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Posted: Fri Aug 18, 2006 7:53 am Post subject: General cable modem questions

I have had DSL for years and never had any experiance with cable modems. I have a few generic questions about them. The one in use is the Motorola SB5100 on what was Aldelphia, now Time Warner cable. There is also a Linksys router in the mix.

1. Adding a broadband amp (US made Winegard 40-1000MHz) seems to kill the modem. Is it because of excessive signal or is there a need for coverage below 40 MHz? Mind you, this isn't one of those cheap no-name Chineese made wonders. It is only a 10db gain, so I doubt it is a excessive signal issue. I would say there is around +5-10 dbmV of signal with the amp.
2. I was told (by TW support) that you have to leave the configuration of your NICs' to "Obtain a IP address automatically". Is this true? I perfer to assign each conputer with a static IP address. With DSL, I have no issue here, so I don't understand why cable would be different. What does the output of the router have to do with what the modem sees??

Posted: Fri Aug 18, 2006 7:59 am Post subject:

Answers (I work for BHN):

1. House amps kill cable modems because whatever you amp on the forward is matched on the reverse. Your reverse signal must be 50dbmv or below, and that amp is not only probably breaking that, but introducing noise to the line. There should be no need to amp it anyway if the levels are decent on the broadband channel.

2. Cable uses DHCP for all modems, meaning the Linksys router will get a dynamic IP, you can still assign internal 192 addresses to your PC's if your using a router. You just have to reconfigure the router itself for DHCP.

Posted: Fri Aug 18, 2006 8:19 am Post subject:

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BHN

??

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whatever you amp on the forward is matched on the reverse

I don't know if it is a two way or not. Reverse as in upstream back to the HE??

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Your reverse signal must be 50dbmv or below

50dbmV??

Don't you mean 15db??

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ntroducing noise to the line.

The customer is probably only a few 'amps' out. The HE is only a mile or two away. Whatever 'noise' is added, would be very little compared to signals at the end of their system.
Are frequencies below 40 MHz required for cable modems??

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Cable uses DHCP for all modems

Router was set for 'auto matic DHCP'. I can't get past the router to the internet. The routers' LED says good and I see the route, but that's it. The card is set for auto ip address. When it was set for static, it couldn't even see the router.

Posted: Fri Aug 18, 2006 8:44 am Post subject:

BHN = Bright House Networks, cable provider in the southeast, amobg other places.

All cable modems are two way, it has to have reverse to send requests for webpages,etc.

Yes 50db. +15 to -15db is the standard level range for the forward path, which on our system is channel 88. the further from the HE you are, the more power you need for the reverse to get back, but the max the modem can put out is 60db, when it gets above 50db on the reverse (26mhz btw) you get probelms with connectivity.

The added noise is cause by the power you place on the line by adding a house amp. Anything with electricity adds noise, which raises the noise floor and thus lowers the SnR, again, causing issues with connectivity.

As far as the router is concerned, as long as the router itself is set for DHCP, you should be getting an external IP to it. something like 65.xxx.xxx.xxx, if you can get that on the router, the computer ips dont matter, as long as they share the gateway IP of the router, which is usually something like 192.168.0.1, so the computers IP's would look like this if you wanted static:

IP: 192.168.0.x (x=different for each PC, 1-2-3-4 etc.)
DNS 1 and 2: provided by the ISP, eg. 24.95.227.34, (DNS for BHN)
Gateway:192.168.0.1

Hope that helps.

Posted: Fri Aug 18, 2006 8:47 am Post subject:

By the way, something to try in order to just get online is to use the hard reset on the router,(little button on the back you may need to use a pen to depress, then reconnect the router to the modem and PC's and powercycle the modem and router. try setting all the PC settings to Automatic DHCP as well, and see if you can get online, then you can work on the statics from there.

Posted: Fri Aug 18, 2006 9:03 am Post subject:

Quote:
Yes 50db. +15 to -15db is the standard level range for the forward path, which on our system is channel 88. the further from the HE you are, the more power you need for the reverse to get back, but the max the modem can put out is 60db, when it gets above 50db on the reverse (26mhz btw) you get probelms with connectivity.

That is a mouthful and it surely made the picture clearer (no pun intended).
When you say when it gets above 50db, are you saying the output of the modem can vary depending on how far out it is?? Are most return paths around 26MHz? Awfull close to that pesky 11 meter band. Wink

The forward path is just one TV channel? In your case ch. 88?

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Anything with electricity adds noise, which raises the noise floor and thus lowers the SnR, again, causing issues with connectivity.

I understand that, this isn't far from the HE and it's not a no-name amp (as in dirty). Shouldn't be that bad.

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use the hard reset on the router

I tried that. I'm going to scope out Linksys site and see what's there.

(Sorry, I have never heard of that cable system.)

Posted: Fri Aug 18, 2006 9:33 am Post subject:

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That is a mouthful and it surely made the picture clearer (no pun intended).
When you say when it gets above 50db, are you saying the output of the modem can vary depending on how far out it is?? Are most return paths around 26MHz? Awfull close to that pesky 11 meter band. The forward path is just one TV channel? In your case ch. 88?

Not the speed of the modem, just the amount of RF signal in DB that it has to send back to the headend. The further it goes, and the amount of splitters, or for that matter, any attenuation, the more it sends, with 50db being the practical limit. Lets say you have a reverse of 30db at the tap, if you then add a DC6 (like a splitter) and put the modem off that, you will have 36db of reverse coming from the modem.

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I understand that, this isn't far from the HE and it's not a no-name amp (as in dirty). Shouldn't be that bad.

Again, it doesnt matter what kind of amp it is, (though a noname would obviously be worse) you simply cant use a house amp on a cable modem unless your reverse is extremely low already, and even then, the noise it causes could still create issues, regardless of how little. The reverse path is much more sensitive than the forward because its cumulative all the way back to the headend. Each house, each amp, each bit of noise is picked up along the way and modulated together.

As far as your IP situation, its kind of hard to troubleshoot without more info. If you set a single PC to automatic DHCP and connect the modem to it, do you get online? Keep in mind that with a cable modem you MUST powercycle the modem with every change you make as far as what its connected to.

Posted: Fri Aug 18, 2006 10:14 am Post subject:

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Not the speed of the modem, just the amount of RF signal in DB that it has to send back to the headend.

Yes, I understand that.

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you simply cant use a house amp on a cable modem unless your reverse is extremely low already, and even then, the noise it causes could still create issues, regardless of how little.

Ok, understood that. I had no idea they were that touchy. Shocked

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If you set a single PC to automatic DHCP and connect the modem to it, do you get online?

Yes.

Posted: Fri Aug 18, 2006 10:18 am Post subject:

Ok then we know the modem is fine, now its just a matter of setting up the ip info for the pcs.

First: do you know the gateway IP of the router, and if you connect the PC to the router, can you browse to the configuration page by that IP?

What is that IP?

Posted: Fri Aug 18, 2006 11:06 am Post subject:

Yes to both, router access isn't a problem.
192.168.1.1

Posted: Fri Aug 18, 2006 11:13 am Post subject:

Ok, here is what you do:

If you dont already have it, connect the modem back to a PC, do an IP config, and write down the DNS ip.

reconnect the router, then set the routers WAN settings to Automatic DHCP, and reset the modem.

plug just one PC into a lan port on the router.

on that pc, chose static, and enter the following ip info:

IP: 192.168.1.10
DNS: The one you found when the modem was direct
DNS2: move the last number of dns1 down. IE: 24.95.227.34 becomes 24.95.227.35
Gateway: 192.168.1.1

save the settings and see if you can connect.

Posted: Fri Aug 18, 2006 11:25 am Post subject:

I'll have to wait till I go there again. I do have the manual off od Linksys site I will look through. I didn't set this up in the first place or I probably wouldn't have most of these issues. Between that and half a** info or no info at all, it's almost the blind leading the blind. You try to help someone and all you get is either "I don't know" or just static.

Anyway, back at the ranch, a couple of questions for you regarding the first part of this thread; cable modems:
You mentioned 26 MHz being the reverse channel and ch. 88 being the forward. Is this typical of many systems? It was my understanding that most or all of the 750 MHz spectrum (or whatever the systems bandwidth is) was used, hence the fact no traps or filters can be used for internet service.

Posted: Fri Aug 18, 2006 11:35 am Post subject:

Pretty much everything on a cable network takes up a small Mhz range. Its all used, but for different things. On our network, 608Mhz, or Ch. 88, is our forward data carrier for High speed Data. Its actually 29Mhz we use for our reverse on HSD, and 26 is what we use for reverse on the digital boxes, for services such as Video on demand.

All the other pieces of the spectrum are segmented out for video channels, as on a digital network each Mhz range can hold multiple channels. Its possible to use traps and filters based on this, for example, all of our digital services can be trapped out with a Basic only trap, which only blocks out from say 200Mhz up (completely made up since I dont remember off hand.) That allows all of the analog channels to pass through since we only modulate them onto 199Mhz and below, for example.

Our entire reverse path is 2-40Mhz, although at the moment I belive we are only using 26Mhz and 29Mhz
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Posted: Fri Aug 18, 2006 11:37 am Post subject:

Oh, and to answer your question, no, Internet does only run on that single frequency of 608Mhz and will work without any of the others. We actually have a data only trap that filters out every other frequency other than 608 and 29

Posted: Fri Aug 18, 2006 12:49 pm Post subject:

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Internet does only run on that single frequency of 608Mhz and will work without any of the others. We actually have a data only trap that filters out every other frequency other than 608 and 29

Very, very interesting. Thank you for that.

Using a SLM, is there anyway you can find what/where the data channels are? Would that channel be at a higher level, lower level or the same than a digital QAM channel? Would there be empty channels on either side to protect that signal?