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DACs and AOCs—Two Important Roles in 40G Migration

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It is well known that 1G and 10G data rates are not adequate to meet the future needs of high-bandwidth applications. Thus 40G and 100G Ethernet are the inevitable trends in the future. And as the market for 40G Ethernet will grow faster than 100G Ethernet in the mid-term for practical reasons, products for 40G Ethernet such as 40G QSFP+ transceivers, 40G QSFP+ direct attach cables (DACs) and active optical cables (AOCs) are very popular in the market. Although 40G transceivers are usually preferred for the advantage of flexibility, DACs and AOCs are more cost-effective alternative solutions in short reach interconnections. This article will give a detailed introduction to DACs and AOCs.

What’s DACs and AOCs ?

DACs and AOCs are two kinds of optical transceiver assemblies, which are terminated with transceiver-style plugs to be used in the same ports where optical transceiver are used. AOCs are active devices, which incorporate active electrical and optical components to boost/receive signal via optical fiber. DACs can be both passive and active. The passive DACs (often called PCCs) have no active circuitry, which means there is a direct connection between copper cable and QSFP+ transceiver’s printed circuit board (PCB) electrical contacts. While the active DACs (often called ACCs) incorporate active component to boost/receive signal via copper cable. Passive DACs are much more often used than active copper cables. The following figure shows the general internal structure of AOCs and DACs.

AOC and DAC

DACs VS AOCs

As ideal solutions for short-distance interconnection, DACs and AOCs are widely used in data center. The following part will analyze the advantages and disadvantages of DACs and AOCs respectively.

DACs

As the development of fiber technology, someone may deem that copper technology is out of fashion. This is not true for DAC cables. In fact, DACs still get their role in the market for the following advantages.

qsfp+ dac

  • Enough data rate for various applications—Today’s DAC cables can support higher data rates than traditional copper interfaces. In fact, DAC cables can offer a highly cost-effective way to establish a 40G link between QSFP+ ports of QSFP+ switches within racks and across adjacent racks.
  • Interchangeability—As the development of copper cable technology, copper DACs are interchangeable and hot swappable with fiber optic modules.
  • Cheap—Since copper cables are much more cheaper than fiber cables, DACs are cost-effective solution over optical transceivers and AOCs for short reach applications.

The defect of DACs is that they are heavy and bulky, making it difficult to be managed. Furthermore, due to the nature of electrical signals, copper DAC is vulnerable to the effects of electromagnetic interference (EMI), such as undesirable responses, degradation, or complete system failure.

AOCs

Providing less expensive, reliable transport for aggregated data rates up to 40 Gbps, the AOCs offer customers the flexibility of traditional optical modules by interfacing to systems via a standard QSFP MSA connector. AOCs are premium products, offering several benefits over DACs.

QSFP+ AOC

  • Greater bandwidth—Needing no equipment upgrades, AOCs have a throughput of up to 40 Gbps with QSFP+ transceivers. AOCs are ideal for the high density signal transmission seen in most data centers and high performance computing applications.
  • Lightweight—Due to the fiber cables, the AOC weighs less than a comparable DAC cable.
  • EMI immunity—Since the optical fiber is a kind of dielectric (not able to conduct electric current), AOCs are immune to electromagnetic energy.

Compared with DAC cables, the drawback of AOCs is that they may be a little expensive for customers.

Conclusion

DACs and AOCs use the same port as an optical transceiver but with significant cost and power savings in short reach applications. This is very important in the 40G migration for the reason that DACs and AOCs can fill the need for short, cost-effective connectivity and provide a power-efficient and cost-effective replacement to 40G optical transceivers. Fiberstore supplies a comprehensive line of high-compatible DACs and AOCs with competitive prices. Each of them is tested in original-brand switch which ensures the 100% compatibility to your device.

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