Troubleshooting guide
5-16
Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
OL-5169-01
Chapter 5 Troubleshooting DOCSIS Networks
Measuring RF Signals
1
DOCSIS specifications are baseline settings for an DOCSIS-compliant, two-way data-over-cable network.
2
Minimum settings are slightly different than the DOCSIS settings to account for cable network variations over time and
temperature. Using these settings should increase the reliability of DOCSIS-compliant, two-way data-over-cable networks.
3
Transit delay is defined as the “round trip” from the cable headend to the furthest customer and back.
4
QAM = Quadrature Amplitude Modulation: a method of modulating digital signals onto a radio-frequency carrier signal
involving both amplitude and phase coding.
5
These settings are measured relative to the digital carrier. Add 6 or 10 dB, as determined by your company's policy and derived
from the initial cable network setup, relative to the analog video signal.
6
dBc = decibels relative to carrier.
7
ns = nanoseconds.
Table 5-2 DOCSIS Cable Downstream RF Specifications
Specification
DOCSIS
Specifications
1
Minimum Settings
2
Your Headend
Settings
System/Channel
RF channel spacing
(bandwidth)
6 MHz 6 MHz
Transit delay
3
0.800 µsec 0.800 µsec
Carrier to noise ratio > 30 dB (64 QAM
4
)
5
> 35 dB (256 QAM)
5
>30 dB (64 QAM)
5
> 33 dB (256 QAM)
5
Carrier to ingress power ratio > 35 dB > 35 dB
Composite triple beat distortion < -50 dBc
6
< -50 dBc
Carrier to second order < -50 dBc < -50 dBc
Cross-modulation level < -40 dBc < -40 dBc
Amplitude ripple 0.5 dB in 6 MHz 0.5 dB in 6 MHz
Group delay 75 ns
7
in 6 MHz 75 ns in 6 MHz
Micro reflections bound for
dominant echo
-10 dBc @ < 0.5 µsec
-15 dBc @ < 1.0 µsec
-20 dBc @ < 1.5 µsec
-30 dBc @ < 1.5 µsec
-10 dBc @ < 0.5 µsec
-15 dBc @ < 1.0 µsec
-20 dBc @ < 1.5 µsec
-30 dBc @ < 1.5 µsec
Carrier hum modulation < -26 dBc (5%) < -26 dBc (5%)
Burst noise < 25 µsec < 25 µsec
Seasonal/diurnal signal level
variation
8 dB 8 dB
Signal level slope (50 to 750
MHz)
16 dB 16 dB
Maximum analog video carrier
level
+17 dBmV +17 dBmV
Minimum analog video carrier
level
-5 dBmV -5 dBmV
Digital Signal Levels
From headend -15 to +15 dBmV -15 to +15 dBmV
Signal as relative to adjacent
video signal
-6 or -10 dBc -6 or -10 dBc