TL;DR
A DAC (Direct Attach Copper) cable is a fixed-length copper cable with modules at each end, used for direct device-to-device communication in data centers. It is cost-effective for short-range, high-speed connections but has limitations at higher speeds and longer distances.
DAC (Direct Attach Copper) cables are fixed-length copper cables with modules at each end, used for direct high-speed communication between network devices within the same rack or data center. They are essential for cost-effective, short-range connectivity as networks push toward higher speeds.
A DAC consists of a twinax copper cable with modules on each end, typically with QSFP+ or QSFP28 connectors. These cables are designed for short distances, generally up to 5 meters, with signal integrity maintained through increased electromagnetic shielding at higher speeds. They are available in passive and active versions; passive DACs consume less power but have shorter reach, while active DACs use more power for slightly longer distances.
DACs are favored for their low cost, simplicity, and reliability compared to optical solutions, especially within racks. They are commonly used in data centers for connecting switches, servers, and storage devices at speeds of 10GbE, 40GbE, and 100GbE. However, as network speeds increase beyond 100GbE, the physical limitations of copper—such as increased cable thickness, stiffness, and reduced flexibility—become more pronounced, restricting their use to shorter distances.
Why It Matters
Understanding DAC cables is crucial for data center infrastructure planning, as they remain a cost-effective choice for high-speed, short-range connections. Their limitations at higher speeds and longer distances influence network design, pushing toward optical solutions for inter-rack or longer-range communication. The choice between passive and active DACs impacts power consumption and reach, affecting operational costs and hardware compatibility.
ASUS Ascent GX10 0.4m 400G QSFP112 DAC Cable, High-Speed AI Cluster Stacking Cable for GX10 Systems
Cable Type/Medium: SAS / Fiber Cable
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Background
DAC cables have been a staple in data center networking for their affordability and ease of deployment at speeds up to 100GbE. As network speeds increase, especially beyond 100GbE, the physical and electrical constraints of copper cables limit their effectiveness to short distances—typically under 5 meters. The industry is gradually shifting toward optical transceivers for longer-range and higher-speed applications, but DACs remain relevant for intra-rack connections.
“DACs are fixed-length copper cables with modules at each end, used primarily for short-distance, high-speed device-to-device communication within data centers.”
— STH (Server Technology Hub)
“As network speeds push beyond 100GbE, DACs face physical limitations, making optical solutions more suitable for longer distances.”
— Industry expert
10Gtek 40G QSFP+ DAC Cable – 40GBASE-CR4 Passive Direct Attach Copper Twinax QSFP Cable for Cisco QSFP-H40G-CU1M, Meraki MA-CBL-40G-1M, Supermicro, Mikrotik, Open Switch Devices, 1-Meter(3.3ft)
Compatible for Cisco QSFP-H40G-CU1M, Meraki, Supermicro, Mikrotik, Open Switch Devices
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What Remains Unclear
It is not yet clear how rapidly DACs will be phased out in favor of optical solutions as network speeds continue to rise beyond 100GbE. The industry is still evaluating the cost-benefit balance at higher speeds and longer distances, and some applications may continue to rely on DACs for years to come.
10Gtek 100G QSFP28 to 4X 25G SFP28 Breakout DAC Passive Direct Attach Copper Twinax Cable for Cisco QSFP-4SFP25G-CU0.5M, Ubiquiti, Arista, Supermicro and More, 0.5-Meter(1.6ft)
100GBASE QSFP28 to 4x 25G SFP28 Breakout Direct Attach Copper Twinax Cable, Passive, 0.5-Meter(1.64ft), 30AWG.
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What’s Next
Next steps involve the adoption of higher-speed optical transceivers for inter-rack and long-distance connections, while DACs will likely remain in use for intra-rack, short-range setups. Industry developments may include new DAC designs that improve reach or flexibility, but the trend toward optical solutions is expected to accelerate.
KEKAELLA QSFP56 200G DAC Twinax Cable, for Q56-200-CU1M Passive Direct Attach Copper Cable for High-Speed Data Center, 0.5m 1m 2m 3m Lengths(3M)
High-Performance 200G Data Transmission – This QSFP56 direct attach copper cable supports 200G Ethernet per port, delivering 2x…
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Key Questions
What is the main advantage of DAC cables?
DAC cables are low-cost, simple, and reliable for short-range, high-speed connections within data centers.
How long can DAC cables reliably operate at 40GbE or 100GbE?
Typically up to 5 meters, with performance decreasing at longer lengths, especially at higher speeds.
Are DAC cables suitable for inter-rack connections?
No, due to physical limitations like cable thickness and stiffness, DACs are generally limited to intra-rack use. Optical transceivers are preferred for inter-rack or long-distance links.
What is the difference between passive and active DACs?
Passive DACs consume less power and are cheaper but have shorter reach, while active DACs use more power to extend the distance slightly.
Will DACs be replaced entirely by optical transceivers?
Not immediately; DACs will likely remain in use for short-range, high-speed connections, but long-term industry trends favor optical solutions for higher speeds and longer distances.
Source: Hacker News
