Working at the limit of optical transmission performance
- Expect to see new optical transmission records at the upcoming ECOC 2023 conference.
- Keysight Technologies’ chart plots the record-setting optical transmission systems of recent years.
- The chart reveals optical transmission performance issues and the importance of the high-speed converters between the analogue and digital domains for test equipment and, by implication, for coherent digital signal processors (DSPs).
Engineers keep advancing optical systems to send more data across an optical fibre.
It requires advances in optical and electronic components that can process faster, higher-bandwidth signals, and that includes the most essential electronics part of all: the coherent DSP chip.
Coherent DSPs use state-of-the-art 5nm and 3nm CMOS chip manufacturing processes. The chips support symbol rates from 130-200 gigabaud (GBd). At 200GBd, the coherent DSP’s digital-to-analogue converters (DACs) and analogue-to-digital converters (ADCs) must operate at at least 200 giga samples-per-second (GSps) and likely closer to 250GSps. DACs drive the optical modulator in the optical transmission path while the ADCs are used at the optical receiver to recover the signal.
Spare a thought for the makers of test equipment used in labs that drive such coherent optical transmission systems. The designers must push their equipments’ DACs and ADCs to the limit to generate and sample the waveforms of these prototype next-generation optical transmission systems.
Optical transmission records
The recent history of record-setting optical transmission systems reveals the design challenges of coherent components and how ADC and DAC designs are evolving.
It is helpful to see how test equipment designers tackle ADC and DAC design, given the devices are a critical element of the coherent DSP, and when vendors are reluctant to detail how they achieve 200GBd baud rates using on-chip CMOS-based ADCs and DACs.
Nokia and Keysight Technologies published a post-deadline paper at the ECOC 2022 conference detailing the transmission of a 260GBd single-wavelength signal over 100km of fibre.
The system achieved the high baud rate using a thin-film lithium niobate modulator driven by Keysight’s M8199B arbitrary waveform generator. The M8199B uses a design consisting of two interleaved DACs to generate signals at 260GSps.
A second post-deadline ECOC 2022 paper, published by NTT, detailed the sending of over two terabits-per-second (Tbps) on a single wavelength. This, too, used Keysight’s M8199B arbitrary waveform generator.
The chart above highlights optical transmission records since 2015, plotting the systems’ net bit rate – from 800 gigabits to 2.2 Tbps – against a symbol rate measured in GBd.
As with commercial coherent optical transport systems, the goal is to keep increasing the symbol rate. A higher symbol rate sends more data over the same fibre spans. For example, the 400ZR coherent transmission standard uses a symbol rate of some 60GBd to send a 400Gbps wavelength, while 800ZR doubles the baud rate to some 120GBd to transmit 800Gbps over similar distances.
“With the 1600ZR project just started by the OIF, this trend will likely continue,” says Fabio Pittalá, product planner, broadband and photonic center of excellence at Keysight.
The signal generator test equipment options include the use of different materials – CMOS and silicon germanium – and moving from one DAC to a parallel multiplexed DAC design.
Single DACs
In 2017, Nokia achieved a 1Tbps transmission using a 100GBd symbol rate. Nokia used a Micram 6-bit 100GSps DAC in silicon germanium for the modulation.
For its next advancement in transmission performance, in 2019, Nokia used the same DAC but a faster ADC at the receiver, moving from a Tektronix instrument using a 70GHz ADC to the Keysight UXR oscilloscope with a 110GHz bandwidth ADC. The resulting net bit rate was nearly 1.4 terabits.
Keysight also developed the M8194A arbitrary waveform generator based on a CMOS-based DAC. The higher sampling rate of this arbitrary waveform generator increased the baud rate to 105GBd, but because of the bandwidth limitation, the net bit rate was lower.
The bandwidth of CMOS DACs can be improved but it tops out in the region of 50-60GHz. “It’s very difficult to scale to a higher baud rate using this technology,” says Pittalá. Silicon germanium, by contrast, supports much higher bandwidths but has a higher power consumption.
In 2020, Nokia reached 1.6Tbps at 128GBd using the Micram DAC5, an 8-bit 128GSps DAC based on silicon germanium. A year later, Keysight released the M8199A arbitrary waveform generator. “This was also based on 8-bit silicon germanium DACs operating at 128GSps, but the signal-to-noise ratio was greatly improved, allowing to generate higher-order quadrature amplitude modulation formats with more than sixteen levels,” says Pittalá.
This arbitrary waveform generator was used in systems that, coupled with advanced equalisation schemes, pushed the net bit rate to almost 2Tbps.
Going parallel
For the subsequent advances in baud rate, parallel DAC designs, multiplexing two or more DACs together, were implemented by different research labs.
In 2015, NTT multiplexed two DACs that advanced the symbol rate from 105GBd to 120GBd. In 2019, NTT moved to a different type of multiplexer, which, used with the same DAC, increased the baud rate to around 170GBd. Nokia also demonstrated a multiplexed design concept, which, together with a novel thin-film lithium niobate modulator, extended the symbol rate to 200GBd, achieving a 1.6Tbps net bit rate.
Last year, Keysight introduced its latest arbitrary waveform generator, the M8199B. The design also adopted a multiplexed DAC design.
Multiplexing two DACs. SR refers to sample rate, BW refers to bandwidth. Source: Keysight.
“There are two 128GSps 8-bit silicon germanium DACs that are time-interleaved to get a higher speed signal per dimension,” says Pittalá. If the two DACs are shifted in time and added together, the result is a higher sampling rate overall. However, Pittalá points out that while the sample rate is effectively doubled, the overall bandwidth is defined by the individual DACs (see diagram above).
Pittalá also mentions another technique, based on active clocking, that does increase the bandwidth of the system. The multiplexer is clocked and acts like a fast switch between the two DAC channels. “In principle, you can double the bandwidth, ” he says. (See diagram below.)
The Keysight’s M8199B’s improved performance, combined with advances in components such as NTT’s 130GHz indium phosphide amplifier, resulted in over 2Tbps transmission, as detailed in the ECOC 2022 paper. As the baud rate was increased, the modulation scheme used and the net bit rate decreased. (Shown by the red dots on the chart).
In parallel, Keysight worked with Nokia, which used a thin-film lithium niobate modulator for their set-up, a different modulator to NTT’s. The test equipment directly drove the thin-film modulator; no external modulator driver was needed. The system was operated as high as 260GBd, achieving a net bit rate of 800Gbps.
Pittalà notes that while the NTT system differs from Nokia’s, Nokia’s two red points on the extreme right of the chart continue the trajectory of NTT’s six red points as the baud rate increases.
OFC’23 O-band record
The post-deadline papers at the OFC 2023 conference earlier this year did not improve the transmission performances of the ECOC papers.
A post-deadline paper published at OFC 2023 showed a record of coherent transmission in the O-Band. Working with Keysight, McGill University showed 1.6Tbps coherent transmission over 10km using a thin-film lithium niobate modulator. The system operated at 167GBd, used a 64-QAM modulation scheme, and used the Keysight M8199B.
Pittalà expects that at ECOC 2023, to be held in Glasgow in October, new record-breaking transmissions will be announced.
His chart will need updating.
Further information
Thin-film lithium niobate modulators, click here
Taking a unique angle to platform design
- A novel design based on a vertical line card shortens the trace length between an ASIC and pluggable modules.
- Reducing the trace length improves signal integrity while maintaining the merits of using pluggables.
- Using the vertical line card design will extend for at least two more generations the use of pluggables with Ethernet switches.
The travelling salesperson problem involves working out the shortest route on a round-trip to multiple cities. It’s a well-known complex optimisation problem.
Novel design that shortens the distance between an Ethernet switch chip and the front-panel optics
Systems engineers face their own complex optimisation problem just sending an electrical signal between two points, connecting an Ethernet switch chip to a pluggable optical module, for example.
Sending the high-speed signal over the link with sufficient fidelity for its recovery requires considerable electronic engineering design skills. And with each generation of electrical signalling, link distances are getting shorter.
In a paper presented at the recent ECOC show, held in Basel, consultant Chris Cole, working with Yamaichi Electronics, outlined a novel design that shortens the distance between an Ethernet switch chip and the front-panel optics.
The solution promises headroom for two more generations of high-speed pluggables. “It extends the pluggable paradigm very comfortably through the decade,” says Cole.
Since ECOC, there are plans to standardise the vertical line card technology in one or more multi-source agreements (MSAs), with multiple suppliers participating.
“This will include OSFP pluggable modules as well as QSFP and QSFP-DD modules,” says Cole.
Shortening links
Rather than the platform using stacked horizontal line cards as is common today, Cole and Yamaichi Electronics propose changing the cards’ orientation to the vertical plane.
Vertical line cards also enable the front-panel optical modules to be stacked on top of each other rather than side-by-side. As a result, the pluggables are closer to the switch ASIC; the furthest the high-speed electrical signalling must travel is three inches (7.6cm). The most distant span between the chip and the pluggable with current designs is typically nine inches (22.8cm).
“The reason nine inches is significant is that the loss is high as we reach 200 gigabits-per-second-per-lane and higher,” says Cole.
Current input-output proposals
The industry is pursuing several approaches to tackle such issues as the issues associated with high-speed electrical signalling and also input-output (I/O) bandwidth density.
One is to use twinaxial cabling instead of electrical traces on a printed circuit board (PCB). Such ‘Twinax’ cable has a lower loss, and its use avoids developing costly advanced-material PCBs.
Other approaches involve bringing the optics closer to the Ethernet switch chip, whether near-packaged optics or the optics and chip are co-packaged together. These approaches also promise higher bandwidth densities.
Cole’s talk focussed on a solution that continues using pluggable modules. Pluggable modules are a low-cost, mature technology that is easy to use and change.
However, besides the radio frequency (RF) challenges that arise from long electrical traces, the I/O density of pluggables is limited due to the size of the connector, while placing up to 36 pluggables on the 1 rack unit-high (1RU) front panel obstructs the airflow used for cooling.
Platform design
Ethernet switch chips double their capacity every two years. Their power consumption is also rising; Broadcom’s latest Tomahawk 5 consumes 500W.
The power supply a data centre can feed to each platform has an upper limit. It means fewer cards can be added to a platform if the power consumed per card continues to grow.
The average power dissipation per rack is 16kW, and the limit is around 32kW, says Cole. This refers to when air cooling is used, not liquid cooling.
He cites some examples.
A rack of Broadcom’s 12.8-terabit Tomahawk 3 switch chip – either with 32, 1RU or 16, 2RU cards with two chips per card – and associated pluggable optics consume over 30kW.
A 25.6-terabit Tomahawk 4-based chassis supports 16 line cards and consumes 28kW. However, using the recently announced Tomahawk 5, only eight cards can be supported, consuming 27KW.
“The takeaway is that rack densities are limited by power dissipation rather than the line card’s rack unit [measure],” says Cole.
Vertical line card
The vertical line card design is 4RU high. Each card supports two ASICs on one side and 64 cages for the OSFP modules on the other.
A 32RU chassis can thus support eight vertical cards or 16 ASICs, equivalent to the chassis with 16 horizontal 2RU line cards.
The airflow for the ASICs is improved, enabling more moderate air fans to be used compared to 1RU or 2RU horizontal card chassis designs. There is also airflow across the modules.
“The key change in the architecture is the change from a horizontal card to a vertical card while maintaining the pluggable orientation,” says Cole.
As stated, the maximum distance between an ASIC and the pluggables is reduced to three inches, but Cole says the modules can be arranged around the ASIC to minimise the length to 2.5 inches.
Alternatively, if the height of the vertical card is an issue, a 3RU card can be used instead, which results in a maximum trace length of 3.5 inches. “[In this case], we don’t have dedicated air intakes for the CPU,” notes Cole.
Cole also mentioned the option of a 3RU vertical card that houses one ASIC and 64 OSFP modules. This would be suitable for the Tomahawk 5. However, here the maximum trace length is five inches.
Vertical connectors
Yamaichi Electronics has developed the vertical connectors needed to enable the design.
Cole points out that, unlike a horizontal connector, a vertical one uses equal-length contacts. This is not the case for a flat connector, resulting in performance degradation since a set of contacts has to turn and hence has a longer length.
Cole showed the simulated performance of an OSFP vertical connector with an insertion loss of over 70GHz.
“The loss up to 70GHz demonstrates the vertical connector advantage because it is low and flat for all the leads,” says Cole. “So this [design] is 200-gigabit ready.”
He also showed a vertical connector for the OSFP-XD with a similar insertion loss performance.
Also shown was a comparison with results published for Twinax cables. Cole says this indicates that the loss of a three-inch PCB trace is less than the loss of the cable.
“We’ve dramatically reduced the RF maximum length, so we had solved the RF roadblock problem, and we maintain the cost-benefit of horizontal line cards,” says Cole.
The I/O densities may be unchanged, but it preserves the mature technology’s benefits. “And then we get a dramatic improvement in cooling because there are no obstructions to airflow,” says Cole.
Vladimir Kozlov, CEO of the market research firm, LightCounting, wondered in a research note whether the vertical design is a distraction for the industry gearing up for co-packaged optics.
“Possibly, but all approaches for reducing power consumption on next-generation switches deserve to be tested now,” said Kozlov, adding that adopting co-packaged optics for Ethernet switches will take the rest of the decade.
“There is still time to look at the problem from all angles, literally,” said Kozlov
BT's IP-over-DWDM move
- BT will roll out next year IP-over-DWDM using pluggable coherent optics in its network
- At ECOC 2022, BT detailed network trials that involved the use of ZR+ and XR optics coherent pluggable modules
Telecom operators have been reassessing IP-over-DWDM with the advent of 400-gigabit coherent optics that plug directly into IP routers.
According to BT, using pluggables for IP-over-DWDM means a separate transponder box and associated ‘grey’ (short-reach) optics are no longer needed.
Until now, the transponder has linked the IP router to the dense wavelength-division multiplexing (DWDM) optical line system.
“Here is an opportunity to eliminate unnecessary equipment by putting coloured optics straight onto the router,” says Professor Andrew Lord, BT’s head of optical networking.
Removing equipment saves power and floor space too.
DWDM trends
Operators need to reduce the cost of sending traffic, the cost-per-bit, given the continual growth of IP traffic in their networks.
BT says its network traffic is growing at 30 per cent a year. As a result, the operator is starting to see the limits of its 100-gigabit deployments and says 400-gigabit wavelengths will be the next capacity hike.
Spectral efficiency is another DWDM issue. In the last 20 years, BT has increased capacity by lighting a new fibre pair using upgraded optical transport equipment.
Wavelength speeds have gone from 2.5 to 10, then to 40, 100, and soon 400 gigabits, each time increasing the total traffic sent over a fibre pair. But that is coming to an end, says BT.
“If you go to 1.2 terabits, it won’t go as far, so something has to give,” says Lord. ”So that is a new question we haven’t had to answer before, and we are looking into it.”
Fibre capacity is no longer increasing because coherent optical systems are already approaching the Shannon limit; send more data on a wavelength and it occupies a wider channel bandwidth.
Optical engineers have improved transmission speeds by using higher symbol rates. Effectively, this enables more data to be sent using the same modulation scheme. And keeping the same modulation scheme means existing reaches can still be met. However, upping the symbol rate is increasingly challenging.
Other ways of boosting capacity include making use of more spectral bands of a fibre: the C-band and the L-band, for example. BT is also researching spatial division multiplexing (SDM) schemes.
IP-over-DWDM
IP-over-DWDM is not a new topic, says BT. To date, IP-over-DWDM has required bespoke router coherent cards that take an entire chassis slot, or the use of coherent pluggable modules that are larger than standard QSFP-DD client-side optics ports.
“That would affect the port density of the router to the point where it’s not making the best use of your router chassis,“ says Paul Wright, optical research manager at BT Labs.
The advent of OIF-defined 400ZR optics has catalysed operators to reassess IP-over-DWDM.
The 400ZR standard was developed to link equipment housed in separate data centres up to 120km apart. The 120km reach is limiting for operators but BT’s interest in ZR optics stems from the promise of low-cost, high-volume 400-gigabit coherent optics.
“It [400ZR optics] doesn’t go very far, so it completely changes our architecture,” says Lord. “But then there’s a balance between the numbers of [router] hops and the cost reduction of these components.”
BT modelled different network architectures to understand the cost savings using coherent ZR and ZR+ optics; ZR+ pluggables have superior optical performance compared to 400ZR.
The networks modelled included IP routers in a hop-by-hop architecture where the optical layer is used for point-to-point links between the routers.
This worked well for traffic coming into a hub site but wasn’t effective when traffic growth occurred across the network, says Wright, since traffic cascaded through every hop.
BT also modelled ZR+ optics in a reconfigurable optical add-drop multiplexer (ROADM) network architecture, as well as a hybrid arrangement using both ZR+ and traditional coherent optics. Traditional coherent optics, with its superior optical performance, can pass through a string of ROADM stages where ZR+ optics falls short.
BT compared the cost of the architectures assuming certain reaches for the various coherent optics and published the results in a paper presented at ECOC 2020. The study concluded that ZR and ZR+ optics offer significant cost savings compared to coherent transponders.
ZR+ pluggables have since improved, using higher output powers to better traverse a network’s ROADM stages. “The [latest] ZR+ optics should be able to go further than we predicted,” says Wright.
It means BT is now bought into IP-over-DWDM using pluggable optics.
BT is doing integration tests and plans to roll out the technology sometime next year, says Lord.
XR optics
BT is a member of the Open XR Forum, promoting coherent optics technology that uses optical sub-carriers.
Dubbed XR optics, if all the subs-carriers originate at the same point and are sent to a common destination, the technology implements a point-to-point communication scheme.
Sub-carrier technology also enables traffic aggregation. Each sub-carrier, or a group of sub-carriers, can be sent from separate edge-network locations to a hub where they are aggregated. For example, 16 endpoints, each using a 25-gigabit sub-carrier, can be aggregated at a hub using a 400-gigabit XR optics pluggable module. Here, XR optics is implementing point-to-multipoint communication.
Lord views XR optics as innovative. “If only we could find a way to use it, it could be very powerful,” he says. “But that is not a given; for some applications, XR optics might be too big and for others it may be slightly too small.”
ECOC 2022
BT’s Wright shared the results of recent trial work using ZR+ and XR optics at the recent ECOC 2022 conference, held in Basel in September.
The 400ZR+ were plugged into Nokia 7750 SR-s routers for an IP-over-DWDM trial that included the traffic being carried over a third-party ROADM system in BT’s network. BT showed the -10dBm launch-power ZR+ optics working over the ROADM link.
For Wright, the work confirms that 0dBm launch-power ZR+ optics will be important for network operators when used with ROADM infrastructures.
BT also trialled XR optics where traffic flows were aggregated.
“These emerging technologies [ZR+ and XR optics] open up for the first time the ability to deploy a full IP-over-DWDM solution,” concluded Wright.
ECOC '22 Reflections - Final Part
Gazettabyte has been asking industry and academic figures for their thoughts after attending ECOC 2022, held last month in Basel, Switzerland. In particular, what developments and trends they noted, what they learned, and what, if anything, surprised them.
In the final part, Dr. Sanjai Parthasarathi of Coherent, Acacia’s Tom Williams, ADVA’s Jörg-Peter Elbers and Fabio Pittalà of Keysight Technologies share their thoughts.
Dr. Sanjai Parthasarathi, Chief Marketing Officer, Coherent
The ECOC event represents an excellent opportunity for us – a vertically-integrated manufacturer selling at all levels of the value chain – to meet with customers, end-customers and partners/ suppliers.
There was a refreshing sense of optimism and excitement for optical communications, driven by relentless bandwidth growth, despite the macroeconomic backdrop.
The roadmap for optical transceivers is dictated by the electrical interface used for Ethernet switch chips. We have seen that play out yet again for 100-gigabit electrical lanes used for 25-terabit and 50-terabit Ethernet switches.
Several transceiver suppliers demonstrated products with 100 gigabit-per-lane electrical interfaces in quad and octal form factors. The optical lane of a transceiver typically begins at the same speed as the electrical lane and then progresses to a faster rate. This transition should be expected for 800-gigabit transceivers as well.
While 100 gigabit-per-lane transceivers, such as the 800G-DR8 and the 2x400G-FR4 devices, there were devices demonstrated that enable the transition to optical 200-gigabit lanes. It was satisfying to see a warm response for the demonstration of Coherent’s 200-gigabit electro-absorption modulated laser (EML) and Semtech’s 200-gigabit EML driver. I am confident that direct detection will play a predominant role in 800-gigabit and 1.6-terabit data centre links.
Despite the great interest in co-packaged optics, nearly all the working demonstrations at the show used pluggable transceiver modules. Industry colleagues are preparing for pluggable transceiver modules using the next 200-gigabit electrical interface. Indeed, at ECOC, there was an OIF-CEI 224G demo by Keysight and Synopsys.
One key topic at the show concerned whether ‘coherent lite’ or direct detect is the preferred solution for data centres and edge aggregation. The debate remains open and no one solution fits all. It will depend on the specific application and architecture. A broad portfolio supported by different technology platforms frees you to select the best approach to serve the customer’s needs.
I saw the industry responding to the need for disaggregation and innovative solutions for access and telecom. Coherent’s 100G ZR announcement is one such example, as well as the extra performance of high-power 400ZR+ coherent transceivers.
We started this trend and we now see others announcing similar solutions.
Arista’s demo, which featured 400ZR connections over a 120km data centre interconnect (DCI) link, enabled by our pluggable optical line system in a QSFP form factor, received much attention and interest.
Tom Williams, Senior Director of Marketing for Acacia, now part of Cisco.
Many of us are still of a mindset where any opportunity to get together and see industry friends and colleagues is a great show.
My focus is very much on the success of 400-gigabit pluggable coherent solutions.
We’ve been talking about these products for a long time, back to the initial OIF 400ZR project starting in late 2016. Since then, 400ZR/ZR+ has been a hot topic at every conference.
The commercial success of these solutions, and the impact that they’re having on network architectures, has been gratifying. These products have ramped in volumes not seen by any previous coherent technology.
The industry has done a great job at 400 gigabits, striking the right balance of power and performance. Now, we’re looking at 800 gigabits and working through some of the same questions. Discussions around 1.6 terabits have even started.
Much work is still required but what we heard from customers at ECOC is that the trend toward pluggable coherent will likely continue.
Jörg-Peter Elbers, Senior Vice President, Advanced Technology, Standards and IPR at ADVA
‘Never say never’ captures well ECOC’s content. There was no one groundbreaking idea but topics discussed in the past are back on the agenda, either because of a need or the technology has progressed.
Here are several of my ECOC takeaways:
- The 130 gigabaud (GBd) class of coherent optics is coming, and the generation after that – 240GBd – is on the horizon.
- Coherent optics continue to push towards the edge. Will there be a Very-High Speed Coherent PON after 50G High-Speed PON?
- Whether co-packaged optics or front-pluggable modules, electro-photonic integration is rapidly advancing with some interesting industry insights shared at the conference.
- Quantum-safe communication is becoming part of the regular conference program.
- Optical Satcom is gaining traction. Optical ground-to-space links are promising yet challenging.
Fabio Pittalà, Product Planner, Broadband and Photonics – Center of Excellence, Keysight Technologies
This was my first ECOC as an employee of Keysight. I spent most of my time at the exhibition introducing the new high-speed Keysight M8199B Arbitrary Waveform Generator.
There were a lot of discussions focusing on technologies enabling the next Ethernet rates. There is a debate about intensity-modulation direct detection (IMDD) versus coherent but also what modulation format, symbol rate or degree of parallelisation.
While the industry is figuring out the best solution, researchers achieved important milestones by transmitting the highest symbol rate and the highest net bitrate.
Nokia Bell-Labs demonstrated record-breaking transmission of 260-gigabaud dual-polarisation quadrature phase-shift keying (DP-QPSK) over 100km single-mode fibre.
Meanwhile, NTT broke the net bitrate record by transmitting more than 2 terabit-per-second using a probabilistic-constellation-shaped dual-polarisation quadrature amplitude modulation (DP-QAM) over different data centre links.
Data centre photonics - an ECOC report
- ECOC 2022 included talks on optical switching and co-packaged optics.
- Speakers discussed optical switching trends and Google’s revelation that it has been using optical circuit switching in its data centres.
- Nvidia discussed its latest chips, how they are used to build high-performance computing systems, and why optical input-output will play a critical role.
Co-packaged optics and optical switching within the data centre were prominent topics at the recent ECOC 2022 conference and exhibition in Basel, Switzerland.
There were also two notable data centre announcements before ECOC.
Tencent announced it would adopt Broadcom’s Humboldt design, a hybrid co-packaged optics version of the Tomahawk 4 switch chip, in its data centres. Tencent is the first hyperscaler to announce it is adopting co-packaged optics.
Google also revealed its adoption of optical circuit switching in its data centres. Google made the revelation in a paper presented at the Sigcomm 2022 conference held in Amsterdam in August.
Optical circuit switching
Google rarely details its data centre architecture, but when it does, it is usually at Sigcomm.
Google first discussed a decade of evolution of its ‘Jupiter’ data centre architecture in a paper at Sigcomm in 2015.
This year, Google gave an update revealing that it has been using optical circuit switching in its data centres for the past five years. As a result, Google can scale its data centre more efficiently using a reconfigurable optical layer.
Upgrading a data centre’s network is much more complex than upgrading servers and storage. Moreover, a data centre is operational far longer than each generation of equipment. It is thus hard for a data centre operator to foresee how equipment and workloads will evolve over the data centre’s lifetime, says Google.
Google would pre-deploy the spine layer when it started operating a data centre. For Google’s Jupiter architecture, 64 spine blocks, each using 40 gigabit-per-second (Gbps) links, would be deployed. Then, Google added newer aggregation blocks with 100Gbps links. But the hyperscaler could not fully benefit due to the pre-existing 40Gbps spine links.
Google wanted to avoid touching the spine switches. A partial upgrade would have limited benefits, while fully upgrading the spine would take months and be hugely disruptive and costly.
Google’s first solution introduced a MEMS-based optical circuit switching layer between the aggregation and spine blocks.
The MEMS-based switch is data rate agnostic and can support multiple generations of optical modules. The switch’s introduction also allowed Google to add new spine blocks alongside new aggregation blocks; the hyperscaler no longer had to pre-deploy the spine.
At some point, Google decided that for new data centre builds, it would use optical circuit switching only and remove the spine layer of electrical switches.
Adopting optical circuit switch-based interconnect changes Google’s data centres from a clos to a direct-connect architecture. However, not all paths are direct; some take two hops to link aggregation blocks.
Google has developed sophisticated control software to best exploit the direct connectivity for traffic flows.
The software also adapts the network topology – the optical links between the aggregation blocks and their capacities. Such topology changes occur every few weeks, with the system first learning the nature of the traffic and workloads.
Removing the spine layer and replacing it with optical circuit switches has reduced Google’s data centre networking costs by 30 per cent and power consumption by 41 per cent.
ECOC reflections about Google’s optical switch adoption
There was much discussion at ECOC of Google’s use of optical circuit switching in its data centres.
S.J. Ben Yoo, a distinguished professor at the University of California, Davis, gave an ECOC talk about new trends in optical switching. “These are expected future trends,” he said. “I don’t have a crystal ball.”
Prof. Ben Yoo stressed the difficulty of scaling up and scaling out data centre networking architectures in the era of artificial intelligence workloads.
He described co-packaged optics as ‘Trend 0’ because it only delivers bandwidth (input-output capacity).
In contrast, introducing a reconfigurable optical switching layer on top of electrical aggregation switches is the first trend in optical switching. This is what Google has done with its optical circuit switch.
The next development in the data centre, says Ben Yoo, will be the introduction of photonic integrated circuit-based optical switching.
Huawei’s Maxim Kuschnerov, in his ECOC talk, said optical switching in the data centre would only grow in importance.
“Are there use cases where we can use optical switching and what are they?” he said. “I like to take a use-case perspective and find a technology that fulfils that use case.”
His view is that with the classical clos architecture, you can’t just rip out a single layer of electrical switches and replace it with optical ones. “There is a reason why you need all these switches and aggregation functionality,” says Kuschnerov.
Kuschnerov views Google’s optical circuit switching as nothing more than an automated patch panel.
“This is not the optical switch which is the saviour of future data centres,” he says.
Mark Filer, optical network architect, systems and services infrastructure at Google, in an ECOC tutorial detailing how Google uses and benefits from standards, multi-source agreements and open-source developments, was asked about Google’s custom optical switch.
How could Google explain such a custom design if the hyperscaler is such a proponent of open standards? And would Google consider contributing its optical circuit switch and software design to an open community framework?
“My guess is over time, we may see that it finds its way into some kind of open framework,” said Filer, adding that right now, Google sees its optical circuit switch as delivering a competitive advantage.
Co-packaged optics
Benjamin Lee, a senior research scientist at Nvidia, in his ECOC address, discussed the high-performance computing market and the role graphics processing units (GPUs) play in accelerating artificial intelligence and machine learning tasks.
Nvidia not only develops processors, GPUs and data processing unit ICs but also networking silicon and systems that the company uses to make high-performance computing systems.
Lee’s talk addressed the role optical interconnect will play in ensuring continuing scaling of high-performance GPU-based computing systems.
Scaled systems
Nvidia’s latest GPU, announced earlier this year, is the 80-billion-transistor Hopper H100. The H100 deliver a six-fold improvement in throughput compared to Nvidia’s existing A100 GPU announced in 2020.
The Hopper is Nvidia’s first GPU that uses the latest generation of stacked DRAM memory, known as high bandwidth memory 3 (HBM3). In addition, Hopper also uses Nvidia’s fourth-generation NVlink interface.
Eight H100 GPUs fit within Nvidia’s DGX box, as do four Nvidia NVSwitches used to interconnect the GPUs. In addition, an Nvidia Superpod connects 32 DGX nodes – 256 GPUs – using an external tier of NVSwitches.
“A paradigm shift we’re seeing is that switched interconnect is becoming important for scale-up,” said Lee. “So when we want to make the node more computationally powerful, those switches are being put inside the box to connect the GPUs.”
Switch ASIC bandwidths are consistently improving, with 51.2-terabit switch silicon being state-of-the-art. But despite such progress, the scaling is insufficient to keep up with bandwidth requirements, said Lee.
Switch ASIC power consumption is also rising, with advanced CMOS scaling having less impact on designs. Lee foresees switch ASICs consuming 2kW if current trends continue.
In turn, ASIC input-output (I/O) accounts for an increasing portion of the chip’s overall power consumption.
This is true for Nvidia’s GPUs and switch chips, so any I/O technology developed for switching will also benefit its GPUs.
Thus, Nvidia sees optical I/O as the key to scaling the processing performance of its ASICs and computing systems.
I/O metrics
Lee outlined various metrics when discussing optical I/O:
- the electrical interfaces used between the ASIC and optics, and their reach
- the power consumption of the module (the chip, and the chip and optics)
- the system power (of the line card or platform)
- interface density: the capacity exiting a millimetre of surface in terabits-per-second-per-mm (Tbps/mm)
For a system using a 102.4-terabit switch IC, half the power is consumed by the ASIC and half by the edge-board pluggable optics. Here the OIF’s long reach (LR) interface links the two.
The chip’s electrical interfaces consume 4.5 to 6.5 picojoule-per-bit (pJ/b) such that the total switch IC I/O power consumed is 450W.
The next step is co-packaged optics. Here, optical chiplets are placed closer to the ASIC (100mm away) such that the OIF’s lower power XSR (extra short reach) interface can be used that consumes 1.24-1.7pJ/s, says Lee.
Again taking a module view, Nvidia views the co-packaged design as comprising two electrical interfaces (the XSR interface between the chip and optical chiplets either side) and one optical interface.
This equates to 250W per chip module, a modest power saving at the chip module level but a significant power saving at the system level, given the optics is now part of the module.
However, bandwidth density is 475-870Gbps/mm, and for beyond 100-terabit switches, a further fourfold improvement is needed: 2Tbps/mm and, ultimately, 10Tbps/mm.
Just achieving a 2Tb/s/mm interface density will be challenging, says Lee.
For that, 2.5D co-packaged optics will be needed with the ASIC and chiplets sharing a silicon interposer that enables higher wire densities.
2.5D integration is already an established technology in the semiconductor industry; Nvidia has been using the technology for its GPUs since 2016.
The technology enables much closer coupling between the ASIC and optics (some 1mm), resulting in sub 1pJ/bit. Nvidia cites research showing a 0.3pJ/b has already been achieved.
Scaling I/O
Lee outlined all the ways I/O can be scaled.
Baud rate is one approach, but the energy efficiency diminishes as the symbol rate increases, from 50 to 100 to 200 gigabaud.
Modulation is another approach, moving from non-return-to-zero to 4-level pulse amplitude modulation (PAM-4) and even higher PAM schemes. The challenge is that the signal-to-noise ratio diminishes the higher the PAM scheme, requiring additional digital signal processing which, in turn, consumes more power.
Another technique, polarization, can be used to double the data rate. Then there is the spatial domain. Here, tighter pitches can be used, says Lee, moving from 250, 127 and even 80 microns before other approaches are needed. These include multi-core fibre, waveguide fan-outs and even bidirectional optics (what Google uses for its optical circuit switch ports, to save on fibre and port count).
All these spatial approaches require considerable development and operational costs, says Lee.
The most promising way to boost throughput and increase interface density is using wavelength division multiplexing (WDM).
Nvidia has produced several generations of test chips that use wavelength parallelism in the O-band based on micro-ring resonators.
Nvidia’s steer
Micro-ring resonator technology already supports 100Gbps modulation rates. The optical circuit is also compact, energy-efficient and supports wavelength scaling.
Lee also outlined other key technologies that will be needed, each bringing their own challenges. One is the external laser source, another is advanced packaging.
Nvidia believes that for future generations of ASICs, dense WDM mirror-ring modulated links offer the most promising approach to meeting both low power and the massive interface density improvements that will be needed.
This will require low-cost lasers while packaging remains a severe challenge.
2.5D integration is going to be an important step in the evolution of switch interconnect, concluded Lee.
ECOC '22 Reflections - Part 3
Gazettabyte is asking industry and academic figures for their thoughts after attending ECOC 2022, held in Basel, Switzerland. In particular, what developments and trends they noted, what they learned, and what, if anything, surprised them.
In Part 3, BT’s Professor Andrew Lord, Scintil Photonics’ Sylvie Menezo, Intel’s Scott Schube, and Quintessent’s Alan Liu share their thoughts.
Professor Andrew Lord, Senior Manager of Optical Networks Research, BT
There was strong attendance and a real buzz about this year’s show. It was great to meet face-to-face with fellow researchers and learn about the exciting innovations across the optical communications industry.
The clear standouts of the conference were photonic integrated circuits (PICs) and ZR+ optics.
PICs are an exciting piece of technology; they need a killer use case. There was a lot of progress and discussion on the topic, including an energetic Rump session hosted by Jose Pozo, CTO at Optica.
However, there is still an open question about what use cases will command volumes approaching 100,000 units, a critical milestone for mass adoption. PICs will be a key area to watch for me.
We’re also getting more clarity on the benefits of ZR+ for carriers, with transport through existing reconfigurable optical add-drop multiplexer (ROADM) infrastructures. Well done to the industry for getting to this point.
All in all, ECOC 2022 was a great success. As one of the Technical Programme Committee (TPC) Chairs for ECOC 2023 in Glasgow, we are already building on the great show in Basel. I look forward to seeing everyone again in Glasgow next year.
Sylvie Menezo, CEO of Scintil Photonics
What developments and trends did I note at ECOC? There is a lot of development work on emergent hybrid modulators.
Scott Schube, Senior Director of Strategic Marketing and Business Development, Silicon Photonics Products Division at Intel.
There were not a huge amount of disruptive announcements at the show. I expect the OFC 2023 event will have more, particularly around 200 gigabit-per-lane direct-detect optics.
Several optics vendors showed progress on 800 gigabit/ 2×400 gigabit optical transceiver development. There are now more vendors, more flavours and more components.
Generalising a bit, 800 gigabit seems to be one case where the optics are ready ahead of time, certainly ahead of the market volume ramp.
There may be common-sense lessons from this, such as the benefits of technology reuse, that the industry can take into discussions about the next generation of optics.
Alan Liu, CEO of Quintessent
Several talks focused on the need for high wavelength count dense wavelength division multiplexing (DWDM) optics in emerging use cases such as artificial intelligence/ machine learning interconnects.
Intel and Nvidia shared their vision for DWDM silicon photonics-based optical I/O. Chris Cole discussed the CW-WDM MSA on the show floor, looking past the current Ethernet roadmap at finer DWDM wavelength grids for such applications. Ayar Labs/Sivers had a DFB array DWDM light source demo, and we saw impressive research from Professor Keren Bergman’s group.
An ecosystem is coalescing around this area, with a healthy portfolio and pipeline of solutions being innovated on by multiple parties, including Quintessent.
The heterogeneous integration workshop was standing room only despite being the first session on a Sunday morning.
In particular, heterogeneously integrated silicon photonics at the foundry level was an emergent theme as we heard updates from Tower, Intel, imec, and X-Celeprint, among other great talks. DARPA has played – and plays – a key role in seeding the technology development and was also present to review such efforts.
Fibre-attach solutions are an area to watch, in particular for dense applications requiring a high number of fibres per chip. There is some interesting innovation in this area, such as from Teramount and Suss Micro-Optics among others.
Shortly after ECOC, Intel also showcased a pluggable fibre attach solution for co-packaged optics.
Reducing the fibre packaging challenge is another reason to employ higher wavelength count architectures and DWDM to reduce the number of fibres needed for a given aggregate bandwidth.
ADVA targets access with its latest pluggable module
- The 25 gigabit-per-second (Gbps) SFP28 is self-tuning and has a reach of 40km
- ADVA’s CEO, Christoph Glingener, in his plenary talk at ECOC 2022 addressed the unpredictable nature of technology adoption.
ADVA has expanded its portfolio of optical modules with an SFP28 for the access market.
The AccessWave25 is a self-tuning dense wavelength division multiplexing (DWDM) pluggable.
The SFP28 is designed to enable communications service providers to straightforwardly upgrade their access networks from 10Gbps to 25Gbps.
ADVA made the announcement just before ECOC 2022.
Features
The SFP28 module links switches and routers to DWDM open-line systems (see diagram below).
The 40km-reach pluggable uses 4-level pulse amplitude modulation (PAM-4) and supports 25 gigabit Ethernet and eCPRI traffic.
The module uses the G.metro self-tuning standard to coordinate with the remote-end transceiver a chosen channel in the C-band, simplifying configuration and removing human error.
The G.metro communication channel also enables remote monitoring of the module.
The SFP28 consumes 3W and works over the extended temperature of -40 to 85oC.
Strategy
ADVA says vertical integration is a critical part of its Optical Engine unit’s strategy.
Saeid Aramideh, ADVA’s Optical Engine’s vice president of business development, says the unit focusses on such technology disciplines as silicon photonics, laser technology and digital signal processing.
The digital signal processing includes aggregation as with ADVA‘s MicroMux module products, PAM-4 used by the AccessWave25, and coherent as with its 100ZR module announced in June.
Advanced packaging is another technology area of interest.
“These are the fundamental innovation areas we focus on,” says Aramideh. “We build our product portfolio based on these platforms.”
ADVA also looks at the market to identify product gaps.
“Not so much every MSA module, but what is happening on the aggregation side,” says Aramideh. “What is it that other people are not paying attention to?”
This is what motivated ADVA’s MicroMux products. The MicroMux module family includes a 10-by-10 gigabit going into 100 gigabits, a 10-by-one gigabit into 10 gigabits, and a four-by-100 gigabit going into 400 gigabits.
“The reality is over 10,000 MicroMux modules are carrying traffic with a top tier-one network provider in Europe,“ says Aramideh. “Not on ADVA equipment but on other network equipment maker, which we haven’t made public.”
For access aggregation, ADVA unveiled at OFC its four-by-10 gigabit MicroMux Edge BiDi with a 40km reach.
“This is for Ethernet, backhaul, and services where fibre is limited and symmetric latency is important,” says Aramideh.
ADVA’s 100ZR module uses a coherent digital signal processor (DSP) developed with Coherent. The 100ZR is a QSFP28 module that dissipates 5W and reaches 300km.
Now, ADVA has added the AccessWave25, a tunable SFP28 that uses direct-detect technology and PAM-4, including ADVA’s IP for distance optimisation.
“The AccessWave25 works on legacy, so if you have a 10-gigabit network, you don’t have to change anything on the physical layer,” he says.
ADVA also looks at metro applications and says it will announce lower-power, smaller form factor coherent designs.
ECOC plenary talk
The CEO of ADVA, Christoph Glingener, gave a plenary talk at ECOC.
Entitled Never say never, Glingener reflected on technology adoption and its timing.
He pointed out how technologies that, at first, seem impractical or too difficult to adopt can subsequently become mainstream. He cited coherent optical communication as one example.
Glingener also discussed how such unpredictability impacts business, citing supply-chain issues, the global pandemic, and sovereignty.
Sovereignty and the influx of government capital for fibre rollout and semiconductors confirm that the optical communications industry is in a good place. But Glingener worries how the industry’s practitioners are ageing and stresses more needs to be done to attract graduates.
Tracing optical communications’ progress, he talked about the 15-year cycles of first direct detect and then fibre amplification. Coherent then followed in 2010.
The industry is thus ripe for breakthrough technology.
Reaching limits
Shannon’s limit means spectral efficiency no longer improves while Moore’s law’s demise continues. Near-term trends are clear, he says, parallelism, whether it is multiple spectrum bands, multiple fibres, or multiple fibre cores. This, in turn, will drive new optical amplifier and wavelength-selective switch designs.
Further optimisation will be needed, integration at the device level and the creation of denser systems. Network automation is also essential and that requires much work.
Glingener also argues for optical bypass rather than electrical packet processing. Large core routers overseeing routing at the IP and optical layer will not aid the greening of the internet.
Next wave
So what is the next technology wave?
Possibilities he cited include hollow-core fibre, photonic computing, and quantum entanglement for communications and the quantum internet.
Will they reach a large scale? Glingener is doubtful.
Whatever the technology proves to be, he said, it is likely already being discussed at ECOC 2022.
If he has a message for the audience, it is to apply their own filter whenever they hear people say, ‘it will never come,’ or ‘it is too difficult.’ Never say never, says Glingener.
ECOC 2022 Reflections - Part 1
Gazettabyte is asking industry and academic figures for their thoughts after attending ECOC 2022, held in Basel, Switzerland. In particular, what developments and trends they noted, what they learned, and what, if anything, surprised them.
In Part 1, Infinera’s David Welch, Cignal AI’s Scott Wilkinson, University of Cambridge’s Professor Seb Savory, and Huawei’s Maxim Kuschnerov share their thoughts.
David Welch, Chief Innovation Officer and Founder of Infinera
First, we had great meetings. It was exciting to be back to a live, face-to-face industry event. It was also great to see strong attendance from so many European carriers.
Point-to-multipoint developments were a hot topic in our engagements with service providers and component suppliers. It was also evident in the attendance and excitement at the Open XR Forum Symposium, as well as the vendor demos.
We’re seeing that QSFP-DD ZR+ is a book-ended solution for carriers; interoperability requirements are centred on the CFEC (concatenated or cascaded FEC) market; oFEC (Open FEC) is not being deployed.
Management of pluggables in the optical layer is critical to their network deployment, while network efficiency and power reduction are top of mind.
The definition of ZR and ZR+ needs to be subdivided further into ZR – CFEC, ZR+ – oFEC, and ZR+-HP (high performance), which is a book-ended solution.
Scott T. Wilkinson, Lead Analyst, Optical Components, Cignal AI.
The show was invaluable, given this was our first ECOC since Cignal AI launched its optical components coverage.
Coherent optics announcements from the show did not follow the usual bigger-faster-stronger pattern, as the success of 400ZR has convinced operators and vendors to look at coherent at the edge and inside the data centre.
100ZR for access, the upcoming 800ZR specifications from the OIF, and coherent LR (coherent designed for 2km-10km) will revolutionise how coherent optics are used in networks.
Alongside the coherent announcements were developments from the direct-detect vendors demonstrating or previewing key technologies for 800 Gigabit Ethernet (GbE) and 1.6 Terabit Ethernet (TbE) modules.
800GbE is nearly ready for prime time, awaiting completion of systems based on the newest 112 gigabit-per-second (Gbps) serialiser-deserialiser (serdes) switches. The technology for 224Gbps serdes is just starting to emerge and looks promising for products in late 2024 or 2025.
While there were no unexpected developments at the show, it was great to compare developments across the industry and understand the impact of supply chain issues, operator deployment plans, and any hints of oversupply.
We came away optimistic about continued growth in optical components shipments and revenue into 2023.
Seb Savory, Professor of Optical Fibre Communication, University of Cambridge
My overwhelming sense from ECOC was it was great to be meeting in person again. I must confess I was looking at logistics as much as content with a view to ECOC 2023 in Glasgow where I will be a technical programme committee chair.
Maxim Kuschnerov, Director of the Optical and Quantum Communications Laboratory at Huawei
In the last 12 months, the industry has got more technical clarification regarding next-generation 800ZR and 800LR coherent pluggables.
While 800ZR’s use case seems to be definitely in the ZR+ regime, including 400 gigabit covering metro and long-haul, the case for 800LR is less clear.
Some proponents argue that this is a building block toward 1.6TbE and the path of coherence inside the data centre.
Although intensity-modulation direct detection (IMDD) faces technical barriers to scaling wavelength division multiplexing to 8×200 gigabit, the technological options for beyond 800-gigabit coherent aren’t converging either.
In the mix are 4×400 gigabit, 2×800 gigabit and 1×1.6 terabit, making the question of how low-cost and low-power coherent can scale into data centre applications one of the most interesting technical challenges for the coming years.
Arista continues making a case for a pluggable roadmap through the decade based on 200-gigabit serdes.
With module power envelopes of around 40W at the faceplate, it shows the challenge that the industry is facing and the case co-packaged optics is trying to make.
However, putting all the power into, or next to, the switching chip doesn’t make the cooling problem any less problematic. Here, I wonder if Avicena’s microLED technology could benefit next-generation chip-to-chip or die-to-die interconnects by dropping the high-speed serdes altogether and thus avoiding the huge overhead current input-output (I/O) is placing on data centre networking.
It was great to see the demo of the 200-gigabit PAM-4 externally modulated laser (EML) at Coherent’s booth delivering high-quality eye diagrams. The technology is getting more mature, and next year will receive much exposure in the broader ecosystem.
As for every conference, we have seen the usual presentations on Infinera’s XR Optics. Point-to-multipoint coherent is a great technology looking for a use case, but it is several years too early.
At ECOC’s Market Focus, Dave Welch put up a slide on the XR ecosystem, showing several end users, several system OEMs and a single component vendor – Infinera. I think one can leave it at this for now without further comment.
Ciena's multi-format 400G coherent QSFP-DD pluggable
Ciena showcased a working 400-gigabit Universal coherent pluggable module at the ECOC 2022 conference and exhibition in Basel, Switzerland.
Ciena is using its WaveLogic 5 Nano coherent digital signal processor (DSP) for the Universal QSFP-DD coherent pluggable module.
“We call it universal because it supports many transmission modes – interoperable and high performance; the most in the industry,” says Helen Xenos, senior director of portfolio marketing at Ciena.
The pluggable has custom extended-performance modes and supports three industry formats: the 400ZR interoperable standard, the 400ZR+ multi-source agreement (MSA), and the OpenROADM MSA. (See tables below).
IP over DWDM
Communications service providers (CSPs) want to add pluggable coherent modules to their IP routers, removing the need for a separate transponder card or box linking the router to the optical line system.
The advent of coherent QSFP-DD pluggables has meant the same form factor can be used for client-side and line-side optics, ensuring efficient use of the router ports.
The CSPs want the coherent QSFP-DD module to have sufficient optical performance to meet their demanding networking requirements. For example, the module’s output signal can pass through the filtering stages of reconfigurable optical add-drop multiplexers (ROADMs) along the optical link.
Optical amplification and filtering
Ciena’s coherent QSFP-DD adds a fibre-based optical amplifier and a tunable optical filter to the coherent photonics and electronic ICs.
The optical amplification enables the high-performance mode and the launching of a 4dBm output signal. In contrast, 400ZR and 400ZR+ have a launch power of -10dBm.
“This is the industry’s highest [QSDP-DD] transmit power,” says Xenos.
The tunable optical filter improves the optical performance of the coherent receiver.
In an optical line system with colourless ROADMs, the Erbium-doped fibre amplifiers (EDFAs) generate out-of-band transmission noise – amplified spontaneous emission (ASE). The noise sources superimpose and become significant, impairing wavelength and system performance dramatically.
The tunable optical filter eliminates this effect and simplifies deployment over any photonics line system. In addition, Ciena says the pluggables can now work alongside high-baud rate transponders in existing ROADM applications.
The QSFP-DD’s tunable optical filter means its optical performance closely matches that of the CFP2-DCO, aiding the two classes of pluggables working together.
Modes of operation
400ZR defines the module’s baseline coherent performance. The OIF developed the 400ZR standard so hyperscalers can link their equipment in two separate data centres up to 120km apart.
The 400ZR specification delivers just enough optical performance to meet the optical link budget. The OIF produced a low-cost, interoperable, pluggable coherent specification.
400ZR supports a single baud rate – 60 gigabaud (GBd), and modulation scheme – dual-polarisation 16-QAM, and carries Ethernet frames.
Google, Meta, Microsoft and Alibaba were all involved in the OIF development, with the 400ZR Implementation Agreement published in early 2020.
400ZR supports two-channel widths: 75GHz and 100GHz, while the forward error correction scheme used is CFEC.
The 400ZR+ MSA enhances the performance by supporting other data rates – 100, 200 and 300 gigabits-per-second (Gbps) – as well as 400Gbps. In addition, it uses several modulation schemes and the enhanced O-FEC error correction scheme that extends reach.
Ciena’s module also meets the OpenROADM MSA, supporting Ethernet and OTN and an enhanced reach at 400Gbps.
Ciena’s Universal module’s extended performance modes up the symbol rate to 65 and 70 gigabaud (GBd) and uses probabilistic constellation shaping (PCS).
PCS maps the bitstream onto the constellation to maximise the data recovery at the coherent receiver, thereby improving overall optical performance. The scheme also allows the fine-tuning of the data rate sent.
At ECOC, Ciena showed the module implementing the high-performance mode at 70GBd and PCS.
ECOC innovation award
The ECOC Exhibition Market Focus Advisory Committee awarded the most innovative product award to Ciena’s WaveLogic 5 Nano 400G Universal QSFP-DD.