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crapslock

join:2012-11-06
Clearwater, FL

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?


LazMan
Premium
join:2003-03-26
canada
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.


Jahntassa
What, I can have feathers
Premium
join:2006-04-14
Conway, SC
kudos:4
said by LazMan:

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.


dnoyeB
Ferrous Phallus

join:2000-10-09
Southfield, MI
kudos:1

1 recommendation

reply to crapslock
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.
--
dnoyeB
"Then said I, Wisdom [is] better than strength: nevertheless the poor man's wisdom [is] despised, and his words are not heard. " Ecclesiastes 9:16


cablegeek01

join:2003-05-13
USA
kudos:1

1 recommendation

reply to crapslock
Click for full size
lots of modems
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),


jeffmoss26

join:2002-07-22
Beachwood, OH
Holy modem...that is...wow!


cablegeek01

join:2003-05-13
USA
kudos:1
Click for full size
another view of the DOCSIS lab
I'd be a liar if I said I didn't like coming into work every day.


dnoyeB
Ferrous Phallus

join:2000-10-09
Southfield, MI
kudos:1
reply to crapslock
Wait! Are those all plugged in!?


cablegeek01

join:2003-05-13
USA
kudos:1
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.


Nightfall
My Goal Is To Deny Yours
Premium,MVM
join:2001-08-03
Grand Rapids, MI
kudos:1
reply to crapslock
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.
--
My domain - Nightfall.net


cablegeek01

join:2003-05-13
USA
kudos:1

1 recommendation

said by Nightfall:

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.


Nightfall
My Goal Is To Deny Yours
Premium,MVM
join:2001-08-03
Grand Rapids, MI
kudos:1
Reviews:
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·Comcast
·Callcentric
said by cablegeek01:

said by Nightfall:

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 domain - Nightfall.net


cablegeek01

join:2003-05-13
USA
kudos:1
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.


jeffmoss26

join:2002-07-22
Beachwood, OH
reply to crapslock
So what does the ethernet side of all those modems connect to?


cablegeek01

join:2003-05-13
USA
kudos:1
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.


jeffmoss26

join:2002-07-22
Beachwood, OH
reply to crapslock
very cool, thanks for explaining!

cramer
Premium
join:2007-04-10
Raleigh, NC
kudos:9

1 recommendation

reply to cablegeek01

Believe it or not, there are a decent number of customers who refuse to upgrade their old modems because "they still work".

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.)

Kearnstd
Space Elf
Premium
join:2002-01-22
Mullica Hill, NJ
kudos:1
reply to cablegeek01
said by cablegeek01:

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.

That is a lot of power, I guess gives an idea of how much a whole block's worth of modems is using.
--
[65 Arcanist]Filan(High Elf) Zone: Broadband Reports