Cisco DPC2203C, Installation And Operation Manual

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Theory of Operation
72-4017315-01 Rev B 7
Theory of Operation
This section summarizes the theory of operation for cable modems and provides a
high-level overview of the operational stages for the cable modem. Reading this
chapter provides a better understanding of how cable modems operate.
Note: This section is not intended to be a specification for the cable modem.
Cable Modem Initialization
A cable modem must establish a communication link with the headend before it
becomes fully operational. This section describes the eight DOCSIS-required
operational stages through which a cable modem progresses in establishing this
communication link.
This section provides a detailed explanation of each of the following operational
stages.
1 Scan for Downstream Channel
2
Obtain Upstream Parameters
3 Adjust Timing Offset and Power Level
4
Establish IP Connectivity
5 Establish Time of Day
6
Transfer Operational Parameters
7 Register with the Cable Modem Termination System (CMTS)
8
Initialize Baseline Privacy
Scan for Downstream Channel
When a cable modem powers on, the cable modem starts to scan the network for the
CMTS downstream channel. The downstream channel is the channel used to send
data from the CMTS to the cable modem. The cable modem identifies a valid
downstream data channel as a channel that has QAM signal timing, forward error
correction (FEC) framing, MPEG packets, and downstream media access control
(MAC) messages. The CMTS terminates the cable modem signal at an upstream
location and provides the cable modem with a network connection.
This section discusses the cable modem downstream scanning routine along with
two features that speed up the downstream scanning process: the Valid CMTS
Frequency Table and the WebWizard Gscan function.