Fastpath on Sky FTTC

My line is still on interleaved after 10 days (I know the DLM on FTTC is different, but still it's been 10 days), I have a feeling it was supposed to start on fastpath but it did not. The openreach engineer unplugged the modem to test the line after it was already connected up and working fine (D & E sides were originally mixed up at the cab), is that the cause? And I'm wondering if/when it's going to go to fastpath?

My line is stable at 9.65MB/s (using internet download manager and a 1GB tbb test file).

Useful Info:
Sync:-
Downstream: 79996
Upstream: 20000
That's according to the sky router, I'm now using a Buffalo WZR-HP-G300NH router instead as it out performs the sky router (the sky router can't handle those speeds, it tops out at around 9.3MB/s I think).

Ping from PC to the bbc is 15ms,

I'm pretty sure the openreach engineer said the downstream SNR was 16 and up was 12.

I haven't touched the modem.

How do you know 15ms is interleaved?

Where are you located?

I'm on BT Infinity. I am not interleaved, and have a ping to bbc of 20ms.

Best indication is to unlock the modem. Failing that, try 'tracert bbc.co.uk'. The first hop will give a better indication. Mine is 7ms even without interleaving. Some luckier BT users get 5ms or 6ms pings all the way to bbc.

Here's a traceroute that I've just taken. I'm in lancashire.
Tracing route to bbc.co.uk [212.58.241.131]
over a maximum of 30 hops:

1 <1 ms <1 ms <1 ms DD-WRT [192.168.11.1]
2 * * * Request timed out.
3 19 ms 17 ms 16 ms ip-84-38-37-12.easynet.co.uk [84.38.37.12]
4 18 ms 19 ms 19 ms te0-1-0-0.er11.thlon.ov.easynet.net [89.200.135
97]
5 17 ms 15 ms 19 ms ntl-ge2-9.prt0.rbsov.bbc.co.uk [212.58.238.189]

6 * * * Request timed out.
7 15 ms 15 ms 15 ms ae1.er01.rbsov.bbc.co.uk [132.185.254.46]
8 16 ms 17 ms 15 ms 132.185.255.60
9 15 ms 15 ms 15 ms 212.58.241.131

Trace complete.

So I went ahead and "hacked" the modem, they are below. From memory I think it's the D line that refers to interleaving. Am I on fastpath or interleaved?

xdslcmd: ADSL driver and PHY status
Status: Showtime
Retrain Reason: 0
Max: Upstream rate = 32417 Kbps, Downstream rate = 134704 Kbps
Path: 0, Upstream rate = 20000 Kbps, Downstream rate = 79999 Kbps

Link Power State: L0
Mode: VDSL2 Annex B
VDSL2 Profile: Profile 17a
TPS-TC: PTM Mode
Trellis: U:ON /D:ON
Line Status: No Defect
Training Status: Showtime
Down Up
SNR (dB): 20.9 13.1
Attn(dB): 0.0 0.0
Pwr(dBm): 13.1 -1.9
VDSL2 framing
Path 0
B: 239 236
M: 1 1
T: 23 5
R: 0 16
S: 0.0955 0.3771
L: 20107 5410
D: 1 1
I: 240 255
N: 240 255
Counters
Path 0
OHF: 94337 25644
OHFErr: 0 0
RS: 0 1666041
RSCorr: 0 0
RSUnCorr: 0 0

Path 0
HEC: 0 0
OCD: 0 0
LCD: 0 0
Total Cells: 23988724 0
Data Cells: 1226 0
Drop Cells: 0
Bit Errors: 0 0

ES: 0 0
SES: 0 0
UAS: 40 40
AS: 158

Path 0
INP: 0.00 0.00
PER: 1.64 6.12
delay: 0.00 0.00
OR: 116.56 203.67

Bitswap: 0 0

Total time = 3 min 21 sec
FEC: 0 0
CRC: 0 0
ES: 0 0
SES: 0 0
UAS: 40 40
LOS: 0 0
LOF: 0 0
Latest 15 minutes time = 3 min 21 sec
FEC: 0 0
CRC: 0 0
ES: 0 0
SES: 0 0
UAS: 40 40
LOS: 0 0
LOF: 0 0
Previous 15 minutes time = 0 sec
FEC: 0 0
CRC: 0 0
ES: 0 0
SES: 0 0
UAS: 0 0
LOS: 0 0
LOF: 0 0
Latest 1 day time = 3 min 21 sec
FEC: 0 0
CRC: 0 0
ES: 0 0
SES: 0 0
UAS: 40 40
LOS: 0 0
LOF: 0 0
Previous 1 day time = 0 sec
FEC: 0 0
CRC: 0 0
ES: 0 0
SES: 0 0
UAS: 0 0
LOS: 0 0
LOF: 0 0
Since Link time = 2 min 37 sec
FEC: 0 0
CRC: 0 0
ES: 0 0
SES: 0 0
UAS: 0 0
LOS: 0 0
LOF: 0 0

Interleave with a depth of just 1, i.e. hardly on at all.

Isn't that adding around 7ms?

In reference to en.wikipedia.org/wiki/Asymmetric_digital_subscriber_line#Interleavi...:

ISPs (rarely, users) have the option to use interleaving of packets to counter the effects of burst noise on the telephone line. An interleaved line has a depth, usually 8 to 64, which describes how many Reed�Solomon codewords are accumulated before they are sent. As they can all be sent together, their forward error correction codes can be made more resilient. Interleaving adds latency as all the packets have to first be gathered (or replaced by empty packets) and they, of course, all take time to transmit. 8 frame interleaving adds 5 ms round-trip-time, while 64 deep interleaving adds 25 ms. Other possible depths are 16 and 32.

"Fastpath" connections have an interleaving depth of 1, that is one packet is sent at a time. This has a low latency, usually around 10 ms (interleaving adds to it, this is not greater than interleaved) but it is extremely prone to errors, as any burst of noise can take out the entire packet and so require it all to be retransmitted. Such a burst on a large interleaved packet only blanks part of the packet, it can be recovered from error correction information in the rest of the packet. A "fastpath" connection will result in extremely high latency on a poor line, as each packet will take many retries.

I think that depth of 1 is the best that Openreach run at, some sources claim modem with D=1 is actually fastpath.

What is the problem you are trying to solve?

As that source is ADSL specific I am wary of quoting it for use with VDSL2

Fastpath is rare for any VDSL2 deployment, seems if you want lower latency then you need to wait for Fibre on Demand

15ms to London from Lancashire doesn't seem entirely unreasonable. I reckon you're on fastpath already but the first hop visible on the traceroute isn't a local one.

I get around 6ms first hop on Infinity in London - so assuming you were the same, that'd be about 9ms from Lancashire to London. I think it makes sense?

Edited by deleted (Thu 30-Aug-12 15:08:18)

Ah, I see now, so what makes my ping 7ms higher than other users? The routing? The MSAN? I wouldn't of thought the distance of fibre would add any ping, well at least within the UK. I suspect if it was the MSAN then changing ISP would make a difference.

As the link says, any instability on a "fastpath" line will result in increased latency.

I remember pinging bbc 5-7ms from college a couple of years ago and that's about a mile from here.

Very roughly (looking at a map) the distance from Lancashire to London looks about the same as the distance from London to Paris. London to Paris latency is a touch under 10ms? So 15ms to London for you seems about right, I think.

In reply to a post by Anonymous:

I remember pinging bbc 5-7ms from college a couple of years ago and that's about a mile from here.

I can't speak for a few years ago but a traceroute from a VPS in London to www.lancs.ac.uk shows roughly 8-9ms from London to Lancs

I know someone who lives in Durham and he pings 9ms to the bbc so I really can't see how the distance makes a difference.

Everyone has focused on the fibre and is ignoring the switches that link all the bits of fibre, these are where most of the latency impact is and Sky may not even be showing a big portion of their network.

Hopefully someone on FTTC who gets a ping of around 8-9ms could check to see if it says the depth is 1 (D: 1).

least how it works on sky if the cabinet detects you line working perfectly with no errors it will love you over to fastpath.

Sky can't change it, or even check it directly, found this out in passing when talking to the sky network team to try and resolve a speed issue problem

In reply to a post by Anonymous:

Hopefully someone on FTTC who gets a ping of around 8-9ms could check to see if it says the depth is 1 (D: 1).

I can take a look tonight - about 3 hours from now.

That would be great .

I use a draytek Vigor 2750n and so it reports the interleave by actual delay so I cannot tell what the d value is as I don't use the OR modem.

To get fastpath I didn't get the 80/20 upgrade and left it at 40/10 which then gives my line a noise margin in the 20's which means my line is extremely stable amd so the DLM granted me fastpath.

I wish there was a working way of adjusting the target SNR margin on the modem, e.g. instead of syncing with around 6dB I could specify to sync with around 15dB. I haven't discovered a mentioned and working way of this being done though, unlike ADSL, so I'm guessing it's not possible with VDSL at this time.

I've altered the noise margin on my Huawei and it was counter-productive. The behaviour of the sync's down and up was very strange, on three-six plays. (I forget exactly, but it was in that range). I decided I was better not messing.

It's also the case that in the latest Openreach documentation, which gives the specifications for modems' and modem/routers' behaviour when wires-only comes along, it specifically says:-

Note : It is the DLM system that sets the line profile, and this should not be interfered with by CPs/users setting rates, SNR margins etc. at the Modem.

That's the line profile by the way, not the IP Profile which comes along in the WBC DLM but not on Sky or TT.

Why would you want to sync FTTC at 15dB noise margin?

To try and encourage DLM to put me on fastpath (depth 1) as a result of minimal errors due to higher SNR. My depth is currently between 1100 to 1600, changes every few days typically.

The other future alternative I suppose is, when the contract has finished, choose a 40/10 package. Unfortunately that would mean losing the 20 upload, but by the time my contract finishes perhaps there will be more options .

Edited by Ixel (Thu 30-Aug-12 22:54:16)

In reply to a post by RobertoS:

I've altered the noise margin on my Huawei and it was counter-productive. The behaviour of the sync's down and up was very strange, on three-six plays. (I forget exactly, but it was in that range). I decided I was better not messing.

How did you achieve that Bob?

I have tried various "xdslcmd configure --snr" values with my HG612, but nothing seemed to change.

Now my connection has been repaired, I never see less than around 5.5dB SNRM. It usually hovers around 6.5dB to 6.9dB, so I would like to try it out at say 3dB.

My attainable rate is now always around 33Mb to 34Mb, but sync speed is always around a maximum of 28Mb (currently 28075 K with 6.5dB SNRM, so apparently not hard-cap banded by DLM).

DS Interleaving depth is around 450.
DS INP is at 3.00
DS delay isa at 8.00

I'm 12 hours later than promised, but here we are. I get 5-6ms to my first hop.

1 <1 ms <1 ms <1 ms 192.168.1.254
2 6 ms 5 ms 5 ms 217.32.147.97

# xdslcmd info --stats
xdslcmd: ADSL driver and PHY status
Status: Showtime
Retrain Reason: 0
Max: Upstream rate = 33119 Kbps, Downstream rate = 88236 Kbps
Path: 0, Upstream rate = 20000 Kbps, Downstream rate = 79999 Kbps

Link Power State: L0
Mode: VDSL2 Annex B
VDSL2 Profile: Profile 17a
TPS-TC: PTM Mode
Trellis: U:ON /D:ON
Line Status: No Defect
Training Status: Showtime
Down Up
SNR (dB): 8.3 14.8
Attn(dB): 0.0 0.0
Pwr(dBm): 12.4 6.8
VDSL2 framing
Path 0
B: 239 237
M: 1 1
T: 23 45
R: 0 16
S: 0.0955 0.3782
L: 20107 5373
D: 1 1
I: 240 127
N: 240 254
Counters
Path 0
OHF: 115012215 3943928
OHFErr: 170658 796
RS: 0 4293740
RSCorr: 0 17195
RSUnCorr: 0 0

Path 0
HEC: 906477 0
OCD: 0 0
LCD: 0 0
Total Cells: 405528636 0
Data Cells: 531377316 0
Drop Cells: 0
Bit Errors: 0 0

ES: 50126 243
SES: 83 40
UAS: 17 17
AS: 7292543

Path 0
INP: 0.00 0.00
PER: 1.64 4.25
delay: 0.00 0.00
OR: 116.56 60.17

Bitswap: 521020 17724

These are the bits I said I was trying to sort out on my logs, but I got myself in a mess trying to make the log run on from the one I uploaded for you.

I can probably upload the area I was doing it, but I won't remember the --snr settings I used. IIRC the relative movement of the down/up sync's may be useful, but I think I concluded that they were inversely related.

I'll try to look again today or tomorrow. The differnce was very small compared to what we would expect, as you just said. Which is why I stopped messing.

I'm guessing that the --snr is partially a percentage value, for example 100 being default, 50 being -3dB or some decrement below the DSLAM's target SNR margin, and 150 being +3dB. Perhaps I'm wrong but that's a guess after all. I haven't tried this as I don't want to keep doing unnecessary syncs to bump up my depth.

Talking ADSLx!

Yes it's a percentage of the DSLAM setting. Bear in mine that the "dB" is a logarithmic figure and that a change of 3dB is approximately a halving or doubling.

So if the DSLAM setting is 6dB, --snr 50 will result in around 3dB. If the DSLAM is 9dB then --snr 50 will result in around 6dB.

If you want to add stability, then to get from 6dB to 9dB it is --snr 200. To get to 12dB from 6dB it is --snr 400.

On FTTC/VDSL2 the Openreach DSLAM/DLM doesn't seem to work that way, but I'm saying that based on just a few tries, as I said earlier.

Fair enough, well I might give it a try, but given DLM is in the back of my mind I don't really want to upset it so I probably won't experiment at all.

In reply to a post by driz:

I'm 12 hours later than promised, but here we are. I get 5-6ms to my first hop.

1 <1 ms <1 ms <1 ms 192.168.1.254
2 6 ms 5 ms 5 ms 217.32.147.97

Text

1 23 45 67 89 1011 1213 1415 1617 1819 2021 2223 2425 2627 2829 3031 3233 3435 3637 3839 4041 4243 4445 4647 4849 5051 5253 5455 5657 58

# xdslcmd info --stats xdslcmd: ADSL driver and PHY statusStatus: Showtime Retrain Reason: 0Max:    Upstream rate = 33119 Kbps, Downstream rate = 88236 Kbps Path:   0, Upstream rate = 20000 Kbps, Downstream rate = 79999 Kbps  Link Power State:       L0Mode:                   VDSL2 Annex B VDSL2 Profile:          Profile 17aTPS-TC:                 PTM Mode Trellis:                U:ON /D:ONLine Status:            No Defect Training Status:        Showtime                Down            Up SNR (dB):        8.3             14.8Attn(dB):        0.0             0.0 Pwr(dBm):        12.4            6.8                        VDSL2 framing                         Path 0B:              239             237 M:              1               1T:              23              45 R:              0               16S:              0.0955          0.3782 L:              20107           5373D:              1               1   <----------------------------------- I:              240             127N:              240             254                         Counters                        Path 0 OHF:            115012215       3943928OHFErr:         170658          796 RS:             0               4293740RSCorr:         0               17195 RSUnCorr:       0               0                          Path 0HEC:            906477          0 OCD:            0               0LCD:            0               0 Total Cells:    405528636       0Data Cells:     531377316       0 Drop Cells:     0Bit Errors:     0               0  ES:             50126           243 SES:            83              40UAS:            17              17 AS:             7292543                          Path 0INP:            0.00            0.00   <--------------------------------- PER:            1.64            4.25delay:          0.00            0.00   <--------------------------------- OR:             116.56          60.17  Bitswap:        521020          17724

D = 1 = fast path (no interleaving)
delay = 0 = delay from interleaving (in microseconds)
INP = 0 = impulse noise protection (a result of interleaving)


See the excellent work of "JustAnother" here: http://forum.kitz.co.uk/index.php?topic=11113.0

cheers, a

Edited by deleted (Fri 31-Aug-12 15:22:45)