All default operations of MAC are done by using PMAC. One more MAC, which is identical to PMAC is used, termed as EMAC, which is used when IEEE 802.3br is configured. IEEE 802.3br Interspersing Express Traffic is one of the TSN standards, which defines a mechanism to allow an express frame to be transmitted with minimum delay at the expense of delaying completion of normal priority frames.
This standard has been implemented by instantiating two separate MAC modules with related DMA, a MAC Merge Sub Layer (MMSL) and an AXI arbiter. One MAC is termed the express or eMAC and the other is a pre-emptable or pMAC. The eMAC is designed to carry time sensitive traffic, which must be delivered within a known time.
Figure 1. IEEE 802.3br Support
