Testing and Troubleshooting Media Converters

Installers and integrators often test media converters and other network equipment,  to ensure the equipment functions properly, before installing them at the end customer’s site.  Having invested in bench test equipment, integrators and installers  test the media converters for valid link and passing data.  If the equipment is not linking/passing data, it’s a lot easier to troubleshoot  in a lab. Generally, when new equipment is tested, 85% of the time problems occur due to a mismatch of speed, duplex, protocol, or fiber type. So it usually isn’t a case of bad equipment.  If the equipment is installed at the customer’s site without testing first, and it is not connecting correctly, the integrator has a few choices: replace the device (if he carries a spare), pull out the equipment and bring it back to his bench test location, or troubleshoot it onsite.

Once the considerations of mismatch are resolved, and the conclusion is that possibly there is a hardware issue, there are simple ways to test media converters.  And the best place to start is at the hardware level, the OSI Layer 1. For an Ethernet copper to fiber media converter, make sure all diagnostic features are set to OFF (if available) and connect the copper port to an active port on a switch, establish link, and then loop the fiber port back onto itself by using a single strand of fiber. If the device is sound, a link LED is established, and it will glow green.

MC Basic Test

Single strand of fiber, looped back on the XMT to RCV port. Note green LED indicating link.

A Telco media converter, such as a T1 (or DS3), requires a different kind of test. The integrator will need  something like T-Bird test equipment, if it is in a bench test environment.  Telco media converters will usually offer copper and fiber loopback tests, via a DIP Switch setting on the device.  This allows the integrator to test each segment, one at a time, to determine where the fault is occurring. Like Ethernet media converters, Telco converters have link LED indicators.  But they go one step further: if there are errors, LED indicators will help point the tester in the right direction of the fault. The LEDs often include LOS (Loss of Signal), ER (error), and RAI (Remote Alarm Indication); some of them appropriate for the copper port and some for the fiber port. Telco cards must be set up in pairs, and as a Host/Remote configuration (think Master/Slave).

A Power over Ethernet (PoE) media converter, which is able to source power to a PD (Power Draw) device, may be tested just like a standard Ethernet media converter. By connecting the copper port to an active switch or a PD device, the fiber port can be looped back onto itself to verify link.  Typically, PoE devices will have link LEDs but also a fault (FLT) LED. This LED will glow red if a fault has been detected. The PoE ports may also flash, to indicate a fault condition.  A solid state indicates the link is good, and without errors.

There are some caveats to consider when faced with testing media converters. Sometimes installers don’t want to establish connectivity on the all ports to test connectivity.  While that was acceptable to do when 10Mbps was king, it is unwise to assume this is still a good idea, so it is important to set up connections on the relevant interfaces.  PoE products most certainly will need connections. Without the proper connections, a fault LED will glow red, indicating there is a problem. The reason for this is that the PoE copper port is expecting a connection to a PD device, and if that does not occur, it displays a fault condition. There is no way for the device to determine that you simply have not put a connection on that port. Finally, I have a rule of three. If three media converters are all exhibiting the same fault, chances are the fault is not with the converters…it is something else.

While network books and vendors will encourage installers and integrators  to buy cable testers, fiber optic meters and other test devices,  a simple approach can be very useful.  It’s easy to fall into the trap of looking at the failure from a higher level.