Broadband Forum TR-115 Testing at the UNH-IOL

The UNH-IOL DSL consortium offers testing based on the Broadband Forum TR-115 test plan. This test is an excellent starting point in understanding the overall quality and functionality of the modem or DSLAM’s (Digital Subscriber Line Access Multiplexer) VDSL2 transceiver. The test cases within this test plan cover a variety of different functions and capabilities that the transceivers should be able to perform. There are counters that track a wide variety of information such as code violations, errored seconds, full initializations, unavailable seconds, etc. Some of the test cases have been designed to test these individual counters by isolating them with specific noise injection methods. These tests will increment the counters and the tester can then verify whether or not they are accurately recording the occurrences that the tester has forced.

When a DSL modem is deployed into the field to be used in homes, it is important the modem does not interfere with other communications that are occurring in close proximity. Crosstalk will always occur when multiple modems are sharing one of the multi-pair lines on the telephone poles so it is important that each modem can “play nice” with the other modems using nearby pairs. There are a number of power limitation masks defined by the DSL specifications / standards. If the connection is trained up using a specific mask, it is crucial that the mask be obeyed. If these masks are not obeyed, the power transmitted by the modem will be too high at frequencies where other technologies may be trying to communicate. In order to deal with these possible issues, there are custom power limit masks that can be configured to limit transmitted power in certain frequency bands. We use a MATLAB (Matrix Laboratory) tool with a spectrum analyzer to view the transmitted power from the modem and compare the measured power to the pre-defined power limitation masks that have been defined by the ITU-T (International Telecommunication Union Standardization Sector).

There are many sources of interference that can couple onto the DSL line and lower the signal to noise ratio. In order to ensure the connection is successful in transmitting data as fast as possible without any lost information, the data rates will adapt without the loss of synchronization to optimize the signal to noise ratio (SNR). When there is too much noise and not enough signal strength, the data rate should be lowered to increase the SNR and make sure that no data is lost because of the compromised signal strength. When there is minimal noise on the line, the data rate will be increased which in turn lowers the SNR to the pre-set value that was set in the profile used for the connection. This is an important performance function for the modem because it guarantees the fastest possible data rate will always be used without allowing any data loss for the current state of the line.

When I was first hired at the IOL, I didn’t have a full understanding of what the responsibilities of my position would entail. Shortly after my hire date, the previous TR-115 tester gave his two weeks which was when my boss came to me and said, “Raise your hand” so I did, which prompted him to say “Congratulations, you have volunteered to be the 115 tester, you have two weeks to learn the TR-115”. From this point roughly a year since I’ve been testing TR-115 and to-date, I have trained a second TR-115 tester, assisted in the development of automation for some of the test cases, updated the PSD collection tool, and I’m currently in the process of training another TR-115 tester.

Over the past year, we have seen a large increase in the demand for the BBF TR-115 test which is why it has been so important to not only have multiple testers trained to run the test by hand, but to also begin the process of automating as many of the tests as possible. With the recent advances in test efficiency as well as the currently implemented automation, we are now able to meet the demand for this test without any delays. I have noticed that for attended testing, it is nice to be able to actually show the engineers, that are coming to observe, the exact testing procedures as well as detailed explanations for any changes made to the test procedures. This TR-115 test plan is a good way to ensure that the CPE or DSLAM will function properly when deployed into the actual field. I believe that with our latest advances in automation and our additionally trained students, we will continue to provide both timely and accurate test results for TR-115.

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