This method combines at least two signals at different wavelengths for transmission along one fiber-optic cable. CWDM uses fewer channels than DWDM, but more than standard WDM. In 2002, a 20-nm channel spacing grid was standardized by ITU (ITU-T G.694.2) ranging from 1270 to 1610 nm. However, since wideband optical amplification is not available, this limits the optical spans to several tens of kilometers.
CWDM is a cost-effective option for increasing mobile backhaul bandwidth, but it does come with some network characterization and deployment challenges, including a limited maximum distance. However, a major advantage is that it can be easily overlaid on existing infrastructure.
The most basic configuration is based on a single fiber pair, where one fiber is used to transmit and the other to receive. This configuration often delivers eight wavelengths from 1471 nm to 1611 nm. However, networks are now deploying in the O-band, which doubles the capacity to 16 wavelengths (1271 nm to 1451 nm), excluding the 1371 nm and 1391 nm water peak wavelengths.
CWDM architecture is only comprised of passive components, namely multiplexers and demultiplexers; no amplifiers are used. This means there is no amplification, and therefore, no noise. The main advantage of this is that there is no need to measure the optical signal-to-noise ratio. Upon activation, barring improper fiber characterization, only the following elements can prevent proper transmission:
- Transmitter failure
- Sudden change in the loss created in an OADM
- Human error (e.g., connection to the wrong port or splicing to the wrong ﬁlter port)
Links can be tested end-to-end and fully characterized with a specialized CWDM OTDR. Thanks to its CWDM-tuned wavelengths, this tool will drop each test wavelength at the corresponding point on the network (e.g., customer premises, cell tower, etc. This means that each part of the network can be characterized at the head-end, which will save time and avoid travelling to hard-to-get-to sites.
Once the wavelength is active, a channel analyzer must be used at the customer premises or cell tower to validate that it is present and that the power level received is within budget. This OTDR and channel analyzer combo is also useful when a single customer is experiencing issues. If the channel analyzer cannot confirm that the channel is present and within power budget, the CWDM OTDR can be used to test at either a specific or out-of-band wavelength to detect issues. The advantage of using an out-of-band wavelength (1650 nm) is that the OADM will ﬁlter it out.