Networking → Two cable modem connections one coax run

Can anyone give me the technical details on how or why you can just take one cable run and split it and use two DOCSIS modems simultaneously. In my experience (Bright House Tampabay, 50/5mb service) it works flawlessly but recently someone expressed their doubts about that working properly and I was unable to change their mind and could only insist that in my experience it just worked. What about splitting it into three modems provided you have the db signal strength after the spliting is done?

Cable's a shared medium anyways... Basically, every modem on the node / segement is on a common cable. There's no difference between the modems being side by side in the same room, or one modem in your house, another in your neighbors.

As long as the signal is strong enough, there's no issues having two (or more) modems on the same drop.

This has been my experience. The tap that everyone in your neighborhood is connected to is pretty much nothing more involved than a splitter.

being that its shared, it won't matter unless and until you are hitting the maximum bandwidth of the medium. The modems are probably capped lower than the cable network itself. However, if you have some business plan with a very fast modem, then you may already get the max through a single modem. Adding a second will just divide the bandwidth.

I work in a lab with over 800 modems that can be switched onto a single coax feed (lots of splitters and forward/return amps). There is no problem with this and it will work fine.
An HFC (hybrid fiber coaxial) cable plant is nothing more than a large splitter/combiner arrangement with amplifiers placed throughout the system.

If your friend was talking about the network aspect of it, it's hard to load balance a single data connection across two modems (say a single FTP download) like per-packet load balancing. You can easily do per-connection load balancing though, where each data stream gets a different path. I've done this in the past for business customers that had multiple modems.

The bandwidth is another point. a single 4 channel modem can use 100% of the available bandwidth on a cable plant, (if load balancing isn't set up properly by the provider) so there is an argument that there is no difference between 1 modem, and 2 if the modems are uncapped as far as downstream uncapped traffic goes. Upstream uncapped traffic can be limited by the upstream scheduler on the CMTS (around 8-9Mbps on most Cisco and ARRIS CMTS if upstream channel bonding is not enabled),

Holy modem...that is...wow!

I'd be a liar if I said I didn't like coming into work every day.

Wait! Are those all plugged in!?

Every last one of them. They're drawing about 8Kw of power, and we're pushing 10Mb symmetrical across each one (for the upstream channel bonding capable modems, 5Mb for the narrowband units) for about 8Gbps of bandwidth through the CMTS.

Wow, thanks for the picture. I see some older modems in there. A couple DOCSIS 1 3com sharkfin modems. You don't retire out older modems? Just curious.

The goal of this specific lab configuration is to emulate what's actually out in the field. Believe it or not, there are a decent number of customers who refuse to upgrade their old modems because "they still work". This forces us to test every firmware version of every modem in the field against different CMTS configurations before they're pushed into production.
/on a side note, that's one of the reasons that it takes so long to get modem firmware updates to the field. Testing testing testing, and more testing.

Damn man, nice info. Thanks!

My pleasure. I work with HFC / DOCSIS systems for a living, so if anyone has any cable questions, feel free to drop me an IM and I'll be happy to try and help.

So what does the ethernet side of all those modems connect to?

They run to a series of switches (VLAN tagged) that in turn connect to several pieces of equipment that generate and monitor traffic. It allows us to simulate a small town, or neighborhood, and test things like RFC2544 compliance.

very cool, thanks for explaining!

Until you stop providing DOCSIS 1.0 and 1.1 profiles for them, they will "still work." Pull those profiles (and repurpose the QAMs they "waste"), they'll stop working, and upgrade.

(I don't know if TWC has pulled the old profiles, but they did demand everyone replace the SB4100 many years ago. I'm always surprised when anyone says they're using one.)

That is a lot of power, I guess gives an idea of how much a whole block's worth of modems is using.