how passive Dense Wavelength Division Multiplexing (DWDM) can help network operators achieve
very high per-fiber capacity using inexpensive passive DWDM filters up to 80 channels.
This is where it gets a bit complicated. When mentioning DWDM to data networking professionals, even some service provider engineers may associate it with large, complex, and costly DWDM systems.For example, Reconfigurable Optical Add-Drop Multiplexing (ROADM) systems are fully automated and perform optical switching, sub-signal aggregation, and some Layer 2 functions. However, DWDM is simply the combination and separation of circuits by wavelength, and it constitutes only a small part of these larger systems.
DWDM enables users to place over 40 distinct circuits on a given fiber and then separate them at the opposite end to connect to individual switch ports. As mentioned, this can often be done passively, requiring no electrical power, software, or annual maintenance agreements, and it can be implemented at a fraction of the cost of more complex active systems.
For point-to-point Data Center Interconnects (DCI) on leased or owned fiber connections between campus facilities and network facilities—whether between rooms or floors—using passive DWDM can reduce or eliminate the need for leased or new fiber builds, maximize the data rate per fiber on installed plants, drastically cut capital expenditure (CapEx) costs for high-capacity switches and complex DWDM systems, reduce reliance on service providers for maintenance, and enhance the speed of DCI connections.
To accomplish this, a simple optical link engineering approach is needed. By examining the physical map of your network and zooming in on a specific span where a capacity bottleneck exists, the task becomes more manageable. For simplicity, we will focus on connecting 10 Gbps switch ports across spans of 2 to 50 kilometers.
The essential factors to consider are link budget versus link loss and dispersion. Every optical transceiver has a minimum transmit power and a minimum receiver sensitivity. By subtracting these values, you obtain the link budget, which represents the total power loss the signal can experience while remaining detectable by the receiver.
In a standard connection, you would calculate or measure the total loss of the fiber patch, channels, cassettes, and splices between the two optics. If that loss is less than the link budget, the connection should work. However, passive DWDM introduces additional considerations into link engineering.
The optics at each end will need to be specific DWDM optics, and the filters will introduce more insertion loss at each end. Nevertheless, the calculations remain similar. For 10 Gbps DWDM optics, the link budget typically falls in the 23 decibel range. Therefore, if a fiber span with DWDM filters has less than 23 decibels of loss, the link should function properly.
In summary, If your network requires increased fiber capacity, lower fiber infrastructure costs, or the flexibility for high-speed circuits, adopting passive and amplified DWDM networks could be the ideal solution.
