Low Latency 40- and 100-Gbps Ethernet MAC and PHY MegaCore Function User Guide

ID 683628
Date 12/28/2017
Document Table of Contents Bus Quantization Effects With Adapters

The TX custom streaming interface allows a packet to start at any of two or four positions to maximize utilization of the link bandwidth. The TX Avalon-ST interface only allows start of packet (SOP) to be placed at the most significant position. If the SOP were restricted to the most significant position in the client logic data bus in the custom streaming interface, bus bandwidth would be reduced.

Figure 16. Reduced Bandwidth With Left-Aligned SOP Requirement Illustrates the reduction of bandwidth that would be caused by left‑aligning the SOP for the 100GbE IP core.

The example shows a nine-word packet, which is the worst case for bandwidth utilization. Assuming another packet is waiting for transmission, the effective ingress bandwidth is reduced by 25%. Running the MAC portion of the logic slightly faster than is required can mitigate this loss of bandwidth. Additional increases in the MAC frequency can provide further mitigation, although increases in frequency make timing closure more difficult. The wider data bus for the Avalon-ST interface also helps to compensate for the Avalon-ST left‑aligned SOP requirement.