Twinaxial cabling, or "Twinax", is a type of cable similar to coaxial cable, but with two inner conductors instead of one. Due to cost efficiency it is becoming common in modern very-short-range high-speed differential signaling applications. It comes in either an active or passive Twinax (twinaxial) cable assembly and connects directly into an SFP+ housing. An active Twinax cable has active electronic components in the SFP+ housing to improve the signal quality; a passive Twinax cable is mainly just a straight "wire" and contains few components. Generally, Twinax cables shorter than 7 meters are passive and those longer than 7 meters are active, but this may vary from vendor to vendor. SFP+ Direct Attach has a 10 meter distance limitation, thus the target application is interconnection of top-of-rack switches with application servers and storage devices in a rack with low latency and low cost.

Passive and Active SFP+ DAC Cables Comparison

Passive cables are much less expensive but require the host to do the work of driving it properly.
Benefits:
• Lower Costs
• Higher Reliability
• Components (No Active Tx /Rx Components)
• Only Capacitors, Resistors, EEPROM, Cable

Tradeoffs:
• No LOS
• No TX Disable
• No Interrupts
• Limited Management Interface
• Host must drive Cu cable Active cables just like a optical module.

Benefits:
• Enhanced Signal Integrity
• Longer Cable Lengths
• Transmit Pre-emphasis
• Active/Adaptive Receive Equalization
• Tx Disable • Loss of Signal (LOS)
• Interrupts • Management Interface
• Looks like an optical module
• Don’t have to worry about host Tx/Rx for Cu cables

Tradeoffs:
• Higher Cost