Fragmentation; Interoperability - Cisco cBR 8 Configuration Manual

DOCSIS 1.1 for the Cisco CMTS Routers
• Support for multiple service flows per cable modem allows a single cable modem to support a combination
• Greater granularity in QoS per cable modem in either direction, using unidirectional service flows.
• Upstream service flows can be assigned one of the following QoS scheduling types, depending on the

Fragmentation

DOCSIS fragmentation allows the upstream MAC scheduler to slice large data requests to fit into the scheduling
gaps between UGS (voice slots). This prevents large data packets from affecting real-time traffic, such as
voice and video.
Fragmentation reduces the run-time jitter experienced by the UGS slots when large data grants preempt the
UGS slots. Disabling fragmentation increases the run-time jitter, but also reduces the fragmentation reassembly
overhead for fragmented MAC frames.
DOCSIS fragmentation should not be confused with the fragmentation of IP packets, which is done to fit
Note
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.

Interoperability

DOCSIS 1.1 cable modems can coexist with DOCSIS 1.0 and 1.0+ cable modems in the same network. The
Cisco CMTS provides the levels of service that are appropriate for each cable modem.
of data, voice, and video traffic.
type of traffic and application being used:
◦Best-effort—Data traffic sent on a non-guaranteed best-effort basis. This type of service flow is
similar to the method used in DOCSIS 1.0 networks.
◦Real-time polling (rtPS)—Real-time service flows, such as video, that produce unicast, variable
size packets at fixed intervals.
◦Non-real-time polling service (nrtPS)—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 grants (UGS)—Constant bit rate (CBR) or committed information rate (CIR) traffic,
such as voice, that is characterized by fixed-size packets at fixed intervals, providing a guaranteed
minimum data rate.
◦Unsolicited grants with activity detection (USG-AD)—Combination of UGS and rtPS, to
accommodate real-time traffic that might have periods of inactivity (such as voice using silence
suppression). The service flow uses UGS fixed grants while active, but switches to rtPS polling
during periods of inactivity to avoid wasting unused bandwidth.
Cisco cBR Series Converged Broadband Routers Quality of Services Configuration Guide for Cisco IOS XE Fuji
Fragmentation
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