[time-nuts] CATV assymetries

[Eric Scace]

   CATV plant in inherently asymmetric in delays. Referring to the DOCSIS (data over cable service interface specification) specs for North America:

Downstream to the customer:
   6 MHz channels with 64- or 256-level QAM modulation. If 256-QAM is used, the raw usable bandwidth is about 38 Mbit/s per channel.

Upstream from the customer:
   6.4 MHz channels (with fallback to channel widths between 0.2 and 3.2 MHz when talking with DOCSIS v1 equipment)
   QPSK, 8-, 16-, 32- or 64-level QAM
   DOCSIS v3.1 allows up to 1024-level, optionally also 2048 or 4096. Max rate with 4096-QAM and 25 kHz sub channel carrier spacing is 1.89 Gbit/s per 192 MHz OFDM channel.

DOCSIS 3.0 support channel bonding, allowing multiple channels to be used simultaneously to/from the same customers.

Downstream channels are shared and therefore traffic queuing may occur because of activity of other customers. Similarly, upstream channels may be shared by customers via TDMA or S-CDMA. Some contention can occur if customers transmit simultaneously.

Signal-to-noise ratios may force specific customers to use lower speeds.

These represent a large set of combinations that yield different downstream and upstream throughputs.

More generally, asymmetric behavior is almost guaranteed on CATV plant between the customer site and the cable head end. Upstream packets take longer to send then downstream, and may have to contend for their channel(s). Downstream packets are sent faster but may queue.

Between the cable head end and the public NTP server, symmetrical delays may be more prevalent.

   — Eric

> On 2019 Oct 20, at 19:31 , Hal Murray <hmurray at megapathdsl.net> wrote:
> 
> 
> fio at cattaneo.us said:
>> Maybe I should double check the routing from both ends again and make sure
>> they are really symmetric...... I can also ask my coworkers in the networking
>> group and hear what they think about it. 
> 
> We need a cable TV geek.
> 
> It wouldn't surprise me if the upstream traffic went through a couple of extra 
> hoops/hops that aren't visible at the IP level using tools like ping and 
> traceroute.