BIN Bits, Gain, SNR

My Draytek 2600 router has an ADSL spectrum readout giving the
following data

BIN Bits
BIN Gain
BIN SNR

Re SNR, my status screen on the router shows SNR margin as around 10db
whereas the BIN SNR spectrum reading indicates between 20 & 30 over a
range of 30 to 170 BIN

My Status SNR Margin appears to differ from the Spectrum reading or is
the word MARGIN significant.

Also, what are BIN bits and BIN gain.

Geoff Lane

"Geoff Lane" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> My Draytek 2600 router has an ADSL spectrum readout giving the
> following data
>
> BIN Bits
> BIN Gain
> BIN SNR
>
> Re SNR, my status screen on the router shows SNR margin as around 10db
> whereas the BIN SNR spectrum reading indicates between 20 & 30 over a
> range of 30 to 170 BIN
>
> My Status SNR Margin appears to differ from the Spectrum reading or is
> the word MARGIN significant.

The word margin is significant. With everything expressed in dB, the
following holds:

SNR margin = measured SNR - theoretical minimum SNR

The theoretical minimum SNR can be computed knowing the modulation scheme.

> Also, what are BIN bits and BIN gain.

ADSL (more precisely, G.DMT) uses many low bandwidth subchannels to carry
the data; the bins are those subchannels and each bin is centred on a
particular frequency (bin number x 4.something kHz). A given line has
different characteristics at different frequencies (eg due to interference),
which affects the achievable bitrate per bin. So the modem and exchange
equipment come to an agreement on the bins to use and bits per (sub)symbol
for each used bin (the bin bits).

I'm not sure about the bin gain; my best guess is that some equalisation of
the signal levels of different bins is necessary (or at least helps).

Alex

On Sun, 12 Nov 2006 10:00:32 -0000, "Alex Fraser" <(E-Mail Removed)>
wrote:

>> Re SNR, my status screen on the router shows SNR margin as around 10db
>> whereas the BIN SNR spectrum reading indicates between 20 & 30 over a
>> range of 30 to 170 BIN
>>
>> My Status SNR Margin appears to differ from the Spectrum reading or is
>> the word MARGIN significant.
>
>The word margin is significant. With everything expressed in dB, the
>following holds:
>
>SNR margin = measured SNR - theoretical minimum SNR

Thanls Alex for a good explanation, I've heard 10db mentioned as a
minimum practical SNR so I suppose the 10db SNR margin corresponds
with the BIN SNR spectrum reading of between 20 and 30, probably
taking the lowest reading.

Geoff Lane

"Geoff Lane" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> On Sun, 12 Nov 2006 10:00:32 -0000, "Alex Fraser" <(E-Mail Removed)>
> wrote:
>>> Re SNR, my status screen on the router shows SNR margin as around 10db
>>> whereas the BIN SNR spectrum reading indicates between 20 & 30 over a
>>> range of 30 to 170 BIN
>>>
>>> My Status SNR Margin appears to differ from the Spectrum reading or is
>>> the word MARGIN significant.
>>
>>The word margin is significant. With everything expressed in dB, the
>>following holds:
>>
>>SNR margin = measured SNR - theoretical minimum SNR
>
> Thanls Alex for a good explanation,

I probably should have said "theoretical minimum /required/ SNR" above, and
pointed out that this is a per-bin calculation.

> I've heard 10db mentioned as a minimum practical SNR so I suppose the
> 10db SNR margin corresponds with the BIN SNR spectrum reading of between
> 20 and 30, probably taking the lowest reading.

The margin for a particular bin depends on the SNR and bin bits, and I
reckon the allocation of bits to bins is probably designed to make the bin
margins roughly equal (and as large as possible) - ie, to make the signal
equally noise resistant across all used bins. If this is true, there will be
a clear correlation between SNR and bin bits (a linear relationship I
think), and the single SNR margin figure might be an average of the per-bin
figure over all used bins.

Alex