Multimode — the OM ladder
ISO/IEC 11801 grades multimode fiber by bandwidth: OM1 (62.5 µm, legacy, orange), OM2 (50 µm, orange), OM3 (50 µm laser-optimized, aqua, 300 m at 10G), OM4 (aqua/violet, 400–550 m at 10G, standard for 40/100G parallel optics) and OM5 (lime green, adds shortwave WDM for multi-wavelength 850–953 nm transmission). New installs should never specify below OM4.
Single-mode — the OS and G.65x world
OS2 cabling uses ITU-T G.652.D fiber — the global workhorse. G.657 adds bend-insensitivity: class A1 tolerates 10 mm radius, A2 7.5 mm, B3 down to 5 mm — essential for FTTH drops, wall boxes and crowded closures. Specialty types you should recognize: G.655 NZDSF for legacy DWDM long-haul, G.654 ultra-low-loss for submarine and backbone.
Choosing in practice
Distance and data rate decide: inside a rack or hall, multimode with VCSELs is economical; between buildings, across cities or in any PON, single-mode always. When in doubt specify single-mode — the fiber is cheap; the electronics set the cost, and glass in the ground must survive 25+ years of upgrades.
Compatibility rules
Never mix 50 and 62.5 µm in one channel. Never connect multimode equipment to single-mode plant without conditioning cords designed for it. And record the exact fiber class in the as-built documents — the next technician depends on it.