Bridgetaps

What is a bridge tap?

A bridge tap BT is an extraneous length of dangling unterminated cable in a communications line (local loop). BTs are the result of poor planning and maintenance usually left over from previous installations and configurations. BTs alter the physical characteristics of the line by changing the impedance, capacitance and introducing reflections in the line therefore adversely degrading the quality of the ADSL signal.

How do I know my line have bridge taps?

Large variations in SNR (signal to noise ratio), attenuation and loss of adsl signal can be the effects of BTs in the phone line. The phone company can verify the presence of BTs by using dedicated tools like TDRs (time domain reflectometer) and other types of equipment. If you suspect the presence of BTs in your line contact your local telephone company for a complete physical and metalics analysis.

Some ADSL modems like the efficient networks 51xx, 41xx, Westell 51xx, and others have additional circuitry (hybrid components) designed to handle telephone lines with unusual electrical characteristics. These hybrids sections compensate for capacitance, inductance and reflections in phone lines caused by old copper, bridge taps and other factors. If your modem reports that it is using hybrid 2 or 3 or else instead of hybrid 1 this can be an indication of the presence of bridge taps in your phone line. Other types of modems like the 2wire will report telephone phone line problems in the detailed diagnostics page.

The location of BTs can be anywhere from inside your house or office to the external line connecting you to the phone company. Every time old homes and offices are remodeled new phone wires are usually attached to the existing phone wiring. Over the years changes to phone wiring topologies can add considerable amount of BTs to homes or offices.

Discussion:

One very important tool in the detection of BT is the TDR. TDRs can detect line anomalies to the inch! To better understand how line anomalies affect adsl we will discuss this instrument in detail. We will run some tests using a home made TDR and discuss how to make one for electronic enthusiast.

The TDR is basically a radar. TDRs emit a high amplitude short pulse of energy (1ns ~ 10 μs or more) and then wait a selected period of time for an echo return. The theory is that un-terminated bridge taps reflect a large portion of the emitted pulse causing reflections in the line. Terminated or shorted bridge taps will dissipate or absorb the energy pulse.

The applet above simulates the travel of a TDR generated pulse from a house to the phone company. The first BT is open and the second BT is terminated or shorted.

1. The TDR transmits a high amplitude pulse from the home or office.
2. As the pulse travels from the home to the phone company the pulse encounters the first unterminated BT.
3. At this point in time the energy will be split in two, one part is diverted to the BT and the rest of the energy continues to travel over the line.
4. A short time after the diverted pulse will be reflected back to the home, the second BT and to the phone company.
5. The lef over energy from the first BT will be split in two again when it reaches the second BT.
6. The energy diverted to the second BT will dissipate because the BT is terminated.
7. The rest of the energy will reach the phone company.
8. The reflected energy from the first BT will reach the home the second BT and phone office.

Needles to say reflections (delayed and out of phase original signals) and power loss can cause havoc in adsl lines. Reflected signals are like ripples in a pond every time it encounters an obstacle an new interfering wave is generated. Over time the wave interference pattern becomes chaotic.

TDRs can be build with a handful of components. Tomi Engdahl's site shows how to build a simple TDR. To use Tomi Engdahl's design you need a digital storage oscilloscope to measure the reflected pulse.

Other ways to generate high amplitude short pulses is by using pulse generators of function generators like the Tektronix PG-508 or FG-504 and many others. Some of these instruments have manual triggering systems and trigger out connectors making it ideal for TDR use.

We used a Tektronix FG-504 to generate a 0-40 Vp-p amplitude pulse for our TDR. The timing can be from 5ns and up. We used the symmetry feature in the FG-504 to modify a square wave to a pulse. As a measuring device we used a Tektronix 2440 digital oscilloscope. The layout of the TDR is exactly as described in Tomi Engdahl web site.

Custom made TDR.

Test conditions:

Three tests were made to test the TDR. A bridge tap was constructed from phone grade twisted wire to form two bridge taps of 47 ft (14.32 m) and 26 ft (7.92 m) the cable connecting the TDR to the BT is 5 ft (1.52 m).

One important parameter related to transmission lines is the vf (velocity factor) Different transmission line types have different velocity factors. The vf for a phone grade twisted pair cable with air as the dielectric is approximately 64 ~ 68 which means that signals will travel at 64% ~ 68% of the speed of light which is approximately 186,000 Mi/s (300,000 Km/s) We will need this parameter for our TDR calculations.

For the first test we used the 47 ft unterminated BT located 5 ft away from the TDR. A 2 Vp-p 30 ns wide pulse was used. The reflected pulse was detected 161 ns later.

From the formula d = v·t where d = distance, v = velocity and t = time. and using a 66 vf for the twisted pair we have 66% of 300,000,000 m/s = 198,000,000 m/s.

Solving for d = 198,000,000 · 161^-9 we obtain d = 31.87 m or 104.56 ft.

The anomaly was detected 104.56 ft away but why? Our bridge tap is 5 ft + 47 ft = 52 ft long so what happened? We need to take in to consideration that signals travel 52 ft to reach the unterminated end of the BT and 52 ft to return to the TDR so 52 ft + 52 ft = 104 ft!

For the second test we used the 47 ft shorted BT located 5 ft away from the TDR. Again we detected the anomaly 104 ft away but the reflected pulse is different and considerable smaller why? We need to understand that a shorted BT will put an excessive load on the phone line. Basically the line electrical resistance is the only obstacle to the signal therefore reducing the adsl energy and dissipating it as heat.

For the third test we used the 26 ft unterminated BT located 5 ft away from the TDR. Again solving the formula for d = v*t we get d = 18.81 m or 61.71 ft away.

52 ft open bridge tap.

52 ft shorted bridge tap

31 ft open bridge tap

8400ft no bridge taps

Conclusion:

From the tests above we observe that open or unterminated BTs generate reflections in transmission lines. These reflections severely interfere in the communication process. Shorted BTs excessively load transmission lines and severely affect received power levels in the line.

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