Service Class - Cisco cBR 8 Configuration Manual
Cbr series converged broadband routers
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DOCSIS 1.1 for the Cisco CMTS Routers
Every cable modem establishes primary service flows for the upstream and downstream directions, with a
separate SFID for the upstream and the downstream flows. The primary flows maintain connectivity between
the cable modem and CMTS, allowing the CMTS to send MAC management messages at all times to the
cable modem.
In addition, a DOCSIS 1.1 cable modem can establish multiple secondary service flows. The secondary service
flows either can be permanently created (by configuring them in the DOCSIS configuration file that is
downloaded to the cable modem), or the service flows can be created dynamically to meet the needs of the
on-demand traffic, such as voice calls. Permanent service flows remain in effect, even if they are not being
used, while dynamic service flows are deleted when they are no longer needed.
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 network. Every service flow is identified by an SFID, while
upstream service flows in the admitted and active state have an extra Layer 2 SID associated with them. The
SID is the identifier used by the MAC scheduler when specifying time-slot scheduling for different service
flows.
Service Class
Each service flow is associated with a service class, which defines a particular class of service and its QoS
characteristics, such as the maximum bandwidth for the service flow and the priority of its traffic. The service
class attributes can be inherited from a preconfigured CMTS local service class (class-based flows), or they
can be individually specified when a cable modem dynamically requests a service flow and the CMTS creates
it.
The DOCSIS 1.1 service class also defines the MAC-layer scheduling type for the service flow. The schedule
type defines the type of data burst requests that the cable modem can make, and how often it can make those
requests. The following types of schedule types are supported:
• Best-effort (BE)—A cable modem competes with the other cable modems in making bandwidth requests
and must wait for the CMTS to grant those requests before transmitting data. This type of service flow
is similar to the method used in DOCSIS 1.0 networks.
• Real-time polling service (rtPS)—A cable modem is given a periodic time slot in which it can make
bandwidth requests without competing with other cable modems. This allows real-time transmissions
with data bursts of varying length.
• Non-real-time polling service (nrtPS)—A cable modem is given regular opportunities to make bandwidth
requests for data bursts of varying size. This type of flow is similar to the rtPS type, in that the cable
modem is guaranteed regular opportunities to request data bursts of varying length, except that the CMTS
can vary the time between its polling of the cable modem, depending on the amount of traffic and
congestion on the network.
• Unsolicited grant service (UGS)—A cable modem can transmit fixed data bursts at a guaranteed minimum
data rate and with a guaranteed maximum level of jitter. This type of service flow is suitable for traffic
that requires a Committed Information Rate (CIR), such as Voice-over-IP (VoIP) calls.
• Unsolicited grant service with activity detection (UGS-AD)—Similar to the UGS type, except that the
CMTS monitors the traffic to detect when the cable modem is not using the service flow (such as voice
calls when nobody is speaking). When the CMTS detects silence on the service flow, the CMTS
temporarily switches the service flow to an rtPS type. When the cable modem begins using the flow
again, the CMTS switches the flow back to the UGS type. This allows the CMTS to more efficiently
support VoIP calls.
Cisco cBR Series Converged Broadband Routers Quality of Services Configuration Guide for Cisco IOS XE Fuji
DOCSIS 1.1 Quality of Service
16.7.x
39
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