GOV: TALAN Switch Matrix Failures and Solutions

The Problem Exhibited

If you’ve played with the TALAN 3.0 to any extent, and paid attention, you’ll find that some of the core functionality was broken in the upgrade process. The specific problem is how an instrument under test reacts to the TALAN being in line in a Power-over-Ethernet (PoE), Gigabit switch environment. The setup would look as follows.

Phone under test in PoE, Gigabit switch environment.

When using the DMM, Audio, or RF Spectrum manual tests on the TALAN while in the above setup, the TALAN will kick a gigabit, PoE phone instrument off the network when the user is cycling through the different available pairs. Not every pair exhibits this behavior, but it happens often enough that during an automated check, the phone instrument will remain offline longer than it is online. Second hand accounts have surfaced that indicate that sometimes the whole phone may lose power while checks are occurring.

In the RF spectrum check, the behavior displays itself in the following manner.

TALAN running RF Spectrum check in PoE, gigabit environment.

As the RF spectrum check jumps from pair to pair and sweeps through the full frequency range, the phone will constantly disconnect and reconnect. This will cause the spectrum to be noisy one moment and quiet the next. This can make an investigators job particularly difficult as the phone is never in a stable, usable state.

The Likely Cause

To achieve gigabit speeds, network infrastructure requires a fairly pristine environment. Imbalances and mismatches can cause the switch to automatically downshift to 10/100 mode. Throw in the demand of PoE, which uses resistance levels to set initial power levels, and you end up with a highly sensitive system.

It appears that since the TALAN was updated, it is introducing an unwanted line loading to the system. This line loading causes PoE, gigabit switches to reassess its connection. Usually these switches can deal with a strangely loaded line, if that line doesn’t change. However, when the user of the TALAN switches between lines, the switch sees a loading change. This causes the switch to step back and reacquire the connection.

The same sort of behavior may happen to a phone instrument and network switch if you are introducing a monitoring computer to an inline network tap. With the TALAN this weird line loading introduction happens every couple of seconds for the length of the test.

It must also be noted, that the same connection reset behavior being caused by the TALAN can be also caused by using a low to mid grade, hand held DMM in conjunction with a modapt.

The Gigacure Solution

The solution is to create a buffer so that the network switch does not see or is not effected by the TALAN’s switching. This can be accomplished by using some well placed resistors.

Schematic of TALAN line loading solution.

The above schematic has 3 ports:

  • Analyzer – port that connects to the TALAN (Phone or Line jack)
  • Switch – port that connects to the PoE, gigabit switch
  • Phone – connects to the PoE complaint VoIP phone

A resistance of 470Ω is usually sufficient to prevent the problems shown in the RF example above while a resistance of 200Ω will suffice for the Audio and DMM manual tests.

These resistors provide a line loading buffer so that when the TALAN switches internally, the line running from the phone to the switch is only minimally disturbed. The overall effect is that the phone remains on the network and the check can continue unabated. A fully functional buffering device is displayed below and has been christened the Gigacure.

The Gigacure with 470Ω resistors.

Concerns & Use Recommendations

Several concerns an investigator should raise about using the Gigacure will be addressed below.

First, the resistors do dampen a suspected signal. However, the dampening effect is only slight. It’s true that any amount of resistance will attenuate a signal. In this case I would argue the attenuation is acceptable as the alternative is to possibly not see a signal at all because the item under test is never in a stable state.

Secondly, one might argue that the TALAN is no longer in line with the phone and is only tapping the line. Because of this connection, it would no longer be possible to take accurate DMM readings like current, resistance, and capacitance. My answer to that is that resistance and capacitance checks shouldn’t be taken on the instrument when it is powered anyway, so when you are doing these offline checks, remove the Gigacure. As far as current checks, that check does not work anyway. For real, it doesn’t work on TALAN 3.0. Try it for yourself. It’s wildly inaccurate.

Lastly, I would recommend that when you line drive an instrument, take the Gigacure out of the system. The TALAN allows for very little current to be pulled down the line. The resistors in the Gigacure would only eat that small amount of current away.

Gigacure Extras

There are many different ways in which to make a similar device. Please feel free to take the above schematic and make your own. I’d love to see pictures.

For those of you that have taken the TSCM 210 class, you received the board shown above. Recently, a motivated student put in some great work and has created a nice 3D printed box.

If you are interested in the STL files, they have been posted in the Government References section of the webpage.

Your Feedback is Appreciated

If you’ve noticed this same problem in the TALAN and have devised a different way of approaching the issue, I’d love to hear about. Also, if you make the device outlined above, I’d love to see. Let me know if you have any questions or comments.

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