Limitations of intra datacentre communications exploiting optical PCIe interconnections

To support the huge intra-datacentre traffic growth, network architectures based on Peripheral Component Interconnect Express (PCIe) communication protocol have been proposed in alternative to Ethernet ones. To cover the entire datacentre area and to promote the recently released PCIe Gen4, operating at 16 Gb/s, the employment of optical fibre communication is mandatory. We demonstrate, by physical layer measurements, the compliance to the PCIe standard in terms of eye diagram mask and jitter for both Gen3 and Gen4 optical PCIe links, considering up to 10-km standard single mode fibre (SMF) propagation and transmission in the second window of optical communications. On the other hand, we perform an analysis to evidence that the maximum fibre reach is limited by the propagation latency due to the PCIe protocol ACK/NACK operation. For 1-lane PCIe Gen3 link we experimentally verify, through a protocol analysis, the reach limits expected by our evaluation; moreover, for longer distances, we detect retransmissions which reduce the actual transmitted capacity, without error introduction.