Troubleshooting guide

5-5
Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
OL-5169-01
Chapter 5 Troubleshooting DOCSIS Networks
Understanding Initialization States
This ranging process occurs in the initial maintenance or broadcast regions of the MAP, because the
CMTS has not assigned the modem a service identifier (SID) for unicast transmissions in the MAP. Thus,
broadcast ranging is contention based and subject to collisions. To compensate for this the modems have
a ranging backoff algorithm to calculate a random backoff time between RNG-REQ transmissions.
When the transmit power has reached a sufficient level for the CMTS, it responds to the RNG-REQ with
a RNG-RSP containing a temporary SID. This SID is used to identify unicast transmission regions in the
MAP for unicast ranging.
If a modem cannot proceed out of ranging_broadcast_state, the likely cause is an insufficient transmit
power level. Transmit power can be adjusted by adjusting attenuation at the low frequency port of the
diplexer. Increased attenuation will result in increased transmit power levels. Roughly 20 - 30 dBmV of
attenuation is a good place to start. This can be refined once the modem is reaching
ranging_unicast_state more reliably.
Ranging Unicast State
MAC State --->>> 'ranging_unicast_state'
With help from the CMTS, the goal of unicast ranging is to have the modem configure the transmit
timing offset and power level, to insure that transmissions from the modem are received at the correct
time and are at an acceptable input power level at the CMTS receiver. This is achieved through a
conversation of unicast RNG-REQ and RNG-RSP messages. The RNG-RSP messages contain power
and timing offset corrections the modem must make. The modem continues to transmit RNG-REQ and
perform adjustments per RNG-RSP, until the RNG-RSP message indicates ranging success.
If a modem cannot proceed out of ranging_unicast_state, transmit power needs to be refined. That is
what this state is for, so it is a lot easier to find the right attenuation levels for the proper CMTS input
power level. A log mask of 0xffff00bf should be adequate to view ranging messages and power levels.
Use the following command:
-> cmSetLogMask 0xffff00bf
Turning on extensive debugging may severely impact performance and functionality.
Continue? [y|n] y
Full debugging enabled.
cmWriteFlashFile("CM_LOGCONFIG", 0x82f0168, 0x18) by TID 0x82f0400 (tShell)
usrEraseSysFlash(1, 0x1e0000, 0x18) by TID 0x82f0400 (tShell)
.........value = -65473 = 0xffff003f = __func_taskRegsShowRtn + 0xf7ed9d5f
->
For Killerbee reference design modems, a reasonably good transmit power is roughly 40 - 50 dBmV
(based on a CMTS input power of 0 dBmV.) Other hardware may vary. Like the downstream channel,
the carrier in the upstream channel should be sufficiently strong for the CMTS receiver to discern the
symbols, yet not too high to prevent increased bit error-rates.
A modem which encounters T4 timeouts in this state is indicative of a upstream frequency error. The IF
(intermediate frequency)— the output frequency from the uBR before upconversion—is 44 MHz. With
every signal on the cable, there are weaker harmonic products of the signal present on the cable. In most
cases, the power-ratio between the signal and the harmonic products should be large enough such that
the harmonic products can be dismissed. However, the upstream is more sensitive to harmonic products
so an upstream frequency which lies at a harmonic frequency of the IF is a difficult channel to range on.
Harmonics of the IF occur at 11 MHz steps on either side of the 44 MHz IF so upstream frequencies of
11, 22, and 33 MHz should be avoided.
Additionally, on linecards with multiple upstreams, the upstream frequencies on a single linecard must
be unique if the upstream channels are combined on the same physical trunk. The upstreams may be on
the same frequency if the upstream channels are not physically combined.