DOCSIS 3.1 upstream

Lorell Hathcock lorell at
Fri Apr 15 11:52:50 UTC 2016

In our small, aging plant very near the Mexican border in south Texas, the SNR for <~30MHz is ~20 dB so we can only use two upstream channels. It works okay for our 150 cable modem customers. They can get 40 Mbps upstream throughput. 

The downstream channels are around 300MHz with much better SNR so we can bond 8 channels.  Depending on load, customers can get up to 80 Mbps downstream throughput. 

This is on a DOCSIS 3.0 Cisco CMTS network with a 10 year old cable plant. 


Sent from my iPhone

> On Apr 15, 2016, at 5:07 AM, Nick Hilliard <nick at> wrote:
> 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.
> Nick

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