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

Enhanced QoS

Extensive scheduling parameters allow the CMTS and the cable modem to communicate QoS

requirements and achieve more sophisticated QoS on a per service-flow level.

Different new time-slot scheduling disciplines help in providing guaranteed delay and jitter bound on

shared upstream. Activity detection helps to conserve link bandwidth by not issuing time slots for an

inactive service flow. The conserved bandwidth can then be reused for other best-effort data slots.

Packet classification helps the CMTS and the cable modem to isolate different types of traffic into

different DOCSIS service flows. Each flow could be receiving a different QoS service from the CMTS.

Provisioned QoS

Provisioned QoS (PQoS) allows the cable modem to create service flows for voice calls and other

real-time traffic at the time it registers with the CMTS, without actually using the bandwidth for those

flows. When such a service flow is specified in the DOCSIS configuration file, the cable modem creates

a flow that uses the DOCSIS 1.1 unsolicited grant service (UGS). The service flow, however, is not

activated until the cable modem signals the voice call using the DOCSIS 1.1 Dynamic Service Change

Request (DSC-REQ) message. Bandwidth is used only when the voice call is actually in progress.

Fragmentation

The MAC scheduler fragments data slots to fill the gaps in between UGS slots. Fragmentation reduces

the jitter experienced by voice packets when large data packets are transmitted on the shared upstream

channel and preempt the UGS slots used for voice. Fragmentation splits the large data packets so that

they fit into the smaller time slots available around the UGS slots.

Multiple Subflows per SID

This feature allows the cable modem to have multiple calls on a single hardware queue. This approach

scales much better than requiring a separate SID hardware queue on the cable modem for each voice call.

Payload Header Suppression

Payload header suppression (PHS) allows the CMTS and the cable modem to suppress repetitive or

redundant portions in packet headers before transmitting on the DOCSIS link. This helps to conserve

link bandwidth, especially with types of traffic, such as voice, where the header size tends to be as large

as the size of the actual packet.

Service Classes

The QoS attributes of a service flow can be specified in two ways: either explicitly by defining all

attributes, or implicitly by specifying a service class name. A service class name is a string that the

CMTS associates with a QoS parameter set.

The service class serves the following purposes:

• It allows operators to move the burden of configuring service flows from the provisioning server to

the CMTS. Operators provision the modems with the service class name; the implementation of the

name is configured at the CMTS. This allows operators to modify the implementation of a given

service to local circumstances without changing modem provisioning. For example, some

scheduling parameters might need to be set differently for two different CMTSs to provide the same

service. As another example, service profiles could be changed by time of day.

• It allows CMTS vendors to provide class-based-queuing if they choose, where service flows

compete within their class, and classes compete with each other for bandwidth.

• It allows higher-layer protocols to create a service flow by its service class name. For example,

telephony signaling might direct the cable modem to instantiate any available provisioned service

flow of class G.711.