back to article Gigabit duplex DOCSIS 3.1 passes feasibility study, kind of

CableLabs has popped over to Amazon and ordered a few dozen ten-gallon back-patters, with the announcement that its standardisation boffins have applied the ACME firehose expander to hybrid fibre-coax networks to get full-duplex gigabit transmissions happening. For that, you have to strip out blather both misleading and …

  1. Kevin McMurtrie Silver badge

    The Y axis is time?

    1. hplasm

      The Y axis is time?


  2. Mage Silver badge

    Always been in theory possible

    In practice this is really really hard the longer the cable, the more variable the cable quality is, connectors and the wider the bandwidth.

    Speech on copper pair uses cancellation and there is significant crosstalk. Gigabit ethernet uses this technique and as a result needs much better cable and connections than 100M bps, as well as strict distance limits. it uses all four pairs in parallel, in duplex, versus original cat5 Ethernet which actually only used one separate pair in each direction.

    I imagine only part of the 110MHz to 862MHz band will be used and often only on cable that is good quality (no nips, sharp bends, corrosion, poor connections and accurate impedance). At the minute 5MHz to 65 MHz (or sometimes 88 or even 110MHz) is used for upstream which is lower QAM and and TDMA. That's 0.8, 1.6, 3.2, or 6.4MHz channels with virtual TDMA channels in them. Downstream is 2, 4, 6 or 8MHz channels between 110MHz and 560 (old cable), 862MHz or even up to 1200MHz, essentially using DVB-C, but multiplexed data rather than TV.

    The problem also is that downstream is really One to Many broadcast (though modems ignore data for other users) and upstream is Many to One, with potentially only 10 simultaneous users if you dedicate a physical channel rather than TDMA. Non-HFC is very much longer cables shared to more users. Hence the typical 8:1 or worse ratio Download/Upload with DOCSIS, though DSL is often worse. Also the QAM is typically lower to reduce modem TX power, this means lower speed. The symmetrical idea would mean very much higher modem power.

    A simpler solution on cables not passing too many houses is to use 5 to 500 MHz upstream and 550MHz to 1200MHz downstream. Much more robust than full duplex on the same channels.

    1. Cynic_999 Silver badge

      Re: Always been in theory possible

      AFAIAA Ethernet over UTP has separate pairs for Tx and Rx (two of each in the case of 1000baseT), and so while the link as a whole is full-duplex, data does not pass in opposite directions over the same physical link (pair).

  3. Mage Silver badge

    100BaseT vs 1000Base T

    1000BASE-T uses all four pairs bi-directionally and the standard includes auto MDI-X; however, implementation is optional. With the way that 1000BASE-T implements signaling, how the cable is wired is immaterial in actual usage.

    It does for common Gigabit, but not for 100Mbps.

    The reason is that it's easier to do the cancellation for up to 100m than get twice as fast signalling over Cat5e.

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