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.

Images of different submarine cable network services

Figure 1: Different submarine cable network services

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.

Channel planning, what you need to know

If you’d like to learn more about key applications and considerations associated with channel planning, especially as you migrate from traditional “fixed grid” to modern “flexible grid” architectures, check out the updated Open Submarine Cable Handbook Bundle where we added a new Open Submarine Cable Channel Planning Handbook that covers what you need to know before lighting up new channels. By understanding the key considerations related to flexible grid channel planning, you’ll maximize wet plant bandwidth utilization and its return on investment.