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

If a cable modem has not been granted the decryption keys for a particular multicast service flow, it does

not forward the traffic to its CPE devices. This ensures that only authorized subscribers can receive the

multicast traffic, and prevents cable modems from loading down their local networks by forwarding

unnecessary multicast traffic.

DOCSIS 1.1 uses the concept of Security Associations (SA), which are dynamically created and

maintained to provide the service flows required to transmit IP multicast traffic on the downstream. A

cable modem sends an SA Map Request message to request the SA for the downstream service flow that

is carrying the desired multicast traffic.

If the cable modem is not authorized to receive the multicast traffic, or if the traffic is not available on

BPI+ encrypted SA, the CMTS sends an SA Map Reject message. The cable modem then does not repeat

any further SA Map Requests for this particular multicast traffic. However, if the traffic is available on

an unencrypted service flow, it begins forwarding that traffic to its CPE devices.

If the cable modem is authorized to receive the multicast traffic, and if the traffic is available, the CMTS

replies with an SA Map Reply message to provide the information that allows the cable modem to

receive the multicast traffic. The SA Map Reply message contains the SA identifier (SAID) for the traffic

and the cryptographic suite that is necessary to decrypt the multicast traffic.

If the cable modem supports the cryptographic suite being used, it sends a Key Request to the CMTS,

requesting the public keys it needs to decrypt the multicast service flow. If the CMTS replies with a Key

Reply that contains the requested public keys, the cable modem begins decrypting the multicast traffic

and forwarding it to its attached CPE devices.

The multicast traffic can be mapped to the cable modem’s primary SA, a static SA, or a dynamically

created SA. One service flow can support multiple multicast traffic flows, each with its own SAID.

Multicast traffic mapped to a primary SA can be received only by the cable modem that is assigned the

associated primary service flow. Multicast traffic mapped to static and dynamic SAs can be received by

all cable modems that are assigned the associated secondary service flows.

Payload Header Suppression and Restoration

The PHS feature is used to suppress repetitive or redundant portions in packet headers before

transmission on the DOCSIS link. This is a new feature in the DOCSIS1.1 MAC driver. The upstream

receive driver is now capable of restoring headers suppressed by cable modems, and the downstream

driver is capable of suppressing specific fields in packet headers before forwarding the frames to the

cable modem.

Migrating from Earlier Versions of DOCSIS

DOCSIS 1.1 cable modems have additional features and better performance than earlier DOCSIS 1.0

and 1.0+ models, but all three models can coexist in the same network. DOCSIS 1.0 and 1.0+ cable

modems will not hamper the performance of a DOCSIS 1.1 cable modem, nor will they interfere with

operation of DOCSIS 1.1 features. There is full forward and backward compatibility in the standards.