User's Manual

14 Cable System Basics
This Section reviews the basic system operation between the CMTS and the Cable Modem.
A hybrid fiber coax cable system looks like Figure 14-1.
Off-Air
TV Stations
Satellite-Fed
Services
Cable Modem
System (CMTS)
C
o
m
b
I
n
e
r
Fiber
Transmitter
Neighborhood
Fiber Node
Coax Distribution - Two Way
Subscribers
Downstream to other Neighborhood Nodes
NOTE:
- Many neighborhood nodes are fed
from common downstream
- Each neighborhood node is a
separate upstream
Fiber
Receiver
Upstream
Fiber
Receiver
Upstream
Figure 14-1: Typical Hybrid Fiber Coax cable system.
As a cable modem enters the system, it has to accomplish a number of tasks:
1. Tuning
2. Ranging
3. Connection
4. Configuration
5. Registration
6. Maintenance
These are briefly described below.
14.1 Tuning
The cable modem steps through the standard 6 MHz downstream channels listening for a
signal. If it finds one, it looks for the DOCSIS frames. If it doesn’t find one, it continues the
scan.
If it finds a DOCSIS frame, then it synchronizes to it. This includes the modulation (QAM),
error correction (FEC), etc.
The model extracts the information it needs, which includes the Upstream channel
characteristics: channel, bandwidth, modulation, error correction, encoding, etc.
June 2003 Page 14-1