4.8.1 PLCA Burst Mode
Some applications, such as sensors or audio, may require the transmission of frequent small frames with a limited latency. As PLCA specifies only one transmit opportunity for each node in each bus cycle, these applications may experience significant latency when they are connected onto a multidrop mixing segment with applications that transmit large packets. For example, an audio application may require the transmission of eight stereo 16-bit audio samples as 64 byte packets every 167 μs with minimal latency. When another node on the segment transmits a 1500 byte packet it will occupy the channel for 1.2 ms. The audio application will therefore buffer seven audio packets during the time that the channel is occupied. With standard PLCA, the audio application will only be able to transmit one of its audio packets during the next PLCA bus cycle. The result is that each successive audio packet the audio application needs to transmit is delayed with increasing latency.
One solution to this problem is to allow specific nodes to transmit more than one packet during its transmit opportunity. This ability to transmit a burst of multiple packets allows the audio application in the above example to empty its buffers and transmit all audio packets that it has queued, preventing the latency of the audio packets to grow beyond a tolerable limit.
The ability to transmit packets in a burst is configurable individually for each node on the segment. The Maximum Burst Count (MAXBC) field in the PLCA Burst Mode (PLCA_BURST) register configures the maximum number of additional packets allowed to transmit in each of the node’s transmit opportunities. This is in addition to the initial packet that may be transmitted by the node in its transmit opportunity. Additionally, the Burst Timer (BTMR) field configures the amount of time the node may transmit (COMMIT) to maintain a hold on its current transmit opportunity after transmitting a packet to allow the MAC to transmit an additional packet. Once this timer expires, the node will then yield the transmit opportunity to the next node.