DAC - Solution for Short Distance Connectivity

Posted byVijay Gupta13/05/20200 Comment(s)

Direct attach cable (DAC) is a communication cable commonly used to realize short distance high-speed interconnected communication. It is mainly used to realize the interconnection between servers, switches, memory and other devices. It is a low-cost high-speed data communication solution. The common wire specifications are 24AWG, 28AWG and 30AWG, which can meet a variety of applications.

A Direct Attach Copper link or a DAC link is a Twinax copper cable that firmly links the ports (or line cards) inside a piece of dynamic hardware, such as switches, network equipment, servers or any other type of device that stores -or connects- information.

DAC forms a direct electrical connection hence the name, Direct Attach Copper cabling.  A DAC is simply two wires where the 1,0 electrical signal is the voltage difference between two wires. A wire pair is used to create one directional lane; so, two pairs creates a single-channel, bi-directional interconnect. Similarly, eight wire pairs for four-channels. Wrap it all up in multiple layers of shielding foil, and solder the wires onto a tiny PCB with an EPROM chip that contains identity data about the protocol, data rate, cable length, etc. Then, put it all in an industry standard plug shell such as SFP or QSFP to create the complete cable with connector ends.

At high signal rates, the wires act like radio antennas. This means the longer the reach and higher the data rate, resulting in requiring more EMI shielding and the cable becomes thicker, more difficult to bend. IEEE and IBTA sets the cable standard specifications for Ethernet and InfiniBand applications. 

Characteristics of DAC cable:

  • High performance, low cost

Real bandwidth from 1G to 10G data transmission IEEE 802.3ae and fiber channel standards are widely used, strong Integrated scheme switching capability. 

  • Anti-radiation and interference

The mechanical design and EMI design of the circular hand can effectively organize EMI electromagnetic radiation and interference.

  • Energy saving and environmental protection

Support hot plug, low power consumption, copper has the function of natural radiator.

  • EEP storage signature design

Distinguish the copper wire integration scheme and optical module cable integration scheme.


DAC Popularity

Popularity of DAC can be summed up in two words “low price” Copper cabling is the least expensive way to interconnect high speed systems together. It is hard to beat the cost of a copper wire, a solder ball and tiny PCB all built on an automated machine.  More complex technologies such as optical fibers, GaAs VCSEL lasers, SiGe control ICs, InP lasers or Silicon Photonics, which all require sub-micron alignment tolerances, manual labor and a vast assortment of technology piece parts to assemble, cost much more than DACs.


DAC in Data Center

These low-cost, low-power consumption and high-performance capabilities has led DAC to becoming extremely popular in hyperscale, enterprise and many HPC systems as they are being used to interconnect servers and storage to top-of-rack switches using network adapter cards.  Because the number of cables needed in only one rack can be 32-56 or more, even small performance or cost difference become especially important. This is especially true when large data centers deploy tens or hundreds of thousands of cable links.


DAC Applications

DAC links are produced using twinax copper and are processed through module-based plants that give an electrical association that is well-founded into dynamic gear. The modules cannot be expelled from the link thus all DAC links are built with a fixed length. This is one of the disadvantages of utilizing DAC links, compared to using a blend of fiber links and optical handsets.

There are various kinds of fast DAC links: direct and breakout, uninvolved and dynamic. A direct interface DAC link will associate one port/line card to another. A breakout DAC link will permit one port to be associated with up to 4 different ports. A functioning DAC link will offer marginally longer transmission separation than a latent DAC link as it utilizes gadgets inside the modules at each conclusion to support the signs.

  • 10G Ethernet Connections

Likely, the most common usage of DAC links is interfacing hardware located inside information racks where transmission separations range from 1 to 2m. At the time of composing, 10G ports are the most well-known and this is where the biggest volume of DAC links are typically used. An SFP+ to SFP+ DAC link will plug appropriately into the two bits of gear and right away empower a 10G Ethernet association.

  • 40G Ethernet Connection

The proportion of 10G ports to 40G ports in a system is diminishing at a regular pace. DAC links can be used to conveniently associate a 40G port to another 40G port, however, they can likewise be utilized to interface one 40G port to up to four 10G Ethernet ports. During the current second application, DAC breakout links are required. Once again, the fundamental requirement is the separation between the four SFP+ ports.

  • 100G Ethernet Connection

DAC links can be employed to rightly interface a 100G Ethernet port to another 40G port, however, they can likewise be used as above and employed to associate one 100G port to up to four 25G Ethernet ports. During the current second application, DAC breakout links are also required. Again, the main requirement is the separation between the four SFP+ ports.


Active Vs Passive DAC

If a DAC contains the signal conditioning circuitry, it is an “Active DAC”. If the DAC does not contain this circuitry, it is a “Passive DAC”. For a Passive DAC to work, the host networking equipment must support the signal processing functions.

More than 80% of all transceiver based switches on the market today support Passive DACs. The remaining less than 20% perform no signal conditioning and, therefore, must be equipped with substantially more expensive Active DACs if copper interconnection is desired. However, these switches, since they do not contain the signal conditioning on each SFP+ port, are considerably less expensive.



  • Material

The interior of the DAC cable is made of copper core. copper has the function of natural radiator, good heat dissipation, energy saving and environmental protection.

  • Power consumption

DAC cable has low power consumption. Because the passive cable does not need power supply, the power consumption is almost 0. The power consumption of active optical cable is generally about 440mW, DAC cable has a certain advantage in energy consumption.

  • Low cost

The copper price is far lower than the fiber, therefore, use DAC cable will also reduce the wiring cost for the entire data center.



  • Short transmission distance, heavy weight, large volume, difficult to manage.
  • Susceptible to electromagnetic interference, such as adverse response and degradation etc.


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