Specifications

DOCSIS 1.1 for Cisco uBR905 and Cisco uBR925 Cable Access Routers and Cisco CVA122 Cable Voice Adapters
Restrictions for DOCSIS 1.1 Support
4
Cisco IOS Release 12.2(15)CZ
Maximum Burst Size
Previously, the maximum concatenated burst size parameter could be set to zero to specify an unlimited
value. In a DOCSIS 1.1 environment, this parameter should be set to a nonzero value, with a maximum
value of 1522 bytes for DOCSIS 1.0 cable modems.
If a cable modem attempts to register with a maximum concatenation burst size of zero, the DOCSIS 1.1
CMTS refuses to allow the cable modem to come online. This avoids the possibility that a DOCSIS 1.0
cable modem could interfere with voice traffic on the upstream by sending extremely large data packets.
Since DOCSIS 1.0 does not support fragmentation, transmitting such data packets could result in
unwanted jitter in the voice traffic.
In addition, DOCSIS 1.1 requires that the maximum transmit burst size be set to either 1522 bytes or the
maximum concatenated burst size, whichever is larger. Do not set the maximum concatenation burst size
to values larger than 1522 bytes for DOCSIS 1.0 cable modems.
Note This change requires you to change any DOCSIS configuration files that specify a zero value for the
maximum concatenation burst size. This limitation does not exist for DOCSIS 1.1 cable modems unless
fragmentation has been disabled.
Provisioning
The format and content of the TFTP configuration file for a DOCSIS 1.1 cable modem are significantly
different from the file for a DOCSIS 1.0 cable modem. A dual-mode configuration file editor is used to
generate a DOCSIS 1.0 style configuration file for DOCSIS 1.0 cable modems and a DOCSIS 1.1
configuration file for DOCSIS 1.1 cable modems.
Registration
A DOCSIS 1.1 CMTS is designed to handle the existing registration TLVs from DOCSIS 1.0 cable
modems as well as the new type TLVs from DOCSIS 1.1 cable modems. A DOCSIS 1.0 and DOCSIS 1.1
cable modem can successfully register with the same DOCSIS 1.1 CMTS.
A DOCSIS 1.1 cable modem can be configured to make an indirect reference to a service class that has
been statically defined at the CMTS, instead of explicitly asking for particular service class parameters.
When this registration request is received by a DOCSIS 1.1 CMTS, it encodes the actual parameters of
the service class in the registration response and expects a DOCSIS 1.1-specific
registration-acknowledge MAC message from the cable modem.
When a DOCSIS 1.1 cable modem registers with a DOCSIS 1.0 CMTS, it responds with DOCSIS 1.0
style registration messages and does not use the DOCSIS 1.1 feature set.
Performance
DOCSIS 1.0 cable modems lack the ability to explicitly request and provide scheduling parameters for
advanced DOCSIS 1.1 scheduling mechanisms, such as unsolicited grants and real-time polling.
DOCSIS 1.1 cable modems on the same upstream channel can benefit from the advanced scheduling
mechanisms and a DOCSIS 1.1 CMTS can still adequately support voice traffic from DOCSIS 1.1 cable
modems with DOCSIS 1.0 cable modems on the same upstream channel.