"spacing" definition in design_bands in ROADM

[tttlz]

In multiband_example_network.json topology example, spacing is used in design_bands for ROADM.

Could you please explain how this spacing is defined?
My understanding is, flexible grid ROADM should support flex spacing. The slot width value is defined in spectrum.json file (when using spectrum.json to replace SI in eqpt_config.json). Why do we need to include "spacing" in ROADM?

Spacing is defined as optional in YANG model, However, when spacing is not defined in topology json input file, Throw an error like this:

  File "xxx\gnpy\core\network.py", line 1363, in set_per_degree_design_band
    common_range = find_common_range(amp_bands, None, None, equipment['SI']['default'].spacing, node.design_bands)
  File "xxx\gnpy\core\utils.py", line 744, in find_common_range
    spacing = calculate_spacing(first, second, default_spacing, default_design_bands, f_min, f_max)
  File "xxx\gnpy\core\utils.py", line 684, in calculate_spacing
    temp = get_spacing_from_band(default_design_bands, f_min, f_max)
  File "xxx\gnpy\core\utils.py", line 812, in get_spacing_from_band
    return band['spacing']
KeyError: 'spacing'

Thank you

[EstherLerouzic]

design bands serves as reference to design the optical line starting from this node (ROADM typically). This is used to determine the maximum amount of power to be supported by amplifiers in this OMS. By default the values for equipment_config/SI are used. The user can specify different values perOMS thanks to this parameter, for example to model a particular link in the network that would be designed for 48 channels instead of 96 (reference channels), by setting the spacing to 100GHz instead of 50GHz. Links with less channel number can support more per channel power and still reach the optimum per channel power for lossy spans (eg > 30dB loss).
So in short if you want to define this per_degree_design_band, then you need to define the spacing to properly design the link.

But yes, you are right spacing is optional in yang, but should be mandatory if the container exists, or we should use the default value from SI

[tttlz]

Thank you. The explanation is very helpful.
Do you think it will be better to define parameter-used-for-design as mandatory if design-bands exist?