Enhanced Transmission Selection
The PFC solves the challenges of marking and controlling which traffic can be stopped with a pause frame to avoid dropped frames, but on the same unified link, which has a certain bandwidth, different types of traffic are still flowing, and there is the need for a mechanism to manage that bandwidth. IEEE 802.1Qaz Enhanced Transmission Selection (ETS) is a specification of a network scheduling algorithm that allows different bandwidth to be allocated based on the different traffic classes. The standard allocates bandwidth for the traffic classes, but if the bandwidth is not utilized for a specific class, the algorithm allows the unused bandwidth to be given to another traffic class that needs more for a certain period of time. This borrowing of unused pipe capacity provides the flexibility to support traffic that is bursty by nature. At the same time, the minimum bandwidth for each traffic class is guaranteed. This means that if a traffic class is loaning some of its bandwidth to another traffic class and suddenly needs it back, that bandwidth will be immediately returned to cover its needs.
The ETS defines two types of classes: the bandwidth allocation classes and the strict-priority classes. This allows for time-sensitive or low-latency traffic to be assigned strict priorities and not to be affected by the bandwidth-sharing mechanism.
In summary, the IEEE 802.1Qaz ETS, shown in Figure 14-4, allows for the following:
Figure 14-4 IEEE 802.1Qaz Enhanced Transmission Selection
- Bandwidth allocation and sharing: Enables intelligent sharing of bandwidth between traffic classes. Each traffic class is guaranteed a minimum bandwidth, but if that bandwidth is not used, it is shared among the other traffic classes until it is needed again.
- Traffic class management: Uses strict-priority traffic classes, which guarantees compliance with special requirements, and solves the challenge of bursty traffic.