Signal loss is the total sum of all losses due to attenuation across the fiber span. This value should be within the power budget to maintain a valid connection between devices. To calculate the maximum signal loss across an existing fiber segment, use the following equation:
Signal Loss = (Fiber Attenuation/km * Distance in km) + (Connector Attenuation) + (Safety Margin)
Figure 2 provides average optical loss characteristics of various fiber types that can be used in this equation, although loss may vary depending on fiber type and quality. It’s always better to measure the actual optical loss of the fiber with an optical power meter.
Some receivers may have a maximum receiver sensitivity. If the optical signal is greater than the maximum receiver sensitivity, the receiver may become oversaturated and not be able to decode the signal, causing link errors or even total failure of the connection. Fiber attenuators can be used to resolve the problem. This is often necessary when connecting FC switches to DWDM equipment using single mode FC transceivers.
FC Transceivers for Extended Distances
Optical Small Form-factor Pluggable (SFP) transceivers are available in short- and long-wavelength types. Short-wavelength transceivers transmit at 850 nm and are used with 50 or 62.5 μm multimode fiber cabling. For fiber spans greater than several hundred meters without regeneration, use long-wavelength transceivers with 9 μm single-mode fiber. Long-wavelength SFP transceivers typically operate in the 1310 or 1550 nm range.
Optical transceivers often provide monitoring capabilities that can be viewed through FC switch management tools, allowing some level of diagnostics of the actual optical transceiver itself.
Distance Connectivity Options
FC SANs can be extended over long-distance optical networks in different ways. Any of the following technologies can provide a viable long-distance connectivity solution, but choosing the appropriate one depends on a number of variables—including technological, cost or scalability needs.
Note that terms are often misused or used in a generic way. In addition, products can be configured and used in any of the different ways discussed in the following sections. Ensure there’s no confusion or uncertainty about the type of equipment being used. If connectivity is being provided by a service provider, in addition to equipment deployed at your site, it’s important to understand all devices in the network.
Native FC Over Dark Fiber
The term “dark fiber” typically refers to fiber optic cabling that has been laid but remains unlit or unused. The simplest, but not necessarily most cost-effective or scalable method for extending SANs over distance, is to connect FC switches directly to the dark fiber using long-wavelength SFP transceivers. An optional Brocade Extended Fabrics license can be used to provide additional buffer credits to long-distance E_Ports in order to maintain FC performance across the network.
Wave Division Multiplexing
DWDM is optimized for high-speed, high-capacity networks and long distances. DWDM is suitable for large enterprises and service providers that lease wavelengths to customers. Most equipment vendors can support 32, 64 or more channels over a fiber pair with each running at speeds up to 10 Gbit/sec or more. Fiber distances between nodes can generally extend up to 100 km or farther. DWDM equipment can be configured to provide a path protection scheme in case of link failure or in ring topologies that also provide protection. Switching from the active path to the protected path typically occurs in less than 50 ms.
Coarse Wavelength Division Multiplexing (CWDM) provides the same optical transport and features of DWDM, but at a lower capacity, which allows for lower cost. CWDM is generally designed for shorter distances (typically 50 to 80 km) and thus doesn’t require specialized amplifiers and high-precision lasers (lower cost). Most CWDM devices support up to eight or 16 channels. CWDM generally operates at a lower bit rate than higher-end DWDM systems—typically up to 4 Gbit/sec.