AT&T fiber-to-the-home cable type?
December 11, 2020 11:20 AM   Subscribe

Got my new AT&T fiber connection installed today. Because I am paranoid and also klutzy, I'd like a spare fiber optic cable for between the router/gateway and the wall. Turns out there's quite a few kinds of these cables! What kind am I looking for, please?
posted by humbug to Technology (10 answers total)
Do you have a box on the wall, or is it just a plate with some kind of jack on it? If it's a box/thing on the wall, the thing on the wall is called an ONT (optical network terminal). Its job is to take the fiber cable from outdoors and convert the signals on it into signals on non-fiber cables for use within your house/apartment.

I don't have AT&T fiber so I am not sure what they normally set up for the in-house wiring between the ONT and gateway, but it is very likely either Ethernet or coaxial (as in cable TV) cable.
posted by Juffo-Wup at 12:47 PM on December 11, 2020 [1 favorite]

I have AT&T fiber and the connection from the wall to the router is standard Ethernet.

Can you look up the router model? It's spec sheets will tell you the connection types.
posted by pdoege at 1:32 PM on December 11, 2020 [1 favorite]

If you could take a picture of the cable and what it is plugged into on your wall, that would be helpful for answering this.

Back when I had AT&T fiber, the connection from the wall to the router was normal Ethernet, like pdoege above.
posted by zsazsa at 1:43 PM on December 11, 2020

Response by poster: The router is a Humax BGW320-500. Pretty sure the wall connection is not the actual ONT. The actual gizmo is definitely not connected via Ethernet -- the only Ethernet cable plugged in is the yellow one that goes to my Ethernet dongle. (Sorry about the weird angle and blurriness; the Sooper Seekrit Device Passwords -- which I have changed, but still -- are on a sticker right next to the ports.)
posted by humbug at 1:53 PM on December 11, 2020 [1 favorite]

Best answer: No promises here, but if I search on 'humax SFP', their PON (cable network) SFPs (the silver bit sticking out of your router) says its connection is SC/APC. (SC is the connector size, APC says the end of the connection is polished at an angle to avoid reflections.)

Technically the other end might be a different kind of connector, but eyeballing it I think it's also an SC connector - and if so there's a colour code to the plugs and you should be able to do your own research to double check exactly which subtype it is. If you look at both ends together you'll see if my guess is right because they should look similar.

Warning: I am a networking person and I know what PON is, but I have never played with the physical bits involved so this is mainly be doing Google research. YMMV.
posted by How much is that froggie in the window at 2:47 PM on December 11, 2020

Best answer: That manual does show that the router has a place for the SFP module that does the optical interfacing, so it certainly does look like you really do have optical straight into the router, and an optical wall jack. I don't know what kind of cable it is, but some things to look for are:

- SC vs LC (the exact termination, including what kind of cut was made to the end of the glass fiber)
- Single mode vs multimode (related to glass fiber diameter and how the laser bounces inside it)
- Simplex vs Duplex (a single fiber carrying both directions, or one fiber for each direction)

Look at the fine print on your existing cable, it might be stated there. Some googling of your router and "AT&T" points in the direction of it being an LC-LC single-mode duplex. The manual doesn't say anything about it.

Ah, I miss communications engineering.
posted by intermod at 2:48 PM on December 11, 2020 [1 favorite]

Best answer: We may have done this the hard way: Amazon gives this for 'FTTH cable', it claims it works with AT&T and it's SC/APC both ends.
posted by How much is that froggie in the window at 2:55 PM on December 11, 2020

Response by poster: aha! thank you! This appears to be the Monoprice equivalent, which is what I was hoping to find. :)
posted by humbug at 3:27 PM on December 11, 2020 [1 favorite]

It's LC. Small form-factor pluggable transceiver. SC are like 1cm square for each of Tx/Rx. LC are smaller and both sorta fit into a fat rectangle because they have little latching things on them. The SCs are just snap-n-fit. Two SCs won't fit in the form factor of an SPF. You need like an FPIM for those. I forget. You can get copper 10G SPFs or Ethernet SPFs or LC SPFs but it's too small for two SCs.

Unplug the cable and look at the ends.
posted by zengargoyle at 3:51 PM on December 11, 2020

OP, the cable you've found is UPC (flat ends). The cable I pointed at is APC (angled ends). They're not quite the same.

zengargoyle, I only see a single cable to the SFP there (which makes sense, given this is PON not Ethernet). What makes you say it's LC?
posted by How much is that froggie in the window at 11:15 PM on December 11, 2020 [1 favorite]

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