Have you ever encountered the problem that the link cannot be up when the 1310nm optical module is transmitted on G.655 optical fiber? I encountered this kind of problem recently, and now I want to share it with you.

Failure symptom

Module： 100G QSFP28 1310nm 80km optical transceiver

Switch： Cisco NCS540

Optical fiber type:  G.655, distance 50km

Failure symptom:  Transmission link cannot go up.

Analysis:

1.Change another switch, the link continues flap

2.Use G.652 optical fiber test in lab, Link works ok. Optical module Tx power and Rx power are in spec. That means optical transceivers performance are ok.

3.DWDM optical modules work well on G.655 link，that means G.655 link no break.   

4.Deduce：The optical module wavelength and fiber type don’t match

Root Cause:

Optical modules with a working wavelength of 1310nm cannot use G.655 optical fiber for long-distance transmission, G.655 Optical cable cutoff wavelength ≤ 1480nm, work window is C band and L band (1530nm~1625nm) , which means if wavelength smaller than 1480nm，cannot use on  G.655 

Summary:

1. G.655 optical fiber cannot transmit 1310 nm signals because the cut-off wavelength of G.655 optical fiber is 1480 nm.

2. If the input power and receive power are normal but the link cannot be up, it is usually because the optical module type does not match the fiber type.