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Advantages and Disadvantages of Direct Attach Cable (DAC Cable)

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Driven by need for high speed Ethernet and increasing services like Cloud and virtual data center, many data centers have been loaded with numerous cables and various equipment. Due to this case, shortage of data center space is common but it’s getting difficult and expensive to build new or expand existing locations. Today this post intends to introduce a type of cable – DAC cable (direct attach cable), which can solve this problem effectively.

Overview of Direct Attach Cable (DAC Cable)

Direct attach cable is a type of copper cable similar to coaxial cable, but it has SFP+, QSFP+ or QSFP28 plus at each end integrated. DAC cable is usually used in data centers for short transmission between racks. Generally, according to the construction differences, the DAC cable comes in two types: passive and active DAC. Passive cable is suitable for short distances up to 10 m and active DAC has a little longer transmission distance up to 15m at 10Gbps or 40Gbps, because it has an active component to boost/receive signal. And passive DAC cable requires no power for internal electronics, which avoid consuming power or produce heat. While active DAC comes in reverse, it needs power to make its internal circuits work properly.

On today’s markets, in order to satisfy the ever growing need for cost-effective delivery of more bandwidth, there are mainly three kinds of DAC according to the transmission rate: 10G SFP+ DAC, 40G QSFP+ DAC and 100G QSFP28 DAC.

10G SFP+ cable offers the high density, low cost and low power solutions for today’s 10G Gigabit Ethernet connections. With its SFP+ interface, this twinax cable allows interconnects with hot-pluggable optical transceivers and Ethernet switches in data centers. As shown in the picture below, two Cisco catalyst switches are connected with a 10G SFP+ DAC.

two Cisco Catalyst 4948E-F Switches directly with 10G sfp+ Direct Attach Cable

40G QSFP+ cable offers a way for 40G Gigabit short distance connections between QSFP ports switches. As its name shows, 40G QSFP+ DAC has QSFP+ connectors on both ends. And it uses integrated duplex serial data links for bidirectional communication. 40G QSFP+ DAC is a little different from 10G DAC cable. There are two types of 40G QSFP breakout DAC cables: 40G QSFP+ to 4XSFP+ DAC and 40G QSFP+ to 4xXFP DAC. These breakout DAC cables can be used to migrate from 10G to 40G.

two Cisco switches connected by 40 QSFP+ to QSFP+ DAC cable

The last one is 100G QSFP28 DAC. The 100G QSFP28 DAC cables are high speed cable to meet and exceed 100G Gigabit Ethernet, providing connectivity between devices using QSFP28 ports. The 100G QSFP28 DAC includes QSFP28 to QSFP28 and QSFP28 to 4xSFP28 breakout DAC. These interconnect cables provide four channels of high speed differential signals with data rates ranging from 25Gbps up to potentially 40Gbps.

Advantages and Disadvantages of Direct Attach Cable (DAC Cable)

Twinax DAC cables come into play in data centers mainly because of two factors which also show its superiority. We all know that denser equipment in the data center or server rooms will consume more electrical power and produce more heat. That means more cooling systems are required. However, DAC cables have the potential to reduce the overall power consumption and heat dissipation, which help network operators save cost. Another factor is that DAC cable is robust and does not need patch panels or additional cables when connected to devices, as is the case with an optical module. The modules on both ends make them sturdy and reliable as well as space-saving.

40gbase-qsfp+-to-4-sfp+-breakout-dac

Nothing can be perfect, so do the DAC cables. Although they can save space and cost for data center managers, the drawbacks still exist. As the main element of DAC cable is copper, it is heavy and bulky, causing difficulties for management. What’s more, if DAC cables are deployed in high volume, the cable diameter and cable stiffness are another problem that should be considered. In this case, active optical cables (AOC cables) seem to be a better choice, for they are made of thinner and more pliable optical cable.

Summary

DAC cable provides a low power consumption and high-speed solution for data center interconnections. With the ability to support data rate of 10G, 40G and 100G, they are now widely used in optical links. All the DAC cables mentioned above are supplied in FS.COM. If you need to know more details, please visit www.fs.com or directly contact us via sales@fs.com.

Related Article: Use High Speed Direct Attach Cable for Data Center Interconnection

                         SFP+ DAC Twinax Cable Deployment Considerations 

40G Fanout Solution for Data Center

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With the requirement of high-speed Ethernet in data center, the migration from 10G to 40G is beginning. Fanout technology has been widely applied in 40G data center to get higher data rate and higher port density. The principle of fanout technology is just like the water pipeline in a building. Water is transferred from the trunk pipeline, and then trunk pipeline fans out into several pipelines that have smaller diameters to bring the water to every house.

A device needing to be connected to two or several devices with different physical interface is very common. So the superiority of fanout technology is brought into full play, especially in the distribution layer of 40G data center and adapting lower data rate to 40G in cabling. Several widely used fanout/breakout assemblies in 40G data center will be introduced in this article.

40G MPO Fanout Cables

A MPO fanout cable is a multi-fiber optical cable containing several individual tight buffered optical fibers with one end terminated with a male or female MPO connector and the other end usually terminated with several LC connectors.

MPO fanout cable

Figure 1

Various MPO fanout cables are available in the market now. The fanout/breakout cable which is able to fan out into 12 or 24 fibers is most commonly used in 40G cabling deployment.

Figure 1 shows a typical 12-fiber MPO breakout cable (also called MPO harness cable) with OM3 optical fiber as the transmission media. This 12-fiber MPO fanout cable is terminated with a male MPO connector on one end and 6 duplex LC connectors on the other end. It can work from MPO trunk backbone assemblies to LC fiber rack system in high density backbone cabling from 40G device to 10G devices.

There is also a little bit smaller MPO fanout cable, of which the fibers fan out directly from the MPO connectors. This mini MPO harness cable can be easily put into patch panel and increase the cabling density effectively (see Figure 2).

MPO cassette

Figure 2

MPO cassette is another special type of MPO fanout cable. It is designed for those who want to have everything in neat and tidy. The MPO cassette breaks the traditional design of the fanout cable. It can offer better cable protection and management by housing one or several mini size MPO fanout cables in a cassette. For a 12-fiber MPO cassette, there will be a 12-fiber mini direct fanout MPO cable inside the cassette. With one MPO in the back side of the cassette and six duplex LC connectors in the front (see Figure 3).

This type of cassette can be installed in the standard rack in data center. To achieve higher cable density, more 12-fiber or 24-fiber mini MPO fanout cables are installed for 40G transmission. For instance, Figure 3 is a 24-fiber MPO cassette containing two 12-fiber fanout cables, thus there are two 12-fiber MPO connectors on the cassette. A 24-fiber MPO cassette can also have a mini size 24-fiber fanout cable inside the cassette. This cassette will only have one 24-fiber MPO connector in the back side. No matter what the fiber count and connector type are, these MPO cassettes can be customized in Fiberstore according to your requirements.

24 fiber cassette

Figure 3

40G Fanout Direct Attach Cable

By offering direct interconnection for devices in data center, direct attach cable also has fanout design. Converting one form factor to a different form factor is necessary in many cases. For instance, a 40G device may be connected to one or several 10G devices for distribution or adapting. With fanout direct attach cable, this process would be much easier. This pre-terminated components can also increase the reliability of data center effectively.

A 40G direct attach cable usually has a 40G QSFP+ connector on one end, no matter it uses copper or fiber as the transmission media. And four 10G XFP connectors or four 10G SFP+ connectors are terminated on the other end of the 40G direct attach cable.

QSFP-8LC AOC

Figure 4

Sometimes it also needs to convert QSFP+ to LC interface. There is also another type of 40G DAC can satisfy this requirement. This kind of DAC is attached with a QSFP+ on one end and several LC connectors on the other end (see Figure 4). Figure 4 shows an active optical cable (AOC) with one end plugged into a QSFP+ switch and the other end attached with four duplex LC connectors which are separately linked to four 10G SFP+ transceivers. These 10G transceivers are then plugged into 10G switch with SFP+ ports. In this way, 40G cabling to 10G cabling is achieved.

Conclusion

Fanout technology is playing an important role in 40G data center. Products like 40G break out cable, cassette and 40G break out direct attached cable can all be found and customized in Fiberstore. Different connectors, cable length, fiber count etc. can all be specially designed according to your application. Please contact sales@fs.com for more details about fanout products in 40G data center.