Do You Know The Difference Between 40G QSFP+ DAC VS AOC Cables?

What Is QSFP Direct Attach Cable

The Definition of QSFP DAC

The QSFP+ DAC is an integrated high-speed Twinax cable that has a QSFP+ connector at both ends of the cable linked for bi-directional communication. The wire portion of the QSFP 40G direct attach cables includes silver-plated conductors and foam insulated cable, this Direct Attach Cable copper transmit electrical signals with a twisted pair shield cable. The characteristics of copper transmission also cause the DAC cable to incur a small amount of transmission loss when transmitting data, and the wire gauge of the QSFP 40G DAC ranges from 24 to 30AWG.

The Type of QSFP DAC

Two different types of direct attach copper cables are available, including passive DAC and active DAC, which differ in that the active direct attach cable has an additional driver chip inside than passive DAC, which can be used for signal conditioning. Since a passive direct attach cable has no electrical components, it does not amplify or modulate the signal in any way; the transmission mode is through the host network device and is regenerated by it. The passive QSFP direct attach cable has a maximum transmission distance of 7 meters and the power consumption is nearly zero, while the active direct attach cable has a maximum transmission distance of about 10 meters or more, and it has more internal devices than the passive DAC, so the power consumption is about 440 mW. Currently, data centers generally choose passive DACs for component base cabling systems

The Application of QSFP DAC

Two types of 40G QSFP+ DACs are available in the market, 40G QSFP+ to QSFP+ DAC, 40G QSFP+ to 4×SFP 10G DAC. The 40G QSFP+ DAC copper wire is designed for connecting 40 Gigabit links over a short range of up to 7 meters, among the QSFP interfaces of in-rack switches, and across adjacent racks. The 40G QSFP+ DAC Twinax cable is widely deployed in data centers for SAN (storage area network) and high-performance computing connections because of its cost savings, reduced power consumption, and high performance.

What Is QSFP Active Optical Cable 

The Definition of QSFP AOC

QSFP AOC cable is designed to transmit signals by using duplex multimode fiber. Both ends of the QSFP active optical cable are factory designed with two already fabricated QSFP+ connectors. QSFP AOC cable has more electronic components inside than DAC cable, and these components make QSFP active optical cable more expensive than direct attach cable. At the same time, QSFP 4x10G AOC needs external power to convert electrical and optical signals, and its power consumption is approximately 2 W. Due to the use of fiber optic material, 40G active optical cable supports a much longer transmission distance than DAC cable, and which can reach 100 meters.

Application of QSFP AOC 

The AOC cables offer more advantages than copper DAC cables, including signal integration, a longer transmission distance, managed interfaces, and lower transmission loss. But in terms of cost, active optical cables are designed with optical fiber, and cables are sold at a higher price, so it costs more to build a cable system with AOC cables than with a DAC. QSFPTEK offers a variety of QSFP+ DAC and AOC cables, including 40G QSFP+ to QSFP+ AOC, 40G QSFP+ to 4x SFP + breakout active optical cable, and 40G QSFP+ to 4x duplex LC breakout cables. 40G active optical cable can be used in consumer applications such as cameras, high-definition TVs, and data center networks supporting advanced data communication systems. The image below shows the QSFP AOC in a switch application.

What Are The Differences Between AOC and DAC?

After comparing these two high-speed cables, the next section will focus on the differences between these two, including transmission distance, cost, etc.

Cost

Generally speaking, when building a cable system, engineers often need to take into account the cost factor. In comparing the two types of cables, as we mentioned earlier, the direct attach cable costs less because it contains one less electronic component than the Active Optical Cable, for instance, at QSFPTEK, the price of 5M Cisco QSFP H40G CU5M compatible 40G QSFP+ passive direct attach copper cable is only $28, whereas the price of 5M Cisco QSFP H40G AOC5M compatible 40G QSFP+ Active Optical Cable is 64.8 USD.

Power Consumption

DAC active copper cables and AOCs are able to provide longer transmission distances than passive copper cables, primarily because they provide support for active electronics. The passive copper cables have no power requirements. Typically in 40G data transmission, an active copper cable requires about 440mw of power, which is significantly lower than the 2W power consumption of an AOC. In addition, this affects the cooling system of the data center. A higher power consumption creates a higher amount of heat in the data center, which places a bigger strain on the data center's cooling system. Slimmer QSFPTEK fiber optic cable will have a greater cooling performance in the data center as well.

Transmission Distance

Due to the physical properties of copper, the maximum transmission distance of a QSFP direct attach cable varies with the data rate. As the transmission rate increases, that distance decreases; for instance, a 40G QSFP+ DAC copper cable is limited to a maximum transmission distance of 7 meters. This results in DAC Twinax cables being used only in a few restricted conditions: typically in the same rack in a data center, linking servers and TOR switches. The AOC cables were then introduced to surmount this challenge of long transmission distances. Since active optical cable uses fiber optic cables, the maximum transmission distance can achieve up to 100 meters, providing a much longer transmission distance than that of passive copper cables.

Conclusion

With this post, you may have an idea about how to choose the right high-speed cable. When the transmission distance is less than 10 meters, for example in the same rack of a data center, QSFP+ DAC cables are suggested. For transmission distances between 10 meters and 100 meters, QSFP+ active optical cables are suggested, although the cost will be higher. If you want to know more about 40G DAC cables and 40G AOC cables, please contact QSFPTEK via sales@qsfptek.com. QSFPTEK offers a wide range of optical fibers, as well as optical module-related products.