Moving to optical transmission speeds beyond 100 Gbit/s -- whether at 400 Gbit/s or 1 Tbit/s -- will require ROADMs with a channel width wider than the ITU standard 50GHz grid.
Since operators want to use such lightpaths alongside existing 40Gbit/s and 100Gbit/s traffic, the question is -- how to accommodate such wider channel widths alongside the existing 50GHz and 100GHz ITU grid without wasting fiber spectrum?
And what a question...
The answer, of course, is to go off the grid -- in other words, go gridless!
Operators and vendors are exploring the use of finer-increment channels that can be deployed to create intermediate channel widths such as 75MHz (using 25GHz increments), or even allowing adaptive widths that disregard the ITU grid altogether. Such ROADM capability is referred to as gridless.