Kitz Forum
Broadband Related => Broadband Technology => Topic started by: Experimentor on July 30, 2008, 11:04:51 AM
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Forgive me if this question has been asked before.
Do BT Wholesale for exchanges close to powerfull MW transmitters program their equipment
not to use certain 'frequency bins' for ADSL1 because they anticipate
strong interference from Radio Station signals.
The reason I am asking is that DMT and RouterStats report my modem as not using bins
in the range 681-712 kHz.(day or night).
Is this because
a. BT not using this frequency range?
b. My line to the exchange, even though all underground (except for one or more street cabs!), is still acting
like an aerial and picking up 'noise' from a radio station transmitting on this frequency range (can't remember which station).
If the answer is a. do ADSL2+ LLU operators also exclude bin ranges for similar reasons.
(Sure I read somewhere that BE don't use certain 'troublesome' frequencies)
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I'm not an expert, but I think this is an automatic part of the negotiation between your router and the DSLAM when the connection is established. Bins which are seen to have high levels of interference are not used, or only used in a small way.
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I was told that as dsl was being developed, the possible effect of mw radio transmitters were very closely looked at and after a whole series of trials it was found the transmitters have a supprisingly indisernable effect on dsl.
It's not just the frequency but the type of noise being tranmitted on that frequency (which why tracing REIN can become so difficult)
It's rather like having a conversation with someone whilst there's some backround noise at the same volume. Some types of noise/music will have little effect on your being able to hear/understand each other. the wrong type of noise and your conversation is drowned out. It's the same with dsl
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I'm not aware of BT restricting any particular channels.
AFIAK Its more down to the modem element in ther outer which monitors the SNR level in each frequency bin. If a particular frequency of a particular channel is too low, then the modem can ignore that particular frequency and instead use another one with a stronger SNR. The modem will monitor each channel and select only those frequencies with a strong enough signal,
IIRC its the DMT standard which allows the modem to utilise frequency bins with a strong enough SNR.
Theres also something called bit swapping which is done "on the fly" and on several occasions using DMTtool Ive seen my own router performing "bit swapping" on certain channel if the SNR dips too low.
>> Sure I read somewhere that BE don't use certain 'troublesome' frequencies
Yeah - bit of a strange one - 2053kHz-2156kHz, AFAIK none of the other operators appear to block that frequency. At first Be waffled about it having something to do with maritime, but have since backed down on that and have said at some point in the future they may review.