Nokia’s PSE-2s delivers 400 gigabit on a wavelength

Nokia has unveiled what it claims is the first commercially announced coherent transport system to deliver 400 gigabits of data on a single wavelength. Using multiple 400-gigabit wavelengths across the C-band, 35 terabits of data can be transmitted.

Four hundred gigabit transmission over a single carrier is enabled using Nokia’s second-generation programmable Photonic Service Engine coherent processor, the PSE2, part of several upgrades to Nokia's flagship PSS 1830 family of packet-optical transport platforms.

Kyle Hollasch“One thing that is clear is that performance will have a key role to play in optics for a long time to come, including distance, capacity per fiber, and density,” says Sterling Perrin, senior analyst at Heavy Reading.

This limits the appeal of the so-called “white box” trend for many applications in optics, he says: “We will continue to see proprietary advances that boost performance in specific ways and which gain market traction with operators as a result”.

The 1830 Photonic Service Switch

The 1830 PSS family comprises dense wavelength-division multiplexing (DWDM) platforms and packet-OTN (Optical Transport Network) switches.

The DWDM platform includes line amplifiers, reconfigurable optical add-drop multiplexers (ROADMs), transponder and muxponder cards. The 1830 platforms span the PSS-4, -8, -16 and the largest and original -32, while the 1830 PSS packet-OTN switches include the PSS-36 and the PSS-64 platforms. The switches include their own coherent uplinks but can be linked to the 1830 DWDM platforms for their line amps and ROADMs.   

The 1830 PSS upgrades include a 500-gigabit muxponder card for the DWDM platforms that feature the PSE2, new ROADM and line amplifiers that will support the L-band alongside the C-band to double fibre capacity, and the PSS-24x that complements the two existing OTN switch platforms.      

100-gigabit as a service  

In DWDM transmissions, 100-gigabit wavelengths are commonly used to transport multiplexed 10-gigabit signals. Nokia says it is now seeing increasing demand to transport 100-gigabit client signals.

“One hundred gigabit is becoming the new currency,” says Kyle Hollasch, director, optical marketing at Nokia. “No longer is the thinking of 100 gigabit just as a DWDM line rate but 100 gigabit as a service, being handed from a customer for transport over the network.” 

Current PSS 1830 platform line cards support 50-gigabit, 100-gigabit and 200-gigabit coherent transmission using polarisation-multiplexed, binary phase-shift keying (PM-BPSK), quadrature phase-shift keying (PM-QPSK) and 16 quadrature amplitude modulation (PM-16QAM), respectively. Nokia now offers a 500-gigabit muxponder card that aggregates and transports 100-gigabit client signals. The 500-gigabit muxponder card has been available since the first quarter and already several hundred cards have been shipped. 

“The challenge is not just to crank up capacity but to do so profitably,” says Hollasch. “Keeping the cost-per-bit down, the power consumption down while pushing towards the Shannon limit [of fibre] to carry more capacity.”

 Source: Nokia

Modulation formats

The PSE2 family of coherent processors comprises two designs: the high-end super-coherent PSE-2s and the compact low-power PSE-2c.

Nokia joins the likes of Ciena and Infinera in developing several coherent ASICs, highlighting how optical transport requirements are best met using custom silicon. Infinera also announced its latest generation photonic integrated circuit that supports up to 2.4 terabits.

The high-end PSE-2s is a significant enhancement on the PSE coherent chipset first announced in 2012. Implemented using 28nm CMOS, the PSE-2s has a power consumption similar to the original PSE yet halves the power consumption-per-bit given its higher throughput. 

The PSE-2s adds four modulation formats to the PSE’s existing three and supports two symbol rates: 32.5 gigabaud and 44.5 gigabaud. The modulation schemes and distances they enable are shown in the chart.

The 1.4 billion transistor PSE-2s has sufficient processing performance to support two coherent channels. Each channel can implement a different modulation format if desired, or the two can be tightly coupled to form a super-channel. The only exception is the 400-gigabit single wavelength format. Here the PSE-2s supports only one channel implemented using a 45 gigabaud symbol rate and PM-64QAM. The 400-gigabit wavelength has a relatively short 100-150km reach, but this suits data centre interconnect applications where links are short and maximising capacity is key.

Nokia recently conducted a lab experiment resulting in the sending of 31.2 terabits of data over 90km of standard single-mode fibre using 78, 400-gigabit channels spaced 50GHz apart across the C-band. "We were only limited by the available hardware from reaching 35 terabits," says Hollasch.

Using the 45-gigabaud rate and PM-16QAM enables two 250-gigabit channels. This is how the 500-gigabit muxponder card is achieved. The 250-gigabit wavelength has a reach of 900km, and this can be extended to 1,000km but at 200 gigabit by dropping to the 32-gigabaud symbol rate, as implemented with the current PSE chipset.

Nokia also offers 200 gigabit implemented using 45 gigabaud and 8-QAM. “The extra baud rate gets us [from 150 gigabit] to 200 gigabit; this is very valuable,” says Hollasch. The resulting reach is 2,000km and he expects this format to gain the most market traction.  

The PSE-2s, like the PSE, also implements PM-QPSK and PM-BPSK but with reaches of 3,000-5,000km and 10,000km, respectively.

The PSE-2s introduces a fourth modulation format dubbed set-partition QPSK (SP-QPSK). 

Standard QPSK uses amplitude and phase modulation resulting in a 4-point constellation. With SP-QPSK, only three out of the possible four constellation points are used for any given symbol.  The downside of the approach is that a third fewer constellation points are used and hence less data is transported but the lost third can be restored using the higher 45-gigabaud symbol rate.

The benefit of SP-QPSK is its extended reach. “By properly mapping the sequence of symbols in time, you create a greater Euclidean distance between the symbol points,” says Hollasch. “What that gives you is gain.” This 2.5dB extra gain compared to PM-QPSK equates to a reach beyond 5,000km. “That is the territory most implementation are using BPSK and also addresses a lot of sub-sea applications,” says Hollasch. “Using SP-QPSK [at 100 gigabit] also means fewer carriers and hence, it is more spectrally efficient than [50-gigabit] BPSK.”  

The PSE-2c

The second coherent DSP-ASIC in the new family is the PSE-2c compact, also implemented in 28nm CMOS, designed for smaller, low-power metro platforms and metro-regional reaches.

The PSE-2c supports a 100-gigabit line rate using PM-QPSK and will be used alongside the CFP2-ACO line-side pluggable module. The PSE-2c consumes a third of the power of the current PSE operating at 100 gigabit. 

“We are putting the PSE2 [processors] in multiple form factors and multiple products,” says Hollasch.

The recent Infinera and Nokia announcements highlight the electronic processing versus photonic integration innovation dynamics, says Heavy Reading's Perrin. He notes how innovations in electronics are driving transmission across greater distances and greater capacities per fibre and finding applications in both long haul and metro networks as a result.

“Parallel photonic integration is a density play, but even Infinera’s ICE announcement is a combination of photonic integration and electronic processing advancements,” says Perrin. “In our view, electronic processing has taken a front seat in importance for addressing fibre capacity and transmission distance, which is why the need for parallel photonic integration in transport has not really spread beyond Infinera so far.”

The PSS-24x showing the 24, 400 gigabit line cards and 3 switch fabric cards, 2 that are used and one for redundancy. Source: Nokia

PSS-24x OTN switch

Nokia has also unveiled its latest 28nm CMOS Transport Switch Engine, a 2.4-terabit non-blocking OTN switch chip that is central to its latest PSS-24x switch platform. Two such chips are used on a fabric card to achieve 4.8 terabits, and three such cards are used in the PSS-24x, two active cards and a third for redundancy. The result is 9.6 terabits of switching capacity instead of the current platforms' 4 terabits, while power consumption is halved.

Nokia says it already has a roadmap to 48-terabits of switching capacity. “The current generation [24x] shipping in just a few months is 400-gigabit per slot,” says Hollasch. The 24 slots that fit within the half chassis results in 9.6 terabits of switching capacity. However, Nokia's platform roadmap will achieve 1 terabit-per-slot by 2018-19. The backplane is already designed to support such higher speeds, says Hollasch. This would enable 24 terabits of switching capacity per shelf and with two shelves in a bay, a total switching capacity of 48 terabits.

The transport switch engine chip switches OTN only. It is not designed as a packet and OTN switch. “A cell-based agnostic switching architecture comes with a power and density penalty,” explains Hollasch, adding that customers prefer the lowest possible power consumption and highest possible density.

The result is a centralised OTN switch fabric with line-card packet switching. Nokia will introduce packet switching line cards next year that will support 300 gigabit per card. Two such cards will be ‘pair-able’ to boost capacity to 600 gigabit but Hollasch stresses that the PSS-24x will not switch packets through its central fabric.

Doubling capacity with the L-band

By extending the 1830 PSS platform to include the L-band, up to 70 terabits of data can be supported on a fibre, says Hollasch.

Nokia has developed a line card that supports both C-band and L-band amplification that will be available around the fourth quarter of this year. The ROADM and 500-gigabit muxponder card for the L-band will be launched in 2017.

Once the amplification is available, operators can start future-proofing their networks. Then when the L-band ROADMs and muxponder cards become available, operators can pay as they grow; extending wavelengths into the L-band, once all 96 channels of the C-band are used, says Hollasch.