  Optical Spectrum as a Service (OSaaS) spanning over multiple transparent
optical network domains, can significantly reduce the investment and
operational costs of the end-to-end service. Based on the black-link approach,
these services are empowered by reconfigurable transceivers and the emerging
disaggregation trend in optical transport networks. This work investigates the
accuracy aspects of the channel probing method used in Generalized Signal to
Noise Ratio (GSNR)-based OSaaS characterization in terrestrial brownfield
systems. OSaaS service margins to accommodate impacts from enabling neighboring
channels and end-of-life channel loads are experimentally derived in a
systematic lab study carried out in the Open Ireland testbed. The applicability
of the lab-derived margins is then verified in the HEAnet production network
using a 400 GHz wide OSaaS. Finally, the probing accuracy is tested by
depleting the GSNR margin through power adjustments utilizing the same 400 GHz
OSaaS in the HEAnet live network. A minimum of 0.92 dB and 1.46 dB of service
margin allocation is recommended to accommodate the impacts of enabling
neighboring channels and end-of-life channel loads. Further 0.6 dB of GSNR
margin should be allocated to compensate for probing inaccuracies.
