OS1a: Single-mode fiber type 1 dB/km attenuation at 1310, 1383, and 1550 nm. It is designed to address utilisation of OS1a and OS2 with OS1a being better suited for indoor use and OS2 for outdoor (underground). This is brought about in part by the effect on the glass in the construction process with tight buffering of the fiber cores producing a higher attenuation within a constructed fiber cable.