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.

Routes to 400 gigabit 

The first 400-gigabit modules using the CFP8 form factor support the 2km-reach 400Gbase-FR8 and the 10km 400Gbase-LR8; standards defined by the IEEE 802.3bs 400 Gigabit Ethernet Task Force. The 400-gigabit FR8 and LR8 employ eight 50Gbps wavelengths (in each direction) over a single-mode fibre.

There is significant investment going into the QSFP-DD and OSFP modules

But while the CFP8 is the first main form factor to deliver 400-gigabit interfaces, it is not the form factor of choice for the data centre operators. Rather, interest is centred on two emerging modules: the QSFP-DD that supports double the electrical signal lanes and double the signal rates of the QSFP28, and the octal small form factor pluggable (OSFP) MSA.

“There is significant investment going into the QSFP-DD and OSFP modules,” says Stanley. The OSFP is a fresh design, has a larger power envelope - of the order of 15W compared to the 12W of the QSFP-DD - and has a roadmap that supports 800-gigabit data rates. In contrast, the QSFP-DD is backwards compatible with the QSFP and that has significant advantages.

“Developers of semiconductors and modules are hedging their bets which means they have got to develop for the QSFP-DD, so that is where the bulk of the development work is going,” says Stanley. “But you can put the same electronics and optics in an OSFP.”    

Given there is no clear winner, both will likely be deployed for a while. “Will QSFP-DD win out in terms of high-volumes?” says Stanley. “Historically, that says that is what is going to happen.”

The technical challenges facing component and module makers are achieving 100-gigabit-per-wavelength for 400 gigabits and fitting them in a power- and volume-constrained optical module.

The IEEE 400 Gigabit Ethernet Task Force has also defined the 400GBase-DR4 which has an optical interface comprising four single-mode fibres, each carrying 100 gigabits, with a reach up to 500m. 

“The big jump for 100 gigabits was getting 25-gigabit components cost-effectively,” says Stanley. “The big challenge for 400 gigabits is getting 100-gigabit-per-wavelength components cost effectively.” This requires optical components that will work at 50 gigabaud coupled with 4-level pulse-amplitude modulation (PAM-4) that encodes two bits per symbol.

That is what gives 200-gigabit modules an opportunity. Instead of 4x50 gigabaud and PAM-4 for 400 gigabits, a 200-gigabit module can use existing 25-gigabit optics and PAM-4. “You get the benefit of 25-gigabit components and a bit of a cost overhead for PAM-4,” says Stanley. “How big that opportunity is depends on how quickly people execute on 400-gigabit modules.”

The first 200-gigabit modules using the QSFP56 form factor are starting to sample now, he says.  

100-Gigabit

A key industry challenge at 100 gigabit is meeting demand and this is likely to tax the module suppliers for the rest of this year and next. Manufacturing volumes are increasing, in part because the optical module leaders are installing more capacity and because of the entrance of many, smaller vendors into the marketplace.

End users buying a switch only populate part of the ports due to the up-front costs. More modules are then added as traffic grows. Now, internet content providers turn on entire data centres filled with equipment that is fully populated with modules. “The hyper-scale guys have completely changed the model,” says Stanley.

The 100-gigabit module market has been coming for several years and has finally reached relatively high volumes. Stanley attributes this not just to the volumes needed by the large-scale data centre operators but also the fact that 100-gigabit modules have reached the right price point. Another indicator of the competitive price of 100-gigabit is the speed at which 40-gigabit technology is starting to be phased out.

Developments such as silicon photonics and smart assembly techniques are helping to reduce the cost of 100-gigabit modules, says Stanley, and this will be helped further with the advent of the new SFP-DD MSA.

SFP-DD

The double-density SFP (SFP-DD) MSA was announced in July. It is the next step after the SFP28, similar to the QSFP-DD being an advance on the QSFP28. And just as the 100-gigabit QSFP28 can be used in breakout mode to interface to four 25-gigabit SFP28s, the 400-gigabit QSFP-DD promises to perform a similar breakout role interfacing to SFP-DD modules.  

Stanley sees the SFP-DD as a significant development. “Another way to reduce cost apart from silicon photonics and smart assembly is to cut down the number of lasers,” he says. The number of lasers used for 100 gigabits can be halved from four using 28 gigabaud signalling and PAM-4). Existing examples of two-wavelength/ PAM-4 styled 100-gigabit designs are Inphi’s ColorZ module and Luxtera’s CWDM2. 

The industry’s embrace of PAM-4 is another notable development of the last year. The debate about the merits of using 56-gigabit symbol rate and non-return-to-zero signalling versus PAM-4 with its need for forward-error correction and extra latency has largely disappeared, he says.    

The first 400-gigabit QSFP-DD and OSFP client-side modules are expected in a year’s time with volumes starting at the end of 2018 and into 2019

Coming of age

Stanley describes the coherent technology used for line-side transmissions as coming of age. Systems vendors have put much store in owning the technology to enable differentiation but that is now changing. To the well-known merchant coherent digital signal processing (DSP) players, NTT Electronics (NEL) and Inphi, can now be added Ciena which has made its WaveLogic Ai coherent DSP available to three optical module partners, Lumentum, NeoPhotonics and Oclaro.

CFP2-DCO module designs, where the DSP is integrated within the CFP2 module, are starting to appear. These support 100-gigabit and 200-gigabit line rates for metro and data centre interconnect applications. Meanwhile, the DSP suppliers are working on coherent chips supporting 400 gigabits. 

Stanley says the CFP8 and OSFP modules are the candidates for future pluggable coherent module designs.

Meanwhile, the first 400-gigabit QSFP-DD and OSFP client-side modules are expected in a year’s time with volumes starting at the end of 2018 and into 2019.

As for 800-gigabit modules, that is unlikely before 2022.

“At OFC in March, a big data centre player said it wanted 800 Gigabit Ethernet modules by 2020, but it is always a question of when you want it and when you are going to get it,” says Stanley.