A. fiber failure between Sites A and D
B. EDFA failure at Site A （clockwise direction）
C. EDFA failure at Site C （clockwise direction）
D. 15216 multiplexer at Site A （counter-clockwise direction）
E. OADM pass-through （West "OUT"） failure at site D （clockwise）
F. wavelengths 1 and 3 transmitter failures at Site A （clockwise direction）

A. All "Add" channels require uniform power levels.
B. The flexibility of wavelengths per OADM is limited.
C. It is easier to balance "Pass-Through" and "Add" channels.
D. There is lower pass-through loss using multi-channel OADMs.
E. You can extract a single channel without dropping other channels.