What is Direct Attached Cables(DAC)

What are DAC(Direct attached cables) cables

Direct Attach Cables(DAC) are a form of shielded high-speed cable with SFP connectors on either end(SFP, SFP+, QSFP, QSFP+ modules). These are generally used to connect switches to routers, storage or servers within the same rack or adjacent racks. DAC cables are becoming increasingly popular, not just within data centers but within wider areas of the industry, as they offer a very cost-effective(almost half than using transceiver and fiber cables separately), efficient and easy way of making these connections.  The modules cannot be removed from the cable and so all DAC cables are a fixed length. This is one of the drawbacks of using DAC cables compared to using a combination of fiber cables and optical transceivers. Mainly there are two types of DAC cables and those are

·      Direct Attached Copper Cables(DAC cables)

·      Active Optical Cables(AOC cables)

Direct Attached Copper Cables(DAC cables)

DAC cables are made from Twinax copper cables and are factory terminated with modules that provide an electrical connection directly into active equipment. Since high-speed cables are sensitive to electromagnetic fields, the shielding is an important quality factor for the copper cables. This is written on the cables in the form of AWG ratings(American Wire Gauge (AWG) system which defines wire size in diameters and it's showing the quality of shielding) like 28AWG and 30AWG. When AWG values increases it will increase the protection against electromagnetic waves. The longer the distance, the higher the AWG rating should be.

There are different types of high-speed DAC cables; direct and breakout, passive and active. A direct connect DAC cable will connect one port/line card to another(overall single cable). In fact, direct connect DAC cables are the most commonly seen in data centers. A breakout DAC cable will allow one port to be connected up to 4 other ports(DAC cable where there is a QSFP+ transceiver with 40Gbps transfer speed can be terminate into 4 x SFP+ cables where each terminal will provide 10Gbps transfer speed).

The DAC cable's termination can be either SFP transceiver or QSFP transceivers. So depending upon the characteristics of these transceivers the DAC cables properties will also change.

Even though we have classification direct and breakout cables, the main differentiation is based on the classification Passive and Active cable structure which is a vital part in defining the cables to be used.

Passive DAC cables

The most basic version of this cable is the ‘Passive Assembly’ where the data cables are directly connected to the Transmitting and Receiving signal. Hence, it has a minimal power consumption of no more than 0.15w and generally for short reach up to 7meters. This is the most commonly used ones also due to the reason that DAC cables are used mainly for the interconnection between devices in the same rack or adjacent racks.

Active DAC cables

The active cable version contains more sophisticated electronics where the transmitting power is amplified to give a better data-pulse. You can see this in the product naming like ACU and not CU (e.g. SFP-H10GB-ACU5M vs SFP-H10GB-CU5M). An active DAC cable will offer slightly longer transmission distance than a passive DAC cable as it uses electronics within the modules at each end to boost the signals. It allows a little longer transmission distance up to 15m and ensures better transmission quality. whereas it has a higher price and consumes more power around 0.5-1w.

As a summary of DAC cables, we can say that it's used to establish the connection between devices when the speed is at 10G we will choose SFP+ to direct DAC cables(passive or active depends on distance). When the speed rate is up to 40G we will choose QSFP+ DAC cables(passive or active depends on distance).

Why Choose DAC Cables Over Optical Transceivers?

When considering whether to use DAC cables or optical transceivers, one may be confused. Nowadays DAC cable is very popular with the following advantages and some limitations,

Less Power Consumption 

Compared to optical transceiver modules, DAC cables, especially passive copper cables consume less power.

Transmission Distance

They can only be used where the physical distance between these ports(interconnecting devices) is small – less than 15m. However, the maximum distance a signal can be transmitted via a DAC cable changes depending on the data rate. As data rates get higher, from 10G to 40G and then to 100G the distance gets smaller and smaller. A 100G DAC can only transmit up to 5m.

These distance limitations mean that the most common use of DAC cables is connecting equipment located within the same data rack or adjacent racks, for example, connecting servers to a top of rack switch.

Costs

Using a DAC cable will always be more cost-effective than using two optical transceivers and a fiber cable. Again the cost savings will depend on the data rate being transmitted, but it is not unusual for them to be 2 or 3 times cheaper. Where their use is permitted by distance, deploying large amounts of DAC cables can lead to a considerable cost saving.

Reliability

DAC cables are extremely dependable. The modules that plug directly into the equipment ports make a physical electrical contact which ensures a solid connection. The cables are made from 26 to 28 AWG Twinax copper and so are physically strong and so are unlikely to be damaged by handling. Both these factors mean that it possible for connections to be made swiftly, reliably and affordably in all kinds of applications.

There is a lot more to say about the second category of DAC cables which is ‘Active Optical Cables(AOC)’ and hence I have added a different article to this. Look at this for information about that.

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