What is a DAC (Direct Attach Cable) and when to use it?

DAC (Direct Attach Cable) is a fixed cable assembly with SFP+/QSFP+ modules on both ends, using copper twinax for short-distance connections.

Types of DACs

• Passive DAC: Straight copper, up to 5-7m
• Active DAC: Includes signal conditioning, up to 10-15m
• SFP+ DAC: 10G, single-lane
• QSFP+ DAC: 40G, 4-lane
• QSFP28 DAC: 100G, 4-lane at 25G
• Breakout DAC: QSFP+ to 4x SFP+ for aggregation

Advantages

• Very low cost compared to optical modules ($30-150 typical)
• Low power consumption (passive DACs use no additional power)
• Zero latency (no optical conversion)
• No fiber cleaning required
• Simple installation

Limitations

• Short distance (5m passive, 10-15m active)
• Fixed length — cannot be spliced
• Heavy for very short runs (bulky twinax)
• EMI susceptibility (copper)

Ideal Use Cases

• Top-of-Rack (ToR) to server connections
• Adjacent switch stacking
• Same-rack aggregation
• Storage system connections

DAC vs AOC (Active Optical Cable)

DAC vs Optical Modules + Fiber

• DAC cheaper for <10m
• Optical + fiber cheaper beyond 10-20m
• Optical + fiber more flexible (reroute, splice)
• DAC lower latency

Compatibility

• Vendor coding applies — some switches require branded DACs
• Verify DAC speed matches switch port
• Passive DACs work with more switches than active