The many paths to 400 gigabits

The race is on to deliver 400-gigabit optical interfaces in time for the next-generation of data centre switches expected in late 2018.

The industry largely agrees that a four-wavelength 400-gigabit optical interface is most desirable yet alternative designs are also being developed.

Optical module makers must consider such factors as technical risk, time-to-market and cost when choosing which design to back.

Rafik Ward, FinisarUntil now, the industry has sought a consensus on interfaces, making use of such standards bodies as the IEEE to serve the telecom operators.

Now, the volumes of modules used by the internet giants are such that they dictate their own solutions. And the business case for module makers is sufficiently attractive that they are willing to comply.

Another challenge at 400 gigabits is that there is no consensus regarding what pluggable form factor to use. 

“There is probably more technical risk in 400 gigabits than any of the historical data-rate jumps we have seen,” says Rafik Ward, vice president of marketing at Finisar.

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Has coherent optical transmission run its course?

Feature: Coherent's future

Three optical systems vendors share their thoughts about coherent technology and the scope for further improvement as they look two generations ahead to symbol rates approaching 100 gigabaud   

Optical transmission using coherent detection has made huge strides in the last decade. The latest coherent technology with transmitter-based digital signal processing delivers 25x the capacity-reach of 10-gigabit wavelengths using direct-detection, according to Infinera.

Since early 2016, the optical systems vendors Infinera, Ciena and Nokia have all announced new coherent digital signal processor (DSP) designs. Each new generation of coherent DSP improves the capacity that can be transmitted over an optical link. But given the effectiveness of the latest coherent systems, has most of the benefits already been achieved?

Source: Infinera

“It is getting harder and harder,” admits Kim Roberts, vice president, WaveLogic science at Ciena. “Unlike 10 years ago, there are no factors of 10 available for improvement.”

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Inphi unveils a second 400G PAM-4 IC family

Inphi has announced the Vega family of 4-level, pulse-amplitude modulation (PAM-4) chips for 400-gigabit interfaces.

The 16nm CMOS Vega IC family is designed for enterprise line cards and is Inphi’s second family of 400-gigabit chips that support eight lanes of 50-gigabit PAM-4.

Its first 8x50-gigabit family, dubbed Polaris, is used within 400-gigabit optical modules and was announced at the OFC show held in Los Angeles in March.

“Polaris is a stripped-down low-power DSP targeted at optical module applications,” says Siddharth Sheth, senior vice president, networking interconnect at Inphi (pictured). “Vega, also eight by 50-gigabits, is aimed at enterprise OEMs for their line-card retimer and gearbox applications.”  

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SFP-DD: Turning the SFP into a 100-gigabit module

Part 2: New optical transceiver MSA

An industry initiative has started to quadruple the data rate of the SFP, the smallest of the pluggable optical modules. The Small Form Factor Pluggable – Double Density (SFP-DD) is being designed to support 100 gigabits by doubling the SFP’s electrical lanes from one to two and doubling their speed.

Scott SommersThe new multi-source agreement (MSA), to be completed during 2018, will be rated at 3.5W; the same power envelope as the current 100-gigabit QSFP module, even though the SFP-DD is expected to be 2.5x smaller in size.

The front panel of a 1-rack-unit box will be able to support up to 96 SFP-DD modules, a total capacity of 9.6 terabits. 

The SFP-DD is adopting a similar philosophy as that being used for the 400-gigabit QSFP-DD MSA: an SFP-DD port will support legacy SFPs modules - the 25-gigabit SFP28 and 10-gigabit SFP - just as the QSFP-DD will be backward compatible with existing QSFP modules.

“Time and time again we have heard with the QSFP-DD that plugging in legacy modules is a key benefit of that technology,” says Scott Sommers, group product manager at Molex and the chair of the new SFP-DD MSA. Sommers is also a co-chair of the QSFP-DD MSA.

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Heavy Reading’s take on optical module trends

Part 1: Optical transceiver trends 

The industry knows what the next-generation 400-gigabit client-side interfaces will look like but uncertainty remains regarding what form factors to use. So says Simon Stanley who has just authored a report entitled: From 25/100G to 400/600G: A Competitive analysis of Optical Modules and Components.

Implementing the desired 400-gigabit module designs is also technically challenging, presenting 200-gigabit modules with a market opportunity should any slip occur at 400 gigabits.

Simon StanleyStanley, analyst-at-large at Heavy Reading and principal consultant at Earlswood Marketing, points to several notable developments that have taken place in the last year. For 400 gigabits, the first CFP8 modules are now available. There are also numerous suppliers of 100-gigabit QSFP28 modules for the CWDM4 and PSM4 multi-source agreements (MSAs). He also highlights the latest 100-gigabit SFP-DD MSA, and how coherent technology for line-side transmission continues to mature.

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Elenion's coherent and fibre-to-the-server plans

• Elenion’s coherent chip - an integrated modulator-receiver assembly - is now generally available. 
• The company has a silicon photonics design library that includes over 1,000 elements. 
• Elenion is also developing an optical engine for client-side interfaces.

Elenion Technologies has given an update on its activities and strategy after announcing itself eight months ago. The silicon photonics-based specialist is backed by private equity firm, Marlin Equity Partners, which also owns systems vendor, Coriant. Elenion had already been active for two and a half years and shipping product when it emerged from its state of secrecy last December. 

Larry SchwerinElenion has since announced it is selling its telecom product, a coherent transceiver PIC, to Coriant and now other companies.

It has also progressed its optical engine design for the data centre that will soon be a product. Elenion has been working with Ethernet switch chip maker, Cavium, and data centre player, Microsoft, as part of its datacom work.

“We have moved forward,” says Larry Schwerin, the CEO of Elenion.

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Meeting the many needs of data centre interconnect

High capacity. Density. Power efficiency. Client-side optical interface choices. Coherent transmission. Direct detection. Open line system. Just some of the requirements vendors must offer to compete in the data centre interconnect market.

“A key lesson learned from all our interactions over the years is that there is no one-size-fits-all solution,” says Jörg-Peter Elbers, senior vice president of advanced technology, standards and IPR at ADVA Optical Networking. “What is important is that you have a portfolio to give customers what they need.”

 Jörg-Peter Elbers

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