HP StorageWorks 2/24 FW 07.00.00/HAFM SW 08.06.00 McDATA Products in a SAN Env - Page 184

Implementing SAN Internetworking Solutions 4 • Creates one logical Fibre Channel fabric through a stretched E_Port connection. The connection is vulnerable to disruptions caused by events at each site or to disruptions caused by problems with the extended-distance dark fiber link. Figure 4-8 Dark Fiber Extended-Distance Connectivity Due to the high cost of burying cables, dark fiber has limited physical availability relative to other WAN link options. Because dark fiber is usually buried, it is often susceptible to damage from excavating equipment. For these reasons, dark fiber is used on a limited basis for metropolitan-distance SDR applications. Dark fiber is not practical for ADR applications that span large distances. WDM Optical networks consist of fibers transmitting laser-generated flashes of light, and more information is transmitted by increasing the number of flashes per second (increasing the bit-rate). Using multiple lasers to simultaneously transmit different colors (wavelengths) of light also increases the capacity of optical fibers. Assigning laser light to designated frequencies, multiplexing (combining) the result to one signal, and transmitting the signal one fiber is called wavelength division multiplexing. At the receiving end, combined wavelengths are separated (demultiplexed). Each wavelength requires a discrete detector to convert light pulses to useful information. The number of wavelengths used is a power of two (2, 4, 16, 32, 64, or eventually more). Technology that provides 64 wavelengths or more per fiber is called dense wavelength division multiplexing (DWDM). Technology that provides 32 or fewer wavelengths per fiber is called coarse wavelength division multiplexing (CWDM). CWDM is less complex and expensive to deploy. 4-40 McDATA Products in a SAN Environment - Planning Manual