User guide

Canopy System User Guide Understanding Bandwidth Management
pmp-0229 (Mar 2013)
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Low Latency Bit
Bit 3 is set by a device outside the system. In the uplink frame, the SM monitors Bit 3. If this bit is
set, then
• the SM prioritizes this traffic in its high-priority queue according to AP configuration settings
for the high-priority channel.
• the system sends the packet on the high-priority channel and services this channel before any
normal traffic.
802.1P Field
See Priority on VLANs (802.1p) on Page 169.
DSCP Field
Like Bit 3 of the original IPv4 ToS byte, the DSCP field (Bits 0 through 5) in the redefined ToS
byte is set by a device outside the system. A packets 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.
RECOMMENDATION:
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 270. 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.