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DOCSIS and Cable Modems – How it works :: Cable Modem Registration

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DOCSIS cable modem registration

DOCSIS Cable Modem Registration

Through DOCSIS tutorial seminars, I have found the most effective way to bring someone up to speed on DOCSIS communications is by teaching the DOCSIS cable modem registration process. During the registration process, we will cover the RF physical layer, the DOCSIS Media Access Layer (MAC) and the Internet Protocol (IP) layer. Yes, the cable modem exercises the first three layers of the Open Systems Interconnection Reference Model (OSI Reference Model or OSI Model).

First let's consider the picture below to understand conceptually the DOCSIS network as it exists in a standard cable TV (hybrid fiber-coax or HFC) network.

DOCSIS in an HFC Network

On the left is a Cable Modem Termination System (CMTS) that is connected to an Internet back bone.  To the CMTS is also connected a number of servers which run a Dynamic Host Configuration Protocol (DHCP) server, a Trivial File Transfer Protocol (TFTP) server, and a Time of Day (ToD) server.  The CMTS transmits downstream data via 64- or 256-QAM signals over the RF Plant to the subscriber's home cable modem.  The cable modem transmits data to the CMTS on the upstream over the RF Plant via QPSK, 8-, 16-, 32, or 64-QAM modulation.  It is at this point that we must make the assumption that the RF Plant (essentially miles of coaxial cable, fiber optic cable, RF amplifiers, a fiber optic node, Erbium-doped fiber optic amplifiers and countless RF passive devices) is capable of supporting the RF modulations being transmitted by the DOCSIS devices without significant degradation to stop higher layer communications.

Downstream DOCSIS Channel Lock

Downstream DOCSIS Channel Lock

Once a cable modem is powered on and connected to the RF cable of a CATV network, it will begin a "hunt" for a valid downstream DOCSIS channel.  First the cable modem looks for and locks to any 64- or 256-QAM digital channel.  Fortunately for the modem, the CMTS is sending out a "Sync" broadcast at least every 200 msec, which is used for system timing.  In addition, the CMTS sends out an Upstream Channel Descriptor (UCD) every two seconds, which tells modems the upstream frequency to transmit on, symbol rate, modulation profile, and other parameters necessary to communicate on the network.  Finally the CMTS is sending out Media Access Protocol (MAP) messages to allocate "talk time" to each cable modem.  You see, since there are many cable modems and only one upstream frequency, the cable modems must "time-share" the upstream channel, which is called Time Division Multiple Access (TDMA).  Now, when the cable modem successfully locks to a QAM channel, it looks for the Sync, UCD and MAP messages from the CMTS.  If it finds these it knows it is on an active DOCSIS channel.  If they are not present, the cable modem assumes the DOCSIS channel is offline or it is on a video QAM channel and continues its search.

Assuming a successful lock above, the cable modem is now ready to begin ranging with the CMTS.  The ranging process begins with Initial Ranging, which is a process in which the cable modem begins by sending a Range-Request at a power of 8 dBmV (very low power).  If it does not receive a Range-Response from the CMTS the cable modem retransmits the Range-Requests at a 3 dB higher power level and continues the process until a Range-Response is received.  See the following diagram:

Ranging with CMTS

Ranging with CMTS

All initial ranging occurs during a "contention" window, which means that the CMTS does not have prior knowledge of the cable modems existence.  Therefore it is possible that when multiple cable modems are attempting to register during the contention window that they could interfere with each other and cause collisions.  DOCSIS has a built-in back-off window for just such an occurrence which will help alleviate collisions in the contention window.  This is most prevalent when there is a system outage and many cable modems are trying to come back online at the same time.  Once the modem has received its first Range-Response from the CMTS it will moved from Initial Ranging to Station Maintenance.  The cable modem will also be instructed by the CMTS to make adjustments to its transmitting frequency, amplitude, timing offset and optionally pre-equalization.  Station Maintenance (Ranging) will occur at least once every 30 seconds for each cable modem on the DOCSIS network to continue making these adjustments and so that the CMTS knows the modems are online.

Next the cable modem is ready to move from DOCSIS protocol communications to IP layer communication and perform DHCP to get an IP address and the addresses of other devices in the network.  Now that the cable modem is operating within its TDMA parameters, it must first ask for permission to transmit data to the CMTS by sending a bandwidth REQUEST.  The CMTS will prioritize the request in its queue and issue a MAP for the specific cable modem.  When the cable modem's time slot comes up, it can transmit a DHCP discover to find a DHCP server.  The DHCP server on the network will respond and offer an IP address to the cable modem along with a number of other network addresses, gateways and parameters for proper network operation.  A DHCP Request and Acknowledgement are required to complete and confirm the transactions.

DHCP and ToD

DHCP and ToD

Next the cable modem will request the Time of Day from the ToD server.  This is a simple transaction that was a requirement in DOCSIS 1.0, but has now become an option in later release of the specification.  We will see in DOCSIS 3.0 that more emphasis is placed upon the critical nature of over-all network timing accuracy, but this will be addressed in a later post.

Now the cable modem is ready to download a very important file called a configuration file from the TFTP server.  The configuration file contains all of the parameters the cable modems needs for network access speeds, quality of service, advanced service features such as voice-over-IP and much more.  The following diagram illustrates the flow of procuring the configuration file.

TFTP Download and Registration

TFTP Download and Registration

Upon validating the MD5 check-sum to ensure the TFTP file was properly downloaded, the cable modem is now ready to perform the most important step - registration.  The cable will send a Registration Request to the CMTS along with a list of TLV (Type Length Value) parameters that tell the CMTS how the cable modem has been told it is to operate on the network.  The CMTS reviews this information against the parameters it has been programmed with by the system administrator.  Provided the cable modem has not be "hacked", data has been corrupted or there is an interoperability issue, the CMTS will send a Registration Response "Okay" message and assign the cable modem a Service IDentifier (SID).  The cable modem will respond back with a Registration Acknowledgment notifying the CMTS that it has received confirmation of registration and it is now online and ready for subscribers to transmit Internet data.

So as a basic recap, the process goes throught the following steps:

  • Downstream Channel Search and Lock
  • DHCP
  • ToD
  • TFTP
  • Registration

After registration a cable modem could next enter an encryption mode called Baseline Privacy Interface (BPI) protocol, but I will save that topic for a blog of its own.

Limited Time - DOCSIS 3.0 Cable Modem Registration with T3 and T4 Timeouts Poster

Mr. Volpe has over 25 years of communications industry experience. He is focused on the cable and telecom industry with deep technical and business skills. Mr. Volpe is currently the president and chief technologist of the Volpe Firm and holds an MSEE with honors.

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