Subscriber Line Measurements

Techniques for testing the electrical parameters of copper lines: resistance, capacitance, attenuation, and more.

Why Test the Subscriber Line?

Measurements performed during the construction, commissioning, and maintenance of a telecommunication network are essential for verifying its quality and localizing any faults. Statistical data shows that a significant portion of network problems (often around 50%) originate in the physical cabling infrastructure. Therefore, thorough testing of the is crucial.

Testing allows technicians to diagnose a wide range of issues, from simple shorts and opens to more subtle problems like crosstalk and impedance mismatches, which can severely degrade the performance of services like DSL. A specialized device for these comprehensive tests is known as a subscriber loop analyzer or line tester.

Basic DC Measurements: A Line's Vital Signs

The most fundamental tests on a copper subscriber line involve measuring its DC (Direct Current) characteristics. These tests can reveal the line's basic integrity and status.

DC Voltage Measurement (Battery Feed):
A healthy, idle line should have a DC voltage supplied by the central office. This is measured with a high-impedance voltmeter ( $>20 \text{ k}\Omega/V$ ) across the line with the subscriber's equipment disconnected.
Expected Value: Typically -48 Vdc or -60 Vdc (in older exchanges).
Loop Resistance:
This test measures the total resistance of the two wires forming a loop. A low resistance indicates a potential , while an infinitely high resistance indicates an .
Measurement: Often performed indirectly by shorting the far end of the line and measuring the resulting loop current with an ammeter.
Insulation Resistance:
Measures the resistance between each wire and between each wire and ground (shield). Low insulation resistance points to issues like insulation breakdown or moisture ingress.
Expected Value: A good line should have a very high insulation resistance, typically in the range of many megaohms $(\text{M}\Omega)$ . Values below a few megaohms may indicate a fault.

Transmission Parameters: Quality at Higher Frequencies

For services like DSL, DC parameters are not enough. It's crucial to measure how the line behaves at the frequencies used for data transmission.

Attenuation (Insertion Loss):
Measures the loss of signal power as it travels down the line. It is expressed in decibels $(\text{dB})$ and increases with both line length and signal frequency.
Measurement Formula: $\text{Attenuation} = 20 \cdot \log_{10} \left( \frac{U_{in}}{U_{out}} \right) \text{ [dB]}$
Characteristic Impedance ( $Z_c$ ):
The inherent impedance of the line. A consistent impedance (e.g., $100 \text{ } \Omega$ for twisted pair) is vital. Mismatches cause signal reflections, which degrade performance. Measured indirectly by testing the line in open and shorted conditions.
Noise and Signal-to-Noise Ratio (SNR):
Measures the level of unwanted electrical noise on the line. SNR is the crucial ratio of the desired signal power to the noise power, and a higher SNR allows for higher data rates.
Measurement Formula: $\text{SNR} = 10 \cdot \log_{10} \left( \frac{P_{signal}}{P_{noise}} \right) \text{ [dB]}$
Longitudinal Balance (LCL):
Measures the symmetry of the wire pair with respect to ground. A well-balanced line effectively rejects external common-mode noise. Poor balance makes the line highly susceptible to interference.
Crosstalk (NEXT & FEXT):
Crosstalk is the unwanted transfer of signal from one wire pair to another adjacent pair in the same cable.
- NEXT (Near-End Crosstalk): Measured at the same end as the transmitter. It's a critical limiting factor for full-duplex systems like Ethernet.
- FEXT (Far-End Crosstalk): Measured at the opposite end from the transmitter.

Fault Diagnosis: Interpreting Measurement Results

Experienced technicians can diagnose specific line faults by analyzing the results of these electrical measurements.

Symptom / Measurement Result	Likely Fault
Very low loop resistance (e.g., $(0 \text{ to } 50 \text{k}\Omega)$ )	Short Circuit: The two wires of the pair are touching.
Very high loop resistance $(> 5 \text{M}\Omega)$ and low capacitance	Open Circuit: A physical break in one or both wires.
Low insulation resistance between a wire and ground (e.g., $(0 \text{ to } 50 \text{k}\Omega)$ )	Ground Fault: A wire is making contact with the ground or shield.
Moderately low insulation resistance (e.g., $(100 \text{k}\Omega \text{ to } 4 \text{M}\Omega)$ )	Insulation Leakage / Breakdown: Often caused by moisture or cable damage.
High line capacitance (e.g., $(> 6 \mu \text{F})$ )	Poor Audibility: May indicate a short or a bridge tap (an unterminated extension of the line).
High AC/DC foreign voltage $(> 5 \text{V})$ and poor crosstalk values	Crosstalk/Interference: Caused by poor insulation, a poorly grounded shield, or strong external electromagnetic fields.