Wavelength Division Multiplexing (WDM) is a multiplexing method which uses different colors (or wavelengths) of light. Where traditional optics allow a single channel of communication on a fiber pair, WDM deployments allow to carry up to 96 channels on a single fiber pair. Channels can support different technologies and different speeds, for example you can mix 1G Ethernet, 10G Ethernet and 4G Fiber Channel on the same fiber run.
Usage
Since it is a multiplexing technology, we combine (mux) the transmission and we split (demux) the reception. With a typical fiber pairs, this is done by a mux/demux passive box as illustrated below.
The optics used in a WDM setup are called colored optics (tuned to a specific wavelength) therefore the color on each end of the fiber must match.
CWDM vs DWDM
WDM exists in two variety: Coarse (CWDM) and Dense (DWDM), each using its own range of wavelengths. Since their wavelength range overlaps, they can co-exist on the same link to some extent.
| Name | Channels | Wavelength | Cost | Use case |
|---|---|---|---|---|
| CWDM | 18 | 1270-1610nm | Low | Short distance |
| DWDM | 96 | 1520-1577nm | High | Medium/long distance |
Reducing interconnect costs
WDM reduces the interconnection costs by using the same fiber pair for many links. Let’s say we want 60Gbps bandwidth (6x10 Gbps) between two point of presence (POP A and B) with dark fiber between them.
To make this a fair example I will assume the following prices:
- Dark fiber: 100 €/mo/fiber pair
- Traditional LR optic 10Gbps SFP+: 30 €
- CWDM optic 10Gbps SFP+: 100 €/unit
- Mux-Demux: 700 €/unit
Traditional approach
We need 6 dark fiber pairs (600 €/mo) and 12 optics (360 €). Costing a total of 21 960 € over 3 years.
CWDM
Since we need 6 channels CWDM (up to 16 channels) over a few hundred meters (different rooms) will work just fine. Of course we will need only 1 fiber pair (100 €/mo), and 12 CWDM optics (1200 €). We will also use 1 Mux-Demux at each location (1400 €).
Yielding a total cost of 6 200 €, 70% less than the traditional setup.
Total Cost of Ownership calculator
You can use the following calculator to estimate the TCO of a WDM deployment based on the number of channels. The example costs are based on online prices as of March 2017.
Caveats
Using WDM requires additional documentation to keep track of which wavelengths are used by which links. Be mindful of the physical layer and do not put redundant links on the same fiber pair.
Conclusion
WDM is a great tool to deliver high speed (up to 960 Gbps) on a single fiber pair and to keep costs down.