Chapter 4 Connecting the Cisco uBR7225VXR Router to the Cable Headend
Configuring the Digital Signal
Careful system design and operation can prevent potentially serious intermittent performance problems across your cable interface network. Each cable operator should make use of the following guidelines and practices to ensure reliable operation of any 64-QAM based digital network:
•“NCTA Recommended Practices for Measurements on Cable Television Systems” (http://www.ncta.com)
•Part 76 of the FCC Rules and Regulations (http://transition.fcc.gov/oet/info/rules/)
•DOCSIS 1.0 RF Interface Specification (http://www.cablemodem.com)
•DOCSIS 1.1 RF Interface Specification
(http://www.cablelabs.com/specifications/CM-SP-RFIv1.1-C01-050907.pdf)
For example, if your headend overdrives the fiber-optic lasers transmitters, in either the upstream or downstream path, clipping may occur. Laser clipping leads to degraded signal integrity. In minor doses, this signal damage is not immediately visible on an analog video signal, but it can completely disrupt the digital transmission path. (That is, digital signals are more sensitive to clipping than analog signals and will more readily display the negative effects of laser clipping.)
If a digital signal employing forward error correction (FEC) is near its impairment limit, it is very susceptible to changes in signal level—on the order of as little as 0.1 dB. If there is no amplitude margin available in the transmission path between the headend and any one cable modem, the typical signal level variations of a properly functioning cable system (3 to 6 dB) can create intermittent service outages that are difficult to isolate.
Typical CATV measurement equipment, such as digital signal level meters, measure to an accuracy of +/–1 dB. However, some older analog meters only measure to an accuracy of +/–3 dB; therefore, maintaining 6-dB margins above the minimum levels can provide reliable long-term service.
| | Cisco uBR7225VXR Universal Broadband Router Hardware Installation Guide | | |
| | |
| OL-17309-02 | | | 4-41 | |
| | | |
