The TX buffers become packets that are sent through the TX packet buffer. The MAC transmitter uses the descriptors, which provide the necessary information about and a pointer to the TX buffers.
The maximum frame size is 1522 bytes by default and 10,240 bytes (with jumbo frame support) bytes and the minimum frame size is 64 bytes.
In the transmit direction, the DMA (or FIFO interface) continues to provide TX packet data up to a limit of 2048 packets. The interface monitors the TX buffer full condition to avoid overwrites. The maximum useful size of the TX packet buffer is 32 KB.
The DMA uses the packet buffers to hold packets for both transmit and receive paths. While the packet is in the buffer, the controller decides what to do with it. If it is corrupt or does not pass the filters, it is discarded, and left unused in the buffer. This has several performance advantages:
- Retry collided TX frames: the MAC transmitter can re-read the packet directly from the buffer, which saves system interconnect bandwidth (or FIFO interface activity)
- Process the transmit IP/TCP/UDP checksum generation offload
- Generate the checksum before determining the fate of the frame