DWDM and the ITU grid

DWDM channels are defined by frequency on the ITU-T G.694.1 grid, anchored at 193.1 THz. Working in frequency keeps the spacing exact regardless of the small wavelength conversion.

Spacing and channel count

Spacing	Approx. wavelength step	C-band channels (approx.)
100 GHz	0.8 nm	~40
50 GHz	0.4 nm	~80
25 GHz	0.2 nm	~160

Tighter spacing fits more channels but demands more precise, more expensive optics.

Channel numbering

Channels are numbered relative to the 193.1 THz anchor. On the 100 GHz grid a channel's frequency is:

f = 193.1 THz + n × 0.1 THz     (n = …, -1, 0, +1, …)
wavelength ≈ c / f

So channel spacing in frequency is uniform, while the equivalent wavelength step is only approximately constant.

Tunable modules

A tunable DWDM transceiver selects its channel through the management interface. CodingBox can set the channel number and wavelength for tunable DWDM modules and read the current setting back from module memory.