5.2.4 Store-and-Forward versus Cut-Through

The MAC-PHY can transfer Ethernet frames using two methods. By default, the MAC-PHY operates in a store-and-forward method. In this method the MAC-PHY receives the full Ethernet frame, from either the network or the host controller via SPI, and only thereafter forwards it to the destination, the host controller via SPI or the network. Store-and-forward is the most robust transfer method, however it incurs the most latency, especially with large frames, as the frame must be fully received by the MAC-PHY before it can be forwarded.

The second method of transfer is cut-through mode. In cut-through mode, the MAC-PHY does not need to receive the full packet before forwarding it on. Once a threshold of data has been received by the MAC-PHY, either from the network or the host controller via SPI, the MAC-PHY may begin to immediately forward the packet on to the host controller or the network much earlier than in store-and-forward mode.

Cut-through mode can significantly reduce latency in some situations. For packets received from the network, the latency can almost always be reduced when forwarding them to the host controller over SPI. However, in the transmit direction, packets received by the MAC-PHY from the host controller over SPI may need to wait for other devices to finish transmitting and will often need to wait for the local PLCA transmit opportunity to occur before the MAC-PHY can begin to forward the packet onto the network.