Thread: How do cablemodems work?
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Old 15-01-2004, 19:01   #11
Tristan
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Re: How do cablemodems work?
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?
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