$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