Single mode minimum distance

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MichaelNZ

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#302983 8-Jan-2023 21:29

I've read in multiple places the recommended minimum distance for single mode is 2 metres and yet there are 0.5 and 1m patch leads available. I've gone down to 1 metre in production without issues but wondering if I should be using 2+ metres for best practice.

Can someone knowledgeable please shed some light on this?

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BarTender

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#3018792 8-Jan-2023 22:55

I have never had a problem with 10gb short runs. But price wise getting a direct attached copper back to back SFP+ is way cheaper if you don’t need them to be electrically isolated from each other and the SFPs are supported in both devices.

Something like this, but there are many options like this.

https://www.pbtech.co.nz/product/CABLTE1010/Generic-compatible-1m-3ft-10G-SFP-Passive-Direct-A

Tinkerisk

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#3018794 8-Jan-2023 23:10

Same for me for 1m, but for short lengths (30cm upwards) I use passive 10Gb/s DACs as well (when possible). What I can imagine is that with very short optical fibre length, the strength of the light waves reflected from the ends is so strong that optical interference can occur and the receiver cannot cope with it.

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Mehrts

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#3018878 9-Jan-2023 09:20

+1 for using DAC cables for short runs (typically within rack) that don't require electrical isolation.

MichaelNZ

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#3018937 9-Jan-2023 12:10

Thanks for the responses.

I am sticking with optics now due to standardisation and also because the DAC's are apparently not supported on all hardware.

I have decided to get a 2 metre cable.

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cyril7

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#3019008 9-Jan-2023 13:54

Hi, its typically considered a 2m run is the recommended shortest between transcievers. If you have a 1km run and a couple of 0.5 or 1m fly leads at each end, those 0.5 and 1m leads make sense.

Cyril

nitro

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#3019134 9-Jan-2023 16:02

MichaelNZ:

I've read in multiple places the recommended minimum distance for single mode is 2 metres and yet there are 0.5 and 1m patch leads available. I've gone down to 1 metre in production without issues but wondering if I should be using 2+ metres for best practice.

Can someone knowledgeable please shed some light on this?

2m minimum came from how 802.3ae specified the 'minimum range', e.g. minimum '2-300 meters' for devices that can be used for 500m. I must confess I haven't looked at the standards in a while.

there are obviously different types of sfp/sfp+ adaptors, but we have used 0.5m an 0.3m cables in test. We even have a few 0.2m between gigabit SFPs.

FailedWOF

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#3022900 17-Jan-2023 18:27

For short runs you generally use multimode fibre or DACs. Single mode runs less than 2m end to end can work as long as the TX power is lower than the receiver sensitivity. This will normally be the case as long as you're using -LR (<10km) transceivers on both sides. Using -ER or higher over short distances will burn out the optics. But really, why worry? If you want to use SMF (and you have -LR SMF transceivers) just get a 2m OS2 patch cable and loop the excess.

Total cost per metre is the more relevant factor. When you account for the transceivers at either end, plus the cable, I've found the breakover point between MMF and SMF to be about 15m. Less than that, the higher per meter cost of MMF cable is offset by the cheaper transceivers. Above that, SMF transceivers are more expensive, but SMF cable is cheaper per metre. For only a couple of metres, DACs are the cheapest option but requires common compatibility on both ends. In my case, inter switch links are DACs ($19 for 0.5m passive twinax), devices to switches within the rack are MMF, and between the house comms rack and the garage rack I use SMF.

In-rack
- With MM
  - 2x 10GBASE-SR MMF SFP+ = $70
  - 3m OM4 = $11
  - $27/m
- With SM
  - 2x 10GBASE-LR SMF SFP+ = $94
  - 3m OS2 = $8.50
  - $34/m

House to garage
- With MM
  - 2x 10GBASE-SR MMF SFP+ = $70
  - 20m OM4 = $45
  - $5.75/m
- With SM
  - 2x 10GBASE-LR SMF SFP+ = $94
  - 20m OS2 = $19
  - $5.65/m