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Direct Attach Copper (DAC) Cables

Enterprise Resource Center • Networking

Direct Attach Copper (DAC) Cables Explained

Understand Direct Attach Copper cables for high-speed Ethernet, including passive and active DACs, Twinax construction, SFP+, SFP28, QSFP+ and QSFP28 interfaces, breakout cables, distance limitations and hardware compatibility.

Direct Attach Copper Twinax 10GbE 25GbE 40GbE 100GbE
Tech Supply Direct Enterprise Networking Knowledge Base
High-Speed Copper Networking

What Is a DAC Cable?

A Direct Attach Copper cable, commonly called a DAC cable, is a fixed-length high-speed copper cable assembly with network interfaces permanently attached to both ends. DAC cables are commonly used to connect servers, switches, storage systems and other network equipment over short distances.

Unlike a traditional structured copper Ethernet connection, a DAC does not terminate in an RJ-45 connector. Instead, the cable plugs directly into compatible high-speed network ports such as SFP+, SFP28, QSFP+ or QSFP28.

DAC assemblies are especially common inside data-center racks and between adjacent racks where their short reach, integrated design and high-speed capability can make them a practical alternative to separate optical transceivers and fiber patch cables.

DAC Quick Reference

Direct Attach Copper at a Glance

Transmission Medium Twinaxial copper conductors.
Typical Application Short server-to-switch and switch-to-switch connections.
Common Interfaces SFP+, SFP28, QSFP+ and QSFP28.
Common Speeds 10GbE, 25GbE, 40GbE and 100GbE.
Integrated Cable Assembly

How Does a DAC Cable Work?

A DAC cable carries high-speed electrical signals directly between compatible network ports over shielded copper conductors. The network interfaces at each end are permanently integrated into the cable assembly rather than being removable transceivers connected to a separate patch cable.

This integrated design reduces the number of separate components required for a short high-speed link. Instead of installing one transceiver in each device and then connecting those transceivers with fiber, a DAC provides the cable and both interface ends as one assembly.

Switch / NIC Port → DAC Interface → Twinax Copper → DAC Interface → Switch / NIC Port
Copper Construction

What Is a Twinax Cable?

DAC assemblies commonly use Twinax or twinaxial copper construction. Twinax uses paired conductors designed for high-speed differential signaling and is well suited to short high-bandwidth interconnects.

Because the term Twinax describes the cable construction while DAC describes the complete direct-attach assembly, the terms are often used together. You may therefore see products described as Twinax DAC cables, Direct Attach Copper cables or simply DACs.

DAC Cable Types

Passive DAC vs. Active DAC

Direct Attach Copper assemblies can be passive or active. The correct choice depends on link length, signaling requirements and the capabilities of the connected hardware.

Short-Reach Copper

Passive DAC

A passive DAC does not contain active signal-conditioning electronics in the cable assembly. It relies on the connected network equipment to drive and receive the signal across the copper link.

  • Designed for very short links
  • Simple integrated copper assembly
  • Low cable power requirements
  • Common inside racks and between nearby equipment
  • Available for multiple Ethernet speeds
Extended Copper Reach

Active DAC

An active DAC incorporates signal-conditioning electronics into the cable assembly. These electronics help maintain signal integrity over distances that may be difficult for an equivalent passive copper assembly.

  • Contains active electronics
  • Requires some electrical power
  • Can support longer copper runs than comparable passive DACs
  • Compatibility must be verified carefully
  • Availability varies by interface and platform
Interface Families

Common DAC Cable Form Factors

10 Gigabit Ethernet

SFP+ DAC

SFP+ Direct Attach Copper is commonly used for short 10GbE connections between switches, servers and storage systems.

25 Gigabit Ethernet

SFP28 DAC

SFP28 Direct Attach Copper provides short-reach 25GbE connectivity for compatible switches, servers and network adapters.

40 Gigabit Ethernet

QSFP+ DAC

QSFP+ DAC assemblies provide short 40GbE connections and can also be available in breakout configurations for compatible hardware.

100 Gigabit Ethernet

QSFP28 DAC

QSFP28 Direct Attach Copper is used for short 100GbE links and supported 100GbE-to-4x25GbE breakout configurations.

DAC Ethernet Speeds

Common DAC Interfaces and Network Speeds

Interface Common Ethernet Speed Common Application Breakout Potential
SFP+ 10GbE Server / switch connectivity Breakout endpoint
SFP28 25GbE Server / switch connectivity Breakout endpoint
QSFP+ 40GbE Switch uplinks / aggregation 4x10GbE on supported hardware
QSFP28 100GbE High-speed aggregation / core 4x25GbE on supported hardware
Breakout Connectivity

What Is a Breakout DAC Cable?

A breakout DAC divides the lanes of a multi-lane QSFP-family interface into multiple lower-speed connections. This allows one high-density switch port to connect directly to several individual server, storage or switch interfaces when the hardware supports breakout operation.

40GbE Breakout
1 × QSFP+
↓
4 × SFP+
40GbE → 4 × 10GbE
100GbE Breakout
1 × QSFP28
↓
4 × SFP28
100GbE → 4 × 25GbE

Breakout capability is not automatic. The switch port, firmware or operating system, cable assembly and connected endpoints must support the intended breakout mode.

Choosing a Connection

DAC vs. AOC vs. Optical Transceivers

DAC, active optical cables and modular optical transceivers can all provide high-speed connectivity, but they are designed for different cabling distances and infrastructure requirements.

Copper

DAC

  • Twinax copper
  • Fixed-length assembly
  • Designed for short links
  • Integrated interface ends
  • No separate fiber required
Integrated Optical

AOC

  • Optical fiber
  • Fixed-length assembly
  • Active electronics
  • Thinner and lighter than many copper assemblies
  • Useful when DAC reach is insufficient
Modular Optical

Optical Transceivers

  • Separate transceiver modules
  • Separate fiber patch cables
  • Multiple reach options
  • Supports structured fiber infrastructure
  • Useful for longer links
Copper Does Not Mean RJ-45

DAC Cable vs. RJ-45 Ethernet Cable

Although both technologies use copper conductors, a DAC cable is fundamentally different from a conventional twisted-pair Ethernet patch cable.

DAC Cable

  • Integrated pluggable network interfaces
  • Commonly uses Twinax copper
  • Designed primarily for short high-speed links
  • Connects directly to SFP/QSFP-family ports
  • Fixed cable length

RJ-45 Ethernet Cable

  • Uses modular RJ-45 connectors
  • Commonly uses Category twisted-pair cable
  • Supports structured copper cabling
  • Connects to RJ-45 Ethernet interfaces
  • Can be field terminated
Deployment Considerations

Why Use Direct Attach Copper?

Integrated Design

The cable and network interface ends are supplied as a single assembly.

High Bandwidth

DAC technology supports high-speed Ethernet links including 10GbE, 25GbE, 40GbE and 100GbE.

Short-Reach Efficiency

DAC is well suited to short connections where long-reach optical capability is unnecessary.

Breakout Options

Supported QSFP-family DACs can divide a high-speed port into multiple lower-speed connections.

Distance and Infrastructure

When Is DAC Not the Best Choice?

DAC is designed primarily for short-reach connectivity. As required distance increases, active optical cables or separate optical transceivers and fiber cabling may become more appropriate.

Long Distances Fiber is generally better suited to longer network links.
Structured Cabling Modular fiber may be preferable when using patch panels and structured pathways.
Cable Density Copper DAC assemblies can be thicker and heavier than optical alternatives.
Future Reconfiguration Separate transceivers and fiber provide greater modularity when cable lengths change.
Hardware Compatibility

Are All DAC Cables Compatible?

No. Matching the physical connector and network speed does not automatically guarantee that a DAC cable will operate with a particular server, switch or network adapter.

Network equipment manufacturers may validate specific cable assemblies, require particular cable coding or EEPROM information, limit supported cable lengths, or impose signal-conditioning and forward-error-correction requirements.

Compatibility becomes especially important when connecting equipment from different manufacturers or when using breakout DAC assemblies.

Compatibility Checklist

What Should You Check Before Buying a DAC Cable?

Port Type SFP+, SFP28, QSFP+ and QSFP28 are not interchangeable simply because they look similar.
Network Speed Verify the required Ethernet speed and port operating mode.
Cable Length Select a supported length appropriate for the physical cable route.
Passive or Active Verify which cable type the connected equipment supports.
Vendor Support Check the compatibility documentation for both connected devices.
Breakout Mode For breakout DACs, verify the parent port and every endpoint support the configuration.
Troubleshooting

Why Is My DAC Cable Not Working?

Unsupported Cable

The switch or network adapter may not recognize or support the installed DAC assembly.

Speed Mismatch

Both endpoints must support compatible network speeds and port operating modes.

Incorrect Breakout Mode

QSFP-family ports may require explicit configuration before breakout links become active.

Cable Length

Longer copper links can require different cable designs, signal conditioning or FEC settings.

Port Configuration

Verify speed, FEC and other port settings required by the connected hardware.

Cross-Vendor Support

A cable supported by one endpoint is not automatically supported by equipment at the other end.

Frequently Asked Questions

DAC Cable FAQ

What does DAC stand for in networking?
DAC commonly stands for Direct Attach Copper or Direct Attached Cable. It describes a fixed cable assembly used to connect compatible high-speed network ports directly over copper.
Is a DAC cable copper or fiber?
A DAC cable uses copper conductors, commonly Twinax. An Active Optical Cable, or AOC, uses optical fiber instead.
Does a DAC cable need separate transceivers?
No. The interface ends are permanently integrated into the DAC cable assembly, so separate removable transceivers and patch cables are not required.
What is the difference between passive and active DAC?
Passive DAC cables do not contain active signal-conditioning electronics. Active DAC assemblies incorporate electronics to help maintain signal integrity over longer copper links.
What is the difference between DAC and AOC?
DAC uses copper conductors, while AOC uses optical fiber with active optical components integrated into the cable ends. AOCs can provide greater reach and thinner cabling in applications where copper DAC is not ideal.
What Ethernet speeds can DAC cables support?
DAC assemblies are available for multiple Ethernet speeds. Common enterprise examples include SFP+ for 10GbE, SFP28 for 25GbE, QSFP+ for 40GbE and QSFP28 for 100GbE.
What is a breakout DAC cable?
A breakout DAC divides a multi-lane QSFP-family interface into multiple lower-speed interfaces. Common examples include QSFP+ to four SFP+ connections and QSFP28 to four SFP28 connections.
Is DAC the same as an RJ-45 Ethernet cable?
No. DAC cables commonly use Twinax copper and integrated SFP/QSFP-family interfaces. Conventional Ethernet patch cables use twisted-pair Category cable and modular RJ-45 connectors.
Are all DAC cables compatible with every switch?
No. Compatibility can depend on the switch, network adapter, cable coding, interface type, Ethernet speed, cable length, firmware, FEC configuration and whether the link uses normal or breakout operation.
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