  We propose an approach to estimate the end-to-end GSNR accurately in a short
time when a data center interconnect (DCI) network operator receives a service
request from users, not by measuring the GSNR at the operational route and
wavelength for the End-End optical path but by simply applying a QoT probe
channel link by link, at a convenient wavelength/modulation-format for
measurement. Assuming connections between coherent transceivers of various
frequency ranges, modulators, and modulation formats, we propose a new device
software architecture in which the DCI network operator optimizes the
transmission mode between user transceivers with high accuracy using only
standard parameters such as Bit Error Rate. In this paper, we first
experimentally built three different routes of 32 km/72 km/122 km in the C-band
to confirm the accuracy of this approach. For the operational end-to-end GSNR
measurements, the accuracy estimated from the sum of the measurements for each
link was 0.6 dB, and the wavelength-dependent error was about 0.2 dB. Then,
using field fibers deployed in the NSF COSMOS testbed (deployed in an urban
area), a Linux-based transmission device software architecture, and coherent
transceivers with different optical frequency ranges, modulators, and
modulation formats, the fast WDM provisioning of an optical path was completed
within 6 minutes (with a Q-factor error of about 0.7 dB).
