[MLB-WIRELESS] adsl protocol question... - off topic

[Jon Teh]

On Thu, Feb 24, 2005 at 01:36:49PM +1100, Gabrielle Harrison & Paul van den Bergen wrote:
> Hi all,
> 
> <mode = de-lurk>
> does anyone know how the signalling and data transmittion codecs work for  
> adsl cable and satellite?
> 
> e.g.
> The line b/w your house and the exchange is capable of telephone at low  
> frequencies and adsl at high frequencies.  Depending on your adsl plan you  
> get a feed that is somewhat lower in speed than the maximum... which I  
> presume is somewhere around 1.5 Mbps??? (ignoring local conditions and  
> distance to exhchange effects - I'm interested in the max possible output  
> from the exchange.)
> 
> most of us get much less than the 1.5 Mbps.  so how is that achieved. Is  
> it something like a limited frequency band that is used, such that there  
> is, for want of a better term, some dark cable available for another  
> transmitter to access, or is it a codec change... e.g. lower speed, more  
> error checking, greater distance, but still using all the available  
> frequency band (or more likely, using all the frequency band,  
> inefficiently, but not so as one can share).
> 
> basically I want to know if 2 service providers can utilise the copper  
> pair coming ot your home...
> 

Hello,

Basically a POTS (Plain Old Telephone System) connection utilises only
the frequencies between 300 and 3400Hz on your twisted pair cable to the
exchange.

What ADSL does, is it uses frequencies above this standard voice range 
and utilises a modulation technique known as COFDM (Coded Orthogonal
Frequency Division Multiplexing) which, to put simply, means the 
available frequency bandwidth is split up into virtual channels for
parallel data delivery. There is also some form of QAM (Quadrature 
Amplitude Modulation) for the baseband modulation. The complicated
DSP circuitry within an ADSL modem can look at each virtual channel
and analyse the Signal to Noise ratio of each one, and assign data
usage accordingly.

Maximum speed encountered depends basically upon the attenuation, 
noise, and frequency response characteristics exhibited by a particular
twisted pair cable. The most major factor affecting this is cable
length, as the longer the cable, the higher the attenuation and 
induced noise. 1.5Mbit/s is by far not the maximum for the average
twisted pair, but rather an artificial Telstra imposed limit. 
Non-Telstra DSLAMs, such as iiNet's, allow much greater throughput
(eg. 8000/1000 kbit/s up/down speeds), but the ultimate speed is
determined by the quality of your line. ADSL is really just a big
hack to make lines that were only ever intended for analogue voice
conversations, to do high speed data, using modern technology. :)

Mind you, the hack works fairly well, most of the time.

Hope this clears up some issues for you.


Regards,

Jon

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