How do cablemodems work?

Andy C · 15-01-2004, 15:48

I've been trying to explain to a friend that our cablemodems use a MAC address to identify themselves on the network. I described it by saying we're basically on a LAN and allowed MAC addresses are issued with an IP - the DHCP server also sends a config file matching that allowed on the account with the specified MAC address. I also said our data is carried using DOCSIS. Still, I can't seem to get him to understand and he wants to make a wager that a cablemodem still uses a system similar to PPP.

So, can anyone point me to a detailed description on how a cablemodem network works and identifies itself over a CATV network please? Also how a modems MAC address is tied to a user account and config. Possible diagrams showing a users modem, UBRs and headends may be helpful.

Many thanks for any assistance.

Andy.

Ben · 15-01-2004, 16:01

Hi Andy

You might want to take a look at Robin Walker's site

http://homepage.ntlworld.com/robin.d...ips/index.html

might help

Andy C · 15-01-2004, 16:10

Thanks for the reply, Ben.

I have looked at Robin Walkers site, but I was hoping for something more detailed as to what exactly goes on once you power up the modem.

AFAIK, basically the uBR detects the modem and forwards the connection requests to the DCHP server. The DCHP server verifies the MAC address is allowed on the network and what service config file it should be sent. The config is sent and an IP address given. However it's all the techy details I'm unsure about.

Andy.

rdhw · 15-01-2004, 16:13

Quote:

Originally Posted by Andy C

I can't seem to get him to understand and he wants to make a wager that a cablemodem still uses a system similar to PPP.

He loses the wager. He's confusing cable broadband with ADSL, which does indeed use PPPoA, a mechanism derived from PPP. Cable networks are always-on, do not have a dialer, and PCs get their configuration via DHCP.

Andy C · 15-01-2004, 16:15

Quote:

Originally Posted by rdhw

He loses the wager. He's confusing cable broadband with ADSL, which does indeed use PPPoA, a mechanism derived from PPP. Cable networks are always-on, do not have a dialer, and PCs get their configuration via DHCP.

That's what I'm trying to explain, but technical documention as proof would be ideal.

Pritch · 15-01-2004, 16:46

Quote:

Originally Posted by Andy C

That's what I'm trying to explain, but technical documention as proof would be ideal.

But Robin is technical documentation!

This link isn't exactly what I call documentation, but it's about the closest I can get at the moment:

http://www.dslreports.com/faq/3455

icanadvise · 15-01-2004, 17:41

Quote:

Originally Posted by rdhw

He loses the wager. He's confusing cable broadband with ADSL, which does indeed use PPPoA, a mechanism derived from PPP. Cable networks are always-on, do not have a dialer, and PCs get their configuration via DHCP.

Doesn't the AOL service via Cablemodem on NTLs network use PPPoA?

poolking · 15-01-2004, 17:44

Two soup cans and a piece of string.

BBKing · 15-01-2004, 18:20

PPPoA on AOL?

Nope, PPPoE as far as I know. The A in PPPoA stands for ATM, which ntl doesn't use, at least not on the broadband network.

Basic CM details:

CM powers up, starts looking for frequencies on the cable to lock onto, both downstream and upstream (the latter rather lower on the spectrum).

Once it's found the downstream and upstream it locks on and starts looking for a DHCP server using a broadcast. The CMTS (aka UBR) picks this up and forwards it to the DHCP using a process known as DHCP relay.

The DHCP server receives the request (which contains the MAC of the cable modem). It then looks up in a database which CM file name to send back to the CM and which IP range to use for the DHCP lease.

The CM receives the DHCP lease (which contains the CM file name and the TFTP server IP) and sets its IP address details accordingly. It then requests this CM file from the TFTP server and configures itself using the resultant values. Finally the CMTS validates the whole process and if satisfied allows two-way IP communications.

DOCSIS is just the Layer 2 (data) protocol used for two-way communication over HFC networks.

Sociable · 15-01-2004, 18:30

Part of the confusion could come from the fact AOL use both BT and NTL networks so for some people they will be suppliing ADSL and using the same as BT and for some they will be using Cable and so using same as NTL.

So it is possible for both to be correct I think you will find.

Tristan · 15-01-2004, 19:01

Quote:

Originally Posted by BBKing

PPPoA on AOL?

Nope, PPPoE as far as I know. The A in PPPoA stands for ATM, which ntl doesn't use, at least not on the broadband network.

Basic CM details:

CM powers up, starts looking for frequencies on the cable to lock onto, both downstream and upstream (the latter rather lower on the spectrum).

Once it's found the downstream and upstream it locks on and starts looking for a DHCP server using a broadcast. The CMTS (aka UBR) picks this up and forwards it to the DHCP using a process known as DHCP relay.

The DHCP server receives the request (which contains the MAC of the cable modem). It then looks up in a database which CM file name to send back to the CM and which IP range to use for the DHCP lease.

The CM receives the DHCP lease (which contains the CM file name and the TFTP server IP) and sets its IP address details accordingly. It then requests this CM file from the TFTP server and configures itself using the resultant values. Finally the CMTS validates the whole process and if satisfied allows two-way IP communications.

DOCSIS is just the Layer 2 (data) protocol used for two-way communication over HFC networks.

So what happens if you set your PC/router to a fixed IP address (say you've had the same one for months), so that the modem doesn't make the initial DHCP request?

rdhw · 15-01-2004, 19:09

Quote:

Originally Posted by Tristan

So what happens if you set your PC/router to a fixed IP address (say you've had the same one for months), so that the modem doesn't make the initial DHCP request?

You are confusing the cable modem's IP address, which it obtains by DHCP, and your PC's IP address, which your PC obtains by DHCP. You cannot prevent the cable modem obtaining its own IP address by DHCP: it does all this long before your PC is allowed to talk to the network.

If you attempt to set a static IP address on your PC, then NTL will eventually find you out (when your PC fails to renew its DHCP lease), and disconnect you from the network. (This has nothing to do with how cable modems work).