Broadband Related > Telephony Wiring + Equipment
Effects of different ADSL Filters
Pfrog:
Hi
I am currently using a DSL-2740b (having tried many other routers but find, like others, that it holds the line well and has the added benefit of having the SNR margin controlled by DMT TOOl) and was until this morning using an adsl-nation XF-1e filter. For no particular reason I thought I would try replacing the XF-1e with the DLink filter which came with the router.
I was very surprised to see a much broader range of tones showing on the DMT Tool display with the DLink filter, -as attached. The DMT Tool display for the XF-1e shows only x 3 individual tones above the main block ending at around 152. I havn't attached a copy as I am loath to switch back to the XF-1e as the DSLAM seems very keen to knock up the SNR margin (currently 12db).
Am I missing something here? I was under the impression that the XF-1e was the "bee's knees" of filters, and yet it appears to be cutting of a large swathe of useful higher tones.
This test has been this morning at around 09.00, - I have noticed that at night the XF-1e produces no trace at all of tones at all above the main block of tones.
PS Has anyone got RouterStats Lite working properly with the DSL-2740b? I see it is meant to work (I am using the EU_2.89 firmware) but when I can make it start, it loses connection frequently and the readout is full of red dropouts (both via cable and wifi)......
All thoughts appreciated.....
Many thanks!
Pf
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roseway:
That's rather interesting to me, as I use an ADSLNation filtered faceplate, which I believe has the same filter circuitry as the XF-1e. I don't see the same cutoff in the tone allocation as you do, so I don't think that the filter is actually cutting off those higher frequencies. But it's an active filter, so its characteristics are quite different from the normal passive filters, and your experience may just be that the XF-1e isn't so good in your particular set of circumstances.
I did use a DSL-2740b here for a while, and used Routerstats (full version) with it using the telnet method, but I found the same as you that it often failed to collect a sample from the router and displayed what looked like a lot of dropouts. There was short discussion about this on the Routerstats forum, but it wasn't conclusive.
Pfrog:
Hi RW
Interesting. I have in the past noted that the XF-1e introduced a clearly evident 1db (approx) of extra attenuation over simpler filters which I have always put down to the added complications of the active circuit (is it "active" in the normally accepted sense in that it introduces some gain to compensate for the inherent circuit losses, or merely that it uses "active" transistors which could be used for gain?). I am aware that the world of active / passive filtering is quite an advanced topic and would not attempt to debate it in any detailed way.
I shall have to see if I can replicate the results and if so I will copy both sets of traces..... As you say it may be just my particular line which does tend to be quite noisy (in the middle of rural Worcestershire with a lot of overhead line).
The question is, shall I keep the Dlink in circuit with an undoubted lower attenuation effect (albeit very small) or put the XF-1e back in place with its probable superior filtering but more loss??
Thanks for your comments on the DSL-2740b which confirms my findings as I have no problem at all with the Telnet method.
Incidentally, the 2.89 firmware for the DSL-2740b now allows "PPP IP Extension" mode which passes the WAN IP Address to another router and thus can work effectively as a half-bridge modem only. I have used it thus with the WRT54GL with Tomato firmware, and it appears to work well, with the extra benefit of being able to tweak the SNR Margin.....
Pf
orainsear:
--- Quote from: Pfrog on February 28, 2009, 01:19:25 PM ---merely that it uses "active" transistors
--- End quote ---
That's correct.
Passive filters use only passive components; coils, caps resistors etc and only filter when a signal is received.
Active filters will also use active compontents alongside passive; usually transistors, and filter continuously.
--- Quote from: Pfrog on February 28, 2009, 01:19:25 PM ---The question is, shall I keep the Dlink in circuit with an undoubted lower attenuation effect (albeit very small) or put the XF-1e back in place with its probable superior filtering but more loss??
--- End quote ---
The attentuation that the DLink is reporting is based on the frequencies that it can see. If the filters are passing through slightly different frequencies this will have a bearing on the attentuation. It's probably not something to get too hung up over unless it's causing serious issues. You can try and experiment (gently so you don't upset your line profile) with different combinations to find out what suits you best.
Edit: Everything after 'Thats Correct'
roseway:
Yes, I don't know any details, but I think the transistors are there to sharpen up the filtering behaviour, not to provide amplification of the signal.
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