The Grading System Explained
Fiber optic cable grades exist because not all fiber performs equally at high data rates. The grade determines how much modal dispersion the cable introduces — and modal dispersion sets the maximum distance before signal integrity degrades enough to cause errors. Selecting the wrong grade for a high-speed run results in a link that either does not come up at all, or comes up but with elevated bit error rates that cause retransmissions and degraded throughput.
The grades split into two families: OS (Optical Single-mode) grades for single-mode fiber, and OM (Optical Multimode) grades for multimode fiber. Single-mode and multimode are not interchangeable — the cores are physically different diameters (8–10 µm for single-mode, 50 µm for multimode), and the transceivers are designed for one or the other.
Multimode Grades: OM1 through OM5
| Grade | Core | Jacket Color | 10G Max Distance | 40G Max Distance | 100G Max Distance | Status |
|---|---|---|---|---|---|---|
| OM1 | 62.5 µm | Orange | 33 m | Not supported | Not supported | Legacy — do not install new |
| OM2 | 50 µm | Orange | 82 m | Not supported | Not supported | Legacy — do not install new |
| OM3 | 50 µm | Aqua | 300 m | 100 m | 70 m | Minimum for new installations |
| OM4 | 50 µm | Aqua or Erika Violet | 400 m | 150 m | 150 m | Recommended for new structured cabling |
| OM5 | 50 µm | Lime green | 400 m | 150 m | 150 m (400G with SWDM4) | Current for high-density or future 400G |
OM1 and OM2 are 62.5 µm and 50 µm legacy grades respectively, both with orange jackets. They cannot support 10G at useful distances and should never appear in new installations. OM3 is the current floor for new work — it handles 10G across a building and 100G for shorter inter-rack runs. OM4 is only marginally more expensive and adds meaningful headroom at 40G and 100G. OM5 uses wider-bandwidth fiber optimized for SWDM4 (Short Wavelength Division Multiplexing), which carries four 100G channels over a single fiber pair for a total of 400G — relevant for high-density data center backbone upgrades where pulling new fiber is expensive.
Single-Mode Grades: OS1 and OS2
| Grade | ITU Standard | Jacket Color | Max Attenuation | Construction | Primary Use |
|---|---|---|---|---|---|
| OS1 | G.652 | Yellow | 1.0 dB/km | Tight-buffered | Indoor runs, conduit, cable tray |
| OS2 | G.652D | Yellow | 0.4 dB/km | Loose-tube | Outdoor, direct-burial, long-haul |
OS1 and OS2 have the same 8–10 µm core and are optically compatible — transceivers designed for OS1 work with OS2 and vice versa. The difference is cable construction and attenuation specification. OS2's lower attenuation (0.4 dB/km vs 1.0 dB/km) and loose-tube construction make it the correct choice for long outdoor or underground runs where every decibel of optical power budget matters. For short indoor patch cords and equipment-room runs, OS1 is fine and typically less expensive. Both support distances from 2 km (1G) to 40 km (10G LR) to 80 km (10G ER) depending on transceiver type.
Mixing Grades
Within the same fiber family, grade mixing is physically possible but degrades the link to the weaker grade. If an OM3 patch cord is used at each end of an OM4 trunk, the effective performance across the total run is limited by the OM3 sections. In practice, mixed-grade runs are common in existing buildings where some cables are OM3 and newer additions are OM4 — calculate the total link loss budget using the worst-grade segment's specifications to determine if the link will pass.
Between families (single-mode and multimode), mixing is not viable. A single-mode transceiver's narrow DFB laser will overfill a 50 µm multimode core, and a multimode VCSEL cannot efficiently couple into a 9 µm single-mode core. If you inherit a mix of yellow and aqua cables, do not attempt to bridge them with an adapter.
Frequently Asked Questions
Can I use OM3 cable with OM4 transceivers?
Yes — OM3 and OM4 transceivers are physically identical (same LC connector, same VCSEL wavelength at 850 nm). An OM4-rated SFP+ SR module plugged into OM3 cable will link up, but the maximum reliable distance drops to the OM3 limit for that data rate (300 m at 10G, 100 m at 40G, 70 m at 100G). The module does not know which grade of cable it is driving — it will attempt to operate and the link either works or fails based on actual measured loss. For runs well within OM3 limits, this combination is perfectly fine.
Why does my ISP use yellow (single-mode) fiber to my building but the equipment room uses aqua (multimode)?
ISPs use single-mode fiber for outside plant because the runs are measured in kilometers, where multimode would fail entirely. Inside a building, multimode fiber is preferred for runs under 300–400 meters because multimode transceivers are significantly cheaper than equivalent single-mode modules — especially at 10G and above. A media converter or optical demarcation point at the building entry converts between the two fiber types. This is normal and expected; the inside fiber does not need to match the outside fiber type as long as the conversion happens at the demarcation point.
Is OM5 worth the cost for a new office cabling project?
For most office environments, no — OM4 is the correct choice. OM5's advantage (SWDM4 for 400G over a single fiber pair) is relevant only when you need 400G backbone speeds and cannot pull additional cables. At current switch pricing, 400G infrastructure is a data center concern, not a typical office concern. OM4 supports 100G at 150 meters, which is sufficient for any building-scale network for at least the next several years. Install OM5 only if your organization explicitly expects to deploy 400G networking within the lifecycle of the cabling plant and fiber count is constrained.