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Functional Descriptions

VMDS-10492 VSC7440 Datasheet Revision 4.2

3.2.7

Shared Queue System and Hierarchical Scheduler

The analyzer provides the destination port set of a frame to the shared queue system. It is the queue

system’s task to control the frame forwarding to all destination ports.

The shared queue system embeds memory that can be shared between all queues and ports. Frames

are mapped to queues through a programmable mapping function allowing ingress ports, egress ports,

and QoS classes to be taken into account. The sharing of resources between queues and ports is

controlled by an extensive set of thresholds.

Each egress port implements default schedulers and shapers as shown in the following illustration. Per

egress port, the scheduler sees the outcome of aggregating the egress queues (one per ingress port per

QoS class) into eight QoS classes.

3.2.7.1

Class-Based Queuing (Default Configuration)

By default, aggregation is done in a Deficit Weighted Round Robin fashion (DWRR) with equal weights to

all ingress ports within a QoS class, so byte-accurate weighted round robin scheduling between ingress

ports is performed for each QoS class.

The following illustration shows the default scheduler-shaper configuration. Hierarchical scheduling-

shaping is also possible.

Figure 3 •

Default Scheduler-Shaper Configuration

Scheduling between QoS classes within the port can use one of the three following methods:

•

Strict Priority scheduling. Frames with the highest QoS class are always transmitted before frames

with lower QoS class.

•

Deficit Weighted Round Robin (DWRR) scheduling. Each QoS class serviced using DWRR sets a

DWRR weight ranging from 0 to 31.

•

Combination of Strict Priority and DWRR scheduling. The default configuration is shown, where QoS

classes 7 and 6 use strict priority while QoS classes 5 through 0 use DWRR. But any split between

strict and DWRR QoS classes can be configured.

3.2.8

Rewriter and Frame Departure

Before transmitting the frame on the egress line, the rewriter can modify selected fields in the frame such

as Ethernet headers and VLAN tags, IPv4/IPv6 fields, PTP fields, and FCS. The rewriter also updates

Port

Shaper

•

iCPU

Input Port N

Input Port 0

Virtual Input Queues

QoS Class 7

•

iCPU

Input Port N

Input Port 0

Virtual Input Queues

QoS Class 0

Shaper

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