The super coherent drivetrain - balancing capacity and reach to capture the opportunity in every fiber
By offering multiple modulation formats, adjustable baud rate and other advanced techniques, next-gen coherent digital signal processors let Communication Service Providers (CSPs) balance capacity and reach to capture the opportunity in every fiber of their network, no matter the length.
Moore vs. Shannon
Communication Service Providers (CSPs) are looking to maximize the capacity of every fiber in their network. Yet fiber capacity is limited. The Shannon limit dictates a maximum spectral efficiency, and hence capacity, for fiber spans of a given length.
In order to approach the finite capacity of every fiber in an optical network regardless of length, optical interfaces need the ability to optimally trade off capacity and reach within one fully programmable piece of hardware. This is what we as an industry are working on. But how do we get there?
By riding the wave of Moore’s Law.
As silicon processes advance from one generation to the next, numerous benefits are realized. Gate size and power decrease, allowing more processing functions within a single chip. Simultaneously, gate switching speed goes up, allowing faster processing of complex digital functions.
These improvements in silicon translate directly to the capability of the coherent digital signal processors (DSPs) that drive DWDM wavelengths – which ultimately determines the capacity of a fiber. Increased processing speed enables higher baud rates. Greater integration enables higher-order modulation formats, as well as the ability to support multiple different modulation formats and baud rates within the same DSP.
It’s like riding a bike …
In a DWDM wavelength, the modulation format determines how many bits are encoded per analog symbol. The baud rate determines how rapidly these symbols are transmitted. The different modes of operation supported by a state-of-the-art coherent DSP result from unique combinations of baud rate and modulation format. They work together to produce the desired transmission performance over varying fiber impairments, trading off capacity and distance.
In many ways, this is analogous to the drivetrain of a multi-geared bicycle. The rear wheel cogs are analogous to multiple modulation formats, whereas the front cogs can be thought of as the baud rates supported by a modern DSP. It’s the unique combinations of front and rear cog that determine the gear ratios available. The highest gears give you maximum speed downhill, but when the road turns upward you shift to a lower gear, sacrificing speed in order to make it up the hill. Multiple options in between allow you to shift gears as needed to maximize your performance over any kind of terrain.
In an optical network, when distances are short, spectral efficiency (and thus capacity) can be maximized. But when the distance needing to be traversed is long, spectral efficiency must be sacrificed in order to successfully transmit an unregenerated signal. Just as gravity constantly fights the cyclist climbing a hill, the Shannon limit looms as an ever-present ceiling on the spectral efficiency of DWDM wavelengths. And like a bicycle drivetrain, the ability to flexibly trade off capacity and reach depends on having a sufficient number of combinations from which to choose. This is only possible with modern programmable digital signal processors.
First generation DSPs offered one modulation format and operated at one baud rate – resulting in a single “speed”. Today’s second-generation programmable coherent DSPs typically offer three modulation formats operating at one baud rate – resulting in a choice of three “gears”. With state-of-the-art super coherent DSPs, CSPs can take advantage of five modulation formats and now two baud rates: both today’s standard 35 Gbaud and a new 45 Gbaud rate. This gives them seven modes with which to flexibly optimize capacity and reach for every fiber path in the network.
State-of-the-art super coherent DSPs bring more transport options
Multi-modulation with adjustable baud rate
As discussed above, the capacity of a DWDM wavelength is determined by its modulation format and baud rate. By encoding more bits into one analog symbol, higher order modulation formats increase capacity, but compromise on reach — they are limited to metro and regional applications. Lower order modulations encode fewer bits to achieve long-haul reach, but at reduced capacity.
Multiple baud rates provide another parameter with which modern super coherent DSPs can adjust the capacity of a DWDM wavelength, yet unlike high order modulation higher baud rates do not necessarily reduce transmission reach. Higher baud rates combined with a lower-order modulation format can increase channel capacity without sacrificing distance. Alternatively, higher baud rates can be used to increase distance without sacrificing channel capacity. In both cases, an adjustable baud rate delivers a new dimension of fiber capacity optimization that is not possible with adjustable modulation alone. The result is a DSP that can deliver a continuum of operational modes that allow CSPs to maximize the capacity of every wavelength in their networks, regardless of fiber distance. Ultimately, this lowers the cost per bit per kilometer while allowing one device to cost effectively address many applications. This simplifies deployment, and transforms what have traditionally been static interfaces into the foundation of a dynamic programmable wavelength fabric.
Seven modes to balance capacity and reach
Doubling metro and regional capacity
The new wavelength capabilities enabled by state-of-the-art coherent DSPs can lower the cost, increase the capacity, and impact the architecture of new and existing optical networks.
With a new 400G single-carrier wavelength offering the highest available capacity per fiber, CSPs can double their metro capacity. This is essential for meeting rapidly increasing bandwidth demands — especially for the cloud data centers and co-location facilities required by large business customers and internet content providers.
Modern super coherent DSPs also enable 500G superchannels to be built from just two optical carriers. This lets CSPs address both metro and regional distances by combining leading spectral efficiency and 100G client port density.
For long haul transport, CSPs can now nearly double the unregenerated distance of single carrier 100G wavelengths to support ultra-long haul and sub-sea links of greater than 5,000 kilometers. And for the first time unregenerated 200G wavelengths can reach up to 2,000 kilometers, doubling long haul capacity.
New wavelengths are changing how optical networks are built
Expect even more wavelength flexibility and performance
As DSP silicon advances from this generation to the next, CSPs can expect increasing wavelength flexibility and performance. As gate size decreases and more processing can fit onto a chip, speed will climb, allowing higher baud rates and more sophisticated super coherent modulation formats. And as gate power draw goes down, chips can continue to integrate more functionality, or they can be made smaller and more efficient for targeted low-power and low-cost applications. The Shannon Limit will ultimately impose absolute limits on the capacity a DSP can squeeze out of a fiber. But until then, DSP innovations driven by Moore’s law will continue to give CSPs the flexibility to get closer and closer to that limit for the deployment of efficient wavelengths of any distance.