![]() Optical circuits are dynamically created and deleted and this dynamic nature can unintentionally create spectrum fragmentation.įor example, the latest generation of transponders are using 90 Gbaud technology. The emergence of unusable stranded spectrum can actually occur over time. The presence of small slices of unusable spectrum is commonly referred to as “spectrum fragmentation”. An operator should be careful in ensuring that they are not leaving unassigned spectrum components between wavelengths that are not at least 50 GHz. Wavelengths typically require at least a 50 GHz spectral width in order to provide a minimum of 100 Gb/s capacity over a typical metro DWDM network. If channel assignment is not properly planned, it can lead to stranded spectrum within the optical band. ![]() The flexibility of Flex-grid offers a great benefit but it can also be a bit of a double edge sword. double-check the numbers) should be taken to heart. While the ROADM software typically manages spectrum allocation and automatically prevents overlaps, Flex-grid allows for manual spectrum planning so the age-old maxim of “Measure twice, cut once” (i.e. Perhaps the most basic rule is that adjacent channels cannot overlap.The ROADM will then reserve this channel and remove this space from its “spectrum inventory”. Defining a channel is as simple as specifying a center frequency and a spectral width.ROADM systems are being provided with wavelengths of various sizes – all of them wider than 50 GHz … starting with 75 GHz for the last few years and wider wavelengths are on the horizon … What happens when you try to transport a 75 GHz wide wavelength through an older 50 GHz fixed grid system? The boxes are no longer fixed, but may be different sizes. Now, this simple arrangement is being disrupted by the emergence of optical transponders using different baud rates. All the boxes are the same standard size and therefore, the cargo space in the delivery van can be divided into fixed cubbyholes to accommodate boxes of the same size. To reuse the parcel delivery analogy of the previous blog, this is analogous to a van delivering parcels where every parcel is packed using a single box size. Optical spectrum is like cubbyholes, and wavelengths are boxes This was perfectly appropriate for earlier generation of transponders where every wavelength had the same baud rate – each wavelength could neatly fit into one space in the C-band spectrum row of cubbyholes. The ROADM system carved the optical band into 88 or 96 neatly spaced 50 GHz “cubbyholes” where each wavelength could be mapped. This lies in the fact that the first few generations of ROADM systems were built around a fixed grid system where every wavelength was assigned a spectral width of 50 GHz. The increase in spectral width can, in some cases (such as for older ROADM systems), become an obstacle to the successful transmission of the wavelength across the network. ![]() One drawback of this approach is the fact that the spectral width of the wavelength increases from 50 GHz to either 75 GHz, 100 GHz or even 150 GHz depending on the selected baud rate. ![]() My previous blog discussed the advantage of increasing the baud rate of a wavelength in order to increase transport capacity without sacrificing reachable distance. In this blog, I explain the “G” in CDC-G – Gridless – and the operational benefits it provides. In this series of tutorials, I explain the key functionality (and their benefits) that underpin next-generation ROADMs: Colorless, Directionless, Contentionless, and Gridless (CDC-G) functionality.
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