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.
Nokia adds 400G coherent modules across its platforms
Nokia is now shipping its 400-gigabit coherent multi-haul CFP2-DCO. The module exceeds the optical performance of 400ZR and ZR+ coherent pluggables.
Nokia’s CFP2-DCO product follows its acquisition of silicon photonics specialist, Elenion Technologies, in 2020.
Nokia has combined Elenion’s coherent optical modulator and receiver with its low-power 64-gigabaud (GBd) PSE-Vc coherent digital signal processor (DSP).
Nokia is also adding coherent pluggables across its platform portfolio.
“Not just optical transport and transponder platforms but also our IP routing portfolio as well,” says Serge Melle, director of product marketing, IP-optical networking at Nokia.
“This [amplifier and filter] allows for much better optical performance,”
“This [amplifier and filter] allows for much better optical performance,”
Melle is an optical networking industry veteran. He joined Nokia two years ago after a 15-year career at Infinera. Melle started at Pirelli in 1995 when it was developing a 4×2.5-gigabit wavelength-division multiplexing (WDM) system. In between Pirelli and Infinera, Melle was at Nortel Networks during the optical boom.
400ZR, ZR+ and the multi-haul CFP2-DCO
The CFP2-DCO’s optical performance exceeds that of the QSFP-DD and OSFP form factors implementing 400ZR and ZR+ but is inferior to line-card coherent transponders used for the most demanding optical transport applications.
The 400ZR coherent OIF standard transmits a 400-gigabit wavelength up to 120km linking equipment across data centres. Being a standard, 400ZR modules are interoperable.
The ZR+ adds additional transmission speeds – 100, 200 and 300-gigabits – and has a greater reach than ZR. ZR+ is not a standard but there is the OpenZR+ multi-source agreement (MSA).
Implementing 400ZR and ZR+ coherent modules in a QSFP-DD or OSFP module means they can be inserted in client-side optics’ ports on switches and routers.
The OIF did not specify a form factor as part of the 400ZR standard, says Melle, with the industry choosing the QSFP-DD and OSFP. But with the modules’ limited power dissipation, certain modes of the coherent DSP are turned off, curtailing the feature set and the reach compared to a CFP2-DCO module.
The modules also have physical size restrictions.
“You don’t have enough thermal budget to put an optical amplifier inside the QSFP-DD package,” says Melle. “So you are left with whatever power the DWDM laser outputs through the modulator.” This is -7dBm to -10dBm for 400ZR and ZR+ optics, he says.
The CFP2-DCO is larger such that the DSP modes of encryption, OTN client encapsulation, LLDP snooping (used to gather data about attached equipment), and remote network monitoring (RMON) can be enabled.
The CFP2-DCO can also house an optical amplifier and tunable filter. The filter reduces the out-of-band optical signal-to-noise ratio (OSNR) thereby increasing the module’s sensitivity. “This [amplifier and filter] allows for much better optical performance,” says Melle. A 400-gigabit multi-haul module has a 0dBm optical output power, typically.
The different transceiver types are shown in the table.
Nokia’s paper at the recent OFC virtual conference and exhibition detailed how its 400-gigabit multi-haul CFP2-DCO achieved a reach of 1,200km.
The paper details the transmission of 52, 400-gigabit signals, each occupying a 75GHz channel, for a total capacity of 20.8 terabits-per-second (Tbps).
Melle stresses that the demonstration was more a lab set-up than a live network where a signal goes through multiple reconfigurable optical add-drop multiplexers (ROADMs) and where amplifier stages may not be equally spaced.
That said, the CFP2-DCO’s reach in such networks is 750km, says Nokia.
IP-optical integration
Having coherent pluggables enables 400 Gigabit Ethernet (400GbE) payloads to be sent between routers over a wide area network, says Nokia.
“Given this convergence in form factor, with the QSFP-DD and ZR/ ZR+, you can now do IP-optical integration, putting coherent optics on the router without sacrificing port density or having locked-in ports,” says Melle.
Nokia is upgrading its IP and optical portfolio with coherent pluggables.
“In the routers, ZR/ ZR+, and in transponders not only the high-performance coherent optics – the [Nokia] PSE-Vs [DSP] – but also the CFP2-DCO multi-haul,” says Melle. “The 400-gigabit multi-haul is also going to be supported in our routers.”
Accordingly, Nokia has developed two sets of input-output (I/O) router cards: one supporting QSFP-DDs suited for metro-access applications, and the second using CFP2-DCO ports for metro and regional networks.
The choice of cards adds flexibility for network operators; they no longer need to have fixed CFP2-DCO slots on their router faceplates, whether they are used or not. But being physically larger, there are fewer CFP2-DCO ports than QSFP-DD ports on the I/O cards.
While the QSFP-DD MSA initially defined the module with a maximum power dissipation of 14.5W, a coherent QSFP-DD module consumes 18-20W. Dissipating the heat generated by the modules is a challenge.
Nokia’s airflow cooling is simplified by placing a module on both sides of the line card rather than stacking two CFP2-DCOs, one on top of the other.
Nokia is adding its CFP2-DCO to its 1830 optical transport portfolio. These include its PSI-M compact modular systems, the PSS transponder systems and also its PSS-x OTN switching systems.
The 400ZR/ZR+ module will be introduced with all its routing platforms this summer – the 7250 IXR, 7750 SR, 7750 SR-s, and the 7950 XRS, whereas the CFP2-DCO will be added to its 7750 and 7950 series later this year.
Nokia will source the 400ZR/ZR+ from third parties as well as from its optical networks division.
Its routers use QSFP-DD form-factor for all 400GbE ports and this is consistent for most router vendors in the industry. “Thus, our use and supply of 400ZR/ZR+ pluggable DCOs will focus on the QSFP-DD form-factor,” says Melle. However, the company says it can offer the OSFP form-factor depending on demand.
Network planning study
Nokia published a paper at OFC on the ideal coherent solution for different applications.
For metro aggregation rings with 4-5 nodes and several ROADM pass-throughs, using ZR+ modules is sufficient. Moreover, using the ZR+ avoids any loss in router port density.
For metro-regional core applications, the ZR+’s optical performance is mostly insufficient. Here, the full 400-gigabit rate can not be used but rather 300 gigabit-per-second (Gbps) or even 200Gbps to meet the reach requirements.
Using a 400-gigabit multi-haul pluggable on a router might not match the density of the QSFP-DD but it enables a full 400-gigabit line rate.
For long-haul, the CFP2-DCO’s performance is “reasonable”, says Nokia, and this is where high-performance transponders are used.
What the OFC paper argues is that there is no one-size-fits-all solution, says Melle.
800-Gigabit coherent pluggables
Traditionally, the IEEE has defined short-reach client-side optics while the OIF defines coherent standards.
“If we want this IP-optical convergence continuing in the next generation of optics, those two worlds are going to have to collaborate more closely,” says Melle.
That’s because when a form-factor MSA will be defined, it will need to accommodate the short-reach requirements and the coherent optics. If this doesn’t happen, says Melle, there is a risk of a new split occuring around the IP and optical worlds.
The next generation of coherent pluggables will also be challenging.
All the vendors got together in 2019 and said that 400ZR was just around the corner yet the modules are only appearing now, says Melle.
The next jump in pluggable coherent optics will use a symbol rate of 90-130GBd.
“That is very much the cutting-edge so it brings back the optics as a critical enabling technology, and not just optics but the packaging,” concludes Melle.
