Kitz Forum
Broadband Related => Broadband Technology => Topic started by: Weaver on September 15, 2018, 08:35:35 PM
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Consider this line, taken from a recent thread.
xdslcmd: ADSL driver and PHY status
Status: Showtime
Retrain Reason: 0
Last initialization procedure status: 0
Max: Upstream rate = 25970 Kbps, Downstream rate = 86204 Kbps
Bearer: 0, Upstream rate = 19999 Kbps, Downstream rate = 79999 Kbps
Bearer: 1, Upstream rate = 0 Kbps, Downstream rate = 0 Kbps
Link Power State: L0
Mode: VDSL2 Annex B
VDSL2 Profile: Profile 17a
TPS-TC: PTM Mode(0x0)
Trellis: U:ON /D:ON
Line Status: No Defect
Training Status: Showtime
Down Up
SNR (dB): 4.6 15.1
Attn(dB): 13.9 0.0
Pwr(dBm): 13.1 4.3
The SNRM upstream is ridiculously high as the line is capped at 20 Mbps upstream.
Q: Why on earth do they do this?
There is clearly a good bit more upstream performance available from this excellent short line.
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Some of us lucky to be close to the cabinet, if you want another one to ponder over, then here you are:
xdslctl info --stats
xdslctl: ADSL driver and PHY status
Status: Showtime
Last Retrain Reason: 1
Last initialization procedure status: 0
Max: Upstream rate = 25946 Kbps, Downstream rate = 100163 Kbps
Bearer: 0, Upstream rate = 20000 Kbps, Downstream rate = 79987 Kbps
Link Power State: L0
Mode: VDSL2 Annex B
VDSL2 Profile: Profile 17a
TPS-TC: PTM Mode(0x0)
Trellis: U:ON /D:ON
Line Status: No Defect
Training Status: Showtime
Down Up
SNR (dB): 11.3 9.6
Attn(dB): 8.8 0.0
Pwr(dBm): -9.9 -10.0
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Why cap it?
In fact, in that latter example, why cap the downstream or the upstream ?
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I do not know the exact reason why Openreach have capped it to 20 Mbps and nor will I pretend to know why. This may be something for the more experienced and Openreach/ex-Openreach people. Though I would guess that it is a mix of the following:
- In the absence of vectoring it may not be possible for many lines to reliably sync at above 20 Mbps on the upstream (lines longer then 300-400m on cabinets with a high degree of subscribers probably wouldn't see much above 20 Mbps).
- It is a technical thing to reduce crosstalk as shorter lines will need less bits loaded and lower power levels to achieve headline speeds.
- It is product consistency since FTTP is also subject to the same upstream speeds on products with the same downstream as with FTTC.
You could also apply this to the downstream, too. Not just the upstream.
If you think I've got it wrong, please point it out.
Not so much unlike the stats you have posted for the sake example, my modem believes my line would be capable of quite a bit more than 20 Mbps upstream (~28000 Kbps attainable).
Max: Upstream rate = 28010 Kbps, Downstream rate = 86204 Kbps
Bearer: 0, Upstream rate = 19999 Kbps, Downstream rate = 79999 Kbps
Bearer: 1, Upstream rate = 0 Kbps, Downstream rate = 0 Kbps
Link Power State: L0
Mode: VDSL2 Annex B
VDSL2 Profile: Profile 17a
TPS-TC: PTM Mode(0x0)
Trellis: U:ON /D:ON
Line Status: No Defect
Training Status: Showtime
Down Up
SNR (dB): 6.8 15.6
Attn(dB): 9.4 0.0
Pwr(dBm): 13.3 -6.9
It should be noted that VDSL 17a is actually capable of speeds of around 100/50 Mbps (DS/US) with the implementation of G.Vector. It is not unusual to see speeds around that in mainland Europe as Telekom Deutschland has 100/40 Mbps on VDSL which presume to be VDSL 17a G.Vector.
Post edited to remove repeated word.
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I would also like to add that back in the days pre-G.INP and pre-XdB (below 6 dB), attaining the full 80/20 Mbps was even more a rarity. Even some short lines would have been subject to interleaving overheads that would have made it impossible to sync at the full 80/20 (especially with crosstalk on busy cabs). This case still stands with ECI cabinets as you may have noticed as ECI hardware still does not have G.INP or XdB profiles.
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A trivial point, but does anyone have any idea why the line isn't always capped at exactly 80000/20000? e.g. Ian's line above is 79987/20000, and the example Weaver quoted was 79999/19999.
I've two VDSL lines, both currently at 79999/19999, but my logs show that at various points in time, one of the lines has 80000 download, and the other had 20000 upload.
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I have had 80000 Kbps before, as can be seen below:
Max: Upstream rate = 28213 Kbps, Downstream rate = 85162 Kbps
Bearer: 0, Upstream rate = 19999 Kbps, Downstream rate = 80000 Kbps
Bearer: 1, Upstream rate = 0 Kbps, Downstream rate = 0 Kbps
Link Power State: L0
Mode: VDSL2 Annex B
VDSL2 Profile: Profile 17a
TPS-TC: PTM Mode(0x0)
Trellis: U:ON /D:ON
Line Status: No Defect
Training Status: Showtime
Down Up
SNR (dB): 6.9 15.6
Attn(dB): 9.4 0.0
Pwr(dBm): 13.2 -6.8
I think the only time I have had exactly 80000 Kbps sync on the downstream was when I had an INP value of 49.00. Cannot say whether it actually has any bearing on the situation though.
Not sure about the upstream. Can't ever recall seeing it on this line.
TL;DR: I don't know.
Edit:
I did actually ask this question before on the forum (https://forum.kitz.co.uk/index.php?topic=21842.0 (https://forum.kitz.co.uk/index.php?topic=21842.0)), but it didn't attract a lot of attention. There is nothing really conclusive there except educated guesses.
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It's been brought up several times before but we've never been able to fully explain it.
It may have something to do with how the modem calculates the bit rates based on the B/L/T/S Values etc for framing.
Some old stats from my line before x-talk bit me.
No changes in DLM - all INP = 0
Huawei HG612 20000 79999 BCM 6368
TPlink TD-W9980 20000 79995 Lantiq VRX-268
TPlink TD-W9970 20000 79987 BCM 63381
Archer VR900 20000 79987 BCM 63168
Zyxel VMG8324 20000 79987 BCM 63168
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The SNRM upstream is ridiculously high as the line is capped at 20 Mbps upstream.
Q: Why on earth do they do this?
There is clearly a good bit more upstream performance available from this excellent short line.
It's what they decide to sell the product as ie 80/20, 40/10 etc. It's up to the Service Provider to select the max up and downstream rates on their DSLAMs
Same with adsl1 how they used to cap the upstream at 448 (or 832 for premium) when good lines were capable of more.
VM also cap and tend to be less generous with upstream than the DSL offerings.
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I meant that I can’t see why they cap the top speed product.
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Sales. They had to provide a top 10% figure, now median. I guess they decided a range between 10.001 Mb and 40+ Mb for products they were advertising as 'fibre' didn't appeal.
That it may have been bad for private line revenues via fibre to the premises or EFM is purely coincidental.