In the following sections, we shall discuss a variety of field and laboratory measurements on telecommunication fibers, largely excluding sources, detectors and electronics. Certain of these measurements, especially bandwidth and attenuation, will often yield different results for different launching conditions. Therefore, there has been no little attempt to determine a set of launching conditions that will give substantially the same results among laboratories as well as give representative results when compared to field measurements. Because the fibers used for telecommunications tend to be comparatively long, say 1 to 5 km, the industry has largely agreed to make certain measurements using either the 70–70 condition or some sort of equilibrium mode simulator.