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jaytcsd
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fiber cable to connect 2 routers

Fri Feb 03, 2023 2:23 am

I'm new to fiber optics and ordered 2 Mikrtotiks with SFP. I got the S-3553LC20D LC adapter and wanted to make sure this cable would work.

https://www.ebay.com/itm/125665725634?h ... BM8o7xycJh
 
ConradPino
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Re: fiber cable to connect 2 routers

Fri Feb 03, 2023 8:12 am

Not fiber expert here; I just run 10 G fiber between three MikroTik (CRS309, CRS326, CRS326).

As far as I can tell, your cable matches your transceivers; SC (connector style) and single mode (fiber style).
The cable page doesn't state light wave length but I seriously doubt that will be an issue.
A 20 kilometer transceiver set shouldn't have problems with a 25 meter cable. :)
Don't forget to remove white fiber protection caps from connector tips.

At 25 meters, consider dual S-85DLC05D (550 m) at $22 MSRP.
Multi mode fiber cable costs will be similar to single mode.
 
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mkx
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Re: fiber cable to connect 2 routers

Fri Feb 03, 2023 7:36 pm

SFPs S-3553LC20D LC come as complementary pair of modules which then communicate via single fibre strand (so called BiDi modules). This is achieved so that one transmits with wavelength of 1310nm and the other transmits with wavelength of 1550nm ... receiving wavelengths are reversed. So the cable you linked will likely work but only half of it is needed.
A pair of modules S-31DLC20D would do as well (this time using both fibre strands) . But as already @ConradPino suggested, 850nm multimode would do just fine for that short stretch.

A short explanation about SFP "distances" for beginners: optical SFP module has transmitter which can vary transmitted power, but only slightly. And receivers come with certain sensitivity and with maximum received power. A pair of modules (can actually have slightly different specs, in that case it's necessary to do calculation for each direction) can keep link running if received piwer falls between max Rx power and sensitivity ... where Tx power control helps as well.
When calculating maximum range, one has to assume certain fibre loss of about 0.5 dB/km (high quality fibres have it slightly lower; longer wavelengths have it slightly lower, older fibres have a distinctly higher loss at medium wavelengths - around 1400nm), also imperfect fibre welds have additional loss of a few tenths of dB, each connector has insertikn loss of a few tentgs of dB. But assuming perfect fibre with no welds, range is (max Tx power - Rx sensitivity - 2xinsertion loss) / fibre loss and for S-31DLC20D (-3dBm Tx power, -24dBm Rx sensitivity) thats 20dB/0.5dB/km=~40km. With a few imperfect welds, a couple of connectors more (fibre cables are usually terminated on patch pannels and the patch cords are used to connect to SFPs), some sharp corners (which cause many times higher loss) and realistic range drops to 20km.
When it comes to consideration if a pair of modules will work when connected with only short patch cord it is important to compare minimum Tx power to maximum Rx power. As long as mimimum Tx power is lower than max Rx power, this kind of connection is fine. In case of S-31DLC20D this works (minimum Tx power is -9dBm, max Rx power is 0dBm). However in case of S-55DLC80D (80km module) this is not possible: minimum Tx power is 0 dBm and maximum Rx power is -2dBm ... with too short fibre connection transmitter will overpower receiver.
 
ConradPino
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Re: fiber cable to connect 2 routers

Fri Feb 03, 2023 8:33 pm

@mkx, Thank you! You just really stepped up my fiber knowledge.
 
jaytcsd
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Re: fiber cable to connect 2 routers

Sun Feb 05, 2023 9:45 am

My first exposure to fiber was in multimode days when a pair was needed since fiber was one way. It wasn't until last week while working at winter X games that I found out single mode was
bidirectional. The IT vendor used Mikroik routers and switches so I got my first exposure to single mode.

Most of the patch cables I see have dual connectors but from what I can tell the SFP only has 1 jack, so this should work.
https://i.ebayimg.com/images/g/WN4AAOSw ... s-l500.jpg
 
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mkx
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Re: fiber cable to connect 2 routers

Sun Feb 05, 2023 7:22 pm

Fiber is bidirectional in principle. If one wants to use single strand for both directions, then Tx and Rx have to use different wavelengths. In multimode using different wavelengths is not common while in single mode many wavelength tricks are played. BiDi is one of them, CWDM and DWDM are another. The 40Gbps modules (and faster) on single mode fibre use multiple wavelengths to achieve high speeds - it's easier to create "bonds" than to deal with higher symbol rates.

BiDi is generally incompatible with the rest of tricks.
 
jaytcsd
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Re: fiber cable to connect 2 routers

Sun Feb 05, 2023 9:18 pm

I heard about DWDM but BiDi is new, thanks for all your help.

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