There is herein disclosed a method of communication between a client pair, the client pair including an upstream client and a downstream client, the method being performed over a first optical fiber in an optical fiber pair and including: transmitting a downstream DWDM signal over the first optical fiber, wherein the downstream DWDM signal has a first frequency and contains first data originating from the upstream client, transmitting an upstream DWDM signal over the first optical fiber, wherein the upstream DWDM signal has a second frequency and contains second data originating from the downstream client, wherein the first frequency is different to the second frequency, and performing Quantum Key Distribution over the second optical fiber of the optical fiber pair.