On Sun, 4 Feb 2018, William Cheswick wrote:

> One important and sensitive site chose to stick with this technology for 
> a while becauseone could detect unauthorized taps.  I guess TDR isn’t as 
> useful these days.

I believe that optical TDRs exist, for use on fibre.  The ultimate in 
anti-snooping, of course, is quantum; if you so much as *look* at it, 
you've disturbed it (which is how quantum key exchange would work etc).

-- 
Dave Horsfall DTM (VK2KFU)  "Those who don't understand security will suffer."