Using DAC cables in SANs, Datacenters and High-Performance Computing Environments

High-performance Direct Attach Cables (DAC) offer a very cost-effective, efficient and simple way of connecting between network equipment in a storage area network, datacenter or in high-performance computing environments.

What is DAC?

DAC cables are made of end-to-end twinax copper and are pre-fitted at the factory with a transceiver module on each end that provides a direct electrical connection into active equipment. Whilst copper might be seen as out of date in the move to more and more fiber, DAC can still offer a real advantage in different scenarios, with low up-front costs and quick installation, but still with data rates of 10G, 25G, 40G, 100G and 400G.

Due to their copper construction, DAC cables are only suitable for short distances (from 0.5 to 15 meters) and come in fixed lengths with transceivers already attached. However, these features make DAC cables ideal for in-rack and inter-rack connections between routers, servers and switches, with the advantage of low power consumption (<0.3W) and low latency.

Different types of DAC cables

Different DAC cable types are available, including direct, breakout along with passive or active to support active and passive switch ports:

• Direct DAC cables provide a direct connection from one port to another.
• Breakout DAC cables enable one port to be connected to 4 ports. For example, DAC breakout cables can connect one 40G QSFP+ to four 10G SFP+ ports and 100G QSFP28 to four 25G SFP28 ports.
• Passive DAC cables consume no power and enable a direct electrical connection.
• Active DAC cables use electronics within the transceivers at each end of the cable to boost the signal and thereby offer a slightly longer transmission distance than passive DAC.

A wide range of transceivers in different form factors is available for DAC, including SFP+, SFP28, QSFP+, QSFP28.

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Where can you use DAC cables?

The main application for DAC cables is in connecting active equipment together, such as switches, routers, servers and other storage, within or between different data racks.

They can also be used to extend the life of switches and reduce the need to make extensive changes to your existing infrastructure.

The main consideration in deciding which DAC cable to use is the transmission distance alongside the data rate.

The higher the data rate required, the smaller the distance that it can be transmitted over. For example, a 100G DAC can only be used in applications of up to 5 meters, whereas 10G DAC can be obtained in lengths of up to 10 meters.

Advantages of using DAC over fiber optical transceivers and cables

When should you consider DAC cables over fiber optical cabling? DAC cables do offer some real advantages in applications where the limitations of their short distance allow them to be used.

• Cost-effectiveness – DAC is always more cost-effective than purchasing two fiber optical transceivers and cabling. Depending on the data rate, they can be twice or up to three times as cheap as their fiber optical alternatives.
• Reliability – DAC cables are physically strong, made out of twinax copper which is unlikely to be damaged. The transceiver modules plug directly into ports to make a solid electrical connection.
• Simple to install – Pre-installed transceivers mean that DAC can be deployed rapidly.
• Reduced power consumption – Passive DAC copper cables consume less power than optical transceiver modules.

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