Mechanized Loop Test

One interesting thing about shared distribution subloops is that we are able to get information about them through Qwest's Mechanized Loop Test (MLT) system. This page describes the MLT system. We note the following section of the FCC rules:

47 CFR section 51.319(h)(7)(i) Incumbent LECs must provide, on a nondiscriminatory basis, physical loop test access points to requesting carriers at the splitter, through a cross-connection to the competitor's collocation space, or through a standardized interface, such as an intermediate distribution frame or a test access server, for the purposes of loop testing, maintenance, and repair activities.

The MLT system permits us to obtain a real-time actual measurement for one of our subloops without having to go to the line to do the measurements. With MLT, we open a web browser to visit a Qwest server. We specify the telephone number to be tested. The MLT system breaks the connection between the loop and the central office line card (in our case it is a loop carrier box line card). It tests "outwards," measuring resistance, capacitance, impedence and voltage on the loop in the direction of the end user. It also tests "inwards," checking for presence of dial tone at the loop terminals of the line card and the ability to "break" dial tone. When the test is finished the loop is reconnected to the line card. This is all done in an automated way and does not require any manual steps by Qwest personnel.

Interestingly, Qwest says it does not do automated proactive MLT testing for "pair gain" lines.

To use the MLT system it is first necessary to gain access to the Customer Electronic Maintenance and Repair (CEMR) system. This requires being an interconnected carrier with Qwest, and requires getting a Qwest cryptographic certificate if you don't already have one, and requires getting the certificate linked to the CEMR system.

To use MLT, first log into CEMR and select "Non-Design Services," as shown below. Fill in a telephone number (one of your shared lines), select "Send MLT Test Full", and click "Submit".

After a while (3 to 10 minutes), if all goes well, a report will appear.

requesting an MLT test

sample MLT report

A typical test report is at the lower left. Here is a discussion of the fields in the report.

Ver Code. This is a one- or two-character alphanumeric code which indicates the test results.
Ver Desc. This is a text description of the test results, and it corresponds to the "Ver Code." The usual description is "test OK" but if there is something wrong with the line there will be some other description.

The DC resistance-to-ground measurements, which are in kilohms, reveal leakage if any. The values in this report, in excess of 3 megohms, are typical good values. Our loops are all in the range of 2.2 meg to 3.5 meg for tip-to-ground and ring-to-ground.

We have never seen a value higher than 3.5 meg and many readings that we have seen are exactly 3.5 meg. This suggests to us that the MLT ohmmeter tops out at 3.5 megohms. Qwest has confirmed that MLT never reports a figure higher than 3.5M.

Voltage-to-ground values should be zero and in this case they are. All of our loops test "zero" for tip-to-ground and ring-to-ground. Nonzero values suggest a cross to another line.

DC tip-to-ring resistance, for a line that is on-hook, should be quite high. In this case the value is over 1 megohm which is fine. This ohmmeter also seems to top out at 3.5 megohms. Our loops all measure between 1.1 meg and 3.5 meg.

The AC signature results are inadequately documented by Qwest. We speculated that they represent impedence (rather than resistance) at some frequency. Qwest has confirmed that it is an impedence measurement, and has explained that it is measured at 24 Hz. We suspected the units are kilohms, and Qwest confirms this. The tip-to-ground and ring-to-ground numbers for our loops are all in the range of 32 to 40. The tip-to-ring numbers for our loops are all in the range of 7 to 18. One source suggests to us that MLT considers T-R impedence of between 1 and 20 to be okay, and that a REN of 1 or higher yields a T-R figure of 20 or lower.

Capacitive balance compares the capacitance-to-ground for the tip and ring wires and provides a percentage of the smaller to the larger. All of our loops are in the range of 98 to 99. Qwest says that the pass/fail criterion is 95%.

Longitudinal balance is expressed in decibels. Every MLT test we have ever performed that gave a longitudinal balance figure has always said exactly 65 decibels. We suspect the number is a place-holder and doesn't mean anything. Qwest says this is noise-to-ground minus the noise metallic, and that a larger value means less perceived noise. Qwest says the criterion is 50 dB.

The "central office" results in this report are "line ckt OK" and "Dial tone OK." These are inadequately documented by Qwest. We suspect that "dial tone OK" means that the MLT system heard a dial tone coming from the line card and was able to break the dial tone. "Line ckt OK" clearly is an abbreviation for "line circuit OK" but we don't know what Qwest means by this in technical terms.

We choose to perform an MLT full test when each shared distribution subloop is put into service, yielding baseline values which can be compared with readings at later times.

Understanding the "loop length" measurements. The loop length in the MLT reports is said to be in feet. We have done measurements and have determined that the longest copper loop for any home in our neighborhood, from the loop carrier box to the end user, is at most only about 14,000 feet. Yet the MLT results we have received give no loop shorter than about 39,700 feet and gives some measurements as long as about 49,000 feet. So every loop we have tested gives an MLT loop length figure that is tens of thousands of feet longer than the true loop length.

One source of this unexpectedly large MLT-measured loop length may be the distance from the loop carrier box to the Dillon central office. Qwest has not adequately documented this aspect of the MLT test reports, but we gather from other sources that the MLT system may do its job by making a connection at the loop carrier box between (a) the copper loop going to the end user and (b) a copper loop going to the Dillon central office where the MLT measuring equipment (voltmeter, ohmmeter, impedence meter and capacitance meter). In our case the loop carrier box is some 17,000 feet from the central office. Stated differently, we gather that the MLT system may do its tests by adding 17,000 feet of copper to the copper loops which connect each end user to the loop carrier box.

But this still leaves the MLT-measured loop lengths at odds from the actual lengths. For example the loop tested above has an actual length, from end user to loop carrier box, of about 8,000 feet. Adding 17,000 feet of copper from the loop carrier box to the central office would yield a total of about 25,000 feet. Yet the MLT report says the loop is 39,600 feet long. That would leave about 15,000 feet of extra loop length; the MLT figure is some 50% larger than the actual figure.

Qwest says this:

MLT calculates loop length from the AC capacitance measurements. Loop Length is only returned with a Test OK or an Open Out condition. Because MLT is calculating distance based on test measurements, circuits on Pair Gain must be interpreted differently [than loop length measurements for all-copper circuits]. Depending on how the MLT equipment is configured for a particular Remote Terminal (RT), an MLT test on a Pair Gain circuit may return: 1. The length from the RT (this is pretty rare); 2. The length from the Central Office; 3. A length that has no correlation to actual loop length. All loops tested from a single RT will generally have the same type of measurement returned and will be "off" by a similar amount.

We have made baseline MLT measurements and have kept records of the MLT-measured loop lengths. We figure that even if the MLT figures are off by 50% from the actual values, we will still be able to use the MLT-measured values to get some rough indication of the location of the trouble.

We did some experiments, intentionally breaking a few of the POTS lines at our splitter for a brief interval, and running the MLT test while the line was broken, to see what MLT-measured lengths we get. We found that a complete open at the splitter will yield MLT-measured loop lengths in the range of 36,100 to 37,000 feet. This is notably smaller than the number of feet reported by MLT for known good loops, in the range of 39,700 to 49,000 feet. We figure this may help us to remotely diagnose the cause of a POTS outage due, say, to a break at our splitter.

Understanding some of the MLT error messages. For two of our circuits, the MLT results were "34 Possible invalid access." This meant the MLT system was not getting a proper metallic connection to the loops. We complained and Qwest fixed it.

For three of our loops, the MLT results are "55 Pair gain channel failure." According to Qwest, this means either an actual trouble on the line or a failure in the MLT system. We have complained about this but it has not yet been fixed.