Congestion Avoidance - Alcatel-Lucent 7450 Quality Of Service Manual

Congestion Avoidance

Basic Configurations
A basic ATM QoS traffic descriptor profile must conform to the following:
Create an ATM-TD-Profile QoS Policy
Configuring and applying QoS policies and profiles other than the default policy is optional.
To create an ATM QoS traffic descriptor profile, define the following:
Quality of Service Guide
• nrt-VBR unshaped VCs and UBR VCs are scheduled as a common class. Scheduling
among these VCs is done using a WRR scheduler where the weight of each VC is
determined by the configured SIR for nrt-VBR and by the MIR for UBR VCs. The
scheduling is work-conserving, so each VC has access to excess bandwidth in
proportion to its SIR/MIR. Under congestion, the performance of each VC degrades
proportional to the weight of the VC.
• PPD — An ATM cell discarded in the middle of an AAL5 packet makes the entire
packet unusable. The PPD mechanism attempts to minimize the congestion problems
that can occur at the higher layers (TCP) due to ATM cell discards.
With PPD, once a cell is discarded by the ATM policing function, no other cells for
that PDU are accepted, with exception of the "tail cell" which is sent to inform the
far end that the end of a frame has arrived.
PPD is enabled on ATM SAP part of an AAL5 SDU Apipe VLL service and on all
services where the ATM SAP cell stream is re-assembled, i.e., IES, VPRN, VPLS,
Epipe, and Apipe services.
• WRED — Congestion and potential discards are performed on per forwarding class
basis in the SAP queues in the IOM, packets which are re-assembled in the ATM-
capable MDAs take advantage of the application of the WRED congestion avoidance
service queues associated with the ATM SAP.
Depending of the type of XMA or MDA which supports ATM encapsulation, other
XMA or MDA specific packet congestion control mechanisms operating on per SAP
queues in the XMA or MDA are also applied.
• Each policy must have a unique policy ID.
• Default values can be modified but parameters cannot be deleted.
QoS ATM Traffic Descriptor Profiles
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