This picture is a part of the Intel E10GSFPLR (SFP+, Long Reach) datasheet. Can I use them in a short link (less than 1 meter) without any attenuator? Which numbers are important in this datasheet to pay attention to? (To make sure that these SFP+ do not damage without attenuator in short links scenarios)
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2 Answers
Most 10 km PHYs can talk to each other even over very short links.
The key is whether the max launch power overshoots the max receive power, effectively blinding the receiver.
With this transceiver, the max launch power and the max receive power are both .5 dBm, so no additional attenuation is necessary. Additionally, the average lauch power, -8 dBm, is far below the max receive power, so you're really on the safe side.
With higher powered transceivers and short links, running on the upper edge of receive power (when average launch power ≈ max receive power) can reduce the lifetime considerably, but that isn't the case here.
As a side note, the -LR doesn't mean "long reach" but the -L stands for long wavelength (~1300 nm) and the -R for 64b/66b PCS large block code. See this question for more details.
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Actually it does... S, L, and E are short, long, and extended range. Apr 22, 2021 at 1:05
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@alex.forencich That isn't correct. E.g. 10GBASE-LX4 is short range but using ~1300 nm wavelengths. You should check out IEEE 802.3 Clause [edited] 1.4 Definitions.– Zac67 ♦Apr 22, 2021 at 5:54
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That clause doesn't exist, but apparently there is a list in 30.5.1.1.2. Interesting that it has nothing to do with range as per the spec. I have definitely seen 1310 nm transceivers marked 10GBASE-ER, I suppose those are not marked correctly then. Apr 22, 2021 at 6:42
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1I just checked the spec, and 10GBASE-LX4 is 10km range on single mode fiber, same as 10GBASE-LR Mar 31 at 11:14
Launch power max 0.5 dBm, receive power max 0.5 dBm. Should be fine. Also, -8.2 to 0.5 dBm is a huge range for launch power, although this may be specified with aging in mind (launch power reduces with module age, starting closer to 0 dBm and ending up closer to -8 after some number of years of continuous use).
Also, if you have too much signal in an optical fiber, you can add some attenuation by wrapping it around a pen (or other thin round object) a few times, more wraps and tighter wraps for more attenuation.
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