At present, as most companies are planning for disaster recovery scenarios more than ever, many data centers have multiple geographic locations for minimizing disruptions when unexpected events occur. But how to connect these locations? Usually, the dark fibers are used to accomplish the connection. With the growing quantity of data being transferred, it is becoming increasingly popular to maximize the throughput of that dark fiber using WDM (wavelength division multiplexing). There are passive and active WDM systems. This article mainly focuses on active WDM system.
Passive solution is built from passive filters and colored optics plugged into different equipment such as routers, switches. Generally, passive solutions have dispersed management of optical layer and specific high cost optical modules. Moreover, the optical performance monitoring and link power budget and distance are limited. Besides, every service requires a dedicated WDM colored optics increasing the complexity of installation and management. The passive solution is not effective outside of point-to-point topology.
Active solution is in which an active CWDM/DWDM infrastructure is built from transponders and muxponders providing full optical demarcation point agnostic to the routers, switches within the network. The following picture describes the basic working scheme of active WDM system. Different from passive system, the active system has central management system for the optical layer, and uses standard low cost optics and inexpensive vendor specific colored optics. What’s more, it provides centralized optical transport layer demarcation capability enabling optical performance monitoring and swift fault isolation. Additionally, the active system increases spectral efficiency by mapping several services into a single wavelength, thus reducing both deployment and management costs. And it allows for signal amplification and regeneration overcoming the link budget and distance limitations of passive networks.
Passive system may seem to be an immediate solution for solving the backbone connectivity need that is relatively simple. But as the network evolves, passive system imposes significant difficulties. It tends to be very complex to manage as it grows which is due to the dispersion of the optics in several different layer-2 switches, routers at different physical locations and departments of the organizations. This results in challenging maintenance, network expansion and fault isolations of problems that may occur down the road. In addition, it imposes many restrictions. One such restriction is the requirement of using vendor specific optics that are often very expensive and prevent the WDM backbone infrastructure to be vendor agnostic.
Active CWDM/DWDM infrastructure enables organizations to provide managed services with SLA to its partners and customers and to generate revenue from its network infrastructure. It further reduces the operational costs and minimizes the downtime of the services on organization’s backbone infrastructure. Active CWDM/DWDM systems are not limited by the network topology or distances between the sites and provide flexible add/drop capabilities as well as traffic regeneration at different points across the back bone infrastructure. In contrast, passive system is mostly limited to distance and point-to-point topology without the flexible service add/drop capabilities. Additionally, active system enables better spectral efficiency of the backbone network infrastructure using muxponder capabilities (mapping multiple services over single wavelength) resulting in significantly reduced cost and easier management due to fewer wavelengths being used between the backbone sites. Muxponders result in savings of the CWDM/DWDM optics cost and the number of channels/filters within the CWDM/DWDM Mux/DeMux.
Active CWDM systems have lasers that can tune the individual wavelength to whatever wavelengths (color) are required to provide as many discrete channels and types of data as the band space will allow—some even providing bidirectional communication, if desired.
Active CWDM solutions are stand-along AC or DC powered systems separated from the switch. The task of the stand-alone system is to take the short-range optical output signal of the fiber or IP switch and convert it to a long-range CWDM signal. This OEO conversion is handled by a transponder. The converted CWDM signal is then transmitted with the help of transceivers and multiplexers. Due to the separation of the CWDM transport solution from the actual switch, active systems also tend to be more complex than passive, embedded solutions.
In the passive DWDM access networks, each wavelength channel is used to provide one service at a given time regardless of the channel capacity and bandwidth requirement of the service. With the increasing bandwidth capacity of DWDM technology, the bandwidth of one signal channel becomes high enough to carry several or many services even in the access environment. This leads to the thinking of applying TDM in each individual DWDM wavelength channel, resulting in the active DWDM access optical network in which TDM is used within each channel to provide integrated services. The ATM (asynchronous transfer mode) has been proposed as the TDM protocol in the active DWDM access networks.
Active DWDM can take in 8/16/32 channels all on the same wavelength (1310 nm) and then shift them all to different individual frequencies before multiplexing them onto a single fiber. They can be demultiplexed at the other end all back to the same frequency (1310 nm) again. Although an active DWDM access network provides high utilization of the wavelength channels and in return reduces the fiber costs, it adds additional costs due to the ATM devices in the system from CO to user premises. It also increases the complexity of system management and maintenance, which leads to high operating costs. Even so, the extra distance capabilities of optical amplifiers and the immense amount of channels make active DWDM an attractive solution for large capacity optical rings that are servicing hundreds of customers or locations.
Typically, active DWDM system with optical amplifiers are used when building long-distance backbone networks and metropolitan area networks or metro networks. In a longer line, EDFAs are installed at the specified distances. They are able to amplify the signal of any format and, at the same time, to restore a large number of independent WDM channels. Several EDFAs are installed sequentially in the line when creating long-haul DWDM networks. DWDM active systems are typically based on the dual-fiber signal transmission (see the picture above). This occurs due to the fact that EDFAs amplify one-directional signals only. Such harsh conditions are not always feasible within the real data transmission network. In this case, active DWDM system on the single fiber basis was developed (see the picture below). This scheme differs from the standard dual-fiber system by the presence of two additional optical filters. These red/blue filters divide the C-band used when transmitting DWDM signals into two sub-bands: red one (1547.72 to 1564.68 nm), and blue one (1528.77 to 1543.73 nm). Thus, two spectrally separated transmission media are generated within a single fiber.
WDM system increases capacity on the existing fiber infrastructure. It can multiplex multiple optical signals onto a single fiber by using different wavelengths, or colors of light. Active CWDM/DWDM infrastructure can grow with the organization’s needs, and topology can be upgraded from point-to-point to ring or can be increased from 8 wavelengths to 40 wavelengths of 10G capacity step by step with the highest ROI (return of investment).
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