Cable modems 101

dstranathan · October 6, 2003 9:52AM

OK, this sounds dumb, but what IS a cable modem? I mean, what does it do exactly? My guess is that it basically converts/conditions LAN protocols to special WAN protocols. Am I close? Obviously they are more than just a switch. More like a router right?

Do ISPs do anything speical to therir cable/DSL modems, in terms of firmware, hard-coded route info, encryption, etc?

If I buy and replace my existing ISPs (Road Runner) modem with a generic DSL/cable modem (Belkin, etc), do I need to configure it in any way, or will it "just work"?

Can ISPs "log-in" to their customers cable modems via software or terminal connections? Can they be "managed" remotley?

Why are they called "modems" anyway? Why aren't they called routers?

Is there software or hacks out there on the 'net to let me "see inside" my cable modem? I'm curious as to what info it stores, etc...

gargoyle · October 6, 2003 12:51PM

Do a search on google for "docsis cable modem" for the background of how and why they are used... Basically a CM is a transparent learning bridge.

As for replacing your ISP modem with a generic one, I very much doubt that would be possible. Here in the UK (blueonder to be specific) each time the modem boots up (or is reset by the ISP) it requests a config file from a server at their end.

This is normally dependant on the type of modem and your subscription speed. Mine askes for something like sb3100_1024_256.cfg, cos I have a motorola surfboard 3100 and a 1meg/256K line.

There are ways of "tricking" your modem into getting a prepared config from a computer on the "inside" of the network. Normally this is done to uncap the speed of the modem. WARNING doing this will result in your ISP banning you and maybe even blacklisting you or taking you to court - you are effectively stealing from them.

but for education purposes you might want to google a few combinations of the following keywords:-

Code:

cable modem, docsis, snmp, tftp, bootp, md5, docsdiag

ebby · October 6, 2003 1:05PM

Wow. That is technical stuff you want to know. I'll do my best.

Quote:

... what IS a cable modem?

A cable modem converts a LAN signal into the QAM signal used for cable networks. This signal is the same as the signal for your Cable box (for TV) except it can both receive and send data. (Some cable boxes send data too)

Quote:

Obviously they are more than just a switch. More like a router right?

Switches, routers, and modems are very different things. A switch (and hubs) connects and shares data to multiple computer connections. Routers are more "intelligent" with features like port forwarding, firewalls, NAT, statistics, and logs. Routers usually have a switch built into them. Modems simply convert the signals they receive from one network to another kind of network.

Quote:

... do I need to configure it in any way, or will it "just work"? Can ISPs "log-in" to their customers cable modems via software or terminal connections? Can they be "managed" remotely?

Most cable modems use "DOCSIS" to communicate to your ISP. This allows the cable modem to be remotely upgraded and monitored by your ISP. When you get a new cable modem you have to tell your ISP so they can change your account to reflect the new modem.

Quote:

Why are they called "modems" anyway? Why aren't they called routers?

Modem stands for Modulator/Demodulator. Or in other words changing one signal into another, than back again.

Quote:

Is there software or hacks out there on the 'net to let me "see inside" my cable modem? I'm curious as to what info it stores, etc...

There is this... I don't think the modem stores much information at all. Just the firmware, MAC address and the type of service you purchased to limit upload and download speeds. Your computer has much more useful information like your IP address, Subnet mask, search domain, and DNS servers.

graphicump · October 6, 2003 4:54PM

My cable modem, a Linksys, has an ethernet address, as do most:

http://192.168.100.1/

That gives all sorts of information to you, but doesn't let you configure anything. Your ISP does the configuring from their end.

You have to tell your ISP if you replace your modem because the MAC address of the original modem is registered, not the MAC address of your computer. So you won't be able to get online with a new modem unless you've registered it with the ISP.

ebby · October 6, 2003 7:32PM

Quote:

Originally posted by GraphicUmp

My cable modem, a Linksys, has an ethernet address, as do most:

http://192.168.100.1/

Hmmm... Something's not right there. First, modems don't have IP addresses and second that range of IP addresses (192.168.*.*) are reserved for household/general use. (Like behind a firewall)

dmband0026 · October 6, 2003 7:44PM

So would I be able to take the "cable" that runs into my modem and split that to a TV tuner and get the same cable TV that the rest of my house gets?

ebby · October 6, 2003 8:14PM

Maybe. First, it is probably illegal. Second, your ISP probably installed a signal filter on your line that only allows internet data. Third, if you were somehow lucky enough to have this option, there are ways for your ISP to track who is using cable TV/Data and track you down.

dmband0026 · October 6, 2003 9:14PM

Quote:

Originally posted by Ebby

Maybe. First, it is probably illegal. Second, your ISP probably installed a signal filter on your line that only allows internet data. Third, if you were somehow lucky enough to have this option, there are ways for your ISP to track who is using cable TV/Data and track you down.

But if I already had that cable service running to my TV it wouldn't be illegal would it? I just didn't want to have to run another cable line to my computer for my TV tuner card.

reg · October 6, 2003 11:59PM

At my parents house the cable comes in on one line. We then have a 4 way splitter, 3 for TV's and 1 for the cable modem. Most cable companies limit the types of cable modems that you can use. They usually will have a listing of the supported modems on their web sites. We used the company supplied modem (modem with only a usb port) for 1 month just to make sure everything worked properly, then switched to a 4 port wireless linksys unit. We called the cable company when we made the switch and had no down time. if your cable provider has a lot of down time for your cable TV them expect alot of down time for your internet as well. Also check with the cable company if it is a 2 way or 1 way modem. If it is one way then you will still have to use your phone line to upload to your ISP.

reg

ebby · October 7, 2003 12:20AM

Quote:

Originally posted by DMBand0026

But if I already had that cable service running to my TV it wouldn't be illegal would it? I just didn't want to have to run another cable line to my computer for my TV tuner card.

If you already pay for cable TV than it is OK to split it to a TV. Good luck.

graphicump · October 7, 2003 11:07AM

No, cable modems do have ip addresses; and yes, they are for use behind your firewall. I forgot to remove the http:// to remove a clickable link, that's all.

Once there, there are several things to click on a bar at the top. Status, Log, Signal, and a couple others I can't remember right now. It gives the information, such as the caps in upstream and downstream your cable provider has placed on your service, the status of your service and modem, the log which tells you what your ISP is uploading to your modem, etc.

thuh freak · October 7, 2003 1:12PM

Quote:

Originally posted by DMBand0026

But if I already had that cable service running to my TV it wouldn't be illegal would it? I just didn't want to have to run another cable line to my computer for my TV tuner card.

yes, u can split ur cable line, and feed it to both the cable modem and a tv (assuming u have the technical expertise and tools to cut the line and all). infact, when they set my house up for cable tv and internet, they did just that. they laid the line down, from outside to my room's tv. then i reminded them about the internet, so they cut the line, split it 3 ways (in from outside, out to the tv and out to modem), and plugged in coax cables for all the shit.

as for using a new cable modem, it is possible. others have mentioned some of the more technical stuff behind it, but i thought i'd mention that my friend tried just this. he is a bit of a scrummy guy, who found a "perfectly good" cable modem in the garbage. he swapped his with the garbage one, and it worked (so it was `perfectly good' aside from its stink). he then sold it for a little cash on ebay.

Cable modems 101

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