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Networking Fundamentals · Intermediate

Direct Attach Copper (DAC)

A fixed-length twinax cable with transceiver connectors (such as SFP+ or QSFP28) permanently attached on both ends.

Definition

Passive DACs work up to about 5-7 m, while active DACs extend reach somewhat. They are cheaper and use less power than optical transceivers plus fiber. Breakout DACs split one QSFP port into four SFP ports.

Example

A QSFP28-to-4xSFP28 breakout DAC connects a 100G switch port to four 25G servers.

Exam tip

DAC = copper with built-in transceivers, for very short runs.

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