Most of the fiber currently installed in data centers and outside plants was tested and characterized for 10 Gbit/s signals—which was considered high speed at the time. Now the same fiber plants are expected to carry 100 Gbit/s, and possibly 400 Gbit/s, which poses a problem in many cases. These higher-speed signals are more susceptible to fiber impairments such as reflectance and insertion loss, which may not have been noticeable at 10 Gbit/s—especially for intra-data center applications.
Insertion loss and optical return loss are typically measured with an optical loss test set (OLTS), while reflectance requires an optical time-domain reflectometer (OTDR). Though OTDR tests provide more detailed information (including reflectance), they typically take 15–35 seconds wheras an OLTS test can be done in under 5 seconds. In large fiber count situations, like a data center upgrade, this difference can mean days on an overall schedule.
Choosing the right tests is a matter of managing risk—i.e., thoroughness of testing versus time to complete.