How Much Do You Know about CWDM?

As is known to us, CWDM(Coarse Wavelength Division Multiplexing) is a tech, multiplexing wavelengths transmitted in different optic fibers into one fiber to transmit via optical multiplexers. This tech is commonly seen in optical communications. However, What is CWDM? Maybe there still are many people confused about it. This article will have a introduction to us.

Originally, the term "coarse wavelength division multiplexing" was fairly generic, and meant a number of different things. In general, these things shared the fact that the choice of channel spacings and frequency stability was such that erbium doped fiber amplifiers (EDFAs) could not be utilized. Prior to the relatively recent ITU standardization of the term, one common meaning for CWDM meant that two (or possibly more) signals are multiplexed into a single fiber, where one signal was in the 1550 nm band, and the other in the 1310 nm band.

Seen from its literal meaning, CWDM is like that. Of course, the contents that CWDM gets involved in are far more than this. Then the next will introduce what CWDM is from the perspectives of CWDM characteristics, CWDM systems and CWDM applications.

In terms of CWDM characteristics, CWDM transmits few channels and makes use of wider spacing among channels for distances of up to 60 km. Its Transmission is operating via 18 channels with wavelengths between 1270 nm and 1610 nm(shown as the pic). It  is with relatively wide spacing of up to 20 nm, is able to tolerate relatively high temperature fluctuations and works between 1265 nm and 1625 nm.

Besides, it’s a cost-effective solution in metro and regional network, and can provide a capacity boost in the access network. It can meet demands in traffic growth without overbuilding the infrastructure. It is also perfectly alternative for carriers looking forward to increasing the capacity of their installed optical network without replacing existing equipment with higher bit rate transmission equipment, and without installing new fibers.

With respect to CWDM systems, they rely on optical signal regeneration at every node without the use of optical amplifiers. As all channels are regenerated at each node, the link power budget does not depend on the number of channels transported over each span. This simplifies the network design. Signal regeneration implies converting the signal from optical form to electronic form, and then reconverting the signal from electronic back to optical form using OEO (optical-electronic-optical) transponders. With signal regeneration, each wavelength requires its own individual transponder. Signal regeneration makes sense in networks with a limited number of spans and low channel count.

In respects of CWDM applications, it refers to these three aspects:

In LAN and SAN Connection: CWDM has abundant network topology, such as point-to-point, ring, etc. The ring network can provide self-healing protection function, the style of restoring including link breaking protection and node failure separation. CWDM rings and point-to-point links are well suited for interconnecting geographically dispersed LAN (Local Area Network) and SAN(Storage Area Network). Corporations can benefit from CWDM by integrating multiple Gigabit Ethernet, 10 Gigabit Ethernet and fibre Channel links over a single optical fiber for point-to-point applications or for ring applications.

Integrated in 10 Gigabit Ethernet: With the benefits of low implementation cost, relative simplicity of installation and maintenance, Ethernet has been used popularly in the metro/access system now. IEEE 802.3 Ethernet standards spawned a successive upward bandwidth migration from 10 to 100 Mbits to 1 Gbps. And as the bandwidth increases, 10 Gigabit Ethernet with higher data rate was put forward. Ethernet integrated with CWDM is one of the best implementing methods. 10 Gigabit Ethernet in the IEEE 802.3ae is a CWDM solution with four-channel, 1300nm. However, if CWDM were based on 10 channels of 1 Gbps, then 200 nm of the wavelength spectrum would be used.

In PON (Passive Optical Network): PON is a point-to-multipoint optical network that uses existing fiber. It is an economical way to deliver bandwidth to the last mile. Its costs come from passive devices in the form of couplers and splitters, rather than higher-cost active electronics. PON expands the number of endpoints and increases the capacity of the fiber. Although PON is limited in the amount of bandwidth, and CWDM can support. CWDM can increase the bandwidths cost-effectively. When they are combined together, each additional lambda becomes a virtual point-to-point connection from a central office to an end user. If one end user in the original PON deployment  needs his own fiber, he can get a virtual fiber by adding CWDM to the PON fiber. Once the traffic is switched to the assigned lambda, the bandwidth taken from the PON is now available for other end users. So the access system can maximize fiber efficiency.

It’s seen from above that CWDM is an economical and attractive wavelength multiplexing technology for city and access networks. For most users who want to increase their capacity of optical network and satisfy the demands on the growing demands. In addition, with CWDM applied in many networks, it will be more popular in the optical interconnection.