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Implementing SAN Internetworking Solutions
4
McDATA Products in a SAN Environment - Planning Manual
4-40
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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.
Dark Fiber Extended-Distance Connectivity
Figure 4-8
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.
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