[vcf-midatlantic] SX GBIC fiber cable speed

Jameel Akari jakari at bithose.com
Mon Feb 15 18:53:09 UTC 2021

Before somebody says "it's not vintage!" consider that 1Gb Ethernet on 
fiber was spec'd for FDDI cables, and there were optical transceivers 
for even 10Mb Ethernet - I brought a set of AUI to 2xST for the Unix 
Town networking, just because I had them. Also, early 1Gb Ethernet was 
all 1000base-SX with SC connectors, at least that I saw.  It took a 
while for copper UTP to be supported.  I have a few Digital and Sun 1Gb 
NICs that are optical, and so I run them that way.

Now, at 25m, you /really/ don't need fiber for 1Gb Ethernet.  Properly 
installed UTP copper should work reliably for years in normal 
environments.  It's surprising and unusual you had some fail.  
Exception: crimped RJ45 ends on solid conductor UTP cable is likely to 
fail over time if there's any movement or vibration.  Punch on some 
jacks and use premade jumpers.

You can run 10Gb over Cat6A UTP at this distance as well.  This is 
harder to terminate by hand, but you could just get premade patch cables 
(they will be 26-28 AWG stranded, most likely)

But since you've already got optics and fiber is a lot cheaper than it 
used to be... why not?

- You're better off not trying to use the same cabling for 1Gb and 10Gb, 
- You *can* do short-range 1000base-SX with OM3, though it's not spec 
depending on where you look.  It will work.
- By the book, use OM2 for 1Gb and OM3 for 10Gb, assuming short-range 
optics like your SX SFPs. (SFP, not GBIC, if you're talking LC 
- OM2 is usually orange and OM3 is usually aqua-blue.

- As far as I can tell, you don't need mode conditioning for SX 
(short-range) optics.  If you needed to use LR 1Gb optics, or use 
single-mode optics into MMF cabling plant, then you'd use a 
mode-conditioning cable to clean up the noise.

- Bend radius is typically 10x the outside diameter of the cable, unless 
otherwise specified. Here, it's a single strand of a duplex cable.  So 
if it's about 3mm across, bend radius is 30mm.  (but see below on 
bend-insensitive cable). When I dress cables I usually eyeball 3-4" 
diameter loops.  Any smaller just doesn't look or feel right anyway.

I'm assuming that at only 25m you're buying pre-terminated cables with 
LC plugs both ends. This means you need large enough holes in things for 
those plugs to fit through.  Otherwise it's definitely not worth 
splicing your own at this scale.

https://www.fs.com/products/74386.html?attribute=226&id=99219 - for 
$16.72 each at 25m, you get new "bend insensitive" cable (7.5mm radius!)

I love FS.com for all manner of fiber and related cabling:
These have some rule-of-thumb charts.

I think that covers it?

Jameel Akari

On 2021-02-15 12:25, Neil Cherry via vcf-midatlantic wrote:
> I've posted this in the CDL list also. I don't know of a polite way to
> post once to both and
> I'm not sure it's a good idea to try. :-)
> It has been a while since I played with fiber (2010) and I'm trying to
> figure out if I
> should go fiber or copper in my home. Here's what I know from my 
> current switch:
> - LC
> - duplex
> - MMF
> I want to buy about 25M of fiber. I think the price is competitive
> with copper. I'm also
> hoping that the fiber will age better than the copper. I'm seeing a
> few old copper drops
> fail to lower settings. One went to 10/half (wow, that's bad).
> The GBIC & switch I have handle 1G. So I can easily pop in a few
> 1000baseT, put them
> into a LAG and get 4G (4 ports). So I'm covered for a while.
> - Can the same piece of fiber handle 1G or 10G? (in case I upgrade in
> the future).
> - Does anyone know the bend radius of this fiber?
> - What color should I expect for SX fiber. ?
> - What's a "mode-conditioning patch cord" ?
> I'm seeing a $35 price for 25 Meter OM3 LC LC Fiber Patch Cable
> (Corning 50/125um core/cladding)
> and it says 1G but with a "mode-conditioning patch cord" (an 
> attenuator?).
> Any pointers?

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