From what I understand DWDM is characterized by using separation between channels rather small, usually of 0.8nm, and to place all its channels of optical amplifier operation (EDFA), iso is, between 1560nm and 1620nm. It should be used then to the downstream direction. While the CWDM using separation of 20nm channels and does not restrict the EDFA window, which relaxes the tolerance of the components and reduces system costs more also reduces their transmission capacity. CWDM should be used in the upstream direction.