Direct Attach Cable (DAC) assemblies connect host SerDes interfaces directly over passive twinaxial copper conductors, offering ultra-low power consumption, zero latency, and lower procurement costs over short reaches under 2.5 meters. Active Optical Cable (AOC) assemblies integrate electro-optical transceivers permanently onto multimode fiber ends, delivering thin, lightweight, EMI-immune cabling across extended distances up to 100 meters.
High-Speed Cable Media: Direct Attach Copper (DAC) vs. Active Optical Cable (AOC)
Media Mechanics and Electrical Signal Path
Passive Direct Attach Copper (DAC) cables contain no active electronic or optical components in the signal path. The high-speed differential signals from the host SerDes travel directly through precision twinaxial wire pairs, relying entirely on the host ASIC's internal equalization (CTLE, DFE, FFE) to overcome channel insertion loss. Active Optical Cables (AOC) receive electrical signals at the connector shell, process them through internal retimers and VCSEL drivers, convert them to light at 850 nm wavelengths, transmit across OM3/OM4 fiber, and convert back to electrical signals at the remote end.
Physical Handling, Cable Diameter, and Thermal Impact
Power Consumption: DAC consumes essentially 0W at the cable assembly itself (host SerDes power only). AOC requires approximately 1.5W to 5W per end to power the active laser circuitry and optical TIAs.
Cable Mass and Bend Radius: High-speed DAC assemblies (e.g., 100G to 400G) require thick 30 AWG to 26 AWG twinax, making bundles heavy, rigid, and airflow-restrictive in high-density racks. AOC fiber cables are extremely thin (2.0mm to 3.0mm outer diameter), lightweight, and highly flexible.
Electromagnetic Interference (EMI): DAC metal shielding can act as an antenna or ground loop path if compromised. AOC fiber media provides complete galvanic isolation and total immunity to cross-coupling EMI.
Engineering Specification Comparison
Transmission Medium — DAC: Twinaxial copper wire / AOC: Multimode fiber (MMF)
Max Practical Reach — DAC: ~1.5m to 3m (speed dependent) / AOC: Up to 100 meters
Power Consumption — DAC: ~0W per end / AOC: 1.5W to 5+ W per end
Latency Overhead — DAC: ~0 ns added / AOC: ~10 ns to 100 ns (DSP + conversion)
Cable Weight / Flexibility — DAC: Heavy, rigid, large bend radius / AOC: Ultra-light, highly flexible
Cost — DAC: Lowest initial capital expenditure / AOC: Higher component cost
Design Selection Criteria
Deploy Direct Attach Copper (DAC) cables for short, intra-rack connections — such as Top-of-Rack (ToR) switch links to adjacent servers and storage arrays within the same 42U cabinet where distance is short and zero thermal overhead is critical. Deploy Active Optical Cables (AOC) for inter-rack links, row-to-row aggregations, elevated cable tray runs, and high-density liquid-cooled racks where cable routing flexibility, EMI immunity, and reaches over 3 meters are required.