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Originally Posted by kronas
i think someone like BBking should be a better person to answer what bandwidth on the current system upload wise is available, whether it would be viable to increase upload speeds without crippling the system, or a DOCSIS change being the only costly alternative.
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BBKing is asleep so...
Cable networks use 2 types of cards.
MC16 - 1 downstream 6 upstream
MC28 - 2 downstreams, 4 upstreams to each downstream.
The two types of modulation used in upstreams are QPSK (Quadrature Phase Shift Keying) and 16QAM (16 point constellation Quadrature Amplitude Modulation). 16QAM requires insanely clean networks so QPSK is in use in 99% of cable networks around the world.
ntl use 3.2MHz wide upstreams, these allow 2.56 million symbols/second (think of a symbol as a snapshot of an RF wave). QPSK modulation means each symbol can carry 2 bits.
This gives a rate of 5.12Mbit/s per upstream.
Take from this all the overheads (DOCSIS is quite overheads heavy) and it leaves you with maybe 4Mbit/s of usable IP bandwidth.
From there you can do the maths yourself, bearing in mind that each card costs several thousand quid, and that each card needs its' own HFC path downstream and at least 1 if not more HFC paths upstream (these are the expensive part big fibre runs and laying fibre is still amazingly expensive) and it puts into context a bit the complexity and cost.
May seem great having all that upstream there for downstreams, but modems can travel between upstreams, and upstreams don't have to be anywhere near maxed out before pings suffer and downloading gets slowed down.
Hope this helps a bit, apologies to those whose eyes glazed over right at the start
