Wednesday, 06 July 2022

The importance of submarine cable channel planning

posted by Brian Lavallée Senior Director, Solutions Marketing, Ciena
Wednesday 18 May 22

As submarine cables evolve to take advantage of new innovations, complexities in the channel planning process are introduced. Ciena’s Submarine Expert Brian Lavallée explains how we got here and options for simplifying this critical process.

Different submarine cable capacity consumption models When bandwidth demands were comparatively low (compared to today) in the pre-ICP days, sub-lambda services were commonly sold carried by 5Gb/s to 10Gbs wavelengths. As bandwidth demands surged with the growth of the Internet, wavelengths became the currency of network services where end-users purchased entire wavelengths typically operating at 10Gb/s or 100Gb/s rates, still commonly used today.   For end-users with massive capacity demands, such as ICPs, entire fiber pairs were purchased from wholesalers. For routes where an entire fiber pair was unavailable for purchase or for end-users needing more than a few wavelengths but less than an entire fiber pair…

Different submarine cable capacity consumption models

When bandwidth demands were comparatively low (compared to today) in the pre-ICP days, sub-lambda services were commonly sold carried by 5Gb/s to 10Gbs wavelengths. As bandwidth demands surged with the growth of the Internet, wavelengths became the currency of network services where end-users purchased entire wavelengths typically operating at 10Gb/s or 100Gb/s rates, still commonly used today.
 
For end-users with massive capacity demands, such as ICPs, entire fiber pairs were purchased from wholesalers. For routes where an entire fiber pair was unavailable for purchase or for end-users needing more than a few wavelengths but less than an entire fiber pair, optical spectrum is available. In this acquired spectrum, services are managed by the submarine cable operator or end-users themselves. What network services are available on a submarine route will depend on the cables present and the associated portfolio of services offered by wholesalers along the route.
 
Evolving from fixed grid submarine cables
 
Historically, optical submarine networks used a “fixed grid” spacing that dictated channel placements. The available optical bandwidth of the submarine cable was split (initially) based on fixed filter technology limitations where each channel had to match its allocated spectrum slot, which also dictated how much capacity the channel could carry.
 
With the advent of DWDM, the definition of specific channel slots within the available bandwidth was extended to grids based on 12.5 GHz, defined in the ITU-T G.694.1 “Spectral Grids for WDM Applications: DWDM Frequency Grid” Recommendation. This frequency grid, anchored to 193.1 THz, supports a variety of fixed channel spacings ranging from 12.5 GHz to 100 GHz. Smaller channel grid spacing means more channels can be packed into available spectrum, although modem technology and the Shannon Limit ultimately dictate both channel and aggregate cable capacity.
 
The ITU-T G.694.1 Recommendation was enhanced to include a ‘flexible grid’ allowing mixed bit rate or mixed modulation format transmission systems to allocate frequency slots of different spectral widths. For example, a mix 37.5 GHz and 50 GHz channels could be allocated anywhere in the allowable optical spectrum. This allowed for cable operators to use newer Submarine Line Terminal Equipment (SLTE) modems offering new modulation schemes.
 
Early flexible grid systems, actually just fixed grid systems with a new grid, were well-suited to modems based on legacy Intensity-Modulation Direct Detection (IMDD) modems leveraging fixed Multiplexer/Demultiplexer (MUX/DEMUX) structures, which resulted in relatively simple channel planning. However, it was still limited to a specific frequency and a slot width granularity of 12.5 GHz, which quickly became unsuitable once coherent optical detection technology was introduced into newer SLTE modems.
 
Coherent modems changed everything
 
Coherent modems are tunable at the receiver to the desired frequency while rejecting other signals in the electrical domain, without requiring optical filtering. This enables optical SLTE MUX/DEMUX infrastructure to evolve to “gridless” technology allowing modems operating at different bauds (symbols per second) to coexist within the same MUX/DEMUX infrastructure. Once the ability to manage any baud was enabled, modem development sought to maximize baud with each generation to increase the line rate while reducing the cost per bit. Modern coherent modems have selectable baud to improve total capacity or spectral efficiency and further reduce the cost per bit.
 
More options, more complexity
 
While the move to flexible-grid architectures with increasingly higher bauds provides higher line rates, improved spectral efficiency, and a lower cost per bit, it also introduces significantly increased channel planning complexity within available submarine cable optical spectrum. Given that fixed grid channel planning was relatively simple, it could and was often performed manually using spreadsheets. However, flexible grid technology introduces complexities in channel planning related to varying spectral channel occupancy requirements from different modems and filter impact rules based on MUX structure and photonic equipment.
 
The recent introduction of higher fiber count Spatial Division Multiplexing (SDM) cables accelerates channel planning challenges because the number of fiber pairs goes from 4 to 8 on traditional cables to 16 to 24, and even higher, modern SDM cables. Subsequently, channel planning on modern submarine cables requires far more sophisticated tools to maximize spectral efficiency and validate rules imposed by the SLTE, as well as faster time-to-market for new submarine network services, internally and externally.
 
Want to know more?
 
We’re hosting our pioneering “Ciena’s Vision of Submarine Networking” workshop on Wednesday, May 18, from 2:00PM to 5:00PM in the Business Design Center as part of Submarine Networks EMEA, where you can learn about other hot submarine network topics.
 
If you’re interested in this free workshop, space is limited, so please register here to reserve your spot.
 

Since you're here...

...the Telecoms industry is characterised by constant change and evolution. That's why it's crucial for telecoms professionals to keep up-to-date with what is happening. Join 35,000+ of your peers and sign up to our free newsletter service today, to be in the know about what is going on. PLUS, as a member you can submit your own press releases!

See all membership options

Please enable JavaScript to view the comments powered by Disqus.

Newsletter signup

Quickly get on board and up to date with the telecoms industry