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Quick: 1 and 4.5 Gig DOCSIS Explained
Thursday, 16 June 2011 14:03
octoI've been reporting since 2005 that (shared) gigabit cable would come so this is no surprise. John Chapman and Anton Wahlman provided the roadmap at Fast Net Futures. ARRIS at the 2011 cable show demo'd 4.5 gig down by eliminating any TV programming and using the full bandwidth (128 6 MHz channels) for data. Comcast bonded more than 25 channels and showed 1 gig down and 300 meg up. These are improvised demos, not close to production equipment. It will take two to five years for gear that would make this practical for regular customers; lots of other obstacles beside the technical mean you will not see these speeds at home for a very long time. 
Here's how it works.
Coaxial cable by design carries robust signals, divided in the U.S. into typically 128 channels. In DOCSIS 3.0, every 6 MHz channel delivers about 40 megabits downstream. 
  • 4 channels bonded give you the shared 160 megabits now available to 70% of the U.S. It's all done on standard chips and needs no special hookup, so 160 meg gear costs little more than 10 meg gear.
  • 8 channel bonded chips are now coming from TI and Broadcom and were used in the demonstrations. They will cost little more in modems and soon will allow 320 meg (shared) connections. 
  • 25 channels, bonded, yield a gigabit. It would take extra chips today but is completely possible. When in 2008 Singapore demanded a gigabit broadband system, both Cisco and CableLabs put top engineers to work on a gigabit over cable. The conclusion: perfectly practical but too expensive at the time. Moore's Law has brought down the chip cost, and commercial cable modems should be capable of a gigabit in 2014-2015. 
  • 100 channels bring capacity to 4 gigabits. ARRIS used 16 8 channel bonded modems to demo 4.5 gigabits in Chicago.
  • 300 channels and over 10 gigabits is practical with better amplifiers and other field components. Cox did some experiments with 3 MHz cable equipment and they were a technical success.
  • EuroDOCSIS, based on European television standard, uses 8 MHz channels and hence carries 1/3rd more data per channel.
Takeaway: Coax can carry gigabits, although that will rarely be a commercial product. The gigabit (shared) capacity will become important later this decade as data usage grows. Raising the shared capacity to a gigabit can maintain solid performance for all users at 50 and 100 megabits well. My guess is some nodes will need upgrading 2015-2018 and the increased capacity will support 50 meg service well into the 2020's.