Kitz Forum
Broadband Related => Telephony Wiring + Equipment => Topic started by: sheddyian on July 02, 2012, 12:42:41 PM
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Some months back, I happened to try swapping the RJ11 lead from the modem to the phone socket with a different one. The original was very thin, a flat 2 wire thing, marked 28AWG. With this cable I typically had an UP S/N Margin of around 8. The replacement, was a similar but thicker flat cable, marked 26AWG, and with this I consistently got an UP S/N Margin of around 9.5 - this seemed a significant jump, although synch speeds seemed largely unaffected.
Curious, I ordered an ADSL Nation twisted pair RJ11 modem lead. It certainly looks good, nice thick cable, quality RJ11 plugs (with screening).
Using this extra-super cable, I get a S/N Margin of around 8, even if I earth the screened plugs rather than having them float.
Anyone have any idea why a seemingly superior twisted-pair cable would give me a lower S/N Margin than a cheap flat one?
Cable S/N Margin UP
Original flat RJ11 28AWG 8.1
Similar RJ11 26AWG 9.9
With ADSL Nation T/P RJ11 8.0
I'm assuming here that a higher S/N Margin indicates a generally better line/signal quality. Thoughts?
The ADSL Nation cable has 2 pairs, all 4 RJ11 pins connected - so one pair is probably floating. Acting as an aerial? But then the cable is foil screened as well. The other two flat RJ11 cables only have the centre two pins connected to one pair. All are broadly the same length, from about 1 metre to 1.5 metres, the ADSL Nation lead being the shortest.
Ian
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A snapshot of the noise margin on its own doesn't really tell you much. To make a proper comparison you need to look at the connection speed and also how the noise margin varies over time.
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Lower AWG = larger diameter which = more conductor area to use (skin effect and all that) which should in turn lead to lower insertion loss - all else being equal. 26AWG is 130Ω/km, 28AWG is 210Ω/km or thereabouts.
Twisted pair cabling radiates RF less than flat cable and is less prone to induced noise.
Screening isn't going to do anything unless the screen is grounded at both ends and in fact if its not then it makes for a nice antenna, especially where foil/centre earth is involved.
I wouldn't expect to see much of a difference over 1-1.5 metres. The 26AWG should be marginally better than 28AWG is about all that can be said. All much of a muchness really.
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I was always taught that the grounding at BOTH ends of a screened cable leads to earth loops and makes things worse, and was told to only ground the foil/drainwire at one end.
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I'm a bit rusty on such matters, but I think that earth loops are a problem at audio frequencies, and can lead to hum in a sound system. I don't think that they are a problem at RF frequencies.
On the other hand, screening can be effective without the screen being earthed at all, because it acts as a Faraday cage (although once again I'm unsure in the case of a screened cable, in which the screen is very close to the wires). I suppose it could be the case that earthing it at one end only is the worst option.
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Been a bit busy, will post up a bit more info on line speed etc (though I don't think it changed much, even though the S/N margin did).
What I've noticed is that the S/N margin doesn't vary very much, but the readings I get are consistent with the specific cables. The thicker cable always gives me a higher S/N margin than the thinner one, and the fancy screened twisted pair ADSL Nation one always gives me around 8db. ???
A quick look now. my S/N Margin UP is 8.0 and DOWN is 6, which is where it was when I originally connected this particular cable.
The modem *does* refresh the readings, so I don't think they're stuck - I have very occasionally seen the DOWN S/N Margin drop to 3.8 ish for a while, before creeping back to 6.
I will see if I can get some Routerstats graphs from it in due course.
I can confirm that I have grounded only one end of the ADSL nation shielded RJ11 cable, and it's connected to the earth that runs up the unused pairs of the CAT5 telephone wire, grounded to an earth rod in the garden.
Ian
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Grounding the screen at only one end leads to the cable looking like an antenna to RF signals.
Given the length of the cable in question I don't see what the point of screening is. I guess it may be useful if you have a noisy DECT base unit beside the cable but I wouldn't imagine it'd make any difference for a 1.5m cable.
The ADSLNation cable sounds rather like the smoke and mirrors "audiophiles" come out with when they're wittering on about gold connectors and oxygen-free cable - ie makes no difference at all :D
FWIW I use a cable that came with a 56k Motorola modem to connect the router. Its 2m long & I've never seen any difference in stats/sync rate when I've been swapping routers about (using the cable which they came with).
Unless its a long run of cable (10m+) and/or its got stranded wire/is mega-cheap flat cable I don't think it makes much odds what you use.
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Grounding the screen at only one end leads to the cable looking like an antenna to RF signals.
Thank you for explaining the above. That is my new thing, learnt today. :)
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Grounding the screen at only one end leads to the cable looking like an antenna to RF signals.
Well yes and no, in that it's an earthed low-resistance antenna and so still shields the cables inside.
Unless you've very close to a transmitter the "earth-loop" induction from mains equipment is likely to be much higher in magnitude, which is why only one end is earthed.
Have a read: http://en.wikipedia.org/wiki/Ground_loop_(electricity)
(something rather odd about copying that link - click on it then follow the "did you mean"? ) ???
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Grounding the screen at only one end leads to the cable looking like an antenna to RF signals.
Well yes and no, in that it's an earthed low-resistance antenna and so still shields the cables inside.
Unless you've very close to a transmitter the "earth-loop" induction from mains equipment is likely to be much higher in magnitude, which is why only one end is earthed.
Have a read: http://en.wikipedia.org/wiki/Ground_loop_(electricity)
(something rather odd about copying that link - click on it then follow the "did you mean"? ) ???
I wasn't talking about antenna reception - try grounding/earthing a coax at one end and pump RF down it, see what happens ;) It's the classical half/quarter wave dipole.
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Grounding the screen at only one end leads to the cable looking like an antenna to RF signals.
I wasn't talking about antenna reception
???
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Grounding the screen at only one end leads to the cable looking like an antenna to RF signals.
Given the length of the cable in question I don't see what the point of screening is. I guess it may be useful if you have a noisy DECT base unit beside the cable but I wouldn't imagine it'd make any difference for a 1.5m cable.
The ADSLNation cable sounds rather like the smoke and mirrors "audiophiles" come out with when they're wittering on about gold connectors and oxygen-free cable - ie makes no difference at all :D
FWIW I use a cable that came with a 56k Motorola modem to connect the router. Its 2m long & I've never seen any difference in stats/sync rate when I've been swapping routers about (using the cable which they came with).
Unless its a long run of cable (10m+) and/or its got stranded wire/is mega-cheap flat cable I don't think it makes much odds what you use.
Agree re the audiophile cables, though there can be a benefit in having a good quality cable compared to a nasty cheap thing - recently swapped a friends scart cable for a nice thick chunky one, and the picture was immediately much sharper, far less blurred. But that's more down to the old scart being cheap and nasty, I think! Reading some of the nonsense on expensive digital interconnects (this HDMI cable gives better colour definition than that HDMI cable... :lol: )
Te reason I'd bought the ADSL Nation cable was curiosity - if swapping from a basic 28AWG lead to a basic 26AWG lead had gained me 1db extra noise margin, what would putting a twisted pair lead in it's place do? Answer, strangely, drop me back by 1db !
As I said, this is all repeatable - if I put the basic 26AWG lead back, I will get around 9db S/N margin once more, even though the lead is only about a metre long. It doesn't make a lot of sense, but that's the numbers I get.
Grounding, or not, the screen on the RJ11 doesn't appear to make any difference. It's a metallic plug that's evidently intended to connect with a metal socket, but since the ADSL faceplate and the modem have plastic sockets, the shielding will usually float, which is how it'll be with most people who buy one of these leads. I deliberately connected a stiff copper ground wire to the RJ11 outer to connect it to ground - with no perceivable difference in stats.
Ian
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Grounding the screen at only one end leads to the cable looking like an antenna to RF signals.
I wasn't talking about antenna reception
???
I knocked out our Freeview DVB-T channels and everyone else's channels within 50m before the switchover by grounding one end of CAT5e cabling and shoving gigabit ethernet down it. It took 3Mbps off the ADSL speed too.
I should probably be clearer on what I say - sorry. To me an antenna is an antenna, it has gain and maybe some directional ability and that's it - screening will make bugger all difference to reception in this case as the conducted/induced noise from the E/D-side cabling will be much greater.
You can radiate some local noise yourself as I found out, should have known better but I haven't worked on MF/HF stuff for 25 years - recent stuff is all above 6GHz, usually nearer 40GHz these days.
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As I said, this is all repeatable - if I put the basic 26AWG lead back, I will get around 9db S/N margin once more, even though the lead is only about a metre long. It doesn't make a lot of sense, but that's the numbers I get.
Well reducing the AWG - ie increasing cable diameter and more importantly circumference - is likely to reduce insertion loss. I don't know about 1dB/metre but that seems to be what you've got.
When you increase cable circumference (within reason) then RF signals* suffer less insertion loss. That means your attenuation figure has reduced. It doesn't mean the noise on the line has reduced.
So putting in 26AWG cable and getting a better result is perfectly logical - I'm just somewhat surprised the difference over a metre is noticeable.
*google skin effect
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grounding one end of CAT5e cabling and shoving gigabit ethernet down it.
Surely Cat5e is normally unshielded twisted pair...... so what did you ground?
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Now, here's a thing.
Since around 4pm yesterday I've had Routerstats running, and I've just checked it. The tx and rx noise margin stayed the same throughout the entire run, not one glimmer of change! Is that normal?
I wondered if it might be stuck, so I unplugged the RJ11 lead and replaced it with the one that gives me the higher S/N margin, and the graph showed this improvement once synch was restored. I didn't restart the modem nor did I do anything to routerstats, just swapped the cables.
Attached are plots - they don't show the whole of the run (I hadn't set it to log it all) but I scrolled the graph all the way to the start to look at it, and take my word for it, from 16:00 yesterday until I changed the cable at around 13:20 just now, the plots were entirely flat line. And you can see the small improvement in noise margin and synch speed when swapping from the fancy twisted pair screened RJ11 to the 26AWG flat untwisted RJ11 cable.
Ian
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Now, here's a thing.
Since around 4pm yesterday I've had Routerstats running, and I've just checked it. The tx and rx noise margin stayed the same throughout the entire run, not one glimmer of change! Is that normal?
No, definitely not. I suspect a glitch in the recording software (Routerstats). :-\
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I just paused routerstats, looked at the the s/n figure on the modem web interface, and it reported a higher figure than routerstats had been plotting.
Resumed routerstats and it started plotting the higher figure.
So it does look like a bug, or something is getting cached somewhere.
Ian
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Are you using the latest version of Routerstats? Do you think it would be worth mentioning to John Owen, creator and maintainer of the package?
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I've had some difficulty on occassions like this
Only on R.Stats using telnet - and never on R.Statslite.
Either it won't record and runs at zero, or it records and although the data seems to be getting to R.Stats in the terminal window on telnet - its not being correctly plotted so I get the straight line syndrome...seemingly from the last point some time before just re-plotted.
HOWEVER in my case after only a bit it rights itself and starts behaving normally. If it was running at zero I see a sudden jump up to the correct value of SNRm, sync etc.
It is possible its linked to a router or computer reboot. If I switch off R.Stats and put the computer on standby ovenight I never get any problems, but if I've switched computer totally off then sometimes I get the hassle.
I did a router re-boot today (computer on standby) and sure enough I got the zero plot to start with.....
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Maybe I am missing something but I am a bit puzzled by all this. When ever you change anything such as a cable that means a reconnect and resync. The modem will aim at its set SNR margins for upstream and downstream. Usually it connect close to but not exactly at these margins. To evaluate a component I think you have to take it in a out at least a few times in succession at a time of day when the SNR seems be stable and then look at mainly the Sync rates and not so much the SNR. Generally the better component will give a higher Sync with the SNR's being roughly fixed by the target values. If the SNR varies a bit after resyncs it would be better to estimate what the sync should have been to give the target of perhaps 6db or 9db (depending on your DSLAM settings). This can be done with a rough ratio of sync speed increment to an SNR increment of order 0.5Mhz per db of SNR.
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Are you using the latest version of Routerstats? Do you think it would be worth mentioning to John Owen, creator and maintainer of the package?
I was using the previous version, but have just downloaded the latest. Also downloaded Routerstats lite, which I've just been running, but it doesn't plot the upstream S/N margin, which is the one that seems to vary more then the downstream. The upstream is also the figure that changes when I use different cables, and which I can roughly predict what it'll be, based on which cable I use ::)
And yes, I will have a look on the author's site and see about contacting them.
Ian
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Maybe I am missing something but I am a bit puzzled by all this.
So am I ;D
My S/N margin is pretty steady (regardless of routerstats showing it as a completely flat line, from looking at the figures, it doesn't change that much)
I've just been trying another cable, an RJ11 twisted pair lead I made myself a while back. This consistently gives me around 8.3db noise margin UP.
The "magic" flat RJ11 cable consistently gives me around 9.5db noise margin UP. Having just put that cable back again,. I'm currently getting 9.9db UP.
My up synch speed is always 1020, never higher, never lower.
My down synch speed fluctuates slightly at every resynch, but never by much.
It does seem odd that one particular cable gives me better figures than others, especially given it's relatively short length compared with everything else. ???
I shall stick with this "magic" basic flat RJ11 cable, since it's clearly what the modem likes :)
Ian
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On SOME modems RSlite will plot the US SNR
Go to the SNR graph and with mouse over graph area - right click
Drop down menu appears - tick plot US noise.
However it may not actually work on some router model - it plots a zero line.
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That clarifies things a bit. I assume the upstream sync is pretty well at its max assuming you are on an ADSL2+ connection. In that case as you say, the sync is fixed and the SNR varies and for Upstream a bigger SNR should give less errors.
What happens down stream? Again assuming you are on ADSL2+ your sync is excellent but does that Sync and the matching SNR vary?
I also by the way always do a little better with an old flat cable (it came with a D-Link). I suspect it is impedance matching that is making one better than another.
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On SOME modems RSlite will plot the US SNR
Go to the SNR graph and with mouse over graph area - right click
Drop down menu appears - tick plot US noise.
However it may not actually work on some router model - it plots a zero line.
Thank you, I hadn't spotted that! It does work on my modem (Dlink DSL-2780) and I'm now successfully plotting a slightly fluctuating U/S SN margin, along with a constantly steady D/S!
I shall watch that with interest :)
Ian
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What happens down stream? Again assuming you are on ADSL2+ your sync is excellent but does that Sync and the matching SNR vary?
I also by the way always do a little better with an old flat cable (it came with a D-Link). I suspect it is impedance matching that is making one better than another.
The downstream is pretty stable on 6db S/N margin. It occasionally drops to around 3.5, but not for long. I thought I'd discovered why - it seemed to be when the sun was heating the drop cable on the west facing wall in the afternoon, but it's not consistant, so :no: But it doesn't drop often enough to bother me. I think I've only ever had 1 resynch since November 2011!
Impedance matching seems a very plausible answer to why the S/N margin is better with one cable over another.
Ian
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Just for interest, here's a plot from Routerstats lite, which seems to work correctly on my modem. Couldn't get full version Routerstats to plot anything other than the first sample value for noise - just gave me straight lines.
Top trace is upstream, bottom trace is downstream. The resynchs are where I unplugged the RJ11 cable to try different ones, the final cable being my preferred "magic" RJ11 cable that gives best performance.
Having sung the praises of my Dlink DSL-2780 modem as being reliable, Routerstats Lite broke it (temporarily). After around 4 hours of monitoring, the stats all dropped to zero, and the web interface on the modem wouldn't respond. Modem and router function carried on working though. A reboot cleared the problem.
Ian
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Interesting, Ian.
Just to confirm...are the various cables exactly the same length, and follow exactly the same route (ie, does each exactly overlay the other)?
If not, it introduces another uncertainty. The cable acts an antenna picking up interference, and the slightest variation in position or orientation can have a significant effect on that antenna's efficiency in picking up interfering signals.
I'm probably not alone in remembering early 60s VHF TV reception, when the only way to get a decent picture was to persuade Gran to stand at some very specific point in the corner of the lounge with the Aerial held aloft, and a teapot in her hand. Actually the teapot may not have made much difference but it suited her image, so we never let on.
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Ok, out of curiosity, I've done a few more tests.
Firstly, the cable runs : the modem is on a shelf about 80 cm above the NTE/5 with a filtered faceplate. The cables do differ a bit in length, see the table below. Each cable followed the same route, namely straight up from socket to modem, although any excess was left to dangle downwards.
Moving the cables about, or even winding them through a ferrite core whilst the line was synched didn't seem to make any difference to the data - I watched the graph as moved the wire or wound it, then left it a bit.
Cable Length Description
1 203cm "Magic" RJ11 flat untwisted cable that gives me best S/N ratio. 26AWG
2 100cm ADSL Nation Screened Twisted pairs RJ11 cable
3 124cm Home-made RJ11 single twisted pair (from old solid core CAT5 cable)
4 100cm Original flimsy RJ11 flat pair supplied with modem, 28AWG
5 184.5cm RJ11 to BT plug from old 56K modem. (Plugged into NTE/5 test socket)
(Lengths are measured plug tip to plug tip)
View the attached graph in conjuction with the cable descriptions above.
You will see that there isn't a significant synch speed change, but that cable 1 consistently gives the highest upstream S/N ratio. The unlabelled start of the plot was from previous days synch, still connected using cable 1.
I was also interested in the amount of ADSL noise radiated from the different cables. I used a MW radio, held at a distance from the vertical RJ11 cable, and moved it around to determine the distance where I could no longer detect ADSL noise from the background hiss.
cable distance
1 21cm
2 5cm
2 (earthed screen) 1cm
3 5cm
4 10cm
5 omitted test - oops!
Unsurprisingly, the screened cable (no.2) radiated the least noise, which was even further reduced when the screen was earthed at one end. Even at 0cm proximity in contact with the MW radio, the ADSL noise was noticeably muted compared with other cables, and I could instead detect some processor noise from the modem.
Make of that what you will, I know which cable is my favourite, and which sees to give me fairly consistent synch speeds :)
Ian