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
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Cisco IOS Release 12.2(15)CZ
• Packet classifier—A set of packet header fields used to classify packets onto a service flow to which
the classifier belongs. When a packet is presented to the DOCSIS MAC layer at the CMTS or cable
modem, it is compared to a set of packet classifiers until a matching classifier is found. The SFID
from this classifier is used to identify the service flow on which the packet will be sent.
• PHS rule—A set of packet header fields that are suppressed by the sending entity before transmitting
on the link, and are restored by the receiving entity after receiving a header-suppressed frame
transmission. Payload header suppression increases the bandwidth efficiency by removing repeated
packet headers before transmission.
In the upstream direction, the output queues at the cable modem get remotely served by the CMTS MAC
scheduler, based on DOCSIS 1.1 slot scheduling constraints such as grant-interval and grant-jitter. In the
downstream direction, the CMTS packet scheduler serves the flow queues depending on the flow
attributes like traffic priority, guaranteed rate, and delay bound.
DOCSIS 1.1 adds several new MAC scheduling disciplines to provide guaranteed QoS for real-time
service flows on the multiple access upstream channel. Multiple grants per interval helps in supporting
multiple subflows (such as voice calls) on the same SID. Multiple subflows per SID reduces the
minimum SID requirement in cable modem hardware.
The CMTS is responsible for supporting QoS for all cable modems in its control. The traffic in the
downstream is assumed to be a combination of voice, committed information rate (CIR) data, and excess
burst best-effort data. To provide QoS support, the following functions must be performed:
• Packet classification—Mapping packets to service flows based on header information
• Policing (rate limiting) the individual flows
• Queuing packets into appropriate output queues based on the type of service
• Serving the output queues to meet delay and rate guarantees
The admission control block helps the overall downstream QoS block to track the current bandwidth
reservation state on a per-downstream basis. Decisions can be made whether to admit or reject a request
for a new service flow on that DS channel, based on this reservation state and the QoS guarantees
requested by the new service-flow.
IP packet classifiers help in filtering out unique service flows on an interface for differential QoS
treatment. Rather than doing per-cable modem downstream rate shaping, DOCSIS 1.1 software provides
rate shaping at a much more granular level of individual service flows of the cable modem.
Note Cisco uBR905 and uBR925 cable access routers and Cisco CVA122 cable voice adapters running
Cisco IOS Release 12.2(15)CZ can transparently interoperate with CMTS routers running DOCSIS 1.0,
DOCSIS 1.0+ extensions, or DOCSIS 1.1.
Service Flows and Packet Classifiers
Every cable modem establishes a primary service flow in both the upstream and downstream directions.
The primary flows maintain connectivity between the cable modem and the CMTS at all times.
In addition, a DOCSIS 1.1 cable modem can establish multiple secondary service flows. The secondary
service flows either can be permanently created (they persist until the cable modem is reset or powered
off) or can be created dynamically to meet the needs of the on-demand traffic being transmitted.
A service flow gets created at the time of cable modem registration (a static service flow) or as a result
of a dynamic MAC message handshake between the cable modem and the CMTS (a dynamic service
flow). At any given time, a service flow might be in one of three states (provisioned, admitted, or active).
Only active flows are allowed to pass traffic on the DOCSIS link.