Usage Guidelines
To display ERBA
settings as applied to DOCSIS 1.0 cable modems and QoS profiles on the Cisco
CMTS, use the
show
cable
qos
profile command in Privileged EXEC mode.
The following
example of the
cable
qos
profile command in global configuration mode
illustrates changes to the
cable
qos
profile command. Fields relating to the ERBA
feature are shown in bold for illustration:
Router(config)# cable qos pro 10 ?
grant-interval Grant interval
grant-size Grant size
guaranteed-upstream Guaranteed Upstream
max-burst Max Upstream Tx Burst
max-ds-burst Max Downstream Tx burst (cisco specific)
max-downstream Max Downstream
max-upstream Max Upstream
name QoS Profile name string (cisco specific)
priority Priority
privacy Cable Baseline Privacy Enable
tos-overwrite Overwrite TOS byte by setting mask bits to value
The following
example of the
show
cable
qos
profile command illustrates that the maximum
downstream burst has been defined, and is a management-created QoS profile:
Router# show cable qos pro
ID Prio Max Guarantee Max
Max TOS TOS Create B IP prec.
upstream upstream downstream
tx mask value by priv rate
bandwidth bandwidth bandwidth
burst enab enab
1 0 0 0 0
0 0xFF 0x0 cmts(r) no no
2 0 64000 0 1000000
0 0xFF 0x0 cmts(r) no no
3 7 31200 31200 0
0 0xFF 0x0 cmts yes no
4 7 87200 87200 0
0 0xFF 0x0 cmts yes no
6 1 90000 0 90000
1522
0xFF 0x0 mgmt
yes no
10 1 90000 0 90000
1522
0x1 0xA0 mgmt
no no
50 0 0 0 96000
0 0xFF 0x0 mgmt no no
51 0 0 0 97000
0 0xFF 0x0 mgmt no no
The following
example illustrates the maximum downstream burst size in sample QoS profile 10
with the
show
cable
qos
prof
verbose command in privileged EXEC mode:
Router# show cable qos pro 10 ver
Profile Index 10
Name
Upstream Traffic Priority 1
Upstream Maximum Rate (bps) 90000
Upstream Guaranteed Rate (bps) 0
Unsolicited Grant Size (bytes) 0
Unsolicited Grant Interval (usecs) 0
Upstream Maximum Transmit Burst (bytes) 1522
Downstreamam Maximum Transmit Burst (bytes) 100000
IP Type of Service Overwrite Mask 0x1
IP Type of Service Overwrite Value 0xA0
Downstream Maximum Rate (bps) 90000
Created By mgmt
Baseline Privacy Enabled no
If a cable
modem registers with a QoS profile that matches one of the existing QoS
profiles on the Cisco CMTS, then the maximum downstream burst size, as defined
for that profile, is used instead of the default DOCSIS QoS profile of 1522.
For example, a
DOCSIS 1.0 configuration that matches QoS profile 10 in the previous examples
would be as follows:
03 (Net Access Control) = 1
04 (Class of Service Encodings Block)
S01 (Class ID) = 1
S02 (Maximum DS rate) = 90000
S03 (Maximum US rate) = 90000
S06 (US burst) = 1522
S04 (US Channel Priority) = 1
S07 (Privacy Enable) = 0
The maximum
downstream burst size (as well as the ToS overwrite values) are not explicitly
defined in the QoS configuration file because they are not defined in DOCSIS.
However, because all other parameters are a perfect match to profile 10 in this
example, then any cable modem that registers with these QoS parameters has a
maximum downstream burst of 100000 bytes applied to it.
For further
illustration, consider a scenario in which packets are set in lengths of 1000
bytes at 100 packets per second (pps). Therefore, the total rate is a
multiplied total of 1000, 100, and 8, or 800kbps.
To change these
settings, two or more traffic profiles are defined, with differing downstream
QoS settings as desired. The table below provides two examples of such QoS
profiles for illustration:
Table 1 Sample QoS
Profiles with Differing ERBA (Maximum Downstream) Settings
QoS
Profile Setting
|
QoS
Profile 101
|
QoS
Profile 102
|
Maximum
Downstream Transmit Burst (bytes)
|
max-burst 4000
|
max-burst 4000
|
Maximum
Downstream Burst (bps)
|
max-ds-burst 20000
|
max-ds-burst 5000
|
Maximum
Downstream Bandwidth
|
max-downstream 100
|
max-downstream 100
|
In this
scenario, both QoS profiles are identical except for the max-ds-burst size,
which is set to 5000 in QoS profile 101 and 5000 in QoS profile 102.
Optimal Settings for
DOCSIS 1.0 Downstream Powerburst
DOCSIS allows
the setting different token bucket parameters for each service flow, including
the token bucket burst size. When burst sizes are closer to 0, QoS is enforced
in a stricter manner, allowing a more predictable sharing of network resources,
and as a result easier network planning.
When burst
sizes are larger, individual flows can transmit information faster (lower
latency), although the latency variance can be larger as well.
For individual
flows, a larger burst size is likely to be better. As long as the system is not
congested, a large burst size reduces the chances of two flows transmitting at
the same time, because each burst is likely to take less time to transmit.
However, as channel bandwidth consumption increases, it is probably that large
burst traffic would exceed the thresholds of buffer depths, and latency is
longer than with well shaped traffic.
For additional
information about the
cable
qos
profile command and configuring QoS profiles, see
the
DOCSIS 1.1 for
the Cisco CMTS document on Cisco.com.