Just to show exactly what’s happening here, line 3’s upstream bit loading is consistently 2 bits less than that of the other lines.
Here’s line 3 (green is upstream):
and for comparison here’s line 4 below:
The surprising thing is that the upstream attenuation of line 3 at 40.5 dB is very slightly better than that of line 4 at 40.7 dB, so it must be higher noise ingress. Or crosstalk? Am I correct in thinking that crosstalk is less likely at these low frequencies given that this large difference is an upstream-only phenomenon?
Summary:
Live sync rates:
#1: down 3015 kbps, up 522 kbps
#2: down 2964 kbps, up 563 kbps
#3: down 2844 kbps, up 386 kbps
#4: down 3012 kbps, up 557 kbps
Notice how the downstream bit loading curves differ in shape slightly, and line 3 is slower than line 4 downstream. But take a look at the speeds of line #2 compared with #4; it has the highest upstream rate and yet its downstream speed is slightly lower than line #4, so there’s no general pattern where upstream speed implies downstream speed.
Line #3 upstream bit-loading:
Bearer: 0, Upstream rate = 386 Kbps, Downstream rate = 2844 Kbps
Tone number Bit Allocation
0 0
1 0
2 0
3 0
4 0
5 0
6 0
7 8
8 9
9 9
10 9
11 9
12 9
13 8
14 8
15 8
16 7
17 6
18 6
19 6
20 5
21 5
22 5
23 4
24 4
25 4
26 4
27 4
28 3
29 3
30 3
31 2
32 0
--
Line #4 upstream bit-loading:
Bearer: 0, Upstream rate = 557 Kbps, Downstream rate = 3012 Kbps
Tone number Bit Allocation
0 0
1 0
2 0
3 0
4 0
5 0
6 0
7 9
8 10
9 11
10 11
11 11
12 11
13 10
14 10
15 10
16 9
17 9
18 9
19 8
20 8
21 8
22 8
23 7
24 7
25 7
26 6
27 6
28 6
29 5
30 5
31 5
32 0