4.8.2 Multiple PLCA Transmit Opportunities

The PLCA burst mode allows a node to transmit multiple packets within its single transmit opportunity in each PLCA bus cycle. While transmitting a burst of packets in a single transmit opportunity bounds the maximum latency to the period of a PLCA bus cycle, the latency may be further reduced by allowing a node multiple transmit opportunities in each PLCA bus cycle. This allows the same node to transmit multiple frames evenly spread in time throughout the PLCA bus cycle. This is accomplished by assigning multiple Local IDs to the node thereby allowing it multiple transmit opportunities. When the transmit opportunity counter matches any one of the multiple Local IDs assigned to the node, the node may then transmit a packet or yield the transmit opportunity.

The LAN8670/1/2 supports the assignment of up to eight additional transmit opportunities per PLCA bus cycle. The additional transmit opportunity Local IDs are configured in the ID1-ID8 fields of the PLCA Multiple ID 0-3 (MULTID0-MULTID3) registers.

The Clause 4 compliant MAC requires an inter-packet gap (IPG) of at least 9.6 μs (96 bits) following the transmission of one packet before it will transmit another packet. Should consecutive transmit opportunities be assigned to the same node, transmit opportunities following a packet transmission will not be used until after the inter-packet gap has expired. For best performance when assigning multiple transmit opportunities to the same node it is therefore recommended that they should be interleaved with transmit opportunities assigned to other nodes.