Are DOCSIS 3.1 Cable Modems truly Backwards Compatible?

DOCSIS® 3.1 cable modems are expected to be backwards compatible. From the title of this post you will already expect this is not fully the case. Let’s go over some important topics to be aware of when using DOCSIS 3.1 cable modems on pre-DOCSIS 3.1 CMTSs!

Downstream lower frequency band edge support

(Euro)DOCSIS 3.0 (and earlier) CMs (cable modems) are required to support a lower downstream frequency edge of 108 MHz. DOCSIS 3.1 CMs are required to support a lower edge of 258 MHz and are only recommended to support a lower edge of 108 MHz. Hence, DOCSIS 3.1 CMs that support a lower edge at a higher frequency than 108 MHz are not truly backward compatible.

A DOCSIS 3.1 CM will not come online on a pre-DOCSIS 3.0 CMTS

During initialization, a DOCSIS 3.1 CM will try to find an MDD message (MAC Domain Descriptor) on the downstream channels that it is able to lock on. If it’s not able to find any MDD message, it will keep on scanning. As only DOCSIS 3.0 and higher CMTSs transmit MDD messages, a DOCSIS 3.1 CM will not come online on a D1.0, D1.1 or D2.0 CMTS.

Bye-bye BPI, CoS, PHS and pre-3.0 concatenation

The DOCSIS 3.0 specification inherited some old features that were of no use anymore. Those were cleaned up in the DOCSIS 3.1 specification.

  • A DOCSIS 3.1 CM no longer needs to support the initial Baseline Privacy Interface (BPI) 1.0 requirements. Instead, only support for BPI+ is required.
  • Class of Service, another DOCSIS 1.0 feature, has been removed from the DOCSIS 3.1 specification. Any DOCSIS 1.0 style configuration file containing Class of Service TLVs cannot be used on a DOCSIS 3.1 cable modem.
  • A DOCSIS 3.1 CM is not required to support pre-DOCSIS 3.0 concatenation. It is required though to support Continuous Concatenation and Fragmentation (CCF) as defined in the DOCSIS 3.0/3.1 specification.
  • Also all Payload Header Suppression (PHS) requirements have been removed from the DOCSIS 3.1 specification.

A new PKI

To provide stronger cryptography, DOCSIS 3.1 introduces a new PKI (Public Key Infrastructure with digital certificates) with increased key sizes. However, for backwards compatibility reasons a DOCSIS 3.1 cable modem still needs to support the legacy PKI. So a DOCSIS 3.1 CM will have two sets of digital certificates: one with the new DOCSIS 3.1 PKI and one with the legacy PKI (which can either be the DOCSIS or EuroDOCSIS PKI, by the way). You can read more about this in a separate blog post.

In addition, due to these two PKI’s, the software update process also got a bit more complex. No worries though, we also have a nice blog post on that topic.

Dynamic operations

DOCSIS 3.1 CMs no longer support UCC (Upstream Channel Change). The CMTS uses DBC (Dynamic Bonding Change) messaging to change any Downstream and/or Upstream channel set that has been assigned to the CM. DCC (Dynamic Channel Change) can be used to instruct the CM to move and re-initialize on a different MAC domain.

IP/LLC filtering support

To simplify the filtering implementation in DOCSIS 3.1 cable modems, downstream filtering is only carried out in the CMTS and upstream filtering is carried out in the CM. A DOCSIS 3.1 CM is no longer required to support IP/LLC filters. All upstream filtering is to be done through Upstream Drop Classifiers (UDC), which also support IPv6 and provide better performance.


DOCSIS 3.1 adds no additional multicast features. The behaviour of a DOCSIS 3.1 CM will be different though if the CMTS is configured to use MDF-disabled mode (Multicast DSID Forwarding disabled). An MDF-disabled DOCSIS 3.1 CM will continue to transparently forward upstream multicast traffic, but it is no longer required to support IGMPv2 proxy functionality as in prior versions of DOCSIS. Thus, if a DOCSIS 3.1 CM is placed into MDF-disabled mode, it will not support the forwarding of multicast traffic. However, the MDF-disabled CM is still required to handle the multicast forwarding necessary for IPv6 provisioning of the CM and embedded devices like the router or settop box functions.


DOCSIS 3.1 cable modems are expected to interoperate seamlessly with (Euro)DOCSIS 3.0 CMTSs, though not with (Euro)DOCSIS 1.0/1.1/2.0 CMTSs. As mentioned in this article, even on a (Euro)DOCSIS 3.0 CMTS you still need to be aware of some important changes and differences.

Excentis offers testing, training and consultancy services on DOCSIS 3.1. Contact us ( to discuss how we can help you.

Source for the information above: DOCSIS 3.1 specifications (

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One Reply to “Are DOCSIS 3.1 Cable Modems truly Backwards Compatible?”

  1. Has Cablelabs defined MIBs for checking Upstream Drop Classifier actions. With the legacy IP/LLC filters we could count the packets processed by the filter to know they are working. How is the equivalent action accomplished with UDCs? By the same token, how can we see what any given UDC is designed to accomplish (in terms of looking at the source/destination ip/port/tos/etc)? Thanks, always useful material found here!


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