# Reason why a coherent system uses fewer OA boards than a non-coherent system

OA boards are used to compensate for insertion loss. Only one OA board is required in case that Gmax (maximum gain) �?Fiber loss + DCM loss; otherwise, two OA boards are required.
The coherent system does not require DCMs, and a single OA board can compensate for larger link loss. When the link loss is within the permitted range, the coherent and non-coherent systems require the same number of OA boards. When the fiber loss is out of the permitted range, the coherent system, however, requires fewer OA boards.
For example, in a system with a 60 km span, 18 dB fiber loss, and 5 dB DCM loss:
The gain required by a coherent system is calculated as follows: Gain = Fiber loss + DCM loss = 18 dB + 0 dB (no DCM) = 18 dB < Gmax. Therefore, one OA board (OAU101) is required. The gain required by a non-coherent system is calculated as follows: Gain = Fiber loss + DCM loss = 18 dB + 5 dB = 23 dB < Gmax. Therefore, one OA board (OAU101) is required.
In a system with a 100 km span, 28 dB fiber loss, and 9 dB DCM loss:
The gain required by a coherent system is calculated as follows: Gain = Fiber loss + DCM loss = 28 dB + 0 dB (no DCM) = 28 dB < Gmax. Therefore, one OA board (OAU101) is required. The gain required by a non-coherent system is calculated as follows: Gain = Fiber loss + DCM loss = 28 dB + 9 dB = 37 dB > Gmax (36 dB for an EDFA board). Therefore, two OA boards (OAU101 and OBU101) are required.