Troubleshooting guide

7-11
Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
OL-5169-01
Chapter 7 Troubleshooting the CMTS
Troubleshooting the Cisco uBR7246VXR
Once a usable UCD is found, the cable modem will begin to listen to MAP (Bandwidth Allocation Map)
messages which contain the upstream bandwidth allocation map of time. A section of time is mapped
out into mini-slots and assigned to individual modems. There are also regions in the MAP for broadcast,
contention based initial maintenance (or broadcast) ranging. It is these regions of the MAP that the cable
modem must send its initial ranging requests until the Cisco uBR7246VXR responds with a ranging
response (RNG-RSP).
If an MTA cannot find an initial maintenance region before a T2 timer expires, most likely the Cisco
uBR7246VXR has a faulty configuration. You should also check the insertion-interval for the cable
interface on the router.
Ranging Process - init(r1),init(r2), and init(rc) state
At this stage, the MTA begins a ranging process to calculate the necessary transmit power level to reach
the Cisco uBR7246VXR at its desired input power level. A reasonably good transmit power is roughly
40 - 50 dBmV (based on a uBR 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 Cisco uBR7246VXR
receiver to discern the symbols yet not too high to prevent increased bit error-rates.
The MTA sends a ranging request (RNG-REQ) message to the Cisco uBR7246VXR and waits for a
ranging response (RNG-RSP) message or a T3 timer expiry. If a T3 timeout occurs, the retry count
increments. If the retry count is less than the maximum number of retries, the cable modem transmits
another RNG-REQ at a higher power level. This ranging process occurs in the initial maintenance or
broadcast regions of the MAP because the Cisco uBR7246VXR has not assigned the cable 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 cable modems have a ranging backoff
algorithm to calculate a random backoff time between RNG-REQ transmissions. This can be configured
using cable upstream range-backoff command. When the transmit power has reached a sufficient level
for the Cisco uBR7246VXR, it will respond to the RNG-REQ with a RNG-RSP containing a temporary
SID. This SID will be used to identify unicast transmission regions in the MAP for unicast ranging.
Below output shows an MTA in init(r1) state indicating the MTA cannot get past the initial ranging stage:
r#show cable modem
The debug below shows how the MTA fails to complete the ranging process and resets after a T3 timer
expiry. Note the CMAC_LOG_ADJUST_TX_POWER messages coming from the Cisco
uBR7246VXR asking the MTA to adjust it's power:
1w3d: 871160.618 CMAC_LOG_STATE_CHANGE ranging_1_state
1w3d: 871160.618 CMAC_LOG_RANGING_OFFSET_SET_TO 9610
1w3d: 871160.622 CMAC_LOG_POWER_LEVEL_IS 19.0 dBmV (comman)
1w3d: 871160.622 CMAC_LOG_STARTING_RANGING
1w3d: 871160.622 CMAC_LOG_RANGING_BACKOFF_SET 0
1w3d: 871160.622 CMAC_LOG_RNG_REQ_QUEUED 0
1w3d: 871160.678 CMAC_LOG_RNG_REQ_TRANSMITTED
1w3d: 871160.682 CMAC_LOG_RNG_RSP_MSG_RCVD
1w3d: 871160.682 CMAC_LOG_RNG_RSP_SID_ASSIGNED 6
1w3d: 871160.682 CMAC_LOG_ADJUST_RANGING_OFFSET 2813
1w3d: 871160.682 CMAC_LOG_RANGING_OFFSET_SET_TO 12423
1w3d: 871160.686 CMAC_LOG_ADJUST_TX_POWER -48
1w3d: 871160.686 CMAC_LOG_STATE_CHANGE ranging_2_state
Interface Prim Sid
Online
State
Timing
Offset
Rec
Power QoS CPE IP address MAC address
Cable2/0/U0 6
init(r1) 2813 12.00 2
0
10.1.1.22 0050.7366.1e01