What Is a Direct Attach Cable (DAC) and When Should You Use It Instead of AOC?
A direct attach cable (DAC) is a factory-terminated copper cable assembly that connects two network ports directly, plugging into the cages at both ends without a separate pluggable transceiver on each side. Inside a DAC, the transceiver electronics are permanently attached to the twinax copper and sealed inside the connector shells, so the module and the cable are one physical part. That is the fundamental difference from a conventional optics link, where two transceivers and a fiber patch cord are bought separately and assembled in the field.
What Is a Direct Attach Cable (DAC)?
A DAC is a fixed, pre-terminated high-speed cable assembly used to link switches, servers, storage arrays and routers inside a data center. Because both ends are terminated, tested and tuned at the factory, the assembly behaves like one long pluggable connector with a guaranteed end-to-end signal path. Like pluggable modules, DACs are built to the electrical interfaces of the IEEE 802.3 Ethernet families they serve, so a port that accepts a standard module accepts the matching DAC as well.
DACs are available in the same form factors as optical transceivers, including SFP+, SFP28, QSFP+, QSFP28 and QSFP56. Hirundo's AOC/DAC series covers the complete 10 Gb/s to 200 Gb/s range, while the wider Hirundo transceiver portfolio extends from 10G up to 800G and 1.6T platforms. That means the same rack can be cabled with copper at short reach and with optics at long reach without changing suppliers.
Three characteristics explain why DACs remain the default choice for short in-rack links: low power consumption, small size and high reliability. They help data center operators increase capacity, improve port density and reduce power consumption per port, which is why they are mainly used for internal interconnection of data centers rather than for long-haul transport.
Passive DAC vs. Active DAC: What Is the Difference?
Both variants use copper twinax as the transmission medium, but they handle signal integrity differently.
Passive DAC. A passive DAC contains no active electronics. The signal travels the length of the copper pair without retiming, equalization or re-driving, which is why it is the cheapest and lowest-latency option available and consumes essentially no power. The trade-off is reach: a passive DAC is practical for a few metres only. In practice, 1 m, 3 m and 5 m assemblies cover the vast majority of top-of-rack to server connections.
Active DAC. An active DAC adds signal conditioning electronics inside the connector housing. Those circuits compensate for the attenuation and inter-symbol interference that copper introduces at high data rates, extending usable reach and improving the eye margin at the far end. An active DAC costs slightly more and draws a small amount of power, but it still costs far less and runs far cooler than a discrete optics link.
Copper gauge matters anyway. Whether passive or active, the conductor size drives the trade-off between reach, weight and airflow. Hirundo's DAC assemblies are offered in 24AWG, 26AWG, 28AWG and 30AWG. Lower AWG numbers mean thicker conductors, better reach and heavier, stiffer cable that resists tight bends. Higher AWG numbers mean thinner, lighter cable that is easier to route and friendlier to airflow, at the cost of shorter practical reach.
What Is an Active Optical Cable (AOC)?
An active optical cable keeps the sealed, factory-terminated, plug-and-play idea of a DAC but replaces the copper twinax with multimode fiber. Electrical-to-optical conversion engines sit inside the connector shells at both ends, so the assembly is effectively two optical transceivers joined by a permanently attached fiber run.
The consequences are physical, not just commercial. Because the link is optical, an AOC is immune to electromagnetic interference and to ground loops, and its cable is thinner, lighter and easier to manage in dense racks. Reach is dramatically longer than copper, typically extending to around 100 m, which is enough to span rack-to-rack and row-to-row connections without deploying patch panels and standalone modules. Hirundo's AOC range supports 40G, 50G, 100G and 400G configurations over OM2, OM3, OM4 and OM5 multimode fiber, with wavelength options including 850 nm, 1310 nm, 1550 nm, 1270/1330 nm and CWDM, LWDM, MWDM and DWDM bands.
DAC vs. AOC: A Side-by-Side Comparison
| Attribute | Passive DAC | Active DAC | AOC |
|---|---|---|---|
| Transmission medium | Copper twinax pair | Copper twinax with in-connector signal conditioning | Multimode fiber with embedded optical engines |
| Typical reach | 1 m to about 5 m | Longer than a passive DAC at the same rate | Up to roughly 100 m |
| Power consumption | Essentially zero | Very low | Low, but higher than copper |
| Latency contribution | Lowest | Very low | Low, with an added electrical-to-optical conversion |
| Relative cost per link | Lowest | Moderate | Moderate |
| EMI and RF immunity | Susceptible | Susceptible | Immune |
| Weight and bend radius | Heavier at low AWG | Heavier at low AWG | Light and flexible |
| Typical deployment | Top-of-rack to server, adjacent ports | High-rate in-rack interconnects | Rack-to-rack and row-to-row links |
When Should You Use a DAC Instead of an AOC?
The decision usually comes down to distance first, then environment, then budget.
- Choose a passive DAC when the link is short and cost matters most. Top-of-rack switches connected to servers inside the same rack, or two switches sitting in adjacent racks, are classic passive DAC territory. You gain the lowest latency available because there is no retiming and no optical conversion in the path, which is attractive for latency-sensitive compute and storage traffic.
- Choose an active DAC when the rate is high but the run is still copper-friendly. At 100G and 200G, copper loss grows quickly. Active conditioning electronics let you keep a copper link where a passive assembly would have run out of margin.
- Choose an AOC once the run leaves the rack. Anything beyond the practical copper limit, and especially anything approaching a 100 m row-to-row span, belongs on fiber. An AOC gives you that reach without buying separate modules and patch cords.
- Choose an AOC in electrically noisy or airflow-constrained spaces. Optical links are immune to EMI, and the thinner cable improves airflow in high-density racks where copper bundles can block cooling.
- Choose an AOC when cabling must be re-routed often. Light, small-diameter AOC assemblies are easier to dress and less likely to be damaged by tight bend radii during moves, adds and changes.
Form Factors and Cable Assemblies in the Hirundo DAC/AOC Range
Hirundo supplies DAC and AOC assemblies across the full 10 Gb/s to 200 Gb/s span, in the breakout and straight configurations that data center topologies actually require:
- QSFP56 to QSFP56 for 200G links
- QSFP28 to QSFP28 for 100G links
- QSFP28 to 4xSFP28 breakout, connecting one 100G switch port to four 25G server ports
- QSFP+ to QSFP+ for 40G links
- QSFP+ to 4xSFP+ breakout, splitting a 40G port into four 10G ports
- SFP28 to SFP28 for 25G links
- SFP+ to SFP+ for 10G links
Assemblies are offered in passive and active data modes with 24AWG, 26AWG, 28AWG and 30AWG copper, in lengths of 1 m, 3 m, 5 m, 15 m, 30 m and 100 m, plus custom lengths on request. Coding options are available for generic use as well as for the specific switch and server platforms commonly deployed in enterprise and cloud data centers.
Key Optical Parameters of a 200G QSFP56-to-QSFP56 AOC
To make the performance envelope concrete, here are the headline per-lane values for a 200G QSFP56-to-QSFP56 active optical cable:
| Parameter (per lane) | Value |
|---|---|
| Signaling speed | 26.5625 GBd, plus or minus 100 ppm |
| Modulation format | PAM4 |
| Center wavelength | 840 nm to 860 nm |
| RMS spectral width | 0.6 nm maximum |
| Average launch power | -6.5 dBm to 4 dBm |
| Transmitter OMA | -4.5 dBm to 3 dBm |
| Optical extinction ratio | 3 dB minimum |
| TDECQ | 4.5 dB maximum |
| Average receive power | -8.4 dBm to 4 dBm |
| Receiver sensitivity (OMAouter) | -6.5 dBm |
| Stressed receiver sensitivity (OMAouter) | -3.4 dBm |
| Receiver reflectance | -12 dB maximum |
Equivalent documentation exists for 10G SFP+ to SFP+ active optical cables and for the 200G QSFP56 assembly, so link budgets can be verified before deployment rather than after.
Where DACs and AOCs Are Used in a Data Center
The two products divide the data center between them along a simple line: distance.
- Top-of-rack to server. Nearly always a passive DAC at 1 m to 3 m, in 10G, 25G, 100G or 200G speeds. This is the highest-volume cabling segment in any facility.
- Switch-to-switch within a row. Passive or active DAC depending on rate and span, with active assemblies taking over as speeds rise.
- Row-to-row and rack-to-rack spine connections. AOC territory, where copper cannot reach and where the lighter cable simplifies overhead routing.
- Breakout applications. QSFP28 to 4xSFP28 and QSFP+ to 4xSFP+ assemblies let a single high-rate port fan out to four lower-rate ports, which is the standard way to connect a 100G leaf switch to 25G server NICs or a 40G port to 10G NICs.
- AI, HPC and GPU clusters. Short-reach accelerator interconnect favours passive copper for its near-zero latency and power, while longer fabric spans move to optics.
Frequently Asked Questions
Is a DAC a cable or a module? It is both. The transceiver electronics are built into the connector shells and permanently attached to the cable, so a DAC is ordered, stocked and replaced as a single part number rather than as a module plus a patch cord.
Do DACs need cleaning or optical inspection? No. There is no exposed optical interface on a copper DAC, so the maintenance routine that applies to fiber connectors does not apply. AOCs likewise arrive sealed and terminated from the factory.
Can a DAC be used with any switch brand? The cable must carry coding that matches the platform it plugs into. Hirundo offers generic coding as well as coding for the major switch and server platforms, and can advise on the correct option before an order is placed.
Which is better for latency-sensitive workloads? A passive DAC, because the signal passes through with no retiming and no optical conversion, giving the lowest possible latency and power for short links.
What is the longest practical DAC run? Passive copper assemblies are intended for a few metres, most commonly 1 m to 5 m. When a link has to be longer, the answer is an active DAC if copper margin allows, or an AOC for anything up to around 100 m.
Why Hirundo
Hirundo builds the AOC and DAC assemblies that hold a data center together at short reach. The series spans 10 Gb/s to 200 Gb/s in the configurations operators actually deploy, including QSFP56 to QSFP56, QSFP28 to QSFP28, QSFP28 to 4xSFP28, QSFP+ to QSFP+, QSFP+ to 4xSFP+, SFP28 to SFP28 and SFP+ to SFP+ assemblies, in passive and active modes, across 24AWG to 30AWG copper and multiple wavelength options. Low power consumption, small size and high reliability are designed in so that operators can raise capacity, improve port density and cut power per port without re-architecting the cabling plant. Alongside the cable range, Hirundo supplies the surrounding optical portfolio — transceivers, WDM modules, polarization-maintaining components, splitters, optical switches and specialty fiber — which means short-reach copper and long-reach optics can be specified and qualified together from one source.
