Troubleshooting guide

5-26
Cisco Broadband Local Integrated Services Solution Troubleshooting Guide
OL-5169-01
Chapter 5 Troubleshooting DOCSIS Networks
Troubleshooting Slow Peformance
Table 5-3 shows the total available downstream bandwidth associated with the four possible downstream
modulation schemes available in DOCSIS networks.
The majority of DOCSIS cable networks currently deploy 64-QAM North American DOCSIS and
therefore have 27Mbps available per downstream channel.
Downstream channel utilization can be determined by executing the show interface cable X/Y command,
where cable X/Y is the cable interface being observed. The displayed output rate in bits per second
should be compared to the available downstream bandwidth as seen in the table above.
In the following example, an interface using North American DOCSIS and 64-QAM digital modulation
is analyzed.
uBR7246-VXR# show interface cable 3/0
Cable3/0 is up, line protocol is up
Hardware is BCM3210 ASIC, address is 0005.5fed.dca4 (bia 0005.5fed.dca4)
Internet address is 10.1.1.1.1/24
MTU 1500 bytes, BW 27000 Kbit, DLY 1000 usec,
reliability 255/255, txload 9/255, rxload 5/255
Encapsulation MCNS, loopback not set
Keepalive not set
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:00, output 00:00:00, output hang never
Last clearing of "show interface" counters 00:45:01
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue :0/40 (size/max)
5 minute input rate 587000 bits/sec, 228 packets/sec
5 minute output rate 996000 bits/sec, 239 packets/sec
85560 packets input, 8402862 bytes, 0 no buffer
Received 1013 broadcasts, 0 runts, 0 giants, 0 throttles
247 input errors, 35 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
65912 packets output, 38168842 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 output buffer failures, 0 output buffers swapped out
The first component of this output to note is the Bandwidth of the interface indicated by the BW
parameter. In IOS releases 12.1(8)EC and later, this value is automatically adjusted according to the
downstream modulation scheme and version of DOCSIS being used. In revisions earlier than 12.1(8)EC,
this value must be manually configured using the cable interface command bandwidth
<bandwidth-in-kilo-bits-per-second> otherwise it will remain at the default value of 27000 Kbps.
The second component to note is the transmission load as indicated by the txload parameter. This
parameter gives a metric out of 255 where 0/255 means that no traffic is flowing in the downstream
direction to 255/255 which means that data is traveling in the downstream at the maximum possible rate
(in this case at 27000 Kbps). If this parameter is consistently running at greater than approximately 75%
during the peak usage time (for example, greater than 191/255) then end users will start to experience
slower Internet access and higher latency.
Table 5-3 Available Downstream Bandwidth for Selected Modulation Schemes
Downstream Modulation Scheme Available Downstream Bandwidth
64-QAM North American DOCSIS 27 Mbps
256-QAM North American DOCSIS 38 Mbps
64-QAM Euro DOCSIS 38 Mbps
256-QAM Euro DOCSIS 54 Mbps