VIAVI Solutions
VIAVI Solutions
Test Challenge
Very high-speed digital subscriber line 2 (VDSL2)
provides data rates and quality of service (QoS) required
for consumer broadband access and video services. It
can be used on existing Access infrastructure and is
a complementary technology for fiber-to-the-home
(FTTH) deployments. However, upgrading the Access
network from asymmetrical digital subscriber line 2+
(ADSL2+) to VDSL2 might be a more significant task
in order to benefit from full VDSL2 data throughput
performance and service stability. VDSL2 uses a
significantly higher frequency spectrum than ADSL2+
and frequency-related issues can occur that were not
present before on ADSL2+ using the same cable pair.
Application Note
Detecting Bridged Tap and
Noise Interference in VDSL2
Access Networks using the
VIAVI SmartClass TPS
The VIAVI SmartClass™ TPS tester is the ideal tool for technicians
who install, troubleshoot, and maintain Triple-Play services.
Detecting Bridged Tap and Noise Interference in VDSL Access Networks using the VIAVI SmartClass TPS
Additional copper faults can impact DSL, such as
shorted pairs, crossed pairs, open pairs; however,
real-world deployments indicate that bridged taps
very often occur in the Access Network that
negatively impact DSL data rates.
Bridged taps will not affect all tones equally. A given
bridged tap may impact some frequencies more than
others. Table 1 shows the critical bridged tap length
for a specific discrete multitone (DMT) tone that will
impact DSL service.
The presence of bridged taps can be identified using the
Hlog(f) data defined in ITU-T G.992.3 as the frequency
response per channel with characteristic “valleys/dips”
at tones that correspond to the bridged tap length
shown in Table 1.This DELT-type measurement requires
that the Digital Subscriber Line Access Multiplexer
(DSLAM) and CPE modem be connected at both ends.
As a simple illustration, the Hlog typically represents the
attenuation per power spectral density (PSD) carrier.
Here are some copper-pair-related faults and issues that can impact VDSL2 performance:
Copper Fault Impact to DSL Service
Bridged taps A bridged tap is every piece of wire not belonging to the main cable between the DSLAM and
the customer premises equipment (CPE) modem. Bridged taps cause undesirable reflections at
the splice point and at the end of the tap. The reflected signal becomes noise in the circuit that
degrades DSL performance. Further, bridged taps can act as antennas and pick up external noise
along the tap.
Poor pair
balance
Twisted copper pair can cancel out some of the external noise caused by other DSL lines or short,
intermittent impulse noise that electrical devices cause in the Home Network. However, if the pair
is unbalanced, noise will bleed into the copper pair and degrade DSL performance.
Corroded
contacts
Corroded contacts act as capacitive faults that especially impact the lower part of the frequency
spectrum resulting in degraded DSL performance.
Bridged TapDSLAM
[m]
Critical bridged tap length
Tone
256
1.00
6.00
11.00
16.00
21.00
26.00
31.00
36.00
41.00
46.00
512 768 1024 1280 1536 1792 2048 2304 2560 2816 3072 3328 3584 3840 4096
Figure 1. Bridged taps can impact DSL performance.
Figure 2. A bridged tap caused a “dip” on the Hlog frequency response curve.
The example shows a bridged tap length of approximately 8 m.
Figure 3. “Clean” frequency response after removing the bridged tap.
Table 1. Critical bridged tap lengths for a selection of transmission channels.*
*All examples are calculated with an assumed velocity of propagation
(VoP) = 200,000 km/s.