DOCSIS 3.1 upstream

Nick Hilliard nick at
Fri Apr 15 10:07:55 UTC 2016

Jean-Francois Mezei wrote:
> Canadian cable carriers seem to have all told the CRTC they can only
> carry 42mhz in the upstream because their amplifiers and nodes only
> amplify that narrow band in the upstream direction.
> Is/was 42mhz common across north america ?

42MHz was the traditional upper limit for annex b docsis.  That limit
was extended up to 85MHz several years ago, but yeah there's probably a
lot of plant out there which can't go above 42MHz for legacy reasons.

> Am trying to figure out realistic bandwidth that a cableco with 42mhz
> limits for upstream will get on 3.1.

If the cableco is limited to 42MHz, there will be 37MHz of upstream
bandwidth (5 to 42), which allows five 6.4MHz upstream channels of
5120ksym/sec.  3.1 improves the upstream modulation from 64qam to
4096qam, which ups the bit throughput rate from 6 bits per symbol to 12
bits.  That gives 5120*5*12 = 307200 of physical layer bit throughput,
and you should budget ~25-ish% for overhead to get usable customer bits
per second.

That's in lab conditions though.  The reality is that you're not going
to be able to use qam4096 unless your upstream path has ridiculously
good SNR.  If the cable network can't go above 42MHz, it's probably
legacy plant which implies older deployments and there's a real
likelihood that the improvements in DOCSIS 3.1 aren't going to make a
blind bit of difference.  It would be probably be easier and more
reliable to do plant upgrades / service retirement to allow 85MHz (12
u/s channels) than clean up the plant so that you get the 30-35dB SNR
required to run 4096QAM.  You can't make extra bandwidth out of nothing.

> Also, have cablecos with such limits for upstream begun to upgrade the
> cable plant to increase the upstream bandwidth ?

I would hope they have.  If they don't, their businesses will be savaged
in the longer term by the introduction of gpon and other fiber technologies.


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