User's Manual

Configuring quality of service
PMP 450 Operations Guide
2-4
pmp-0049 (September 2012)
High-priority Bandwidth
To support low-latency traffic such as VoIP (Voice over IP) or video, the system implements a high-
priority channel. This channel does not affect the inherent latencies in the system but allows high-
priority traffic to be immediately served. The high-priority pipe separates low-latency traffic from
traffic that is latency tolerant, such as standard web traffic and file downloads.
The number of channels available to the AP is reduced by the number of SMs configured for the high-
priority channel. With high priority channel enabled on all SMs, the total sector capacity is reduced
by 50%.
A module prioritizes traffic by
reading the Low Latency bit (Bit 3) in the IPv4 Type of Service (ToS) byte in a received packet.
Bit 3 is set by a device outside the system.
reading the 802.1p field of the 802.1Q header in a received packet, where VLAN is enabled on the
module.
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 2-10. 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.