Network procedures

The Tx-ring is especially important on ATM links (including DSL), in which each PVC has a dedicated driver-level queue (Tx-ring) to ensure adherence to the ATM class-of-service traffic contract of the PVC. Default ATM Tx-rings are typically deep, containing 64 or more particles (typically, each particle is 576 bytes) to ensure that enough particles exist in the buffer to drive the PVC to its full bandwidth utilization. A shallow Tx-ring increases the number of interrupts and the wait states between the driver and the main CPU. Packets are downloaded from the Layer 3 queues to the driver for transmission, which is usually suboptimal at higher speeds.
On the other hand, a deep network procedures can impact voice quality. For example, assume that a 50-packet burst of data passes through the LLQ system and into the FIFO Tx-ring. A voice packet then arrives and is packet number 51 for transmission. The LLQ no longer has the opportunity to prioritize it over the data packets. A shallow Tx-ring pushes packets back into LLQ, where prioritization and packet reordering can be accomplished before the packets are downloaded to the driver. Once in the driver level, the packets have a very short wait time until transmission.