SNLS614A September 2018 – December 2018 DP83869HM
When Full-Duplex operation is selected during priority resolution, the Auto-Negotiation also determines the Flow Control capabilities of the two link partners. Flow control was originally introduced to force a busy station’s Link Partner to stop transmitting data in Full-Duplex operation. Unlike Half-Duplex mode of operation where a link partner could be forced to back off by simply generating collisions, the Full-Duplex operation needed a mechanism to slow down transmission from a link partner in the event that the receiving station’s buffers are becoming full. A new MAC control layer was added to handle the generation and reception of Pause Frames. Each MAC Controller has to advertise whether it is capable of processing Pause Frames. In addition, the MAC Controller advertises if Pause frames can be handled in both directions, that is, receive and transmit. If the MAC Controller only generates Pause frames but does not respond to Pause frames generated by a link partner, it is called Asymmetrical Pause. The advertisement of Pause and Asymmetrical Pause capabilities is enabled by writing 1 to bits 10 and 11 of ANAR (register address 0x0004). The link partner’s Pause capabilities is stored in ANLPAR (register address 0x0005) bits 10 and 11. The MAC Controller has to read from ANLPAR to determine which Pause mode to operate. The PHY layer is not involved in Pause resolution other than simply advertising and reporting of Pause capabilities.