When Fiber “Works” — But Isn’t Installed Correctly

Today on a campus fiber job, we encountered a configuration that was passing traffic but was not engineered according to fiber standards.

The backbone fiber between buildings was single-mode, but the patch cords connecting the fiber panel to the switch were OM3 multimode jumpers.

Single-mode and multimode fiber should not be mixed. However, in some cases, the link can still appear to work.

Here’s why: if the switch is using single-mode optics (1310 nm), light can pass through short OM3 patch cords over very short distances. This can make the connection seem stable.

That does not mean the installation is correct.

OM3 fiber is designed for 850 nm, not 1310 nm. Using multimode jumpers in a single-mode link introduces optical mismatch and reduces signal margin. Over time, this can lead to intermittent issues, packet loss, or failures—especially when equipment is upgraded, optics are replaced, or network speeds increase.

We explained the condition to the client, confirmed whether they were experiencing issues, and documented it so it can be corrected properly.

Fiber infrastructure should be designed for long-term reliability, not just something that “works today.”

#FiberOptic #SingleModeFiber #NetworkInfrastructure #StructuredCabling #CampusNetworks #CommercialIT