crunch was looming.
The session’s consensus was that, despite the challenges associated with the latest OIF 1600ZR and ZR+ standards, which promise to send 1.6 terabits of data on a single wavelength, the industry is confident that it will meet the OIF’s 240-280+ GBd symbol rates.
However, in the discussion about the next generation of baud rate—400-500GBd—the view is that while such rates look feasible, it is unclear how they will be achieved. The aim is always to double baud rate because the increase must be meaningful.
“If the industry can continue to push the baud rate, and get the cost-per-bit, power-per-bit, and performance required, that would be ideal,” says Smith.
But this is where the challenges of Moore’s law slowing down comes in. Achieving 240GBd and more will require a coherent digital signal processor (DSP) made using a 3nm CMOS process at least. Beyond this, transistors start to approach atomic scale and the performance becomes less deterministic. Moreover, the development costs of advanced CMOS processes – 3nm, 2nm and beyond – are growing exponentially.
Beyond 240GBd, it’s also going to become more challenging to achieve the higher analogue bandwidths for the electronics and optics components needed in a coherent modem, says Smith. How the components will be packaged is key. There is no point in optimising the analogue bandwidth of each component only for the modem performance to degrade due to packaging. “These are massive challenges,” says Smith.
This explains why the industry is starting to think about alternatives to increasing baud rate, such as moving to parallel carriers. Here a coherent modem would achieve a higher data rate by implementing multiple wavelengths per channel.
Lumentum refers to this approach as carrier division multiplexing.
Capacity scaling
The coherent modem, while key to optical transport systems, is only part of the scaling capacity story.
Prior to coherent optics, capacity growth was achieved by adding more and more wavelengths in the C-band. But with the advent of coherent DSPs compensating for chromatic and polarisation mode dispersion, suddenly baud rate could be increased.
“We’re starting to see the need, again, for growing spectrum,” says Smith. “But now, we’re growing spectrum outside the C-band.”
First signs of this are how optical transport systems are adding the L-band alongside the C-band, doubling a fibre’s spectrum from five to 10THz.
“The question we ask ourselves is: what happens once the C and L bands are exhausted?” says Smith.
Lumentum’s belief is that spatial division multiplexing will be needed to scale capacity further, starting with multiple fibre pairs. The challenge will be how to build systems so that costs don’t scale linearly with each added fibre pair.
There are already twin wavelength selective switches used for ROADMs for the C-band and L-bands. Lumentum is taking a first step in functional integration by combining the C- and L-bands in a single wavelength selective switch module, says Smith. “And we need to keep doing functional integration when we move to this new generation where spatial division multiplexing is going to be the approach.”
Another consideration is that, with higher baud-rate wavelengths, there will be far fewer channels per fibre. And with growing fibre pairs per route, that suggests a future need for fibre-switched networking not just wavelength switching networking as used today.
“Looking into the future, you may find that your individual routeable capacity is closer to a full C-band,” says Smith.
Will carrier division multiplexing happen before spatial division multiplexing?
Smith says that spatial division multiplexing will likely be first because Shannon’s limit is fundamental, and the industry is motivated to keep pushing Moore’s law and baud rate.
“With Shannon’s limit and with the expansion from C-band to C+L Band, if you’re growing at that nominal 30 per cent a year, a single fibre’s capacity will exhaust in two to three years’ time,” says Smith. “This is likely the first exhaust point.”
Meanwhile, even with carrier division multiplexing and the first parallel coherent modems after 240GBd, advancing baud rate will not stop. The jumps may diminish from the doublings the industry knows and that will continue for several years yet. But they will still be worth having.
Nokia jumps a class with its PSE-6s coherent modem
- The 130 gigabaud (GBd) PSE-6s coherent modem is Nokia’s first in-house design for high-end optical transport systems
- The PSE-6s can send an 800 gigabit Ethernet (800GbE) payload over 2,000km and 1.2 terabits of data over 100km.
- Two PSE-6s DSPs can send three 800GbE signals over two 1.2-terabit wavelengths
Nokia has unveiled its latest coherent modem, the super coherent Photonic Service Engine 6s (PSE-6s) that will power its optical transport platforms in the coming years.
The PSE-6s comes three years after Nokia announced its current generation of coherent digital signal processors (DSPs): the PSE-Vs DSP for the long-haul and the compact PSE-Vc for the coherent pluggable market.
Nokia is only detailing the PSE-6s; its next-generation coherent modem for pluggables will be a future announcement.
Nokia will demonstrate the PSE-6s at the upcoming OFC show in March while field trials involving systems using the PSE-6s will start in the year’s second half.
Reducing cost per bit
In 2020, Nokia bought Elenion, a silicon photonics company specialising in coherent optics.
The PSE-6s is Nokia’s first in-house coherent modem – the coherent DSP and associated optics – targeting the most demanding optical transport applications.
Nokia points out that coherent systems started approaching the Shannon limit two generations ago.
In the past, operators could reduce the cost of optical transport by sending more data down a fibre; upgrading the optical signal from 100 to 200 to 400 gigabit required only a 50GHz channel.
“You were getting more fibre capacity with each generation,” says Serge Melle, director of product marketing, optical networks at Nokia. And this helped the continual reduction of the cost-per-bit metric.
But with more advanced DSPs, implemented using 16nm, 7nm, and now 5nm CMOS, going to a higher symbol rate and hence data rate requires more spectrum, says Melle.
Increasing the symbol rate is still beneficial. It allows more data to be sent using the same modulation scheme or transmitting the same data payload over longer distances.
“So one of the things we are looking to do with the PSE-6s is how do we still enable a lower total cost of ownership even though you don’t get more capacity per wavelength or fibre,” says Melle.
Symbol rate classes
Coherent optics from the leading vendors use a symbol rate of 90-107 gigabaud (GBd), while Cisco-owned Acacia’s latest 1.2-terabit coherent modem in a CIM-8 module operates at 140GBd.
Acacia uses a classification system based on symbol rate. First-generation coherent systems operating at 30-34GBd are deemed Class 1. Class 2 doubles the baud rates to 60-68GBd, the symbol rate window used for 400ZR coherent optics, for hyperscalers to connect equipment across their data centres up to 120km apart.
The DSPs from the leading optical transport systems vendors operating at 90-107GBd are an intermediate step between Class 2 and Class 3 using Acacia’s classification. In contrast, Acacia has jumped directly from Class 2 to Class 3 with its 140GBd CIM-8 coherent modem.
Competitors view Acacia’s classification scheme as a marketing exercise and counter that their 90-107GBd optical transport systems benefited customers for over two years.
Nokia’s 90GBd PSE-Vs can send 400 gigabits using quadrature phase-shift keying (QPSK) over 3,000km. This contrasts with its earlier 67GBd PSE-3s that sends 400GbE up to 1,000km using 16-QAM.
However, with the PSE-Vs, Nokia, unlike its optical transport competitors, Infinera, Ciena and Huawei, decided not to support 800-gigabit wavelengths.
Nokia argued that 7nm CMOS, 90-100GBd coherent optics tops out at 600 gigabit when used for distances of several hundred kilometers, while metro-regional distances are more economically served using 400-gigabit pluggable optics such as the CFP2 implementing 400ZR+.
With the 130Gbd PSE-6s, Nokia has a Class 3 coherent modem with the PSE-6s capable of sending 800 gigabits more than 2,000km.
The PSE-6s also doubles the maximum data rate of the PSE-Vs to 1.2 terabits per wavelength. However, at 1.2 terabits, the reach is 100-plus km, valuable for very high capacity metro transport and data centre interconnect.
Scale, reach and power consumption per bit
Nokia highlights the PSE-6s’ main three performance metric improvements.
First, the coherent modem delivers scaling: two coherent optical engines fit on a line card to deliver 2.4 terabits to transport emerging high-speed services such as 800GbE.
The two PSE-6s are linked using a dedicated interface to share the client-side signals (see diagram).
“We are not the only ones introducing a 5nm solution, but I think we are the only ones that allow two DSPs to work together,” says Melle.
Without the interface, a single 800GbE and up to four 100GbE clients or a 400GbE client can be sent over each DSP’s 1.2-terabit wavelength. Adding the interface, an operator can send three uniform 800GbE clients, with the interface splitting the third 800GbE client between the two DSPs.
“In a single line card, you can stripe the three 800-gigabit services rather than have to deploy three separate line cards in the network,” says Melle.
Nokia is not detailing the interface used to link the DSPs but said that the interface is used for data only and not to share signal processing resources between the ASICs.
“There is an extra amount of circuitry to share the client bandwidth across the two DSPs, but it is not high power consuming, and most transponders have some circuitry between the clients and the DSP,” says Melle. “So the incremental ‘power tax’ is marginal; it doesn’t add any significant power overhead.”
The resulting 2.4-terabit transmission is sent as two 1.2-terabit wavelengths, each occupying a 150GHz-wide channel. Existing systems that operate at 90-107GBd typically use a 112.5GHz channel for an 800-gigabit transmission, so the PSE-6s delivers a fibre capacity benefit.
The two wavelengths can be bonded, as in a two-channel ‘super-channel’, or sent to separate locations.
The second improvement is optical performance. For example, an 800-gigabit payload can travel over 2,000km. Nokia claims this is 3x the reach of existing commercial optical transport systems.
The improved transmission performance is achieved using a combination of the 130GBd baud rate, probabilistic constellation shaping (PCS), and improved forward error correction (FEC). Melle says the contributions to the improvement are 90 per cent baud rate and 10 per cent due to coherent modem algorithm tweaks.
“Baud rate is king; that is what really drives this improved performance,” says Melle.
The third benefit is reduced power consumption at the device and system (networking) levels.
Using a 5nm finFET CMOS process to make the PSE-6s DSP ASIC and developing denser line cards (two modems per card) means systems will consume 60 per cent less power than Nokia’s existing coherent technology.
According to Nokia, the PSE-6s optical engine consumes 40 per cent fewer Watts per bit compared to the PSE-Vs.
Nokia 1830 transport systems
The PSE-6s line cards fit into Nokia’s existing range of 1830 transport platforms.
These include the 1830 PSI-M compact modular data centre interconnect, the 1830 PSS-16 transponder and WDM line system, the 1830 PSS-24x P-OTN and switching chassis, and the 1830 PSI-SUB subsea line-terminating equipment.
For example, the PSI-M platform can hold two line cards, each with two PSE-6s.
Infinera’s ICE6 crosses the 100-gigabaud threshold
Coherent discourse 3
- The ICE6 Turbo can send two 800-gigabit wavelengths over network spans of 1,100-1,200km using a 100.4 gigabaud (GBd) symbol rate.
- The enhanced reach can reduce the optical transport equipment needed in a network by 25 to 30 per cent.
Infinera has enhanced the optical performance of its ICE6 coherent engine, increasing by up to 30 per cent the reach of its highest-capacity wavelength transmissions.
The ICE6 Turbo coherent optical engine can send 800-gigabit optical wavelengths over 1,100-1,200km compared to the ICE6’s reach of 700-800km.
ICE6 Turbo uses the same coherent digital signal processor (DSP) and optics as the ICE6 but operates at a higher symbol rate of 100.4GBd.
“This is the first time 800 gigabits can hit long-haul distances,” says Ron Johnson, general manager of Infinera’s optical systems & network solutions group.
Baud rates
Infinera’s ICE6 operates at 84-96GBd to transmit two wavelengths ranging from 200-800 gigabits. This gives a total capacity of 1.6 terabits, able to send 4×400 Gigabit Ethernet (GbE) or 16x100GbE channels, for example.
Infinera’s ICE6’s coherent DSP uses sub-carriers and their number and baud rates are tuned to the higher symbol rate.
The bit rate sent is defined using long-codeword probabilistic constellation shaping (LC-PAS) while Infinera also uses soft-decision FEC gain sharing between the DSP’s two channels.
The ICE6 Turbo adds several more operating modes to the DSP that exploit this higher baud rate, says Rob Shore, senior vice president of marketing at Infinera.
Reach
Infinera says that the ICE6 Turbo can also send two 600-gigabit wavelengths over 4,000km.
“This is almost every network in the world except sub-sea,” says Shore, adding that the enhanced reach will reduce the optical transport equipment needed in a network by 25 to 30 per cent.
“One thousand kilometres sending 2×800 gigabits or 4x400GbE is a powerful thing,” adds Johnson. “We’ll see a lot of traction with the content providers with this.”
Increasing symbol rate
Optical transport system designers continue to push the symbol rate. Acacia, part of Cisco, has announced its next 128GBd coherent engine while Infinera’s ICE6 Turbo now exceeds 100GBd.
Increasing the baud rate boosts the capacity of a single coherent transceiver while lowering the cost and power used to transport data. A higher baud rate can also send the same data further, as with the ICE6 Turbo.
“The original ICE6 device was targeted for 84GBd but it had that much overhead in the design to allow for these higher baud rate modes,” says Johnson. “We strived for 84GBd and technically we can go well beyond 100.4GBd.”
This is common, he adds.
The electronics of the coherent design – the silicon germanium modulator drivers, trans-impedance amplifiers, and analogue-to-digital and digital-to-analogue converters – are designed to perform at a certain level and are typically pushed harder and harder over time.
Baud rate versus parallel-channel designs
Shore believes that the industry is fast approaching the point where upping the symbol rate will no longer make sense. Instead, coherent engines will embrace parallel-channel designs.
Already upping the baud rate no longer improves spectral efficiency. “The industry has lost a vector in which we typically expect improvements generation by generation,” says Shore. “We now only have the vector of lowering cost-per-bit.”
At some point, coherent designs will use multiple DSP cores and wavelengths. What matters will be the capacity of the optical engine rather than the capacity of an individual wavelength, says Shore.
“We have had a lot of discussion about parallelism versus baud rate,” adds Johnson.
Already there is fragmentation with embedded and pluggable coherent optics designs. Embedded designs are optimised for high-performance spectral efficiency while for pluggables cost-per-bit is key.
This highlights that there is more than one optimisation approach, says Johnson: “We have got to develop multiple technologies to hit all those different optimisations.”
Infinera will use 5nm and 3nm CMOS for its future coherent DSPs, optimised for different parts of the network.
Infinera will keep pushing the baud rate but Johnson admits that at some point the cost-per-bit will start to rise.
“At present, it is not clear that doubling the baud rate again is the right answer,” says Johnson. “Maybe it is a combination of a little bit more [symbol rate] and parallelism, or it is moving to 200GBd.”
The key is to explore the options and deliver coherent technology consistently.
“If we put too much risk in one area and drive too hard, it has the potential to push our time-to-market out,” says Johnson.
The ICE6 Turbo will be showcased at the OFC show being held in San Diego in March.
COBO: specification work nearing completion
The Consortium for On-board Optics (COBO) is on target to complete its specifications work by the year end. The work will then enter a final approval stage that will take up to a further three months.
On-board optics, also known as mid-board or embedded optics, have been available for years but vendors have so far had to use custom products. The goal of COBO, first announced in March 2015 and backed by such companies as Microsoft, Cisco Systems, Finisar and Intel, is to develop a technology roadmap and common specifications for on-board optics to ensure interoperability.
Brad Booth (pictured), the chair of COBO and principal architect for Microsoft’s Azure Global Networking Services, says that bringing optics inside systems raises a different set of issues compared to pluggable optical modules used on the front panel of equipment. “If you have a requirement for 32 ports on a faceplate, you know mechanically what you can build,” says Booth.
With on-board optics, the focus is less about size considerations and more about the optical design itself and what is needed to make it work. There is also more scope to future-proof the design, something that can not be done so much with pluggable optics, says Booth.
COBO is working on a 400-gigabit optical module based on the 8-by–50 gigabit interface. The focus in recent months has been on defining the electrical connector that will be needed. The group has narrowed down the choice of candidates to two and the final selection will be based on the connector's signal integrity performance and manufacturability. Also being addressed is how two such modules could be placed side-by-side to create an 800-gigabit (16-by–50 gigabit) design.
COBO’s 400-gigabit on-board optics will support multi-mode and single-mode fibre variants. “When we do a comparison with what the pluggable people are pushing, there are a lot of pluggables that won’t be able to handle the power envelope,” says Booth.
There is no revolutionary change that goes on with technology, it all has to be evolutionary
On-board optics differs from a pluggable module in that the optics and electronics are not confined within a mechanical enclosure and therefore power dissipation is less of an design issue. But by supporting different fibre requirements and reaches new design issues arise. For example, when building a 16-by–50 gigabit design, the footprint is doubled and COBO is looking to eliminate the gap between the two such that a module can be plugged in that is either 8- or 16-lanes wide.
COBO is also being approached about supporting other requirements such as coherent optics for long-distance transmission. A Coherent Working Group has been formed and will meet for the first time in December in Santa Barbara, California. Using on-board optics for coherent avoids the power constraint issues associated with using a caged pluggable module.
On-board optics versus co-packaging
On-board optics is seen as the next step in the evolution of optics as it moves from the faceplate onto the board, closer to the ASIC. There is only so many modules that can fit on a faceplate. The power consumption also raises as the data rate of a pluggable modules increases, as does the power associated with driving faster electrical traces across the board.
Using on-board optics shortens the trace lengths by placing the optics closer to the chip. The board input-output capacity that can be supported also increases as it is fibres not pluggable optics that reside on the front panel. Ultimately, however, designers are already exploring the combining of optics and the chip using a system-in-package design, also known as 2.5D or 3D chip packaging.
Booth says discussions have already taken place between COBO members about co-packaged optics. But he does not expect system vendors to stay with pluggable optics and migrate directly to co-packaging thereby ignoring the on-board optics stage.
“There is no revolutionary change that goes on with technology, it all has to be evolutionary,” says Booth, who sees on-board optics as the next needed transition after pluggables. “You have to have some pathway to learn and discover, and figure out the pain points,” he says. “We are going to learn a lot when we start the deployment of COBO-based modules.”
Booth also sees on-board optics as the next step in terms of flexibility.
When pluggable modules were first introduced they were promoted as allowing switch vendors to support different fibre and copper interfaces on their platforms. The requirements of the cloud providers has changed that broad thinking, he says: “We don’t need that same level of flexibility but there is still a need for suporting different styles of optical interfaces on a switch.”
There are not a lot of other modules that can do 600 gigabit but guess what? COBO can
For example, one data centre operator may favour a parallel fibre solution based on the 100-gigabit PSM4 module while another may want a 100-gigabit wavelength-division multiplexing (WDM) solution and use the CWDM4 module. “This [parallel lane versus WDM] is something embedded optics can cater for,” says Booth.
Moving to a co-packaged design offers no such flexibility. What can a data centre manager do when deciding to change from parallel single-mode optics to wavelength-division multiplexing when the optics is already co-packaged with the chip? “Also how do I deal with an optics failure? Do I have to replace the whole switch silicon?” says Booth. We may be getting to the point where we can embed optics with silicon but what is needed is a lot more work, a lot more consideration and a lot more time, says Booth.
Status
COBO members are busy working on the 400-gigabit embedded module, and by extension the 800-gigabit design. There is also ongoing work as to how to support technologies such as the OIF’s FlexEthernet. Coherent designs will soon support rates such as 600-gigabit using a symbol rate of 64 gigabaud and advanced modulation. “There are not a lot of other modules that can do 600 gigabits but guess what? COBO can,” says Booth.
The good thing is that whether it is coherent, Ethernet or other technologies, all the members are sitting in the same room, says Booth: “It doesn’t matter which market gets there first, we are going to have to figure it out.”
Story updated on October 27th regarding the connector selection and the Coherent Working Group.
OIF starts work on a terabit-plus CFP8-ACO module
The Optical Internetworking Forum (OIF) has started a new analogue coherent optics (ACO) specification based on the CFP8 pluggable module.
The CFP8 is the latest is a series of optical modules specified by the CFP Multi-Source Agreement and will support the emerging 400 Gigabit Ethernet standard.
Karl GassAn ACO module used for optical transport integrates the optics and driver electronics while the accompanying coherent DSP-ASIC residing on the line card.
Systems vendors can thus use their own DSP-ASIC, or a merchant one if they don’t have an in-house design, while choosing the coherent optics from various module makers. The optics and the DSP-ASIC communicate via a high-speed electrical connector on the line card.
ACO design
The OIF completed earlier this year the specification of the CFP2-ACO.
Current CFP2-ACO modules support single-wavelength transmission rates from 100 gigabit to 250 gigabit depending on the baud rate and modulation scheme used. The goal of the CFP8-ACO is to support up to four wavelengths, each capable of up to 400 gigabit-per-second transmissions.
This project is going to drive innovation
“This isn’t something there is a dire need for now but the projection is that this will be needed in two years’ time,” says Karl Gass of Qorvo and the OIF Physical and Link Layer Working Group optical vice chair.
OIF members considered several candidate optical modules for the next-generation ACO before choosing the CFP8. These included the existing CFP2 and the CFP4. There were some proponents for the QSFP but its limited size and power consumption is problematic when considering long-haul applications, says Gass.
Source: Finisar
One difference between the CFP2 and CFP8 modules is that the electrical connector of the CFP8 supports 16 differential pairs while the CFP2 connector supports 10 pairs.
“Both connectors have similar RF performance and therefore can handle similar baud rates,” says Ian Betty of Ciena and OIF board member and editor of the CFP2-ACO Implementation Agreement. To achieve 400 gigabit on a wavelength for the CFP8-ACO, the electrical connector will need to support 64 gigabaud.
Betty points out that for coherent signalling, four differential pairs per optical carrier are needed. “This is independent of the baud rate and the modulation format,” says Betty.
So while it is not part of the existing Implementation Agreement, the CFP2-ACO could support two optical carriers while the CFP8 will support up to four carriers.
“This is only the electrical connector interface capacity,” says Betty. “It does not imply it is possible to fit this amount of optics and electronics in the size and power budget.” The CFP8 supports a power envelope of 20W compared to 12W of the CFP2.
The CFP2-ACO showing the optical building blocks and the electrical connector linking the module to the DSP-ASIC. Source: OIF
The CFP8 occupies approximately the same area as the CFP2 but is not as tall such that the module can be doubled-stacked on a line card for a total of 16 CFP8-ACOs on a line card.
Given that the CFP8 will support up to four carriers per module - each up to 400 gigabit - a future line card could support 25.6 terabits of capacity. This is comparable to the total transport capacity of current leading dense WDM optical transport systems.
Rafik Ward, vice president of marketing at Finisar, says such a belly-to-belly configuration of the modules provides future-proofing for next-generation lineside interfaces. “Having said that, it is not clear when, or how, we will be able to technically support a four-carrier coherent solution in a CFP8 form factor,” says Ward.
Oclaro stresses that such a high total capacity implies that sufficient coherent DSP silicon can fit on the line card. Otherwise, the smaller-height CFP8 module may not enable the fully expected card density if the DSP chips are too large or too power-hungry.
OIF goal
Besides resulting in a higher density module, a key OIF goal of the work is to garner as much industry support as possible to back the CFP8-ACO. “How to create the quantity of scale so that deployment becomes less expensive and therefore quicker to implement,” says Gass.
The OIF expects the work to be similar to the development of the CFP2-ACO Implementation Agreement. But one desired difference is to limit the classes associated with the module. The CFP2-ACO has three class categories based on whether the module has a limited and linear output. “The goal of the CFP8-ACO is to limit the designs to single classes per wavelength count,” says Gass.
Gass is looking forward to the CFP8-ACO specification work. Certain standards efforts largely involve making sure components fit into a box whereas the CFP8-ACO will be more engaging. “This project is going to drive innovation and that will drive some technical work,” says Gass.