VIAVI Solutions

Test Challenge

Understanding Digital Subscriber Line (DSL) test results

is fundamental for field technicians who must rate the

quality of service (QoS) and reliability of a tested DSL

line. Copper loops carrying DSL signals can be exposed

to many elements and external factors that impact QoS,

including attenuation, environ-mental stress (such as

changing temperature and water ingress to the cable),

and noise inside the cable bundle.

Addressing the Challenge

with SmartClass TPS

DSL rates depend on received signal power in comparison

with the received noise power. The difference between

the two is the signal-to-noise ratio (SNR). The greater

the SNR value per tone, the more bits that can be

carried on that tone (and the better the DSL rate), as

Figure 1 shows. Cable attenuation drives the received

signal power, and longer loop lengths produce higher

cable attenuation—meaning less received signal

power and a lower SNR. The maximum achievable

DSL rate (max rate) is a result of attenuation driving

the SNR values.

Application Note

Understanding DSL

Test Results Using the

SmartClass TPS

The VIAVI SmartClass TPS tester is the ideal tool for technicians

who install, troubleshoot, and maintain Triple-Play services.

Product specifications and descriptions in this

document are subject to change without notice.

smclasstpsdsl-an-acc-tm-ae

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Figure 1. SNR across DSL frequency spectrum.

NOTE: the green area (SNR) can be used for the bit load and the higher the SNR value is per tone,

the more bits that can be carried on that tone. 1.1 MHz is the ADSL frequency, ADSL2+ goes up to 2.2 MHz.

The SmartClass Triple-Play Services (TPS) shows attenuation and margin value. The margin value is the SNR margin

(SNRM), representing the tolerable increase in noise level or decrease in signal level while still guaranteeing a 10(-7)

bit error rate (BER) at a connected rate. A typical margin value is 6 dB or greater. As ADSL2+ provides dynamic

features, such as power-cutback and seamless rate adaptation (SRA), it is highly recommended not to solely rely

on the relation of connect rate vs. max rate, but also factor in the underlying attenuation and margin values.

Displaying attenuation and margin values enables the SmartClass TPS to help technicians quickly test and analyze

DSL lines and identify line disturbers and interferers impacting DSL rate.

Frequency

Bits per tone

Transmitted Signal Power

Received Signal Power

Received Noise Power

1.1 MHz40 kHz

Signal to Noise Ratio (SNR)

SmartClass TPS Bits-Per-Tone screen with an obvious

disturbance around tone 96 (DSL pilot tone at 64)

SmartClass TPS DSL Summary screen