[188779] in North American Network Operators' Group
Re: DOCSIS 3.1 upstream
daemon@ATHENA.MIT.EDU (Lorell Hathcock)
Fri Apr 15 07:52:55 2016
X-Original-To: Nanog@nanog.org
From: Lorell Hathcock <lorell@hathcock.org>
In-Reply-To: <5710BD7B.2060502@foobar.org>
Date: Fri, 15 Apr 2016 06:52:50 -0500
To: Nick Hilliard <nick@foobar.org>
Cc: "Nanog@nanog.org" <Nanog@nanog.org>
Errors-To: nanog-bounces@nanog.org
In our small, aging plant very near the Mexican border in south Texas, the S=
NR for <~30MHz is ~20 dB so we can only use two upstream channels. It works o=
kay for our 150 cable modem customers. They can get 40 Mbps upstream through=
put.=20
The downstream channels are around 300MHz with much better SNR so we can bon=
d 8 channels. Depending on load, customers can get up to 80 Mbps downstream=
throughput.=20
This is on a DOCSIS 3.0 Cisco CMTS network with a 10 year old cable plant.=20=
Lorell
Sent from my iPhone
> On Apr 15, 2016, at 5:07 AM, Nick Hilliard <nick@foobar.org> wrote:
>=20
> 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.
>>=20
>> Is/was 42mhz common across north america ?
>=20
> 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.
>=20
>> Am trying to figure out realistic bandwidth that a cableco with 42mhz
>> limits for upstream will get on 3.1.
>=20
> 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 =3D 307200 of physical layer bit throughput,
> and you should budget ~25-ish% for overhead to get usable customer bits
> per second.
>=20
> 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.
>=20
>> Also, have cablecos with such limits for upstream begun to upgrade the
>> cable plant to increase the upstream bandwidth ?
>=20
> 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 technologie=
s.
>=20
> Nick
>=20
