Overview
692 Quality of Service Guide
QoS Model for ATM-Based Services
Figure 40: Hierarchical Scheduling for ATM-Based Services
This section provides a description of the QoS model used for ATM-based services on the
router ATM MDA. Although slight variations of this model are applied on other ATM
capable MDAs, the principles remain the same. An example of a VPLS Service with ATM
SAP is shown in Figure 40.
When Ethernet frames are sent over an ATM VC, the scheduling of data becomes hierarchical
with two main levels: packet level scheduling and per-VC cell level scheduling.
At the first level, frames are queued on a per-CoS (or forwarding class), per-VC basis in order
to achieve the proper class of service differentiation for the frames in the same VC. Each
Ethernet frame is queued based on the VC dedicated forwarding class queue, as configured
in the service egress QoS policy. The packet level scheduling can make use of HQoS
scheduler policy in order to enforce aggregate bandwidth among a group of queues feeding
an ATM VC or to enforce aggregation of bandwidth across all queues of all VCs at a given
customer site.
The frame level scheduling is the same for other types of SAP (Ethernet, FR, and PPP) and
all the features available on the service ingress and egress QoS policies can be applied.
At the second level, the segmented cells are queued in per-VC queues according to the service
category of the ATM traffic descriptor profile applied to the ATM SAP. Scheduling at the
ATM level enforces the priority and bandwidth sharing desired at the cell level.
VLL/VPLS/
IES/VPRN
Strict Priority
Shaping
CBR High
CBR Low
Cell Level Per-VC Queues
(ATM SAR)
Shaped VC 1
VC 1 Packet
Queues
Non-Shaped VC 2
Packet Level Forwarding
Class Queues
Switch
Fabric
7750SR_001
Strict Priority
Basic/HQoS
Scheduler
Basic/HQoS
Scheduler
EF
AF1
AF2
BE
WRR
Scheduler
nrt-VBR
VC 2 Packet
Queues
EF
AF1
AF2
BE
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