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Author Topic: The Effect of a MOV on an xDSL Circuit  (Read 505 times)

burakkucat

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The Effect of a MOV on an xDSL Circuit
« on: February 12, 2020, 06:50:44 PM »

Late last November (2019) an experiment with a local VDSL2 (ITU-T G.993.2) circuit was performed. The original purpose of the experiment was unsuccessful (and will have to be repeated at a future date with modified conditions) but an unexpected and interesting observation was made.

Equipement Used.

A Planet VC-231 media converter was used as the VTU-C. It was configured to operate the circuit as Profile 17a, with a target SNRM of 6 dB and interleaving.

The metallic pathway consisted of CW1308 specification cable, 100 m in length, with solid copper conductors of 0.5 mm in diameter and a measured attenuation of 1.4 dB per km. (b*cat would like to thank 4c for sanity checking both the measurements and the calculation of the cable's attenuation.)

A Comtrend VR-3030 modem/router was used as the VTU-R. It was used in its default configuration. (VDSL2 mode with ADSL2+ fallback.)

An LJU2/3A was attached to one end of the cable and an LJU2/1A was attached to the other end of the cable. I am unable to find an image of the LJU2/1A used but the discrete components therein were a 1.8 microFarad capacitor & a 470 k resistor, series connected, shunted across the pair and a metal oxide varistor, also shunted across the pair. Both the LJU2/1A & the LJU2/3A dated from the early 1980s and had subsequently been recovered as used.

Both the VTU-C and the VTU-R were connected to the opposite ends of the metallic pathway via 1.5 m patch leads with a 6P2C plug at one end and a BS6312 431A plug (a.k.a. a BT plug) at the other end.

Experimental Details.

(I shall not document the unsuccessful part of the experiment.)

The VTU-C & the VTU-R were powered on, in sequence, and allowed to synchronise. Via a standard Ethernet patch lead, a computer was connected to the 8P8C socket of the VTU-R. A telnet connection, using the default RFC1918 address of 192.168.1.1, was established to the VTU-R and a CLI session was initiated by logging in using the default credentials of root/12345. The usual data harvest was performed (the commands issued are shown in Appendix A, below) so as to allow the four snapshot plots to be created. (Bit Loading, Hlog, QLN and SNR, all versus the sub-carrier index (a.k.a. tone).)

Upon creation and examination of the four plots an unexpected artefact, centred at approximately sub-carriers 3700 - 3750, was observed in each of the four plots. At a quick glance one might consider a bridging tap. However, as the shape of the depression as seen in the Hlog plot was more rounded than that exhibited by a typical bridging tap and as the topography of the metallic pathway is well known, that conclusion must be ruled out.

Fortunately a second LJU2/1A, which (at some time in the past) had had the MOV removed, was available. The equipment was powered off and the two LJU2/1As were interchanged. Following the same steps as detailed earlier, a new data harvest was performed. Creation and examination of the four plots now showed the expected results.

Conclusion.

A metal oxide varistor is detrimental to an xDSL circuit.



Appendix A.

xdslctl info --state
xdslctl info --show
xdslctl info --stats
xdslctl info --SNR
xdslctl info --QLN
xdslctl info --Hlog
xdslctl info --Hlin
xdslctl info --HlinS
xdslctl info --Bits
xdslctl info --pbParams
xdslctl info --linediag
xdslctl info --linediag1
xdslctl info --vendor
xdslctl info --cfg
xdslctl info --webstats
xdslctl info --vectoring
xdslctl profile --show
xdslctl --version
« Last Edit: February 12, 2020, 06:56:44 PM by burakkucat »
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sevenlayermuddle

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Re: The Effect of a MOV on an xDSL Circuit
« Reply #1 on: February 12, 2020, 07:04:23 PM »

I might be missing something, but did the experiment necessarily prove that degradation of the MOV was a factor, rather than just presence of a healthy MOV?    I personally would expect almost any MOV, straight from the factory, to adversely affect DSL.  A degraded one too, but more so.

Preferred surge protection for modern times is, I believe, GDT (Gas Discharge Tubes), as initially fitted to early NTE 5.   Iím not aware of any mechanism by which GDT can significantly degrade DSL, even after degradation (but assuming not total destruction).
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