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Guide to 10G DAC & AOC Cables

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Posted on March 4, 2020
October 26, 2020
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10G SFP+ fiber optic transceivers plus patch cables are considered as a routine solution for structured cabling. However, 10G DAC and AOC are also cost-effective alternatives that offer high-performance, high-efficiency, and low-budget solutions for 10GbE deployment. In the following, 10G SFP+ DAC and 10G SFP+ AOC cables will be illustrated respectively and compared elaborately.

10G DAC/AOC Basics and Types

10G SFP+ Direct Attach Cable (DAC) is comprised of a twinax copper cable terminated with SFP+ connectors on both ends, which can provide an electrical connection directly into 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 cable. The former can deliver without signal conditioning, while the latter has electric components inside the transceivers to boost signals. Normally speaking, 10G SFP+ DACs are used to connect switches, servers, and storage inside racks.

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Figure 1: Passive DAC vs Active DAC vs AOC

10G SFP+ Active Optical Cable (AOC) consists of a multimode fiber optic cable terminated with SFP+ connectors on both ends, which requires external power to complete the conversion of electric and optical signals, from the electric signals to optical ones, and then convert to electric signals finally. Generally speaking, 10G SFP+ AOCs are mostly used to link switches, servers, and storage between different racks inside data centers.

10G DAC/AOC Comparison

After knowing the background info, we can further draw a conclusion that 10G SFP+ DAC & AOC are different in terms of the following several aspects:

EMI Immunity

Electromagnetic interference (EMI), refers to a disturbance generated by an external source that will affect the electrical circuit. Like mentioned before, the 10G 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 with sending electrical signals, 10G 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.

Power Consumption

Normally, the power consumption of 10G SFP+ AOC cables is higher than DAC ones, which is 1-2w. While the power consumption of 10G DAC active cables is less than 1w, and the passive ones cost nearly no power consumption at the value of lower than 0.15w due to the thermal design of 10G SFP+ direct attach copper cables. As a result, the operating expenses on power consumption will be decreased when adopting the DAC options.

Operating Temperature

Influenced by the abovementioned factor: the power consumption of 10G DAC is really low. For the passive DAC, it nearly consumes no power and produces zero heat. Consequently, this kind of DAC product is more versatile and can be used in wider temperature ranges. Compared to the DAC cables, the operating temperature for 10G AOC cables is narrower. However, their thinner bend radius may help to increase airflow cooling in the racks.

Transmission Distance

Adopting optical fiber technology, 10G AOC cable can transmit over longer distances—100 m, while 10G DAC cable link length limit is 10 m (passive DAC: 7 m; active DAC: 10 m). Note, the max. distance of a signal that can be transmitted via a DAC cable change 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. The distance limitations indicate that the most common use of DAC cables is connecting equipment located within the same rack, for example, connecting servers to a top of rack switch. In conclusion, DAC solutions are suitable for short-range transmissions, while AOC solutions are applied in long-range networking cases.

Cost

Roughly speaking, DAC has a relatively simple internal structure with fewer components, and the copper cables are much cheaper than fiber cables. Therefore, the price of 10G DACs is much lower than 10G AOCs. That is to say, when implemented in large-scale data centers, the sum of money will be saved for large quantities of DAC cables compared to AOC options. 10G 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.

10G DAC/AOC Working Scenarios

Influenced by the abovementioned factors, 10G SFP+ DAC and 10G SFP+ AOC cables are normally applied in different working scenarios.

10G SFP+ 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.

Furthermore, adopting DAC solutions in ToR scenarios is suitable for the situation of more access equipment or high-density of single cabinet equipment. In this case, the distributed access will make the connection clear and simple but increase the difficulty of centralized maintenance and management of the switches.

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Figure 2: 10G DAC Connection Scenario

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.

For EoR deployment, the servers are connected to the switch through horizontal routes by 10G AOC cables, designed for a large number of cable connections from multiple server cabinets converged into the network cabinet. Difficult in connection management, but convenient in centralized maintenance. As a compromise between ToR and EoR, the solution of adopting AOC cables in MoR applications is basically similar to the previous EoR mode. The connection is simplified while the management is centralized.

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.

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Figure 3: 10G AOC Connection Scenario

Further Considerations

10G SFP+ DACs and 10G SFP+ AOCs are widely applied in data centers for high-performance computing connectivity. Perhaps you could take the following factors into consideration first before you design the optimal solution for your 10GbE deployment:


  • Application—What is the application scenario? Top of Rack, Middle of Row or End of row?

  • Media Availability—What is the cabling equipment on hand?

  • Cable Management—How much space is available in rack and cable pathways for cabling?

  • Distance—How long are the connections between the ports?

  • Power Consumption—What is the power consumption fee that will cost?

  • Project Total Budget—What's the expected cost and timeline?