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[Weaver]

I have a very small yet puzzling problem. Why is my line 1 downstream a bit slow compared to the others?

Code: [Select]

                                         Status of each link
                                       ───────────────────────

   Link #1: sync rate: downstream: 2.613 Mbps at SNRM 3.3 dB; upstream: 643 kbps at SNRM 6.0 dB
   Link #1: uptime: 9 hours 48 min 9 sec

--
   Link #2: sync rate: downstream: 2.737 Mbps at SNRM 2.8 dB; upstream: 515 kbps at SNRM 6.0 dB
   Link #2: uptime: 14 hours 24 min 41 sec

   Link #2: ES downstream: 'penultimate' period: ES 8 per hr; MTBE 450 s; bucket duration 900 s

--
   Link #3: sync rate: downstream: 2.968 Mbps at SNRM 3.4 dB; upstream: 627 kbps at SNRM 5.9 dB
   Link #3: uptime: 21 days 14 hours 37 min 27 sec

--
   Link #4: sync rate: downstream: 2.741 Mbps at SNRM 3.4 dB; upstream: 355 kbps at SNRM 6.1 dB
   Link #4: uptime: 13 days 6 hours 6 min 55 sec

--
Time now: 2022-09-17T06:14:11 UTC.

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See how line 3 downstream is 350 kbps faster than line 1, i.e. 13.6 %. Why ?
It doesn’t seem to be anything to do with the downstream SNRM. Here are some rather more detailed stats of line 1 below:

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— Line 1
xdslctl: ADSL driver and PHY status
Status: Showtime
Last Retrain Reason: 8000
Last initialization procedure status: 0
Max: Upstream rate = 576 Kbps, Downstream rate = 2988 Kbps
Bearer: 0, Upstream rate = 643 Kbps, Downstream rate = 2613 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): 3.3 6.1
Attn(dB): 65.5 41.1
Pwr(dBm): 18.1 12.4

ADSL2 framing
Bearer 0
MSGc: 63 11
B: 76 78
M: 2 1
T: 1 1
R: 16 16
S: 1.8503 3.8776
L: 735 196
D: 1 8

Counters
Bearer 0
SF: 2575025 472712
SFErr: 1 0
RS: 88838383 3970040
RSCorr: 2581 13832
RSUnCorr: 1 0

ReXmt: 188 0
ReXmtCorr: 188 0
ReXmtUnCorr: 1 0

Bearer 0
HEC: 1 0
OCD: 0 0
LCD: 0 0
Total Cells: 254781490 62749975
Data Cells: 8881037 2241241
Drop Cells: 0
Bit Errors: 57 0

ES: 1054 3035
SES: 122 0
UAS: 6652 6579
AS: 41344

Bearer 0
INP: 29.00 2.50
INPRein: 0.00 0.00
delay: 8 8
PER: 15.95 16.47
OR: 34.58 8.25
AgR: 2635.70 649.42

Bitswap: 9867/9876 18/18


And below, for comparison here’s line 3, which is a truly excellent performer.

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— Line 3
xdslctl: ADSL driver and PHY status
Status: Showtime
Last Retrain Reason: 8000
Last initialization procedure status: 0
Max: Upstream rate = 534 Kbps, Downstream rate = 3288 Kbps
Bearer: 0, Upstream rate = 627 Kbps, Downstream rate = 2968 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): 3.4 5.8
Attn(dB): 64.5 41.4
Pwr(dBm): 18.2 12.4

ADSL2 framing
Bearer 0
MSGc: 52 11
B: 34 78
M: 4 1
T: 3 1
R: 10 12
S: 1.4851 3.9781
L: 808 183
D: 2 8

Counters
Bearer 0
SF: 115199032 654558
SFErr: 193 246
RS: 716190606 1508114
RSCorr: 46614 1130876
RSUnCorr: 1503 0

ReXmt: 27716 0
ReXmtCorr: 25858 0
ReXmtUnCorr: 1548 0

Bearer 0
HEC: 1353 384
OCD: 54 0
LCD: 54 0
Total Cells: 226043307 2771621221
Data Cells: 924443034 154143942
Drop Cells: 0
Bit Errors: 32578 49733

ES: 97 142
SES: 0 0
UAS: 53 53
AS: 1873337

Bearer 0
INP: 26.00 2.00
INPRein: 0.00 0.00
delay: 8 8
PER: 16.15 16.90
OR: 28.72 8.04
AgR: 2983.33 632.99

Bitswap: 311032/311036 669/669

There’s a 1 dB difference in the downstream attenuation between lines 1 and 3. Is that enough to make the difference?

[tubaman]

Higher attenuation will probably mean that less tones are available to use, but I'm surprised that 1dB makes quite so much difference.

[Weaver]

That’s an important point: let’s have a look at the bit-loading (below). Here’s line 1:




And for comparison, here’s line 3, much faster downstream, about 13%, below:



With modem 3, I can see a lot more tones with 12 bits. Also the high end goes down faster, you can more or less draw a straight line eg. from (80, 10) down to (120, 5) for modem 3. Notice how the diagonal lines of the two modems have different gradients. The equivalent diagonal line for modem 1 is (78, 10) down to (120, 4). The left-shift by a couple of tones doesn’t matter (?) but the line is descending down to an endpoint that is a whole tone lower. So averaging it out, we have (120-80)*(5-4)/2 * 4000 bps = 80 kbps. That’s very rough calculation and I haven’t done anything about the effect of the left shift by about two tones that you see when comparing the two slopes. To estimate the effect of the left shift, do we just knock out a column 2 tones wide and 10 bits high ? So that would be 2 * 10 * 4000 = 80 kbps again. So adding the two contributions we get 80 + 80 = 160 kbps. But we still have a long way to go to make up the difference in sync rate; in fact we have not quite made it half way.

Now my guess for the contribution for the very top, the missing 12-bit tones. I think there are about 12 tones up there in the case of modem 3 that are absent in modem 1, and I can see only about 3 12-bit tones for modem 1. So 12 * 4000 = 48 kbps, very roughly.

So adding that contribution in too, we have 80 + 80 + 48 = 208 kbps. So we are now starting to make some progress.

But the question now is the original one: why is the bit-loading so much worse that we lose 350 kbps downstream sync rate?

[Weaver]

Line 1 still remains a bit slower than its companions downstream. Downstream, the next faster line is line 2, at 4.6% faster.

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                       Status of each link
                    ──────────

* There are 4 modems in total. They are:  #1, #2, #3 and #4
* The active, contactable modems are:      #1, #2, #3 and #4
* The modems successfully queried are:    #1, #2, #3 and #4


   Link #1: sync rate: downstream: 2.613 Mbps at SNRM 2.1 dB; upstream: 643 kbps at SNRM 6.0 dB
   Link #1: uptime: 4 days 15 hours 36 min 35 sec

--
   Link #2: sync rate: downstream: 2.733 Mbps at SNRM 2.4 dB; upstream: 519 kbps at SNRM 8.2 dB
   Link #2: uptime: 1 day 1 hour 26 min 7 sec

--
   Link #3: sync rate: downstream: 2.968 Mbps at SNRM 3.3 dB; upstream: 627 kbps at SNRM 5.6 dB
   Link #3: uptime: 25 days 20 hours 26 min 1 sec

--
   Link #4: sync rate: downstream: 2.741 Mbps at SNRM 3.3 dB; upstream: 355 kbps at SNRM 6.0 dB
   Link #4: uptime: 17 days 11 hours 55 min 21 sec

--
Time now: 2022-09-21T12:01:40 UTC.

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