WDM (wavelength division multiplexing) enables carriers to deliver more services over existing optical fiber infrastructure by combining multiple wavelengths on a single fiber. Each service is carried over a separate wavelength, thus increasing the capacity of the fiber by the number of wavelengths transmitted. CWDM (coarse wavelength division multiplexing) and DWDM (dense wavelength division multiplexing) are the two kinds of mature WDM technologies. This article will focus on CWDM which has low cost and simple deployment.
CWDM is a cost-effective solution in metro and regional network, and is able to provide a capacity boost in the access network. It can address traffic growth demands without overbuilding the infrastructure. For example, a typical 8-channel CWDM system offers 8 times the amount of bandwidth that can be achieved using a SONET/SDH system, for a given transmission line speed and using the same optical fibers. It is perfect alternative for carriers who are looking to increase the capacity of their installed optical network without replacing existing equipment with higher bit rate transmission equipment, and without installing new fibers.
CWDM systems 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 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.
Due to the technical characters of the CWDM, CWDM is applied primarily in the two broad areas: metro and access network. There are always two functions. One function is to use each optical channel to carry a distinct input signal at a individual rate. And another one is to use CWDM to break down a high-speed signal into slower components that can be transmitted more economically, such as some 10G transceivers.
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.
With the benefits of low implementation cost, robustly, 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, higher data rate 10 Gigabit Ethernet was put forward. Ethernet integrating with CWDM is one of the best implementing methods. In one of 10 Gigabit Ethernet standards in the IEEE 802.3ae is a four-channel, 1300nm CWDM solution. However, if CWDM were based on 10 channels of 1 Gbps, then 200 nm of the wavelength spectrum would be used. Compared with TDM (transmission time division multiplexing), 10G CWDM technology may have a higher initial cost, but it can offer better scalability and flexibility than TDM.
PON is a point-to-multipoint optical network that uses existing fiber. It is the economical way to deliver bandwidth to the last mile. Its cost savings come from using 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. But PON is limited in the amount of bandwidth it can support. As CWDM can multiple the bandwidths cost-effectively, when combining them 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 grows to the point where he needs his own fiber, adding CWDM to the PON fiber creates a virtual fiber for that user. 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.
CWDM is an attractive solution for carriers who need to upgrade their networks to accommodate current or future traffic needs while minimizing the use of valuable fiber strands. CWDM’s ability to accommodate Ethernet on a single fiber enables converged circuit networks at the edge, and at high demand access sites. With traffic demands continuing to rise, the popularity of CWDM with carriers in the access and metro networks will be akin to the popularity of DWDM in the long haul and ultra-long haul networks.
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