Optical circuit switch
Also known as OCS, optical switching, MEMS optical switch
In plain English
An optical circuit switch uses tiny mirrors to redirect light between fibers, letting Google rewire a TPU pod without touching a cable.
Technical definition
An optical circuit switch is a device that steers optical signals between fiber ports with movable mirrors, creating reconfigurable point-to-point links without converting the signal to electrical form or inspecting packets.
Engineering details
Google uses OCS to stitch 4x4x4 TPU cubes into larger pods while preserving the torus wraparound links across the whole topology. Because the switch redirects light rather than routing packets, it adds negligible latency and no per-packet processing, and it can be reconfigured in software. Ironwood pods scale this way to a 9,216-chip superpod with 42.5 FP8 exaflops of aggregate compute. The same mechanism lets an operator carve a pod into smaller slices of arbitrary shape for different jobs.
Why it matters
The operational payoff is fault tolerance. A dead chip or failed link is bypassed in seconds by moving mirrors instead of sending a technician to re-splice copper in a live datacenter. Reconfigurability also means the physical pod does not dictate the logical topology, so a job can request the torus dimensions it wants.
How to read it in InferenceX
OCS is why TPU results in InferenceX are reported per chip within a pod rather than per fixed rack unit. The Official Preview compares aggregated TPUv7 serving against B200 and B300 nodes, with disaggregated pod-scale comparisons against NVL72 reserved for a follow-up.