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Cisco uBR10012 Universal Broadband Router Software Configuration Guide
OL-1520-05
AppendixA DOCSIS and CMTS Architectural Overview
DOCSIS Specification Summary
Overview of DOCSIS-Compliant Upstream Signals
The upstream channel is characterized by many CMs (or CMs in STBs) transmitting to the CMTS. These
signals typically operate in a burst mode of transmission. Time in the upstream channel is slotted.
The CMTS provides time slots and controls the usage for each upstream interval. The CMTS sends
regular mappings of minislot structure in downstream broadcast MAP messages. The CMTS allocates
contention broadcast slots that all CMs can use, and allocates upstream minislots for unicast or
noncontention data from specific CMs.
The CMTS allocates two basic types of contention slots on the upstream:
•Initial ranging slots that CMs use during their initialization phase to join the network. When the
CMTS receives an initial ranging request from a CM using this kind of slot, the CMTS subsequently
polls the CM, and other operational CMs, in unicast, noncontention station maintenance slots.
•Bandwidth-request minislots that CMs use to request data grants from the CMTS to send data
upstream in noncontention mode. Any CM can use this type of minislot to request a data grant from
the CMTS.
The stream of initial ranging slots and bandwidth request minislots comprise two separate contention
subchannels on the upstream. Cisco IOS Release 12.2XF software uses a “dynamic bandwidth-request
minislots-per-MAP” algorithm to dynamically control the rate of contention slots for initial ranging and
bandwidth requests. The CMTS uses a common algorithm to vary backoff parameters that CMs use
within each of the two upstream contention subchannels. The CMTS uses these algorithms to
dynamically determine the initial ranging slots and bandwidth-request minislots to allocate on the slotted
upstream.
When power is restored after a catastrophic power failure, a large number of CMs attempt to join the
network simultaneously. This represents an impulse load on the initial ranging subchannel. The CMTS
increases the frequency of initial ranging slots so that CMs can quickly join the network.
During high upstream data loads, the CMTS conserves the scarce upstream channel bandwidth resource
and is more frugal in introducing upstream initial ranging slots. The CMTS schedules bandwidth-request
minislots at low loads to provide low access delay. At high upstream loads, the CMTS reduces the
number of contention-based request minislots in favor of data grants, while maintaining a minimum
number of request slots.
Note The system default is to have the automatic dynamic ranging interval algorithm enabled, automatic
dynamic ranging backoff enabled, and data backoffs for each upstream on a cable interface.
Commands to configure the dynamic contention algorithms include:
[no] cable insertion-interval [automatic [Imin [Imax]] in msecs
[no] cable upstream port number range backoff [automatic] | [start | end]
[no] cable upstream port number data-backoff [automatic] | [start | end]
Caution In general, Cisco discourages adjusting default settings. Only personnel who have received the
necessary training should attempt to adjust values.
The CiscouBR10000series equipment periodically broadcasts upstream channel descriptor interface
line card or (UCD) messages to all CMs. These messages define upstream channel characteristics that
include upstream frequencies, symbol rates and modulation schemes, forward error correction (FEC)
parameters, and other physical layer values.