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10 février 2014

Managing Noise and Maximizing xDSL and IPTV Quality of Experience

Andrew Long

Senior Product Line Manager, Access Business Unit

Andy Long joined EXFO in May 2008 in his current role. He brings to EXFO his telecommunications test industry experience in a variety of positions, including design, technical support as well as product and marketing management. Andrew graduated with a degree in Electrical and Electronic Engineering from the University of Plymouth in the UK.

DSL services are more sensitive to interference than traditional phone services. High frequency (or wideband) noise can cause IPTV to tile or freeze and web access to be slow or intermittent. These situations require effective noise source mitigation. To detect wideband noise, and meet performance expectations, appropriate measuring techniques must be used. Also, with recent advancements in xDSL technologies, including bonding and VDSL2 vectoring, getting a full picture of today’s FTTx and hybrid networks is much needed.

Join Russ  Gundrum, MBA PMP SSGB and Andrew Long, Group Manager, Access Business Unit at EXFO, to learn about:

  • The effects of noise on DSL service performance
  • Best practices for copper pair qualification and VDSL2 performance validation
  • How to detect and locate common noise sources on VDSL2 and ADSL services


Some questions were left unanswered during the session. Here are the answers.

Q1. In the past, a balance of 60 db in the voiceband was considered good but, as the frequencies were higher in carrier frequencies, the same pairs would measure around 30 db balance. What balance is required for the vdsl1 and 2?

R1. EXFO recommends >35 dB wideband balance for VDSL2 services.

Q2. It is known that backfeeding DSL in the same sheath can cause issues.  How much cable backfeed can be tolerated (how many feet)?  We have crossconnects with one cable stub going to manholes where the backfeed could occure over 50 to 100 feet from the stub. Can this cause problems?

R2. Yes, this can cause problems because stubs act as bridged taps and cause increased attenuation at specific frequencies. The shorter the stub, the higher the frequency that is affected. For example, a 75 ft stub would hit a frequency of around 2 MHz (ADSL2+/VDSL2 affecting).

Q3. What about Home networking technologies, for example HomePlug Av or; are these technologies a source of noise for VDSL2 or G.Fast?

R3. These technologies overlap a good portion of frequency bandwidth so there could be an impact. Proper grounding of the respective physical cable architecture is required.

Q4. In a single cable sheath, can problems occur when DSL service is fed from 2 different DSLAM manufacturers?

R4. The amount of crosstalk should not be worsened by a mix of DSLAMS.

Q5. Is there a limit of T1/ADSL/VDSL services per 25 pr bunches?

R5. It is not recommended to have T1 service in the bundle if you are offering ADSL2+ and/or VDSL2.

Q6. We currently have a limited number of subcribers on copper IPTV services. As we roll out our FTTH network, some technicians have cut drops at the ground at homes. Could the seizures on the cut lines be attributed to possible issues with dropping sync errors due to impulse noise?

R6. The cut lines would act as bridged taps if the pair continues to another subscriber from the pedestal or crossbox. In that case, there could be a detrimental effect on the DSL service due to the attenuation caused by the tap. Noise could also be induced into the drop, especially if it is unshielded.

Q7. What can DSL modem manufacturers do to mitigate the noise? Can they do anything, or is it an ISP problem?

R7. Using G.INP in the modem would help mitigate noise as well as vectoring as a second step.

Q8. Can you explain a little bit about the frequency distribution of impluse noise?

R8. The frequency distribution of the impulse noise can be across the entire spectrum, from POTS to the upper limits of VDSL2.  Lightning is a good example of this!

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