$AMD Helios networking explained: Scale-up: inside the rack - 72 MI455X GPUs - 12 Broadcom Tomahawk 6 ASICs - 36 parallel network planes - 3.6 TB/s bidirectional per GPU - 260 TB/s aggregate The network is based on two standards: > UALoE defines how GPUs communicate, including memory reads and writes, transaction ordering, and how traffic is distributed across the 36 network planes > ESUN provides the Ethernet fabric underneath, handling forwarding, flow control, congestion management, link-level retries, and lower-overhead transmission for small messages The path is: MI455X → UALoE → ESUN Ethernet → Tomahawk 6 → MI455X The 31 TB of HBM4 stays physically distributed across 72 GPUs. UALoE lets any GPU read or write any other GPU's memory directly, making the whole rack act as a single computer Scale-out: between racks The racks are connected with Ethernet scale-out through 800G Pensando Vulcano AI NICs into a UEC-ready fabric The path is: Helios rack → Vulcano NIC → UEC Ethernet → another Helios rack