Clarification on strict path constraints and “System: input power (dBm)” in GNPy service sheets

[BandarAl-Sadoon]

Hello GNPy team,

I have two questions about the service-sheet format:

1. Strict path that revisits the same ROADM
   I need a light-path to follow a sequence that passes through the same ROADM twice (for example, Node_A -> Node_B -> Node_A).
   When I enter every ROADM in the “routing: path” column and set “routing: is loose?” to “no”, GNPy rejects the request when the path passes through the same node twice. The message I get is:

"WARNING:gnpy.topology.request:Request 0 could not find a path crossing [] in network topology
CRITICAL:gnpy.topology.request:Request 0 could not find a path with user include node constraints.
No path computed"

– Is there a supported way to express a strict path that legitimately revisits the same ROADM?
– If not, what workaround do you recommend (for example, splitting the request into two segments)?

2. Meaning of “System: input power (dBm)” in the service sheet
   Does this field represent:
   A) the launch power into the fibre span (after any booster EDFA), or
   B) the optical power entering the booster EDFA itself (i.e., the transmitter output)?

Thank you for your help.

[EstherLerouzic]

Dear BandarAl-Sadoon,
Thanks for your interest !

On question 1: GNPY only produces or uses simple paths (paths with no repeated node). So it is not possible to force a path with a repeated node. If your purpose is to look for a ring between two nodes in order to define a path and its protected path beween Node_A and Node_B then my suggestion is to split the request and use the "disjoint from" column with the route id of one of the paths.
I do not see any other use case for repeating nodes in a path for optical networks. Could you explain your case ?

On question 2
The field “System: input power (dBm)” represents the case A: the launch power into the fibre span (after any booster EDFA).

Hope this helps
regards
Esther

[akhil-mutike]

just to clarify....this launch power is only for the reference span(20dB loss) ..right?

[EstherLerouzic]

This depends on the "delta_power_range_db" set in "Span" section of the equipment_config. If this does not define a range (eg [ 0, 0, 0]), then this power will be applied as target at every fiber span input. If instead you define a range (eg [-2, 3, 0.5]) then the input power in each span will depend on the span loss using the formula described by J. -L. Auge, V. Curri, and E. Le Rouzic
in "Open Design for Multi-Vendor Optical Networks," OFC 2019, specifically equation 4.
Note that in v2.13 the span loss ref (default 20dB) and the slope (0.3) can be parametrized in the Span section with span_loss_ref and power_slope key words https://gnpy.readthedocs.io/en/master/json.html#span.

[EstherLerouzic]

The reference span loss can now be tuned in SI section of the library : equipment['Span']['default'].span_loss_ref. default is 20dB