Usage Guidelines
The DOCSIS 1.1
and 2.0 specifications allow a CMTS to specify a pre-equalization coefficient
in the DOCSIS ranging response (RNG-RSP) MAC management messages it forwards to
the cable modems (CM)s. When this is enabled, a CM can engage in transmit-side
equalization (pre-equalization) to mitigate the effects of certain impairments
in the cable plant, such as in-channel tilt, and group delay.
These impairments
are more significant with higher order modulation formats such as 64-QAM or
16-QAM, and near the first octave break-point of the diplex filters (34 MHz to
38 MHz). This break-point exists predominantly as a time-domain group delay
issue, and becomes severe for longer amplifier cascades.
Each amplifier
causes the DOCSIS upstream signal to traverse two diplex filters. All diplex
filter frequency splits (5-42 MHz, 5-65 MHz, or 5-88 MHz) share this
characteristic, which is not visible in a frequency response measurement or
sweep test.
For more
information about the use of pre-equalization, see the DOCSIS 2.0 RFI
specification section 6.2.15, Transmit Pre-Equalizer, and section 8.3.6 Ranging
Response (RNG-RSP) MAC Management Messages.
DOCSIS 1.1
specifies the Type 4 type length value (TLV), and an 8-tap equalizer. DOCSIS
2.0 adds the Type 9 TLV, and increases the equalizer to support 24 taps to
compensate for more complex impairments.
The
cable
upstream
equalization-coefficient command configures the
CMTS to send Type 4 TLV or Type 9 TLVs in the DOCSIS RNG-RSP MAC management
messages to the CMs.
The
no form of
the command disables the CMTS from sending Type 4 TLV or Type 9 TLVs in the
DOCSIS RNG-RSP MAC management messages to the CMs.
After the CMTS
stops sending pre-equalization TLVs to the CM, the CM must retain these values
until the modem’s cable interface resets, or a modification is made to the
upstream channel descriptor (UCD), which would make the stored equalization
coefficients invalid.
You can reset the
cable modem’s interface by:
- Power cycling the cable
modem
- Resetting the cable modem
from the CMTS using the
clear
cable
modem
command
Router# clear cable modem aaaa.bbbb.cccc reset
You can reset the
equalizer coefficient of all CMs on an upstream by:
- Changing the upstream
center frequency. A 16-kHz or 32-kHz change is sufficient and unlikely to take
many CMs offline.
- Briefly changing the
periodic ranging messages in the modulation profile between 16-QAM and
Quadrature Phase Shift Keying (QPSK), or 64-QAM and 16-QAM.
A CMTS operating
in a pure DOCSIS 2.0 upstream mode sends equalizer data supporting 24 taps to
all modems during initial ranging.
The
cable
up
0
docsis-mode
atdma
command configures a DOCSIS 2.0-only upstream mode on the CMTS.
A CMTS
operating in a DOCSIS 1.1 or mixed TDMA-ATDMA upstream mode sends equalizer
data supporting 8 taps to all modems during initial ranging. The CMTS cannot
send data supporting 24 taps until it is notified by the CM, that it is DOCSIS
2.0 compliant in the Registration Request (REG-REQ) message, immediately
preceding the online state.
This can be
configured on the CMTS using the
cable
up
0
docsis-mode
tdma-atdma command.
When a CM
receives a Type 4 TLV in the RNG-RSP MAC management message from the CMTS, it
must combine the previously stored values with the new values from the CMTS.
Under certain HFC plant conditions, a dynamic impairment occurs that require
the CM to adjust its main equalizer tap. The DOCSIS standard specifies a
convolution function that is sometimes unable to adjust for dynamic
impairments. The result, is a CM whose signal-to-noise ratio (SNR) value as
received by the CMTS, is quite low compared to other CMs on the same upstream.
In a mixed
modulation profile, where station maintenance is of a lower order modulation
format than the data grants, it is possible for the modem to enter a steady
state where the SNR estimate is:
- Below the required
estimate for data grants to be processed by the CMTS
- Above the requirement
for station maintenance
This results in
a modem that responds to DOCSIS Layer 2 pings, but not to Layer 3 traffic. The
following commands can be used to check status:
- ping
docsis
{cm-mac
address |
cm
ip
address }—succeeds
- ping
ip {cm
IP
address |
CPE
IP
address }—fails
A cable modem
whose SNR estimate is in this condition may enter the expired(pt) state when
baseline privacy interface plus (BPI+) is enabled. A modulation profile where
station maintenance and data grants are of the same order modulation, prevents
this condition.
The CMTS may be
configured to enable cable upstream pre-equalization on a per upstream port
basis, and may be configured to disable the sending of upstream
equalization-coefficients on a per modem MAC address or Organizational Unique
Identifier (OUI) basis, using the related global exclude commands.
Some DOCSIS 2.0
compliant CMs / embedded media terminal adapters (EMTA)s have been known to
report as DOCSIS 1.1 compliant to the CMTS, unless connected to a DOCSIS 2.0
format upstream. These CMs are treated by the CMTS as DOCSIS 1.1, and only send
8-tap equalizer coefficients. A firmware or SNMP set may be used on these cable
modems to enable the proper reporting of DOCSIS 2.0 compliance under a DOCSIS
1.1-style upstream (for example, 3.2 MHz, 16-QAM, TDMA format).
Some older
DOCSIS cable modem firmware versions will incorrectly process the transition
from 8 to 24 taps when in a mixed TDMA-ATDMA mode, and will experience
degrading or low SNR MER as displayed with
show
cable
modem
phy
command . Upgrade the CM firmware, and temporarily
move these modems to a pure DOCSIS 1.1 or 2.0 channel or disable
pre-equalization for these devices with the exclude command.
