The differences between US DOCSIS and EuroDOCSIS, and will DOCSIS 3.1 eliminate them?

If you ask cable people for the differences between DOCSIS and EuroDOCSIS, the only thing they can typically think of is the difference in downstream channel width: 6 MHz for DOCSIS, 8 MHz for EuroDOCSIS. In reality there’s a lot more to it, as we will discuss in this blog post!

Downstream

Let’s first focus on the downstream. The root of most differences is ITU-T.J83, which is the international standard for digital transmission of television signals. EuroDOCSIS is based on ITU-T.J83 annex A, while for DOCSIS that’s ITU-T.J83 annex B.

The best known difference is indeed that EuroDOCSIS uses 8 MHz as downstream channel width, while US DOCSIS uses 6 MHz wide channels. But even the QAM64 and QAM256 modulation is different, as for DOCSIS also Trellis coding is used. The symbol rate for US DOCSIS QAM64 is 5.056941 Msym/s, and 5.360537 Msym/s for QAM256. For EuroDOCSIS that’s 6.952 Msym/s for both QAM64 and QAM256. As a result of this, the raw downstream bitrate of one QAM256 downstream channel is about 42.88 Mbps for DOCSIS versus 55.62 Mbps for EuroDOCSIS.  US DOCSIS also allows for several Interleaver configurations, while EuroDOCSIS works with a fixed (but different) Interleaver config.

Then there is the difference in the used frequency plan. EuroDOCSIS Cable Modems must implement a single but very flexible frequency plan, with center frequencies that start from 112 MHz and that go up to 858 MHz (or optionally 1002 MHz), and this in hops as small as 250 kHz. US DOCSIS however specifies three frequency plans, HRC/IRC/STD, with center frequencies that can go up to 867 MHz (or optionally 999 MHz) and that e.g., only require center frequency hops of 6 MHz to be supported.

Also, some different power requirements are in place. US DOCSIS Cable Modems must support a power range of -15 to +15 dBmV (for both QAM64 and QAM256), while for EuroDOCSIS that’s -17 to +13 dBmV for QAM64 and -13 to +17 dBmV for QAM256.

Finally, at least for the downstream part, there are also different Carrier-to-Noise ratio requirements. For US DOCSIS the minimum C/N ratios are lower compared with EuroDOCSIS.

Upstream

The list of differences in the upstream is not as long as the downstream list, though not less important.

The basic upstream frequency plan goes from 5 up to 42 MHz for US DOCSIS, and 65 MHz for EuroDOCSIS. The extended upstream frequency plan goes up to 85 MHz for both versions.

There are also different requirements on spurious emissions and micro-reflections. Let’s not go into too much detail here, as we would need a few more pages for this topic on itself. But because of these differences it is easier for a EuroDOCSIS modem to make both the legacy (5-65 MHz) and extended frequency (5-85MHz) requirements than it is for a US DOCSIS cable modem (that needs to support both the 5-42 MHz and 5-85 MHz requirements).

Any other differences?

Well yes, the DOCSIS and EuroDOCSIS specifications each specify their own Digital Certificates, which are used for BPI+ and secure software upgrade.

Will DOCSIS 3.1 eliminate all these differences?

As DOCSIS 3.1 includes backwards compatibility with earlier versions, it is clear that a DOCSIS 3.1 Cable Modem will not be the same as a EuroDOCSIS 3.1 Cable Modem. And if you look at Cable Gateways in general (as opposed to standalone Cable Modems), there are also important differences in the voice part (PacketCable versus EuroPacketCable), in the Wi-Fi part (e.g., the channel numbers that are allowed to be used, or different EIRP requirements), etc. But let’s say all this can be part of a separate blogpost!

• Category: Cable Modem, CMTS, DOCSIS