OTDR: Launch Cable Length for Singlemode

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The JDSU Fiber Optic Test Reference Guide,
http://lw.pennnet.com/whitepapers/wp.cfm?id=665 ,
says a launch cable used for singlemode should be 1km to 2km long.

Is that what's used in practice?  What's the consequence of not  using
a launch cable that long (besides the generic answer of unreliable
readings, which I can intuit.)

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