HEC's are a type of CRC error check performed on the ATM cell header. HECs can do a very minimal form of error correction and able correct 1 bit errors, but quite often they trigger a CRC.
http://www.kitz.co.uk/adsl/error_correction.htm#HECIts impossible for me to tell if your router is seeing these in the upstream or downstream because different modems may display up or down first.
>> What I'm not sure about is what distance matters for FTTC? The line between home and the cabinet is still copper so thats the one that matters. Between the exchange and cab its fibre so no need to worry about that distance.
>> I right to assume that, depending on profile, dB will initially be set at 6 or 9 and that the higher the number the lower the speed and vica versa? Is it as simple as, a 'good line' means the value drops whereas a 'bad line' means the value increases?Its initially set at 6dB.. and then things differ from traditional adsl which normally would go through the 6,9,12 etc stages.
The NGA (FTTC) DLM doesn't do this. If it detects a problem, rather than increasing the target SNRm to 9dB, it caps the maximum sync speed.
So say if you were syncing at 40Mbps and it detects the line is noisy, then it will reduce the line rate to say 38Mbps and rate limits it at that speed so you cant sync any higher. You may well see that your SNRm has increased, but because it uses a hard cap/rate limiting, tweaking SNR doesnt work.
>> is there any 'test' I can do ahead of placing the FTTC order to give me an idea of the SNRm value?No. From day one you will sync at 6dB, then any adjustments that DLM thinks needs making will be a reduction of speed cap.
Also, unlike ADSL, you cant turn off interleaving if DLM thinks you need it and why I said NGA DLM is less forgiving than ADSL DLM.
>> I've not done a lot of digging yet but I'm assuming there'd be a charge which we'd like to try and avoid.Most ISPs are happy to upgrade you for free - 80/20 is more expensive product - so they are getting more £ from you each month.
They are not so keen if you downgrade to a cheaper 40/10 (less income for them) and some may pass on the BTw product change fee to you.
Thats why I suggested you start off low as its easier to go up than go down. The only caveat I can think of is watch out for introductory offers.
There is no way anyone can tell you exactly what you will get - even Openreach dont know - until you put it live on the line. The estimates are based on line length and what people near to you currently get. VDSL is more sensitive to cross-talk. Also it uses much higher frequencies why by nature are more sensitive to noise.
Crosstalk is far more of a problem on VDSL, most of us here have seen its effects. A neighbour may get FTTC and suddenly you lose a chunk of speed.
See below for my line.. look what happened at 9am this morning when they turned their router on. That .8 dB drop is a speed loss of ~ 1.5 Mb if I were to resync right now. Different neighbouring lines have different effects. Theres one of my neighbour's cross talk which 'costs' 9Mbps.
Dont wish to scare you, just make you aware of what happens, and why you cant get ever get an exact quote. If I went round with a pair of hedge clippers and cut all my neighbouring lines.. my max sync would immediately shoot up to somewhere in excess of 110 Mbps.