1.13 s13: Increased packet latency and coordinator anomalies in large PLCA segments

Description

The PLCA sublayer includes a FIFO which acts as an elastic buffer to ease the alignment of transmit packets coming from the MAC with the device’s transmit opportunity. If a MAC begins data transmission outside of a valid transmit opportunity, the elastic buffer will store MAC transmit data until it is full. If the transmit opportunity starts before this buffer is full, data will be transmitted onto the mixing segment delayed by the FIFO. If the elastic buffer fills before the transmit opportunity starts, a logical collision is signaled to the MAC by asserting carrier sense. The MAC will stop transmitting the packet, back off, and retransmit the packet at the device’s next appropriate transmit opportunity. The device indicates this transmit opportunity by de-asserting carrier sense.

In the event that the PLCA elastic buffer becomes full at the same time as the assigned transmit opportunity begins, the device will exhibit the following deviations from expected behavior:

  1. The device will assert carrier sense for only a short time.
  2. The device will transmit a single ‘J’ symbol on TRXN/P during the node’s transmit opportunity, instead of a valid Ethernet packet.
Additionally, in a coordinator, when the PLCA elastic buffer becomes full at the same time as a beacon is being transmitted, the beacons is properly transmitted. However, the device will exhibit the following deviations from expected behavior:
  1. In RMII mode, the coordinator will ignore received packets after the transmitted beacon until the device's next valid transmit opportunity.
  2. In MII mode, the COL pin of the coordinator may exhibit a short deassertion pulse during a logical collision coincident with the transmission of a Beacon.
Due to the length of the internal elastic buffer, these deviations can only happen on mixing segments with more than 11 idle transmit opportunities in a row.

End User Implications

The 4 behaviors above have different effects in end user systems, as described below:
  1. The MAC will react to the deassertion of carrier sense and attempt to retransmit the packet earlier than it should following a logical collision. If the short assertion of carrier sense occurs, the end application will see increased packet transmit latency because the MAC will retry transmission. The exact behavior will be dependent on the MAC. Some MACs may retry multiple times with increasing back-off delays. In extreme circumstances the packet may be dropped by the MAC due to excessive retries.
  2. It is theoretically possible that other devices on the mixing segment may detect a short carrier sense due to the transmitted ‘J’ symbol which could cause one packet to be delayed in transmission. No case of this behavior has yet been reported.
  3. An RMII coordinator may miss receiving packets.
  4. The brief deassertion of the COL pin has not been shown to have any negative effects on MACs.

Work Around

Configure the PLCA Node Count to 11 or less.