As a follow up to my last blog regarding whether or not to include patch cords when testing a fiber channel, another question that has long plagued fiber testing is whether to use the 1-, 2- or 3-jumper reference method.
As shown in the graphic, what is being measured in a fiber channel varies greatly depending on the reference method selected. So it’s a good idea to understand why the 1-jumper method is preferred and what the difference is between the three methods.
The 1-jumper reference method recommended by both TIA and IEC standards assesses the condition of the channel end faces against a very high quality multimode connector from the test reference jumper (i.e., 0.1dB per mated pair), and it includes the loss of the connections at both ends of the channel. Because much of the loss is a result of those connections, the 1-jumper reference method provides the highest accuracy.
While many use the 2-jumper reference method because it is the easiest and fastest to set up, and it was one of the methods used before interchangeable test heads were available with test equipment, it is NOT recommended.
With both test reference cords referenced, one end connection is referenced out so that only half the referenced out loss is applied to the connections at the end. This only provides a partial depiction of the total channel loss.
Furthermore, every mated pair is a “random mating” that will show a different loss each time it is connected due to variances in alignment. With the 2-jumper method that destroys a mated pair after reference and assumes only half the referenced out loss is applied to each new connection made, it is not possible to create or destroy a mated pair and maintain the same loss values.
If it is not possible to use the 1 jumper reference method due to limitations of the test equipment, the 3 jumper reference method is the next best method. This is required when the fiber type of the test reference cords does not match the fiber type of the link as is common when testing MPO links with an LC interface on the tester.
Because the 3-jumper method references out both connectors, it does not provide a true indication of the quality of the end faces at either end of the channel. The 3 jumper reference method has a higher level of uncertainty than the 1 jumper method, which becomes critical for shorter channels. Let me explain.
To generate correct loss limits, the 1-jumper method involves adding 4 connectors after the reference while the 3-jumper method involves adding 2. Depending on the test equipment, a 1-jumper reference has a test uncertainty of +/- 0.1dB and a 3-jumper reference has a test uncertainty of +/- 0.2dB.
Using the 1-jumper method, a 1.9dB 300-meter channel therefore has an uncertainty of about 5%, while a shorter 0.9 dB 50-meter channel has an uncertainly of about 11%. Using the 3-jumper method, the same 300-meter channel that now measures 1.4dB has an uncertainty of about 14%. For the 50-meter channel that measures 0.65dB, the uncertainty now pushes upwards of 30%!
While technicians should ensure that the preferred 1-jumper reference method is used, it is up to the designer to ensure that the channel limits are based on the manufacturer’s loss limits and to ensure that those loss limits are within the maximum loss specified by the IEEE standards. This will allow technicians to determine pass or fail of tested channels based on the actual values guaranteed by the manufacturer. Anything higher could indicate an installation problem.
Dwayne Crawford has more than 20 years of experience in the datacomm industry. He has served on several international standards committees to advance high-performance/low-latency protocols (such as IEEE-1394, GigE Vision and CameraLink) used in real-time image processing and utilizing high-performance computing platforms.