Hard Processor System Technical Reference Manual: Agilex™ 5 SoCs

ID 814346
Date 7/19/2024
Public

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5.1.6.10.4. Credit Based Shaper

IEEE 802.1Qav forwarding and queuing enhancements for time-sensitive streams defines traffic shaping based on priority classes, where credit based shaper (CBS) is the algorithm developed to service the traffic queues using credit, also known as credit based fair queuing.

CBS is designed to reduce traffic buffering in receiving endpoints. While a packet is queued for service, credit is accumulated. Credit is then spent during services. This results in queues with different rate of credits accumulation and release which are then used by the CBS algorithm to shape the transmission according to the bandwidth that has been reserved for the TX queue.

The EMAC driver supports offloading of the CBS qdisc. This feature enables audio/video bridging (AVB) on top of local area networks (LANs). Queue 0 is reserved for legacy traffic and cannot be configured for AVB mode. As the driver has a one-to-one mapping with the traffic class, in effect only TX queue 7 (TC7) is able to support AVB.