Cause why VOAs for main optical paths of different directions cannot be configured on the same VA4 board in a coherent system

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This rule prevents signals of multiple optical paths from being concurrently interrupted in case of a VA4 board fault.

Other related questions:
Cause why the VOA embedded on an OA board on the main optical path of a coherent system must be configured on the OUT port of the board
In a coherent system, the VOA embedded on an OA board must be configured on the OUT port of the board. This position change helps adjust the incident optical power and also helps adjust the input optical power of the receive-end OA board in the downstream direction. This requirement decreases non-linear effect and helps improve the system performance.

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.

Differences in commissioning a non-coherent system and a coherent system
The main difference in commissioning a non-coherent system and a coherent system is the position of the transmit-end EVOA. In a coherent system, the transmit-end EVOA is always located after the OA board. In a non-coherent system, the transmit-end EVOA is located after the OA board only in the non-standard fiber access mode.

Cause why TD20/TM20 boards are used at an ROADM site in a coherent system
The OSN 8800 uses the frequency selection technology of coherent boards and works with ROADM boards such as TM20 and TD20 to implement the ROADM solution with a simple structure, greatly reducing the costs for constructing an optical-layer ASON network. For example, when the TN11TM201 and TN12TD201 boards are used, up to 20 colorless wavelengths can be added and dropped, greatly saving board and slot resources.

Difference in the CD and PMD processing mechanism between a coherent system and a non-coherent system
100G/40G ePDM-BPSK systems are coherent systems. They use DSP chips for coherent detection, delivering superior performance in mitigating dispersion. Therefore, no DCM is required in these systems for dispersion compensation. For 40G DQPSK systems and other non-coherent systems, DCMs are required for dispersion compensation. The DCU board can also be used on the line.

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