The Significant Differences Between DOCSIS and EuroDOCSIS

Sercu 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.

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 64-QAM and 256-QAM modulation is different, as for DOCSIS also Trellis coding is used.

The symbol rate for US DOCSIS is 5.056941 megasymbols per second (Msym/s) for 64-QAM and 5.360537 Msym/s for 256-QAM. For EuroDOCSIS it’s 6.952 Msym/s for both 64-QAM and 256-QAM. As a result of this, the raw downstream bitrate of one 256-QAM downstream channel is about 42.88 Mbps for DOCSIS vs. 55.62 Mbps for EuroDOCSIS. U.S. DOCSIS also allows for several interleaver configurations, while EuroDOCSIS works with a fixed (but different) interleaver config.

There also is a difference in the frequency plan. EuroDOCSIS cable modems implement a single but very flexible frequency plan, with center frequencies that start at 112 MHz and that go up to 858 MHz (or optionally 1,002 MHz). Hops are as small as 250 kHz. U.S. DOCSIS, however, specifies three frequency plans: HRC/IRC/STD. Center frequencies can go as high as 867 MHz (or optionally 999 MHz) and only require center frequency hops of 6 MHz to be supported.

There also are some different power requirements in place. U.S. DOCSIS cable modems must support a power range of -15 to +15 dBmV (for both 64-QAM and 256-QAM), while for EuroDOCSIS requires a range of -17 to +13 dBmV for 64-QAM and -13 to +17 dBmV for 256-QAM.

Finally, the downstream has a different carrier-to-noise(C/N) ratio requirements. For U.S. DOCSIS, the minimum C/N ratios are lower than those for EuroDOCSIS.

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

The basic upstream frequency plan goes from 5 MHz up to 42 MHz for U.S. DOCSIS; EuroDOCSIS extends to 65 MHz. 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. But because of these differences, it is easier for a EuroDOCSIS modem to meet both the legacy (5-65 MHz) and extended frequency (5-85MHz) requirements than it is for a U.S. DOCSIS cable modem (that needs to support both the 5-42 MHz and 5-85 MHz requirements).

Are there 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 backward 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. If you also look at cable gateways in general (as opposed to standalone cable modems), there are important differences in the voice element (which, more technically, is PacketCable vs. EuroPacketCable). There also are differences in the WiFi realm, including the channel numbers that are used and different EIRP requirements), etc.

This is the edited version of a post written by Kristof Sercu, who is the testing group manager for Ghent, Belgium-based Excentis. The company offers  ( testing, consultancy and training services for access and home network technologies.

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