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General cable modem questions

Last response: in Networking
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August 18, 2006 3:53:37 PM

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??

Hope all of that makes sense.
August 19, 2006 2:52:45 AM

Cable setup just like your dsl. The only difference is the modem. Not familar with the hardware they are using. DSL does tune but not cable. I'M surprise a amp is required.
August 20, 2006 3:19:52 AM

Your cable company is telling you to use DHCP because you aren't getting a static IP address from them. This doesn't apply to you because you're using a router, so you can set up your IP addresses any way you like as long as the router is getting it's IP address from the ISP via DHCP (default). Their techs are generally not trained to deal with routers, in fact I'm surprised they even talked to you at all. Usually the first thing they ask is if you have a router. They pretty much force you to connect the modem directly to a PC in order to do any real troubleshooting.
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August 21, 2006 2:19:24 PM

Quote:
DSL does tune but not cable.
Huh? :?:
Quote:
I'M surprise a amp is required.
With all the splits, an amp is needed. I understand the issue(s) now. A return path is needed below 40 MHz that the amp doesn't do below 40 MHz, nor is it bi-directional. I had a idea about freq. coverage, but wasn't sure. Anyway, I can work around this.
Quote:
Their techs are generally not trained to deal with routers, in fact I'm surprised they even talked to you at all.
They apparently are in the US and possible right in my area. They apparently are not your average CSRs'.
I was turned off big time by the whole ordeal (two phone calls). I fealt I was being interegated rather than trying to troubleshoot the problem. Apparently, anyone that calls with no internet access is stealing cable. :roll:
Quote:
Your cable company is telling you to use DHCP because you aren't getting a static IP address from them.
I understand that, but I specifically said I wanted to have static addresses for each computer. He apparently thought I wanted a static internet address.

1. Why is the router called the "Gateway" when it is not what 'connects' to the internet? The modem does.
2. If there is no router, whet then is considered the "Gateway"?
3. Why is the MAC address of the NIC what the CC is concerned about? Why isn't it the modem since that is where their 'business' should stop? IOW's it's not of their f***ing business what is on the other end as long as it's not causing network problems and it's not a business.
August 21, 2006 2:57:30 PM

The amp is use to amplify the signal. For all forward frequencies between 40 and 1000 Mhz, all signals are amplied by 10 db. There's probably too many splitters in the node servicing your home.

The router is a gateway in terms of layer three routing. The modem is just a docsis bridge. The cable modem encapsulates your IP traffic with MPEG-2 frame headers and sends it across the HFC DOCSIS network. On the other side of the HFC DOCSIS network, a CMTS (cable modem termination system) strips off MPEG-2 DOCSIS headers, and then forwards the IP packet based on the routing table.

Router and routing are layer three functions. YOu don't necessarily have to have a router, but you have to have a default route (aka. gateway of last resort). If you don't have a router, this only means that you connect your computer directly to the cable modem and you do not have a network at your location; but you still have a default route as provided in the DHCP process, or static IP set up. Without a default route, your traffic would never leave the subnetwork that your computer is on.




Quote:
DSL does tune but not cable.
Huh? :?:
Quote:
I'M surprise a amp is required.
With all the splits, an amp is needed. I understand the issue(s) now. A return path is needed below 40 MHz that the amp doesn't do below 40 MHz, nor is it bi-directional. I had a idea about freq. coverage, but wasn't sure. Anyway, I can work around this.
Quote:
Their techs are generally not trained to deal with routers, in fact I'm surprised they even talked to you at all.
They apparently are in the US and possible right in my area. They apparently are not your average CSRs'.
I was turned off big time by the whole ordeal (two phone calls). I fealt I was being interegated rather than trying to troubleshoot the problem. Apparently, anyone that calls with no internet access is stealing cable. :roll:

1. Why is the router caleed the "Gateway" when it is not what 'connects' to the internet? The modem does.
2. If there is no router, whet then is considered the "Gateway"?
3. Why is the MAC address of the NIC what the CC is concerned about? Why isn't it the modem since that is where their 'business' should stop? IOW's it's not of their f***ing business what is on the other end as long as it's not causing network problems and it's not a business.
August 21, 2006 3:06:57 PM

Quote:
The amp is use to amplify the signal.
I believe that is a 'given'.
Quote:
gateway of last resort
The NIC?
Quote:
The modem is just a docsis bridge.
Then wouldn't a router be considered a "bridge" also?
August 21, 2006 5:05:01 PM

A cable "modem" is a bridge - a layer 2 device with no IP address. Just think of it as a converter, it's sole function is to convert the coax cable to something your PC can use, in this case, ethernet. When you are directly connected to the modem, your gateway is a router outside your premises that the cable company maintains.

When you connect a router, your router is what connects to the Internet. Your PC(s) gateway to your router, and your router in turns gateways to your ISP's customer access router, located either in the cable POP or in one of their datacenters.

The cable company is concerned with whatever device is on the edge of your network. In other words, if you followed the line from their POP back to your house, they would be interested in whatever device immediately follows the cable modem (usually either a router or a single PC).

So, if you have a router, don't even tell them about it - it usually only serves to complicate things unnecessarily. Tell them you have your PC connected directly to the modem, they won't be able to tell any different. Now, behind the router you can use DHCP or static IPs, it makes no difference.

I should note that what I said at the top of my reply about a modem being a bridge is true, but there are also cable routers. When I say router in this sense of the word, I mean something your cable comapny issues to you that has a coax port on one side and one or more ethernet ports on the other. It functions similar to a modem as far as the end user is concerned, except a cable router is actually a real router. It operates on layer 3 and has multiple interfaces and IP addresses. Typically this type of router is used for business cable services when you have one or more static IPs. If you had a cable router, then you would gateway to it. A regular modem is just a dumb device, essentially a media converter.
August 21, 2006 8:07:43 PM

Quote:
ust think of it as a converter, it's sole function is to convert the coax cable to something your PC can use,
Ok,that makes sense. Actually it's a very good analogy.
Quote:
cable POP
POP? You mean their head end?
Quote:
The cable company is concerned with whatever device is on the edge of your network.
With the above analogy, now this makes sense even tough I feel anything past (after) their modem shouldn't be their concern, or more specifically NOTFB (I'll let you figure that one out).
Quote:
a cable router is actually a real router. It operates on layer 3 and has multiple interfaces and IP addresses.
The same as a combo DSL modem/router as a Westell VersaLink327W;
http://www.westell.com/content/products/pdf/versalink_g...
August 22, 2006 1:23:34 AM

Quote:
The same as a combo DSL modem/router as a Westell VersaLink327W;
http://www.westell.com/content/products/pdf/versalink_g...


Actually, no. From what I read in the product PDF that appears to be a modem / router. This is hard to explain. The things Linksys, D-Link, etc make are not routers, they're NAT devices. A *real* router, at least in my opinion, routes traffic both ways between two IP networks. A "Linksys-style" router on the other hand, doesn't route at all really, all it does is NAT.

Without getting into the nitty gritty, a real router is usually used when you have real, non-RFC1918, Internet-facing IPs. For example, a Cisco 2600 series router is a real router. I'll probably get called out on this just for the sheer fact that so many companies market their NAT boxes as routers everyone believes they are.

The Versalink box is the same thing as a regular cable modem plus a wireless router in one box. I know this isn't going to make any sense, so I'm just going to stop trying... :) 
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