Installation Guide

Task 11: Configuring quality of service PMP 450i and PTP 450i Configuration and User
Guide
• comparing the 6-bit Differentiated Services Code Point (DSCP) field in the ToS byte of a
received packet to a corresponding value in the Diffserv tab of the Configuration page
of the module. A packet contains no flag that indicates whether the encoding is for the
Low Latency bit or the DSCP field. For this reason, you must ensure that all elements in
your trusted domain, including routers and endpoints, set and read the ToS byte with
the same scheme.
Modules monitor ToS bytes with DSCP fields, but with the following differences:
• The 6-bit length of the field allows it to specify one of 64 service
differentiations.
• These correlate to 64 individual (CodePoint) parameters in the Diffserv tab
of the Configuration page.
• Per RFC 2474, 3 of these 64 are preset and cannot be changed. (See
http://www.faqs.org/rfcs/rfc1902.html
.)
• For any or all of the remaining 61 CodePoint parameters, you can specify a
value of
o 0 through 3 for low-priority handling.
o 4 through 7 for high-priority handling.
Ensure that your Differentiated Services domain boundary nodes mark any
entering packet, as needed, so that it specifies the appropriate Code Point for that
traffic and domain. This prevents theft of service level.
An example of the Diffserv tab in the Configuration page and parameter
descriptions are provided under DiffServ tab of the AP on Page 163. This tab and its
rules are identical from module type to module type. However, any of the 61
configurable Code Points can be set to a different value from module to module,
thus defining unique per-hop behavior for some traffic.
This tab in the AP sets the priorities for the various packets in the downstream (sent
from the public network). This tab in the SM sets the priorities for the various
packets in the upstream (sent to the public network).
Typically, some SMs attach to older devices that use the ToS byte as originally
formatted, and others to newer devices that use the DSCP field. The
default values
in the Diffserv tab allow your modules to prioritize traffic from the older devices
roughly the same as they traditionally have. However, these default values may
result in more high-priority traffic as DSCP fields from the newer devices are read
and handled. So, after making changes in the Diffserv tab, carefully monitor the
high-priority channel for high packet rates
• in SMs that you have identified as those to initially set and watch.
• across your network when you have broadly implemented Code Point values, such as
via SNMP.
pmp-0957 (April 2015) 157