Coarse WDM systems have been in wide use for metropolitan area backhaul and business access since the very beginning of the twenty-first century. They are based on up to 18 CWDM channels spaced 20 nm in the wavelength region 1270-1610 nm, as defined in ITU-T Recommendation G.694.2. In the beginning, CWDM price had the advantage of being cheaper than DWDM price since CWDM components do not requirement temperature control or stabilization. This advantage is expected to decrease since CWDM transceivers are not wideband tunable. DWDM transmitters, on the other hand, are full-band tunable. This allows cost reduction in manufacturing since only a single type of transmitter needs to be produced. It also allows operational cost reductions, since, for example, sparing and also network planning are greatly simplified.
In many cases, CWDM was used for (DSLAN, GPON, wireless 2G/3G) backhaul, running one or multiple GbE services per wavelength. Up to 4GbE signals can be multiplexed at wire speed (after 10B/8B decoding) onto muxponders running at 4.3Gb/s. In CWDM backhaul, this bit rate is relevant because it has similar cost than 2.5 Gb/s. This low cost ws originally driven by transmission of 4Gb/s Fibers Channels(FC) signals. In general DWDM transmission, 4Gb/s per channel is almost irrelevant due to its lack of spectral efficiency. Capacity increase up to 16 x 10 Gb/s has also been demonstrated with CWDM.
Backhaul topologies are often rings, a four-node CWDM ring with a hub node and three optical add-drop multiplexer (OADM) sites. For monitoring purposes, the OADMs can be connected to a network operations center (NOC) via an embedded communication channel (ECC). The ECC can transport narrowband management information.
CWDM systems often use CWDM-colored small form-factor pluggables (SFPs) as remote interfaces. These can be accommodated directly in the client systems, or on transponders/muxponders. Per-channel bit rates of 1.25-4.3 Gb/s are covered (where 4.3 Gb/s share the transceiver technology with 4G Fiber Channel, which is one of the drivers behind the advantageous economics). Ten gigabit per second CWDM-fixed-wavelength extended form-factor pluggables (XFPs) also exist, but are typically restricted to the wavelengths 1470-1610 nm. In effect, these pluggables are versions of the respective DWDM transceivers, and, hence, unlike their lower bit rate counterparts, do not have significant cost advantages. Consequently, their relevant specifications are similar to the DWDM XFP specifications. So far, dedicated CWDM 40 or 100 Gb/s transceivers have not been built, and it is unlikely this will ever happen. For 4 and 10 Gb/s, reduced chromatic dispersion (CD) allowance and also power budget must be considered. This may limit maximum reach, without added means like CD compensation （not commonly used in CWDM systems）, or forward error correction (FEC) to < 60 km.
Typically, in CWDM systems, effects of polarization-mode dispersion (PWD), polarization-dependent loss (PDL), and nonlinearity are covered within the transceiver specifications, due to distance limitations. If link lengths approach the respective specified CD limits, power budget penalties in the 1dB range may have to be applied.
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