Cable Modem Startup Procedures

A Cisco Router Tutorial

By Chris Bryant, CCIE #12933

The DOCSIS standards specifically outline the steps a cable modem takes to obtain a signal and IP address. Once the cable modem powers up, it's got to find a DOCSIS channel. The cable modem will scan for a particular radio frequency (RF), allowing it to achieve QAM Lock.

The Cable Modem Termination System (CMTS) will send three maintenance messages:

SYNC (Synchronization), a time synchronization message
UCD (Upstream Channel Descriptor), defining parameters of communication with upstream devices
MAP (Bandwidth Allocation Map), telling the modem when it can transmit, for how long

The CMTS will continue to send these messages on occasion, referred to as periodic maintenance.

At this point, the modem has achieved L1 and L2 connectivity. To achieve L3 connectivity, the cable modem will now request an IP address from the DHCP server.

The information contained in the DHCP Response includes the name of a DOCSIS configuration file and the location of a TFTP Server. The cable modem will then download that DOCSIS configuration file from the TFTP server.

At this point, the cable modem has done all of the following:

Completed the QAM Lock process

Acquired an IP address, TFTP server location, and DOCSIS configuration file from the DHCP server

Acquired and processed the DOCSIS configuration file from the TFTP server

The modem must now officially register with the CMTS, listing the communication parameters the modem has successfully negotiated. The CMTS will send a response in return. If the response indicates successful registration, the cable modem can now forward data.

There's one more little detail that DOCSIS doesn't care about - the PC needs an IP address!  At this point, the PC will now request an IP address of its own from the DHCP server.

One step up from the cable modem, we have Digital Subscriber Line, or DSL.  DSL uses a preexisting phone line for broadband delivery. DSL comes in quite a few flavors, though, which you should be aware of when purchasing DSL service. We'll talk about ADSL, G.Lite, SDSL, RADSL, and more in the next free Cisco routing tutorial - just click that link to learn more!

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com