Five types of cable serve AI cluster connections. The choice is determined primarily by distance and secondarily by whether you need field-replaceability of the optical components.

Passive DAC -- Direct Attach Copper

A passive copper cable with OSFP or QSFP-DD connectors at both ends. No electronics inside the connectors. The electrical signal runs directly through copper wire from port to port.

Maximum distance: 7m for 400G, shorter for 800G. Power consumption: below 0.15W (essentially parasitic). Cost: the cheapest cable option by a wide margin. Latency: lowest of all options -- no optical conversion, no active components.

Use for: server-to-ToR switch connections (1-3m typical in a standard rack). This is the standard cable type for all eight DGX NIC to leaf switch connections within the same rack in a BasePOD.

Failure mode: internal copper strand break, usually from excessive bending. When a passive DAC fails, the switch port goes Err-Disabled. The NIC, however, continues reporting the port as Active -- it cannot detect the failure at the electrical level. This two-views-disagree failure mode is exactly what Lab 0 simulates. The diagnostic chain is: NCCL timeout -> ibstat shows rail down -> show interface counters on switch shows Err-Disabled -> ethtool -S on the DGX NIC shows the NIC thinks the link is up -> suspect cable.

Active Electrical Cable / AEC

A copper cable with signal conditioning electronics in the connector heads. The electronics amplify and reshape the signal, allowing slightly longer copper runs than passive DAC at high speeds.

Maximum distance: 7m at 800G. Power consumption: 1-2W per cable. Cost: higher than passive DAC, lower than optical. Use for: intra-rack 800G connections where passive DAC cannot maintain signal integrity (at 800G, passive DAC is typically limited to 3-5m).

Active Optical Cable / AOC

Multi-mode fiber with fixed active optical transceivers built into the connector heads. The transceiver is not pluggable -- it is permanently attached to the cable.

Maximum distance: 100m. Power consumption: 1-3W per cable. Cost: lower than pluggable SR4 for distances under 100m. Weight and bend radius: significantly lighter and more flexible than copper DAC at longer lengths.

Tradeoff: when the transceiver fails, the entire cable must be replaced. In a large cluster with thousands of AOC runs, this is manageable -- a field technician swaps the full assembly. In installations where the fiber run is difficult to access (through conduit, under raised floor), the inability to replace just the transceiver is a maintenance consideration.

Use for: leaf-to-spine connections in BasePOD (typically 10-30m). Also common for ToR-to-spine runs in end-of-row designs.

Pluggable SR4 / SR8

A pluggable OSFP or QSFP-DD transceiver module plus a separate OM4 MMF cable assembly with MPO connectors.

Maximum distance: 100m (SR4 and SR8 both at OM4). The transceiver and cable are independent components. Cost: higher than AOC, but transceiver and cable are replaced separately.

The key advantage: when a transceiver fails in a high-density deployment, you replace the 4kg transceiver, not the cable run through the raised floor. The fiber plant -- which may represent years of structured cabling investment -- stays in place.

Use for: any connection where the cable run has a lifetime significantly longer than the transceiver refresh cycle, or where transceivers are replaced frequently due to operational environment.

Pluggable DR4 / DR8 / FR4

Single-mode fiber transceivers for longer distances. Same pluggable model as SR4/SR8 -- separate transceiver and cable -- but using SMF instead of MMF.

DR4 (Data center Reach): 500m, MPO-12 connector. Cross-row and cross-hall. FR4 (Far Reach): 2km, LC duplex. Cross-building. LR4 (Long Reach): 10km, LC duplex. Campus-scale.

The selection decision tree

Same rack, <=3m:          Passive DAC
Same rack, 3-7m (800G):  AEC
Same row, 7-30m:         AOC or pluggable SR4/SR8
Cross-row, 30-100m:      Pluggable SR4/SR8 (OM4 MMF)
Cross-row, 100-500m:     Pluggable DR4/DR8 (SMF)
Cross-building, <=2km:    Pluggable FR4 (SMF)
Cross-building, <=10km:   Pluggable LR4 (SMF)
Between DCs:             ZR / DWDM coherent (not AI cluster fabric)