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PacketStar
®
PSAX 2300 Multiservice Media Gateway User Guide
, Issue 1
Release 9.0.0
Chapter 3 System Features
PSAX System Software Features
ties. One queue set addresses avoidance of cell loss, which is normally a con-
cern for data traffic, while the other set manages cell transfer delay, which is
critical to voice and some video traffic.
Within each set of queues, The AQueMan algorithm assigns internal priori-
ties even more specialized than the ATM Forum class definitions. Generally,
the lower the assigned priority number, the greater the access to bandwidth
and the less likelihood of loss. For additional information on ATM QoS, see
“ATM Service Categories in the PSAX System” on page C-35.
The AQueMan algorithm classifies traffic based on service-level priorities and
limits congestion by addressing three dimensions of traffic management:
•
Cell loss versus cell delay for cell discard
As Table 3-3 indicates, there are VBR traffic types (for example, network
management data traffic) that are, in fact, higher in priority than some
CBR traffic (for example, off-peak cellular voice calls). The AQueMan algo-
rithm accounts for the service-level priorities of the traffic when determin-
ing which cells to discard during traffic congestion. Thus, CBR does not
necessarily imply a higher priority.
•
Weighted priorities using queue depth ratios
To alleviate congestion in the network caused by lower-priority VBR traffic,
the AQueMan algorithm provides a weighted priority mechanism. This
mechanism allows lower-priority VBR data to be sent ahead of higher-pri-
ority VBR data in cases where there are too many cells in lower-priority
VBR buffers and relatively few cells in higher-priority VBR buffers. The
execution of this algorithm is based on the priority levels the user selects.
•
Cell aging
This capability prevents the lowest-priority data (for example, IP data)
from being buffered in the Multiservice Media Gateway systems indefi-
nitely. The AQueMan algorithm keeps track of how long each cell stays in
the buffer. The lower the priority of the traffic, the longer its cell-aging
time; that is, UBR traffic has a longer cell-aging period than VBR-RT traffic.
This capability allows the Multiservice Media Gateway systems to periodi-
cally send low-priority cells through the network. Doing so prevents
retransmission of IP data traffic while increasing the time-out window for
the TCP/IP sessions. The cell-aging mechanism allows for orderly deconges-
tion of the network without resorting to traffic rerouting and other compli-
cated protocols and procedures.
Table 3-3. CBR and VBR Service-Level Priorities
Priority
CBR
VBR
High
911 voice call
Network management data
Low
Off-peak cellular voice
IP data
