What Is a Direct Attach Copper (DAC) Cable
Recorded: May 27, 2026, 1:21 p.m.
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What is a Direct Attach Copper (DAC) Cable?
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Forums Server Systems 5G Edge Workstation Processors Software Operating Systems Guides Buyer’s Guides Home Tips Buyer's Guides What is a Direct Attach Copper (DAC) Cable? TipsBuyer's GuidesNetworking What is a Direct Attach Copper (DAC) Cable? By Rohit Kumar - October 9, 2021 23 FacebookXPinterestLinkedinReddItEmailPrintCopy URL 40Gb_QSFP+_copper_twinax_cable 10GbE to 10GbE: SFP+ to SFP+ 25GbE/ 50GbE/ 100GbE Generation 25GbE to 25GbE: SFP28 to SFP28 That should be most of the conversions you need to know. This model will work for generations such as QSFP56 and QSFP-DD and beyond as well.
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23 COMMENTS samir October 9, 2021 At 2:33 pm This isn’t perfect and it isn’t really part of your fiber series, but it’s actually a great guide I’ll use to send people. domih October 9, 2021 At 5:24 pm “…One nice thing with DACs, unlike fiber cables, is that they are a lot less sensitive to the bend radius of the cable…/…They help cut costs significantly to the point that they are essential data center gear.” rehgsdfhdaf October 9, 2021 At 6:40 pm DAC can also have slightly less latency compared to fiber, as there’s not conversion steps. kris October 10, 2021 At 4:25 am I have tried using a 5m passive DAC for SOHO use, but it has failed twice in a year. When speaking to tech support, they maintain that 5m cables are for switch to switch use only as the latency is too high for workstation to switch use. They say 3m or less for workstation use. I have had to switch to Fibre as my run is stuck at 5m. Gabriel Ramuglia October 10, 2021 At 12:11 pm I also find DACs to be more reliable than optics. John October 10, 2021 At 12:33 pm Awww i thought this was about digital to analog converters, i get alot of audiophile stuff in my google feed, still an interesting read, we do love cables Nikolay Mihaylov October 11, 2021 At 7:30 am Hey, what about AOC? My understanding is that it’s fixed optical fiber matched to the the trasncievers. So pretty much like DAC but can cover longer distances. Anyone has experience wityh those? Patrick Kennedy October 11, 2021 At 9:20 am Nickolay – we will have one on that coming, probably not this week but it is being worked on right now. Jason Antes October 11, 2021 At 9:43 am Nikolay – I’ve used AOC breakouts, they work fine. We used them across several racks where breakout DAC wouldn’t reach. Ran for years without issue. Jason Antes October 11, 2021 At 9:58 am rehgsdfhdaf – That brief used OM1 fiber? It reads as a marketing brief. Sure DAC has less latency than AOC (maybe, depending on length and type of DAC), but not likely for OM3/4 FC and SPF+. Hans October 12, 2021 At 2:58 am Another useful breakout cable you may want to add to your list is QSFP28 -> 4x SFP+ Michael October 14, 2021 At 8:40 am You couldn’t think of another name to call it? DAC has always meant and will always mean to me, a digital to analogue converter. If you google DAC, that will be what shows up……digital to analogue converter. In fact, that is how I stumbled onto this, by searching for a converter. Patrick Kennedy October 14, 2021 At 8:54 am Michael, just as a heads-up, we did not make up the name. It has been used for decades on the networking side as well. Thomas Mjelva October 15, 2021 At 7:50 am If you use g.652 glass, yes they are somewhat bend sentitive, but for the love of everything Holy, get patch cables with g.657b3 glass, and the bending issues are all but gone. Sure, marginally higher attenuation (0.4dB/kM vs 0.3, or there about). Its down to 5mm bending radius. Not even the best coppercable should be bent that extreme Michael S October 15, 2021 At 12:56 pm Could you speak to the difference between DACs and RJ45 transceivers in terms of signal quality? I see RJ45 transceivers typically advertised for 30 meters of length, but DACs don’t seem to go past 10 meters. I would expect DACs to have better signal quality since there is more end-to-end control, but maybe there’s not many use-cases for running a DAC that far? Brian F October 15, 2021 At 1:48 pm Great write up Rohit! That last part on compatibility is really crucial too!!! Cisco compatible will get you pretty far, but when something doesn’t work and everybody is getting paid to be on site, it suuuuucks to find out your DACs or transceivers aren’t compatible. Brian F October 15, 2021 At 2:02 pm @Michael S: Patrick Murphy October 16, 2021 At 4:13 am From decades of use, I have found twin-ax cables to be the least reliable cabling in the DC. If your budget doesn’t allow optics, that is the only time I’d go with twin-ax. Danny October 16, 2021 At 10:28 am You should give this a different acronym. I bet most people who clicked, were baited here expecting to read an article about digital to analog converter. Cause you know that’s the most popular acronym for DAC Patrick Kennedy October 16, 2021 At 10:50 am Danny – This has been in use in the computer industry for decades. We did not make up this name. That is why we put cable and the spelled-out acronym in the title. Cheezehead October 18, 2021 At 9:17 am I’ll use DACs in ROBO sometimes or for 1/2 meter HA cabling from time to time but otherwise I’ve stopped using them. In larger environments day 1 they are cheaper, but down the road every time changes occur they are the wrong length. There’s also optic and cabling reuse. As a result we ended up going single-mode for everything. OS2 LC-LC is all we need to stock now days and 80% of the optics are 10G 10km variety. Would it have been cheaper to use DACs, AOCs, and OM3/4…sure but looking at larger projects the overall capex premium has been <3% every time with storage space saving and easier training for new hires. Henry J Taylor February 28, 2022 At 1:21 am I was looking for a way to connect my computer and router. I tried using an Ethernet cable but it didn’t work. So, after hearing about DAC cables from a friend, I decided to give them a try. The first time I connected the device with the help of this product, all that happened was blue screen of death (BSOD). But when I removed the power cord from my laptop and then plugged it back in again, it worked perfectly! This thing is amazing because now everytime i need to connect something i just go grab some DAC Cable and plug in without any problems at all! Christian Kohlschütter March 23, 2026 At 6:27 am I found out that you can “uprate” some 10 Gbps DAC cables to 25 Gbps, so you don’t have to them away when upgrading to SFP28 networking gear. LEAVE A REPLY Please enter your comment! Please enter your name here You have entered an incorrect email address! Save my name, email, and website in this browser for the next time I comment. Sign me up for the STH newsletter! Δ Newsletter Your email address: By opting-in you agree to have us send you our newsletter. We are using a third party service to manage subscriptions so you can unsubscribe at any time. ABOUT USServeTheHome is the IT professional's guide to servers, storage, networking, and high-end workstation hardware, plus great open source projects. Advertise on STH DISCLAIMERS: We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites. 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Direct Attach Copper (DAC) cables are copper-based solutions that facilitate direct communication between devices using copper wire, featuring modules at both ends of a twinax copper cable, typically 26-28 AWG. These cables possess fixed lengths and specific connectors, relying on electromagnetic shielding which increases with speed to maintain communication reliability. While optical communication is essential for long-range data transmission, DACs are generally utilized for short-distance connections, particularly within server racks, reflecting a trend where rack-to-rack and longer distances will increasingly utilize optical communication as network speeds evolve into the 400GbE era and beyond. A related concept is the breakout DAC cable, which addresses the need to connect higher-density networking modules, such as QSFP+ or QSFP28, to multiple lower-speed devices. This involves splitting a single, high-density connector to connect to several lower-speed links, for example, using a QSFP+ connector to facilitate four separate 10 Gigabit Ethernet SFP+ connections. Although conceptually useful for increasing port density, the availability of breakout functionality depends on the support of specific network components, and not all devices support this feature. DACs provide a simpler, reliable alternative to fiber optics for connecting devices in dense environments. The practical reach of DACs is dependent on the type of cable and the speed, generally limiting them to runs of 5 meters or shorter when operating at higher speeds like 100GbE. As transmission speeds increase, the necessity for greater electromagnetic shielding forces higher-speed DACs to be thicker and stiffer, which presents challenges for routing cables in racks. Despite these limitations, DACs offer advantages over fiber regarding flexibility, as they are less sensitive to bend radius. Furthermore, DACs can offer lower latency compared to fiber, as they avoid the conversion steps inherent in optical transmission. There are two primary types of DACs: passive and active. Passive DACs consume less power, whereas active DACs can achieve slightly greater distances by using more power. It is important to note that Active Optical Cables exist, which function similarly to DACs as pre-assembled, fixed-length solutions but leverage optical communication to extend reach. The decision to use DACs over optical alternatives often centers on cost considerations. The primary factors driving the continued use of DACs in data centers revolve around minimizing both hardware and operating costs. Optical solutions require additional hardware for the necessary electrical-to-optical and optical-to-electrical conversions on both ends, adding complexity and power consumption. DACs, due to their inherent simplicity, are considered very reliable and reduce complexity in the deployment scenario. When selecting a DAC, users should prioritize both speed and vendor compatibility. Compatible speed combinations, such as 100GbE to 4x 25GbE, or 40GbE to 1x 10GbE, define the necessary cable specifications. Vendor lock-in is another consideration, as some vendors may restrict optic choices, making DAC compatibility a nuanced issue. Ultimately, the quality of the cabling is crucial; specific guidance is provided on selecting the correct cable type based on the required link speeds and understanding the implications for physical routing in data center environments. |