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DAC/AOC Cables Comparison

Apr. 01, 2022

DAC cables and AOC cables are widely used in data center cabling systems for high-performance computing networks because of their low latency, low power consumption, and low cost. DAC cables and AOC cables are available in a variety of configurations to meet network requirements. Each offers 10G SFP+, 25G SFP28, 40G QSFP+, and 100G QSFP28 data rates, as well as additional options from 40G to 4x10G or 100G to 4x25G variants.

 

DAC/AOC Basics and Types

Direct Attach Cable (DAC) is comprised of a Twinax copper cable terminated with SFP+/SFP28/QSFP+/QSFP56/QSFP28 connectors on both ends, which can provide an electrical connection directly to active equipment.

DAC cables can be classified into twofold: passive DAC & active DAC. Both passive and active DAC cables can transmit electrical signals directly over copper cables. The former can deliver without signal conditioning, while the latter has electric components inside the transceivers to boost signals. Normally speaking, DAC cables are used to connect switches, servers, and storage inside racks. 

 Breakout 100g QSFP28 To 4SFP28 DAC Cable

  Breakout 100g QSFP28 To 4SFP28 DAC Cable     

 

Power Consumption

Normally, the power consumption of AOC cables is higher than DAC ones, which is 1-2w. While the power consumption of DAC active cables is less than 1w, the passive ones cost nearly no power consumption at a value of lower than 0.15w due to the thermal design of direct attach copper cables. As a result, the operating expenses on power consumption will be decreased when adopting the DAC options.

 

Transmission Distance

Adopting optical fiber technology, AOC cable can transmit over longer distances—100 m, while DAC cable link length limit is 10 m (passive DAC: 7 m; active DAC: 10 m). To sum up, DAC cabling solutions are suitable for short-range transmissions, while AOC solutions are applied in long-range networking cases.

 

Note: the max. The distance of a signal that can be transmitted via a DAC cable changes depending on the data rate. The link length will decrease as the data rate grows, for example, 100G DAC cables can only transmit up to 5 meters.

40G QSFP TO 4X 10G SFP+ AOC

 40G QSFP TO 4X 10G SFP+ AOC

Cost

Roughly speaking, DAC has a relatively simple internal structure with fewer components, and the copper cables are much cheaper than fiber cables. When implemented in large-scale data centers, the sum of money will be saved for large quantities of DAC cables compared to AOC options. DAC indeed provides a cost-effective solution over AOCs for short-range applications, but for long-range applications, it's wise to have the overall costs list by comparing these two options.

 

EMI Immunity

Electromagnetic interference (EMI), refers to a disturbance generated by an external source that will affect the electrical circuit. As mentioned before, the active optical cable contains optical fibers—a kind of dielectric that can't conduct electric current. Therefore, AOC cables are immune to electromagnetic interference, which can be used in most situations. However, due to the nature of copper in sending electrical signals, direct attach copper cables are vulnerable to the effects of EMI. Thus, the environment is important to avoid undesirable responses, degradation, or complete system failure.


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DAC/AOC Working Scenarios

Influenced by the abovementioned factors, DAC and AOC cables are normally applied in different working scenarios.

 

DAC Cable Typical Application

The major utilization of 10G SFP+ DACs is connecting switches/servers to switches within or adjacent to the rack. In other words, these 10G direct attach cables can be used as an alternative for ToR (Top of Rack) interconnections between 10G ToR switch and server or the stacking of 10GbE switches. Since 10G SFP+DAC typically supports a link length of 7 m with low power consumption, low latency, and low cost, this option is an ideal choice for this short-range server-to-switch connections.


10G SFP+ DAC CABLE 

10G SFP+ DAC CABLE

 

10G SFP+ AOC Cable Typical Application

Without the strict link length limits, 10G SFP+ AOCs are commonly used in several locations in the data center like ToR, EoR (End of Row), and MoR (Middle of Row). Like the DACs, the servers all connect up to a Top of Rack Ethernet switch, and each of them will have one or two Ethernet connections up to the switch and these can be patched by using AOC cables.

 

What's more, the utilization of 10G AOCs in the data center can also be realized in several main networking areas like Spine, Leaf, or Core switching areas. The interconnections are typically fulfilled by adopting these 10G SFP+ AOCs with their theoretical maximum reach being 100 m.