Difference between TWDM and DWDM

Due to increasing amount of applications that require high bandwidth and high speed, standardization currently being conducted for future PON, named next generation Optical access network State 2 (Next Generation Passive Optical Network 2, NG-PON2). The main requirements for NG-PON2 are: support aggregate rate all the ONUs of 40 Gb/s of internet support of mobile devices (mobile backhaul), offer service of 1 Gb/s or more to users, 40 miles of range, 64 units of ONUs and 40 miles of range differential The Group of full service access network (The Full Service Access Network FSAN Group) considered many architectures of networks to meet the requirements of NG-PON2. Among them, were studied: the Division Multiplexing PON wavelength (Wavelength Division Multiplexing PON, WDM-PON), the Orthogonal Frequency Division Multiplexing PON (Orthogonal Frequency Division Multiplexing PON, OFDM-PON), and multiplexing by wavelength-division and Time PON (Time and Wavelength Division Multiplexed PON, TWDM-PON).
I understand now what’s more TWDM now is as follows for example in this article attached it uses DWDM and CWDM in the downstream and upstream respectively, what would be the difference in the case?

now is as follows for example in this article attached it uses DWDM and CWDM in the downstream and upstream respectively, what would be the difference in the case?

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.