Specifications

DOCSIS 1.1 for Cisco uBR905 and Cisco uBR925 Cable Access Routers and Cisco CVA122 Cable Voice Adapters

Information About DOCSIS 1.1 Support

Cisco IOS Release 12.2(15)CZ

• DOCS-SUBMGT-MIB—Describes the subscriber management attributes. This is revision 02 of the

MIB.

• RFC 2933—Describes the IGMP protocol attributes, as defined in RFC 2933.

• DOCS-CABLE-DEVICE-MIB—Describes the operation of the CM and the CMTS, as defined as

RFC 2669.

• DOCS-CABLE-DEVICE-TRAP-MIB—Defines the traps supported by CMs and the CMTS and is

the extension of RFC 2669 (DOCS-CABLE-DEVICE-MIB).

• DOCS-IF-EXT-MIB—Extends RFC 2670 (DOCS-IF-MIB) to provide information about whether

the CMs and the CMTS support DOCSIS 1.0 or DOCSIS 1.1.

Additional DOCSIS 1.1 Features in Cisco IOS Release 12.2(15)CZ

The following sections describe the DOCSIS 1.1 software features that appear in Cisco IOS

Release 12.2(15)CZ.

Concatenation

Concatenation allows the cable modem to make a single time-slice request for multiple packets and send

all packets in a single large burst on the upstream. Concatenation was introduced in the upstream receive

driver in DOCSIS1.0+ releases.

Fragmentation

Grant fragmentation allows the upstream MAC scheduler to slice large data requests to fit into the

scheduling gaps between UGS (voice slots). This reduces the jitter experienced by the UGS slots when

large data grants preempt the UGS slots. The grant fragmentation gets triggered in the MAC scheduler,

and fragment reassembly happens in the upstream receive driver.

Note DOCSIS fragmentation should not be confused with the fragmentation of IP packets, which is done to

fit the packets on network segments with smaller maximum transmission unit (MTU) size. DOCSIS

Fragmentation is Layer 2 fragmentation that is primarily concerned with efficiently transmitting

lower-priority packets without interfering with high-priority real-time traffic, such as voice calls. IP

fragmentation is done at Layer 3 and is primarily intended to accommodate routers that use different

maximum packet sizes.

IP Multicast Support

By default, a DOCSIS CMTS transmits IP multicast traffic without encryption. All DOCSIS cable

modems receiving that multicast traffic must forward it to its attached CPE devices, without regard to

whether any of the devices have requested the traffic. This can waste network bandwidth and require

network devices to waste processor power in forwarding and processing undesired multicast traffic.

A DOCSIS 1.1 CMTS can instead use the Internet Group Management Protocol (IGMP) to maintain the

multicast group memberships of its DOCSIS 1.1 cable modems. BPI+ encryption is used to encrypt the

multicast packets so that only the cable modems with the appropriate public keys can decrypt the packets

and forward them to their attached customer premises equipment (CPE) devices.