RadCom May 2020, Vol. 96, No. 5

Technical May 2020 19 What we need you to do If you are suffering from VDSL interference, we need you to write to Ofcom to complain. It is only if enough numbers of radio amateurs complain to Ofcom that they will be made fully aware of the scale of the problem and hopefully WKH\ ZLOO ÀQDOO\ WDNH WKH SUREOHP VHULRXVO\ )XOO details of the information required and how to PDNH \RXU FRPSODLQW LV RQ WKH 56*% ZHEVLWH at www.rsgb.org/vdsl-reporting. 5DWKHU WKDQ 0HPEHUV VXEPLWWLQJ LGHQWLFDO letters, we would encourage you to submit a complaint to Ofcom in your own words, stressing your particular circumstances and the effect that the interference is having on you. 1HYHUWKHOHVV \RX FDQ PDNH WKH VXEPLVVLRQ using the pro forma you can download from the VDSL web page, www.rsgb.org/vdsl-reporting. We also encourage you to copy your submission WR \RXU ORFDO 03 LQ RUGHU WR UDLVH WKH SURÀOH RI WKH FRPSODLQWV )HHO IUHH WR LQFOXGH D FRS\ RI WKLV DUWLFOH DV EDFNJURXQG It would also be helpful if you can send D FRS\ RI \RXU VXEPLVVLRQ WR WKH 56*% YLD YGVO#UVJE RUJ XN VR WKDW ZH FDQ WUDFN WKH QXPEHU of complaints and pursue them with Ofcom. You can submit your complaint to Ofcom as follows: ‡ by letter to: Ofcom, SpectrumManagement &HQWUH %DOGRFN 5DGLR 6WDWLRQ 5R\VWRQ %DOGRFN 6* 6+ ‡ E\ HPDLO LQWHUIHUHQFH UHSRUW#RIFRP RUJ XN ‡ or by completing the form at https://ofcomforms.secure.force.com/ IRUPHQWU\ 6LWHV)RUP$PDWHXU5DGLR How to identify and measure VDSL (In the UK, the current VDSL service uses a WHFKQRORJ\ NQRZQ DV 9'6/ )RU VLPSOLFLW\ WKH generic term VDSL is used here throughout). VDSL uses the existing unscreened, twisted pair ‘telephone’ cables to carry high speed broadband data signals in the spectrum from N+] WR 0+] EHWZHHQ WKH QHWZRUN provider’s street cabinet and the customer’s premises (typically a distance of up to a few KXQGUHG PHWUHV %HFDXVH 9'6/ LV FDUULHG E\ twisted pair cables designed originally to just FDUU\ DXGLR WHOHSKRQH VLJQDOV XS WR N+] WKH cables are not screened and are often poorly EDODQFHG DQG DV D UHVXOW WKH 9'6/ VLJQDOV OHDN out creating the potential for interference on the +) EDQGV 9'6/ XVHV ZLGHEDQG VRSKLVWLFDWHG GLJLWDO FRGLQJ DQG RQ DQ +) UHFHLYHU WKH interfering VDSL signal appears as wideband noise, with no easily discernible characteristics. 7KH HIIHFW LV WR UDLVH WKH DSSDUHQW EDFNJURXQG QRLVH ÁRRU LQ VRPH FDVHV E\ WHQV RI G% The transmitted VDSL spectrum is divided into six discrete frequency bands; three of which are used to carry upstream data (ie from the VXEVFULEHU WRZDUGV WKH EURDGEDQG QHWZRUN DQG three of which carry downstream data (ie from WKH EURDGEDQG QHWZRUN WRZDUGV WKH VXEVFULEHU These six bands are frequency interleaved and are VHSDUDWHG E\ VPDOO DSSUR[LPDWHO\ N+] ZLGH guard bands (see Figure 1 ). No VDSL signals are transmitted within these guard bands. Searching for these guard bands will enable you to identify the presence of VDSL interference. If a lower EDFNJURXQG QRLVH OHYHO LV REVHUYHG ZLWKLQ WKHVH guard bands when compared to the noise level immediately adjacent to the guard bands, then this indicates the presence of VDSL interference. 7KH EHVW JXDUG EDQGV WR LQYHVWLJDWH DUH DW DQG 0+] :KHQ ORRNLQJ IRU WKH VDSL guard bands, you should be aware that the precise position of any of the guard bands PD\ EH GLVSODFHG E\ XS WR N+] HLWKHU VLGH RI the nominal frequency and that any particular VDSL system may not necessarily use all the up and downstream bands, so it may be necessary WR FKHFN PRUH WKDQ RQH RI WKH JXDUG EDQGV $QRWKHU PHDQV RI LGHQWLI\LQJ ZKHWKHU \RX are suffering from VDSL interference can be made using a specially developed software application called Lelantos. This uses sophisticated digital processing to uniquely identify whether VDSL interference is present in a radio spectrum recording. The instructions as to how to use Lelantos can be downloaded from www.rsgb.org/vdsl-reporting. Using either approach, it is possible to identify that you are suffering fromVDSL interference. This LV DGHTXDWH SURRI WR MXVWLI\ PDNLQJ D FRPSODLQW WR 2IFRP DQG ZH HQFRXUDJH \RX WR GR VR +RZHYHU LI \RX ZLVK \RX FDQ WDNH LW RQH VWHS IXUWKHU %\ PHDVXULQJ WKH VL]H RI WKH VWHS LQ WKH QRLVH OHYHO between the middle of the guard band and the noise level on frequencies adjacent to the guard EDQG HJ N+] EHORZ DQG N+] DERYH WKH guard band) this gives an indication of how much WKH QRLVH ÁRRU LV EHLQJ UDLVHG E\ WKH SUHVHQFH of the interference. Figure 2 shows a spectrum GLVSOD\ RI WKH 0+] 9'6/ JXDUG EDQG VKRZLQJ substantial VDSL interference. This spectrum GLVSOD\ KDV EHHQ DQQRWDWHG WR VKRZ WKH VL]H RI WKH VWHS LQ G% DQ LQGLFDWLRQ RI WKH OHYHO RI WKH interference that is being experienced. If you don’t have a receiver with a spectrum display, then the same observations can be made by carefully recording the noise level as shown by an S-meter as you tune around one RI WKH 9'6/ JXDUG EDQGV 7XQLQJ XS WR N+] either side of the guard band centre should be VXIÀFLHQW WR PDNH WKH QHFHVVDU\ PHDVXUHPHQWV <RX VKRXOG VHH D GLS LQ WKH EDFNJURXQG QRLVH level within the guard band, rising either side as you tune away from the guard band. 1RWH WKDW LW LV QRW QHFHVVDU\ WR PDNH measurements at every one of the guard bands, although it may be interesting to do so in order to assess how the level of VDSL interference YDULHV DFURVV WKH +) VSHFWUXP ,W LV EHVW WR perform the measurements with the receiver $*& WXUQHG RII WR DYRLG WKH ULVN WKDW WKH $*& PDVNV WKH YDULDWLRQV LQ WKH QRLVH OHYHO <RX PD\ ÀQG KLJKHU RU ORZHU OHYHOV RI LQWHUIHUHQFH WKDQ ZDV UHFRUGHG LQ )LJXUH Whatever level you observe, if you can see a GHFUHDVH LQ WKH EDFNJURXQG QRLVH OHYHO LQVLGH one or more of the VDSL guard bands then VDSL LQWHUIHUHQFH LV SUHVHQW DQG ZLOO OLNHO\ EH PDVNLQJ JHQXLQH VLJQDOV ,I \RX ÀQG 9'6/ LQWHUIHUHQFH then please submit a complaint to Ofcom. Further help and support If you need further help in how to identify 9'6/ LQWHUIHUHQFH WR PDNH WKH QHFHVVDU\ measurements, to interpret the results or how to submit the complaint to Ofcom, then members of the EMC Committee are available to provide additional assistance. They can be contacted by HPDLO WR YGVO KHOS#UVJE RUJ XN www.rsgb.org/vdsl-reporting FIGURE 1: VDSL spectrum showing the position of the VDSL guard bands. – Call to action FIGURE 2: (LEFT) Example showing the level of VDSL interference around the 12MHz guard band.