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
Measuring the Correct Parts of the Network
There are a number of ways to gauge the speed and performance of a network, however it is important
to understand exactly what parts of the network are being tested. In the diagram below there are a number
of components:
• The Hybrid Fiber Coax network between the end user and the CMTS
• The local CMTS network segment where the CMTS connects to the cable service provider's network
• The cable service provider's internal network
• The public Internet.
When you perform a speed test between two points, you are measuring the speed of all the network
components between the two points.
For example, if you try to perform a speed test between the CPE and Server 3, which is connected to the
Internet via a 128Kbps ISDN line, then you will never see speeds of greater than 128Kbps, even if the
available bandwidth on the cable segment is greater then 128Kbps.
The most accurate way to measure the performance of the cable segment itself would be to perform a
speed test between the CPE and Server 1 which is connected to the same network segment as the CMTS.
This is because the only path data needs to travel over is the coaxial cable segment. The data must also
travel across the local CMTS Network Segment as well, but it is presumed that this segment is of a high
bandwidth (FastEthernet or greater) and does not have a high level of congestion.
If for some reason, no server can be connected to the local CMTS network segment, then the next most
accurate way to test the performance of the cable segment would be to perform a speed test between the
CPE and Server 2. This will be an accurate measurement as long as there are adequately high speed and
uncongested links within the cable service provider's internal network between the CMTS and the CPE.
The most inaccurate way to determine the performance of the cable segment is to perform a speed test
between the CPE and a server on the public Internet. This is because there may be congested links in the
public Internet between the CPE and the server, or there may be very low speed links in the path between
the CPE and the server on the Internet.
Determining the Download and Upload Rate
It is very important to be able to get an objective measurement of exactly what levels of upload and
download throughput are being achieved before any conclusions can be made about whether a
performance problem exists in a DOCSIS network.
The easiest way to determine the speed at which uploads and downloads may are occurring is to upload
or download a very large file via FTP or HTTP between a CPE device connected to a cable modem, and
a server located somewhere behind the CMTS. Most FTP and HTTP clients are able to display the speed
at which a download or an upload is occurring either during the transfer or once the transfer is complete.
The transfer speed as seen as a result of the FTP or HTTP operation will typically be about 90% of the
true total throughput attained. This is because the displayed FTP or HTTP transfer speed does not take
into account extra IP and DOCSIS overhead that needs to travel between the CPE device and the CMTS.
There are more accurate methods of measuring throughput, such as using dedicated testing equipment
like a Netcom Smartbits or IXIA packet generator, however these systems are not always readily
available or easily connected to a production cable network. It is worth noting however that if throughput
tests are being carried out in a lab environment then using a dedicated device will reveal much more
information than the simple FTP or HTTP download test.