Comcast's Congestion Catch-22

An anonymous reader sends us to Telephony Online for a story about Comcast's second attempt at traffic management (free registration may be required). After the heavy criticism they received from customers and the FCC about their first system, they've adopted a more even-handed "protocol agnostic" approach. Nevertheless, they're once again under scrutiny from the FCC, this time for the way their system interacts with VOIP traffic. By ignoring specific protocols, the occasional bandwidth limits on high-usage customers interferes with those customers' VOIP, yet Comcast's own Digital Voice is unaffected. Quoting: "The shocking thing is just how big a Pandora's box the FCC has appeared to open — and it just keeps getting bigger. When the FCC first started addressing bandwidth usage and DPI issues, it quickly found itself up to its knees in network management minutia. Not long after that, it followed another logical path of the DPI question and asked service providers and Web companies about their use of DPI for behavioral targeting. Now it seemingly has opened up huge questions about what it means to be a voice carrier in the age of IP. It's not hard to imagine the next step: What about video? Telco IPTV services are delivered in roughly the same way as carrier VoIP services — via packets running on the same physical network but a prioritized logical signaling stream. Is that fair to over-the-top video service providers?"

Comments

VOIP!=Internet

[not_anne]

"Comcast Digital Voice uses Internet Protocol and not the Internet. Comcast Digital
Voice calls travel on our private, managed network -- not over the public Internet. That makes
it superior to other 'Best Effort' services delivering phone traffic over the public Internet."

Source (emphasis mine): http://www.comcast.com/MediaLibrary/1/1/About/PressRoom/Documents/ProductsAndServices/digital_voice.pdf

Re:Congestion?

[grumling]

You don't understand cable system design. The reason for the 6:1 ratio between upstream and downstream is not because Cisco (or anyone else) thinks you can oversubscribe the upstream spectrum, but because upstream carrier to noise ratios are much worse than downstream. Because of the lower CNR, upstream modulation has to have a lot more interleaving and error correction (and much lower symbol rates). It also helps isolate noise problems to a smaller service area.

Part of DOCSIS 3 spec is 64QAM upstream. Some operators are trying it now, and finding out that there's a whole new level of plant maintenance necessary to deliver a good upstream bit error rate. Meanwhile, the normal downstream carrier is 256QAM (6.4MSym/s symbol rate), which requires a 3dB improvement in CNR over 64QAM at the same symbol rate. As fiber is driven deeper into the cable network it will be much easier to increase the upstream modulation to 256QAM and downstream modulation at 1024QAM. Typical cable systems today use 16QAM modulation in the upstream, with a 3.2 MSym/s symbol rate.

And, it is fairly common to have multiple upstream carriers in a node (neighborhood). DOCSIS 3.0 adds support multiple downstream carriers* through devices called edge QAMS. The downside of that is most operators have 65 or so analog channels, several dozen digital cable channels, 4-5 VOD carriers, and one DOCSIS 2.0 carriers in the downstream. The push is to get rid of the analog channels, but that's politically unpopular since it would require all customers to get a set top box for every TV (someday tru-2-way TVs and set top boxes will be at Best Buy, but it's a long time coming). Once 3.0 is deployed, the typical system may have 3 or more bonded downstream carriers/service group, about 500 customers. End users will need a new modem to get full use of the channel bonding, but it should be worth it for the much greater increase in speed.

Finally, everyone always gets the "shared bandwidth" argument wrong. Most people think of DOCSIS like classic Ethernet, with a hub or daisy chain cable. This means that Ethernet NICs need to use CSMA/CA to avoid collisions. There is no way for a cable modem to hear another one, so the CMTS assigns a mini-slot to a cable modem when it is provisioned/registered (which essentially makes a TDMA channel). the ONLY time a modem is permitted to transmit is at it's assigned mini-slot. Over the years, CMTS software has improved, and operator's understanding of the configuration has become much more granular, to the point that bandwidth optimization is much better understood than it was 10 years ago, along with moving from 7200 series network engines to VXR and above (in the case of Cisco).

*There is some use of multiple downstreams now, it has been in the spec since DOCSIS 1.1, but isn't needed on much more than a temporary basis. Individual modems can only tune one carrier at at time, so it is typically used to get more customers on a node than it is used to get higher speeds. However, some operators have used multiple downstreams to isolate business class customers from everyone else.