Optical intra-data center connectivity

AI has transformed intra-data center networks, the backend links that connect XPUs (GPUs, TPUs and others) and distribute workloads. Training and inferencing require large processing clusters to work together within tight power and space constraints, transferring terabits per second per port, with nanosecond-to-microsecond latencies. These XPU clusters grow along three axes:

• Scale-up connects XPUs within a rack or pod.
• Scale-out connects top-of-rack switches across racks and rows, into a distributed compute fabric.
• Scale-across extends this fabric between geographically dispersed locations, into unified AI computing systems.

Optical Intra-Data Center connectivity solutions deliver very-high bandwidth over a wide range of distances with reduced power consumption and minimal latency.

Today, any data center connection beyond a few meters uses optical links. As AI fabrics scale, every AI-data center connection will potentially be implemented using optical technologies.

Intra-data center optical architectures range from pluggable optical modules to integrated, non-pluggable design.

• Fully retimed optics (FRO) are mature pluggable modules with transmit and receive retiming and processing. They deliver robust performance, easy deployment and broad support.
• Linear receive optics (LRO), or half-retimed optics (HRO), are pluggable modules that limit retiming to the transmit direction. This reduces power and latency versus FRO.
• Linear pluggable optics (LPO) remove all signal processing from the pluggable module and rely on the host ASIC to perform signal correction. They further reduce power and latency but require compatible switches.
• Near-packaged optics (NPO) place the optical interfaces inside the switch, close to but separate from the switch ASIC.
• Co-packaged optics (CPO) integrate the optical interfaces within the switch-ASIC package. 

NPO and CPO typically require close co-design between optics and switch or ASIC vendors, particularly for CPO, and often use pluggable external light sources (ELS) to improve thermal management, reliability and serviceability.