In November 2011, I posted What Are 10 Gigabit Ultra Short Reach (USR) Optics ? where I tried to what is the actual difference between these optics and the certified 10Gbase-SR products. I recently received the following from an anonymous source. I’ve made some small edits to protect identity and improve the text.
Simply put: USR 10G optics are 10GBASE-SR that are “binned” at lower performance at the manufacturing plant.
As you know, 10GBASE-SR specifies a 300m reach over OM3 fiber. This was pretty tough to achieve in the early days of SFP+, so there was a pretty high yield fallout. Optics that managed 290m reach, but not 300m, for example, could not be sold as “10GBASE-SR” because they don’t meet the spec. They ended up in the trash!
That’s right: an (almost) perfectly good 10GBASE-SR optic that didn’t quite reach 300m, but would have performed just fine on links up to 290m, had to be tossed in the trash!
Someone realized that this was a real shame, because most links aren’t 300m long, so that extra performance was not even needed much of the time, and the “USR” optic was born! Since there is no standard, each of the optics vendors chose a slightly different spec (typically 100m reach) and name. Some call it “Ultra Short Reach” (USR), others call it “Data Center Reach” (DCR), yet others call it “SR Lite” (SRL).
At the specified reach, these USR optics will actually interoperate just fine with SR optics (i.e., if your link is up to 100m OM3, a USR can talk to an SR optic just fine), but, of course, if the link is longer than the 100m (or whatever your specific USR optic specifies), all bets are off — the limiting factor being the USR SFP.
This is more or less what I guessed at during my previous posts. Since they are effectively rejects they have a low commercial value so to recover some of the cost is good business. Therefore they can be sold at cheap prices.
Very few data centres will use 300m cable runs since most cabling plant is built to 100m/330ft lengths for historical reasons. (I’m not sure that many engineers are aware that cable lengths can be longer than 100m)
Thanks to Anonymous for sending me the information/confirmation.