CenturyLink → ADSL2+ vs. VDSL2 8a stats

Just got 20/2 (pair-bonded VDSL2 8a) set up and have it side-by-side with 20/.896 single-pair ADSL2+ (which requires modifying the target SNR to train at the full 20M downstream).

All stats (both VDSL2 and ADSL2+) were taken at the NID. (There's an individual test jack for each pair.) Only one pair was in use at a time, so the setup wasn't completely realistic (potential crosstalk issues with more active pairs). However, after eliminating the rest of the daisy chain of my inside wire (the only active jack now is what is probably the first jack in the chain), the stats on each pair _with all 3 pairs active simultaneously_ are virtually identical.

VDSL2 pair 1

Max:    Upstream rate = 3839 Kbps, Downstream rate = 29580 Kbps
Bearer: 0, Upstream rate = 1018 Kbps, Downstream rate = 10062 Kbps
 
Link Power State:       L0
Mode:                   VDSL2 Annex A
VDSL2 Profile:          Profile 8a
TPS-TC:                 PTM Mode(0x0)
Trellis:                U:ON /D:ON
Line Status:            No Defect
Training Status:        Showtime
                Down            Up
SNR (dB):        23.3            12.8
Attn(dB):        27.0            0.0
Pwr(dBm):        15.8            8.1
 

VDSL2 pair 2

Max:    Upstream rate = 3820 Kbps, Downstream rate = 30554 Kbps
Bearer: 0, Upstream rate = 1018 Kbps, Downstream rate = 10062 Kbps
 
Link Power State:       L0
Mode:                   VDSL2 Annex A
VDSL2 Profile:          Profile 8a
TPS-TC:                 PTM Mode(0x0)
Trellis:                U:ON /D:ON
Line Status:            No Defect
Training Status:        Showtime
                Down            Up
SNR (dB):        24.8            12.3
Attn(dB):        27.1            0.0
Pwr(dBm):        15.8            8.5
 

ADSL2+ (target SNR adjusted for comparison at same downstream rate)

Max:    Upstream rate = 1356 Kbps, Downstream rate = 20160 Kbps
Bearer: 0, Upstream rate = 892 Kbps, Downstream rate = 10090 Kbps
 
Link Power State:       L0
Mode:                   ADSL2+ Annex A
TPS-TC:                 ATM Mode(0x0)
Trellis:                U:ON /D:ON
Line Status:            No Defect
Training Status:        Showtime
                Down            Up
SNR (dB):        20.6            21.2
Attn(dB):        21.0            9.8
Pwr(dBm):        19.3            11.7
 

I'm sure 20M downstream on each pair should be no issue and probably won't even require changing the target SNR. I'm skeptical about the attainable upstream rate though; 3.8M seems very optimistic. As far as getting 40/2 or 40/5 (20/1 or 20/2.5) provisioned, that'll probably be difficult.

I had to call business CS to fix my name (the sales CSR misspelled it), so I thought I'd try my luck at getting the higher rate tested. A very helpful DSL repair CSR confirmed they had no way of doing that; they can restore the rate to the purchased speed if it's dropped but can't manually override for testing. I was told only the programming department can do it and they tend not to, even when field techs call them to ask.

I have no doubt this information is correct (the CSR was obviously very knowledgable, which is a refreshing change when dealing with CenturyLink phone staff these days), and don't intend to pursue that any further. (If TalkToUs sees this and wants to offer assistance on that, I'd take them up on their offer, but I'm not going to reach out to them directly because I suspect their answer will be the same - they'll probably have to go to the same department.) We've heard in the past of Qwest support reps doing this, but I've gotten the impression that CenturyLink has removed various capabilities from the lower-level support CSRs, so this doesn't surprise me. (For example, in the past chat CSRs were able to directly check port utilization on a DSLAM. Now it seems all they can do is check loop qual, like anyone can.)

Also, I think there is a difference between the DSL repair and DSL tech support departments.

This is for a business core connect plan?

Any idea why they didn't put you on the 20/5 plan?

Not Core Connect but it is business service.

They don't qualify this address for anything higher because of the loop length. I will say I've looked at the county records, know where their easement is and have a pretty good idea the exact path the cable takes, and it's not even 2000 ft. (let alone 4500+).

I have no doubt 40/2 is possible at least, but nobody there will consider provisioning anything even with the understanding of "if it breaks, no repairs until you drop down to 20."

Why are they using 2 pairs? Looks like to me they could do 20/2 on a single pair.

Primary reason: it seems bonded tiers are bonded tiers, and even if the same rate can be reliably attained on a single pair, they won't do that. (In other words, the typical lack of flexibility from CL.)

Secondary reason in my case: they won't qualify me for 20M single-pair at all. They're not completely wrong about that as it would be marginal (training at that rate = 2-3 dB SNR on ADSL2+, should be a little higher on VDSL2), but I wish there was more flexibility. It's understandable that they wouldn't want to offer potentially unreliable service to the average user who is probably going to call in, have trucks sent out, etc. at the first sign of trouble, but there's no flexibility for the other side of this.

(I have 20M single-pair on the residential account only because of a bug in Qwest's system several years ago, which allowed me to order any tier in the middle of the night, presumably when certain parts of their system were unavailable. Unfortunately, this doesn't seem to be the case anymore, or I'd have ordered 40/5 that way. I don't think they've ever actually qualified me for higher than 12M per pair.)

Nice to see you around again!

I know one of the reasons you were so specific about getting VDSL2 to begin with was because of the better efficiency than on ADSL2+, where CL's still using ATM transfer mode. Have you seen that actually manifest itself in the speed at which you do things?

Also, the 2M upload is definitely nice, compared to 896k.

One thing about those max rates -- I'm now on a 40/20 line that has happily overprovisioned itself to 50/20, but as a result of that, it's now at the lowest acceptable downstream SNR for VDSL2, of 9dB. (This is on profile 12A.) I may have a few more megs of upstream available, but if CL were to let my line train at 60/30 (another "bonded tiers are bonded tiers and you have to bond them" situation) then I'd probably be at 6dB or below of SNR.

(It would be interesting to know how the modem estimates those rates, if the estimates are based on max training speed at 0dB SNR, then, they simply can't be trusted whatsoever, and based on my experience training up at 50 megs, I bet that's what is happening.)

Without knowing what the SNRs and stability would be like, I'll mirror the sentiments of the many -- your lines look like they should be able to handle 40/2 (an actual tier on the business side) or possible 40/5, but it would almost certainly be pushing it, so it may be best to stick around where you're at until something crazy like quad-bonding comes onboard. (VDSL2 as a standard, and even CL's DSLAMs can do it, but there's really no CPE for it.)

Yes. Using CL's speed test, I'm able to get about 18.5M/1.9M. That's a small difference and on the downstream side, probably not worth paying business rates for, but my primary motivation at this point is upstream.

(I'm aware I could quite possibly obtain more downstream speed for less money by getting two residential lines for $30/mo. each + a VM somewhere, but I have other issues with that approach.)I was wondering the same. In my case, both the ~29M and ~3.8M estimates seem *very* optimistic. The C2000T might be using a 3 dB target SNR for these estimations, based on:

Max:    Upstream rate = 1348 Kbps, Downstream rate = 20140 Kbps
Bearer: 0, Upstream rate = 892 Kbps, Downstream rate = 20125 Kbps
 
Link Power State:       L0
Mode:                   ADSL2+ Annex A
TPS-TC:                 ATM Mode(0x0)
Trellis:                U:ON /D:ON
Line Status:            No Defect
Training Status:        Showtime
                Down            Up
SNR (dB):        3.2             21.1
Attn(dB):        21.0            9.8
Pwr(dBm):        19.2            11.9
 

(Note the very max and actual rates being nearly the same.)

Also of note, I'm not sure how the C2000T is calculating attenuation on VDSL2. See:

 > xdslctl0 info --pbParams
        VDSL Band Status                U0              U1              U2              U3              U4              D1              D2              D3
  Line Attenuation(dB):  7.1     59.0     N/A     N/A     N/A    24.6    70.0     N/A   
Signal Attenuation(dB):  7.0     58.9     N/A     N/A     N/A    24.5    70.4     N/A   
 > xdslctl0 info --show
Attn(dB):        27.4            0.0
 

(Poor formatting, but that's how the modem displays it.)

27.4 dB downstream attenuation on the show command, but D1 is 24.6 and D2 is 70.0. It's not an average of both.

(Which brings up another interesting point: these VDSL2 attenuation numbers will produce misleading results with most or possibly all - if none account for this - loop length calculators, because this is including D1's higher frequencies.)I agree with that. 40/2 was never in question IMO for capabilities, seeing as 20/.896 has been reasonably stable on ADSL2+ with the lower SNR, and some of the added VDSL2 D1 bins are still very usable (which is why the SNR is higher at the same rate). And I'd love the opportunity to try for the 5M upload.

CL's loop optimizer dropped the max DS rate to 18M on the ADSL2+ circuit. I called and had them reset that; will see if it happens again (if it does, I don't intend on calling constantly and will just let them leave it at 18M for as long as we have the VDSL2 PB + ADSL2+ running side-by-side). This is the first time I've had that occur, but I've seen IIRC one or two reports of it here.

I'm unsure what the exact trigger point is to get dropped, but presumably the new active pairs brought the stats to that point. FWIW, there's a very minor difference when I bring the two VDSL2 pairs down - from testing right now, SNR on the ADSL2+ is 3.4 dB with all 3 active and 3.6 dB with only 1.

I'm still kind of shocked...

 > xdslctl0 info --show
xdslctl0: ADSL driver and PHY status
Status: Showtime
Last Retrain Reason:    1
Last initialization procedure status:   0
Max:    Upstream rate = 2753 Kbps, Downstream rate = 31429 Kbps
Bearer: 0, Upstream rate = 1018 Kbps, Downstream rate = 20063 Kbps
 
Link Power State:       L0
Mode:                   VDSL2 Annex A
VDSL2 Profile:          Profile 8a
TPS-TC:                 PTM Mode(0x0)
Trellis:                U:ON /D:ON
Line Status:            No Defect
Training Status:        Showtime
                Down            Up
SNR (dB):        11.9            8.8
Attn(dB):        27.6            0.0
Pwr(dBm):        15.9            7.6
 > xdslctl1 info --show
xdslctl1: ADSL driver and PHY status
Status: Showtime
Last Retrain Reason:    1
Last initialization procedure status:   0
Max:    Upstream rate = 2470 Kbps, Downstream rate = 35997 Kbps
Bearer: 0, Upstream rate = 1019 Kbps, Downstream rate = 20063 Kbps
 
Link Power State:       L0
Mode:                   VDSL2 Annex A
VDSL2 Profile:          Profile 8a
TPS-TC:                 PTM Mode(0x0)
Trellis:                U:ON /D:ON
Line Status:            No Defect
Training Status:        Showtime
                Down            Up
SNR (dB):        16.8            8.8
Attn(dB):        28.4            0.0
Pwr(dBm):        15.9            8.6
 

A huge THANK YOU to the awesome tech who went above and beyond in making this happen! (I'm going to email his supervisor and tell him this too; it's the least I can do.)

When one of those ports (2nd pair/xdslctl1 in the above stats) was uncapped (without data transfer):

 > xdslctl0 info --show
xdslctl0: ADSL driver and PHY status
Status: Showtime
Last Retrain Reason:    0
Last initialization procedure status:   0
Max:    Upstream rate = 2623 Kbps, Downstream rate = 35837 Kbps
Bearer: 0, Upstream rate = 2598 Kbps, Downstream rate = 34624 Kbps
 
Link Power State:       L0
Mode:                   VDSL2 Annex A
VDSL2 Profile:          Profile 12a
TPS-TC:                 PTM Mode(0x0)
Trellis:                U:ON /D:ON
Line Status:            No Defect
Training Status:        Showtime
                Down            Up
SNR (dB):        6.2             6.1
Attn(dB):        27.4            0.0
Pwr(dBm):        13.1            8.8
 

Heh heh, second code box has a different VDSL profile mode set. It might explain the slight difference i've observed in your code boxes for in upstream speed.

Yeah, that was a mistake from someone at CL - uncapped rate and 12a profile. It was training at the highest rate possible at the target margin. The first code box (8a and 20/1 each) shows the current/proper configuration.

If only CL had a flexible speed option. Because 60/4 at 6 dB would be fine with me and I would pay for it.

It'd be nice if CenturyLink went ahead and brought out a new promotion called "CenturyLink Unleashed".

Where said package would allow your xDSL line(s) to train at the highest possible sync. With cost to speed being reflected by the "nearest package" speed tier.

Yeah, that's what I was referring to.

I see why it's not done, but why not dream (of course, let's think bigger, 1000/1000 :P).

6 dB is a really sensible target.