Using Filtered Face Place But Still CRC'S Errors?

hi i'm on sky broadband 8mb connect, i'm using a Filtered face plate with a netgear modem which was supplied, this is connected to a 15metre adsl cable as the router sits on a shelf. Since moving to sky broadband i noticed i'm getting a very slow speed, i was on ukonline 8mb LLU before and had great speeds. I noticed a lot of CRC's Errors, Do You think it could be the modem sky supplied me with?, Anyone know why i have a lot of CRC's Errors? it cant be the 15metre adsl cable could it?

Thank A lot

Edited by j112 (Fri 19-Jan-07 21:45:37)

1] What are your line stats?
2] Difficult to evaluate more than 1 change at a time, so what exactly ahs changed?

/usr/sbin/adslctl: ADSL driver and PHY status
Status: Showtime Channel: FAST, Upstream rate = 448 Kbps, Downstream rate = 4096 Kbps
Link Power State: L0
Mode: G.DMT
Channel: Fast
Trellis: ON
Line Status: No Defect
Training Status: Showtime
Down Up
SNR (dB): 5.9 20.0
Attn(dB): 54.0 31.5
Pwr(dBm): 17.9 11.9
Max(Kbps): 4128 1016
Rate (Kbps): 4096 448
G.dmt framing
K: 129(0) 15
R: 0 0
S: 1 1
D: 1 1
ADSL2 framing
MSGc: 1 1
B: 129 15
M: 1 1
T: 1 1
R: 0 0
S: 1.0000 1.0000
L: 1040 128
D: 1 1
Counters
SF: 188255 188253
SFErr: 2218 0
RS: 0 0
RSCorr: 0 0
RSUnCorr: 0 0

HEC: 539 0
OCD: 1 0
LCD: 0 0
Total Cells: 30918744 0
Data Cells: 1917823 0
Drop Cells: 0
Bit Errors: 0 0

ES: 464 0
SES: 29 0
UAS: 12 0
Total time = 53 min 36 sec
SF = 188255
CRC = 2218
LOS = 2
LOF = 0
ES = 464
Latest 1 day time = 53 min 36 sec
SF = 188255
CRC = 2218
LOS = 2
LOF = 0
ES = 464
Latest 15 minutes time = 8 min 36 sec
SF = 30405
CRC = 666
LOS = 0
LOF = 0
ES = 115
Previous 15 minutes time = 15 min 0 sec
SF = 52931
CRC = 791
LOS = 0
LOF = 0
ES = 160


There you go, thanx

In reply to:

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this is connected to a 15metre adsl cable as the router sits on a shelf

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That's a very high shelf! If you want to make sure that the cable isn't at fault, can you temporarily move the modem so that you can connect directly into the socket with a short lead?

How exacltly do you go about getting those stats from a netgear then ?

Thanks

ok well today i will:
try a diffrent modem
plug directly into the master socket taking the filtered adsl plate off getting rid of all extension wiring.
use a shorter adsl cable

any other ideas? i will post results back soon

thank for all the help

update: tried differnt modem still loads of crc errors
pluged directly in still crc errors
i will try a shorter cabe later on but doubt it could be this as its a new cable and i have always been getting slow speeds even since i joined sky connect however i will give it go. Strangely i just did a speed test and got a good result for once

http://www.speedtest.net/result/76515080.png

connected at 4416 kbps

Edited by j112 (Sat 20-Jan-07 10:29:15)

to get router stats use this link:
http://dg834gt.mognuts.com/NetgearRouterUtility.html

click get detailed modem stats, then press the back button and select get all modem stats, and then download save the file and open it with notepad

just changed the cable still the same, still getting loads of crc errors

Total time = 1 hours 25 min 3 sec
SF = 32491
CRC = 1393
LOS = 31
LOF = 0
ES = 8993

Changed the cable, or moved the router next to the socket with a short cable? If the latter then it may just be that a long cable is picking up lots of noise (perhaps it's not twisted pair, or you've left the ring wire connected, or you're running it near an interference source). If you eliminate the cable entirely then it could be that Sky have put you on a lower SNR margin than UKOnline, or perhaps UKOnline had you on Interleaved? Either of which you will need to contact Sky to change.

ok thanx 4 the help will give it a go, whats the diffrence between interleave and fast?, also i'm on sky connect is that ADSL1?, if i change my router to adsl 2 mode will it have any effect thanx

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whats the diffrence between interleave and fast

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You're stats show that your modem error correction is switched off. Requesting interleaving is the easiest way to get error correction switched on, although you will lose a little line speed and interleaving itself will increase your latency. Alternatively it may be enough just to increase your margin, although this again will lower your speed. In practice if you just tell them you're getting a lot of errors then they'll probably do whatever they think best.

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if i change my router to adsl 2 mode will it have any effect

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It wouldn't do any harm to try. Most routers have a default setting which will attempt ADSL2 first and fall back to ADSL1 if ADSL2 isn't supported on the line, and this is the best setting to use. Most 8Mb services are provided by ADSL1 anyway, but it's just possible that it's a capped ADSL2+ service in which case using ADSL2+ should improve your line a little. Even then I think you'll end up needing interleaving and/or margin increase.

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Requesting interleaving is the easiest way to get error correction switched on, although you will lose a little line speed

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Just to clarify, I thought that you only lost speed if you were able to sync at 7616 or above due to 512 Kb overheads which interleaving uses?

As I am not able to sync that high, I thought that the only penalty I have by having interleaving enabled is the increased latency?

Strictly interleaving and error correction are separate, but they are usually turned on together. Interleaving causes a latency increase, but has no impact on sync speed. However error correction imposes an overhead which reduces the net line speed. The actual overhead varies depending on how much error correction is switched on by the ISP, trading speed for reduced errors. The only exception is when the line can achieve 8Mbps sync with capacity to spare, in which case the correction overhead can be taken from the spare capacity without reducing the net line speed below 8Mbps.

In reply to:

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Strictly interleaving and error correction are separate, but they are usually turned on together. Interleaving causes a latency increase, but has no impact on sync speed. However error correction imposes an overhead which reduces the net line speed. The actual overhead varies depending on how much error correction is switched on by the ISP, trading speed for reduced errors. The only exception is when the line can achieve 8Mbps sync with capacity to spare, in which case the correction overhead can be taken from the spare capacity without reducing the net line speed below 8Mbps.

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Sorry I don't think you're right there see http://kitz.co.uk/adsl/interleaving.htm

BT (and to be fair most UK ISPs) frequently use the term 'interleaving' to refer to both interleaving and error correction, as the BT standard profiles that have error correction all have interleaving as well so it keeps things simple to use a single term. However they are technically quite distinct. It's perfectly possible to have error correction without interleaving (although not the reverse as interleaving has no benefit without error correction).

As for whether you can have error correction whilst syncing at 8Mbps - see these line stats as a recently posted example. This shows 20 bytes of error correction check bytes in every 254 (actually corresponding to an overhead of 640kbps of the line speed), but with the net line speed being 8096kbps.

No he's not, the confusion arises because what BT calls interleaving is not simply interleaving but interleaving with FEC. Interleaving on its own simply rearranges the data, as in your link, so that the transmitted data is no longer contiguous. Thus any errors are not on contiguous blocks when rearranged or deinterleaved at the receive end meaning that it is less likely that there are too many errors for the standard error correction routine to fix.

However, on ADSL MAX when BT applies interleaving it also applies the maximum amount of FEC (Forward Error Correction), which adds an extra 640Kb of error correction information to the data stream. Interleaving adds latency due to the time it takes to interleave and deinterleave the data stream while the extra FEC data added reduces the data rate because some of the data rate is taken up by the extra 640Kb of FEC data transmitted.

Either method can be used in isolation or combined but BT uses them combined with the maximum amount of FEC applied. If interleaving was used without FEC then the only effect would be an increase in latency with the data rate staying the same. If FEC was used on its own then the effect would only be reduced data rate with the latency staying the same. Of course, when FEC is used the actual amount of data moved is the same except that 640Kb of it isn't usable data but error correction data. When the two are used together as in BT's case then you get both an increase in latency and a decrease in data rate.

EDIT: Typo

Edited by deleted (Sun 21-Jan-07 13:49:06)

Sorry, must type faster, I was typing while you posted

No problem - your explanation was more detailed anyway!

Thank you for explaining that so clearly, but I think the confusion about interleaving arises also from the terms used to describe it, including your own. AFAIK a fast ADSL connection has some error correction built in anyway. I assume this is also known as FEC. As you describe BT interleaving includes additional FEC error correction. This could be better described.

Surely the additional error correction introduces latency as well as the interleaving process itself?

I am trying to get my own LLU interleaving turned off. If I eventually succeed, it will be interesting to see whether TT uses a different mix from that deployed by BT.

It occurs to me that a better system to use for correcting transmission errors would be to firstly increase error correction and then secondly switch on interleaving if necessary. ie to make a one stage process into a two stage process.

In reply to:

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a fast ADSL connection has some error correction built in anyway

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Only if it's enabled by the ISP. It's very common for a line not to have error correction enabled, as it wastes line speed if it's not required. On such a line when an error does occur it is uncorrected by the modem and results in a CRC error. On modems that display the ADSL parameters you can see this very clearly - the 'R' paremeter (ie: the number of check bytes) is set to zero. Here's an example.

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I assume this is also known as FEC.

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Correct - Forward Error Correction.

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As you describe BT interleaving includes additional FEC error correction.

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Sorry, but no. BT have error correction either on or off.

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Surely the additional error correction introduces latency as well as the interleaving process itself?

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No. ADSL error correction is performed by hardware and introduces no significant delay. Latency due to interleaving is inherent in the process as you have to wait for all the interleaved frames to arrive before you can recover one.

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It occurs to me that a better system to use for correcting transmission errors would be to firstly increase error correction and then secondly switch on interleaving if necessary. ie to make a one stage process into a two stage process.

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It would certainly be good to have the option, as it would allow errors to be reduced without the latency, which would be adequate for many cases.

In reality both the level of interleaving and the amount of FEC is adjustable it is simply that BT have decided on using maxium FEC when it applies interleaving. Probably on the basis that they appear to be aiming more for stability rather than speed to minimise support costs. LLU operators may well use different amounts of either if they use it at all.

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LLU operators may well use different amounts

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Yes - my LLU line has 6 FEC check bytes out of 254 on the downstream, and FEC disabled on the upstream, whilst still correcting 98% of my line errors. Rather less draconian that BT's 16 bytes each way or nothing at all. It's a shame BT haven't taken their DLM technology a little further so that it sets an appropriate level of error handling automatically for each line, instead of just full on or off.

I have got downstream interleaving on at level 64 and upstream at level 2. I have logged 100 receive CRC errors and 69 HEC errors in the last 8 hours. Would you advise me not to turn interleaving and error correction off? I am on a 9dB target noise margin with a 3-4dB variation every 5 seconds or so.

That really depends on how many errors are being corrected, and of course how much you're interested in the reduced latency and increased speed. The CRC error count is very low, but represents only the errors that are left uncorrected, and gives no reliable indication of corrected error rates. The 64 depth interleaving is giving you around 16msec of latency at 8064kbps. As you're on an LLU line I can't tell how much error correction overhead you have without seeing the check byte figures.

Many modems will display stats including the number of corrected errors - shown as FEC errors or sometimes RS uncorrected errors, although you may need to resort to a telnet commandline interface to get them. Failing that a very very rough estimate might be around 1 uncorrected error out of every 50 corrected, so that would be 5000 in 8 hours, or around 1 every 6 seconds. That's perfectly useable, although speeds and certainly gaming are probably beginning to suffer.

Thank you for that.