TL;DR
A DAC (Direct Attach Copper) cable is a fixed-length copper cable with modules at each end, used for short-range, high-speed device connections within racks. It is favored for cost and reliability over optics in certain scenarios, but has limitations at higher speeds and longer distances.
In 2021, DAC (Direct Attach Copper) cables are widely used in data centers for high-speed, short-range device connections, offering a cost-effective and reliable alternative to optical solutions.
DAC cables are fixed-length copper cables with modules at each end, typically featuring connectors such as QSFP+ or QSFP28. They enable direct electrical communication between devices like switches, NICs, and servers within a rack, often up to 5 meters in length. These cables are distinguished by their simplicity, low power consumption, and affordability compared to optical transceivers.
There are two main types: passive DACs, which use no power and are less expensive but limited in distance; and active DACs, which require power and can extend reach slightly further. As speeds increase beyond 100GbE, DACs tend to become thicker and less flexible, with maximum reliable lengths decreasing to around 5 meters or less. Shielding increases with speed to maintain signal integrity, resulting in bulkier cables.
Breakout DAC cables, which split a high-density port into multiple lower-speed connections, are common in expanding port density. However, support for breakout configurations varies across devices, with most modern switches and NICs supporting them, but some exceptions remain.
Why It Matters
Understanding DAC cables is crucial for network engineers and data center operators, as they influence hardware choices, deployment costs, and network reliability. While cheaper and simpler than optical solutions, DACs are limited in distance and flexibility at higher speeds, impacting scalability and future-proofing strategies.
10Gtek 100G QSFP28 DAC Cable – 100GBASE-CR4 QSFP28 to QSFP28 Passive Direct Attach Copper Twinax Cable for Cisco QSFP-100G-CU0.5M, Ubiquiti, Arista, Supermicro and More, 0.5-Meter(1.6ft)
100GBASE-CR4 QSFP28 to QSFP28 DAC Direct Attach Copper Twinax Cable, Passive, 0.5-Meter(1.64ft) (Note that this length includes two…
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Background
DAC cables have been a staple in data center networking for years, especially at speeds up to 40GbE. With the advent of 100GbE and beyond, their physical limitations and the need for more flexible, longer-range solutions have increased. The industry is gradually shifting toward optical transceivers for inter-rack connections, but DACs remain prevalent within racks for their cost-effectiveness and ease of deployment.
“DAC cables offer a reliable, low-cost solution for short-range high-speed connections within racks, but their physical limitations become apparent at higher speeds.”
— Industry expert
“Support for breakout DACs is widespread, but compatibility varies, so it’s essential to verify device support before deployment.”
— Network hardware manufacturer
100G QSFP28 Parallel AOC – 100GBASE Ethernet Direct-Attach OM3 Active Optical Cable for Cisco QSFP-100G-AOC10M, 10-Meter
High-density 100GbE QSFP28 to QSFP28 full-duplex active optical cable, combines 4 full-duplex 25 Gbps SFP lanes for an…
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What Remains Unclear
It remains unclear how rapidly the industry will transition from DACs to optical solutions for intra- and inter-rack connectivity at higher speeds, or how new DAC designs might evolve to overcome current limitations.
10Gtek 100G QSFP28 to 4X 25G SFP28 Breakout DAC Passive Direct Attach Copper Twinax Cable for Cisco QSFP-4SFP25G-CU2M, Ubiquiti, Arista, Supermicro and More, 2-Meter(6.6ft)
100GBASE QSFP28 to 4x 25G SFP28 Breakout Direct Attach Copper Twinax Cable, Passive, 2-Meter(6.5ft), 30AWG.
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What’s Next
Next steps include monitoring industry adoption of higher-speed DACs, advancements in cable shielding and flexibility, and increased support for breakout configurations across devices. Additionally, the ongoing shift toward optical interconnects for longer distances is expected to accelerate.
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Key Questions
What is the main advantage of DAC cables?
DAC cables are cost-effective, simple to deploy, and reliable for short-range, high-speed connections within data racks.
How long can DAC cables reliably operate at 40GbE?
Typically up to 5 meters, with some active versions reaching slightly further, but limitations increase at higher speeds like 100GbE.
Are DAC cables suitable for connecting devices across different racks?
No, DACs are generally limited to intra-rack use due to their physical and signal integrity constraints. For longer distances, optical solutions are preferred.
What are breakout DAC cables?
Breakout DACs split a high-density port into multiple lower-speed connections, such as one QSFP+ port into four SFP+ ports, increasing port density but with varying device support.
Source: Hacker News
