Troubleshooting guide

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Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
OL-5169-01
Chapter 5 Troubleshooting DOCSIS Networks
Troubleshooting Slow Peformance
Upstream Channel Congestion
Note The measures discussed in this section will not significantly increase the performance of an already
uncongested network.
The upstream channel is normally the most precious resource in a cable network. At present, most cable
service providers use a 1.6MHz channel width and Quadrature Phase Shift Keying (QPSK) modulation
in the upstream path. This equates to approximately 2.5Mbps in total available upstream bandwidth for
all users connected to the one upstream channel. It is important to ensure that the upstream channel does
not become over utilized or congested, otherwise all users on that upstream segment will suffer poor
performance.
The upstream utilization for a particular upstream port can be obtained by executing the CMTS
command show interface cable X/Y upstream <Z>, where cable X/Y is the downstream interface number
and Z is the upstream port number. If Z is omitted then information for all upstreams on interface cable
X/Y will be displayed.
See the Cisco Cable Modem Termination System Command Reference for more information about the
show interface cable X/Y upstream <Z> command.
uBR7246-VXR# show interface cable 6/0 upstream 0
Cable6/0: Upstream 0 is up
Received 71941 broadcasts, 27234 multicasts, 8987489 unicasts
0 discards, 140354 errors, 0 unknown protocol
9086664 packets input, 4394 uncorrectable
122628 noise, 0 microreflections
Total Modems On This Upstream Channel : 359 (354 active)
Default MAC scheduler
Queue[Rng Polls] 0/64, fifo queueing, 0 drops
Queue[Cont Mslots] 0/104, fifo queueing, 0 drops
Queue[CIR Grants] 0/64, fair queueing, 0 drops
Queue[BE Grants] 0/64, fair queueing, 0 drops
Queue[Grant Shpr] 0/64, calendar queueing, 0 drops
Reserved slot table currently has 0 CBR entries
Req IEs 64609697, Req/Data IEs 0
Init Mtn IEs 521851, Stn Mtn IEs 569985
Long Grant IEs 2781600, Short Grant IEs 2067668
Avg upstream channel utilization : 18%
Avg percent contention slots : 77%
Avg percent initial ranging slots : 2%
Avg percent minislots lost on late MAPs : 0%
Total channel bw reserved 37858000 bps
CIR admission control not enforced
Admission requests rejected 0
Current minislot count : 7301855 Flag: 0
Scheduled minislot count : 7301952 Flag: 0
On the upstream port seen in the example, the upstream utilization is currently 18% and there are 359
modems connected to this upstream.
If upstream channel utilization is consistently above 75% during the peak usage time then end users will
begin to suffer issues like latency, slower "ping" times and a generally slower Internet experience. If
upstream channel utilization is constantly above 90% during the peak usage time then end users will be
experiencing an extremely poor level of service because a large portion of end user's upstream data will
have to be delayed or discarded.
Upstream channel utilization will change during the day as different users have an opportunity to use
their cable modem, so it is important to monitor the upstream utilization during the busiest times of the
day rather than at low usage times.