User guide

Understanding Bandwidth Management Link Characteristics
86
pmp-0229 (Mar 2013)
Example download times for various arbitrary tiers of service are shown in Table 64 on Page 441
and Table 65 on Page 442.
Interaction of Burst Allocation and Sustained Data Rate
Settings
If the Burst Allocation is set to 1200 kb and the Sustained Data Rate is set to 128 kbps, a data
burst of 1000 kb is transmitted at full speed because the Burst Allocation is set high enough. After
the burst, the bucket experiences a significant refill at the Sustained Data Rate. This configuration
uses the advantage of the settable Burst Allocation.
If both the Burst Allocation and the Sustained Data Rate are set to 128 kb, a burst is limited to the
Burst Allocation value. This configuration does not take advantage of the settable Burst
Allocation.
If the Burst Allocation is set to 128 kb and the Sustained Data Rate is set to 256 kbps, the actual
rate will be the burst allocation (but in kbps). As above, this configuration does not take advantage
of the settable Burst Allocation.
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.
IMPORTANT!
The number of channels available to the AP is reduced by the number of SMs
configured for the high-priority channel. With this feature enabled on all SMs, an
AP can support 100 SMs (instead of 200).
A module prioritizes traffic by
• reading the Low Latency bit (Bit 3) in the IPv4 Type of Service (ToS) byte in a received
packet.
• 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.