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Author Topic: DSL Symbol Rate - Why?  (Read 2049 times)

WWWombat

  • Kitizen
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  • Posts: 1674
DSL Symbol Rate - Why?
« on: June 02, 2016, 06:20:13 PM »

ADSL, ADSL2+ and VDSL2 all use a "symbol rate" of 4,000 symbols per second.

That means that any one tone, configured with N bits of bit-loading, can transfer (N * 4000) bits per second.

I always wondered why the number of 4,000 was chosen, and why it stayed the same over each iteration of technology, and now I understand why.

G.Fast is designed for 48,000 symbols per second. However, while it uses 106MHz of spectrum, it doesn't use more tones that VDSL2. It uses 2048 tones, each with over 51kHz of spectrum. And that turns out to be the link - the 51kHz of spectrum for each tone is what makes 48,000 symbols per second possible.

In older DSL iterations, the tones were each allocated 4.3175kHz - enough to carry 4,000 symbols per second. I guess the bandwidth allocated to a tone dictates how much time the DSP hardware requires to identify the analogue QAM components of each symbol.

Lightbulb moment  :idea:
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zhadoom

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  • Posts: 40
  • 38/4 Mb/s - HG612
Re: DSL Symbol Rate - Why?
« Reply #1 on: June 02, 2016, 08:09:31 PM »

ADSL, ADSL2+ and VDSL2 all use a "symbol rate" of 4,000 symbols per second.

That means that any one tone, configured with N bits of bit-loading, can transfer (N * 4000) bits per second.

I always wondered why the number of 4,000 was chosen, and why it stayed the same over each iteration of technology, and now I understand why.

G.Fast is designed for 48,000 symbols per second. However, while it uses 106MHz of spectrum, it doesn't use more tones that VDSL2. It uses 2048 tones, each with over 51kHz of spectrum. And that turns out to be the link - the 51kHz of spectrum for each tone is what makes 48,000 symbols per second possible.

In older DSL iterations, the tones were each allocated 4.3175kHz - enough to carry 4,000 symbols per second. I guess the bandwidth allocated to a tone dictates how much time the DSP hardware requires to identify the analogue QAM components of each symbol.

Lightbulb moment  :idea:

The old POTs system used 300Hz-3400Hz range for voice in analog systems. Digital PBX migration consider the voice bandwidth to 4KHz and adopt a 8KHz sampling rate ( 8000 samples per second - minimal for Nyquist-Shannon rule ).
First Dsl systems adopted a 256 sub band over a 0-1.1MHz ( 1105.28MHz to be precise ) range resulting in 4.3175MHz.
The first sub band ( bin 0 ) is used for the voice channel. Since its width is a little larger than 4000Hz its retro compatible with analog channel using a simple low pass filter or a low pass + high pass filter ( splitter ).

The only exception to use non 4k sub bands ( in xdsl technology ) is the VDSL2 30a profile. It uses a 8.625KHz band that is 2x 4.3175KHz.
« Last Edit: June 02, 2016, 11:17:20 PM by zhadoom »
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