Time Domain Reflectometry (TDR) technology is used to detect faults in metallic cables that are connected to the Ethernet electrical interface of a switch. When a pulse signal is transmitted through a cable, part of the energy is reflected back if it encounters a cable end or a fault point. The TDR algorithm measures the time that a pulse travelling through the cable, reaching the fault point, and returning, and then converts the measured time into a distance.
Assume the distance between the interface and the fault point is Distance, the time interval from the pulse signal being sent to when it is received is T, and the transmission rate of the pulse signal in the cable is V. Then the formula for calculating the distance from the fault point to the switch interface is as follows:
Distance = (V x T) / 2
The TDR test command detects the cable connected to the Ethernet electrical interface and display the detection result:
- When the cable is not faulty, the Pair length displayed refers to the total length of the cable.
- When the cable is faulty, the Pair length displayed is the length from the interface to the fault point in the cable.
When employing TDR test on a switch interface, pay attention to the following points:
- TDR test is only used on the Ethernet electrical interface but not suitable for the Optical interface.
- As there is a certain error (refers to the Pair length column in the test results) in the TDR test, so the results are for reference only.
- The recommended cable length for TDR test is less than 100m.
- The interface on which the TDR test is running flaps for a short period of time, this may cause service interruption for a short time.
- Make sure that the interface is enabled when running TDR test.
- The time consumed for TDR test is different on different platforms, from 2 to 8 seconds.
- The combo interface of the switch does not support the TDR function.
Configuring TDR Function
- Run the following command to start the TDR cable test and check the test result from the displayed information.
run show interface diagnostics tdr <interface-name>
After the command is issued, the test result will be displayed. The time consumed for TDR test is different on different platforms, from 2 to 8 seconds.
- Run the following command to start the TDR cable test. The following example shows the cable length (Pair length) is approximately 31 meters long with a possible error of +/- 10 meters.. All four pairs are working fine (Pair status).
Table 1. Description of the run show interface diagnostics tdr output
Indicates the interface name of the switch.
Local four pairs.
Displays the length and error of the cable pair in meters.
Remote four pairs.
The pair status could be "OK", "OPEN", "SHORT", "OPENSHORT", "CROSSTALK" or "N/A".