In my previous article, you have seen what is Direct Attached Cables and Direct attached Copper cables. In this article, we will discuss the second category of Direct attached cables which are active optical cables(AOC). The main physical difference between direct attached copper cables and active optical cables is that the cable which transmits the data in DAC is made of copper and in the case of AOC its optical fiber.

Active Optical Cable (AOC) is a cabling technology that accepts same electrical inputs as a traditional copper cable, but uses optical fiber “between the connectors”. AOC uses electrical-to-optical conversion on the cable ends to improve speed and distance performance of the cable without sacrificing compatibility with standard electrical interfaces. So the basic idea of AOC is to embed the active optical transceiver components into the electronic connector instead of using separate optical transceiver and pluggable fiber cable. (since the cable is permanently attached). Unlike the direct attached copper cables(DAC), AOC cables are available only in active mode and not in passive mode. Hence the signal boosting is happening for data transmission.

Structure of AOC cables

AOC cable mainly consists of two parts- the fiber optic connector and fiber cable. The connection between fiber cable and connectors is not separable. If the connector or cable needs to be changed, they should be removed together. The electrical and optical signal conversion can be achieved right through each end of optical fiber.

Take even a closer examination and we can see that the active optical cable (AOC) assembly is actually composed of 4 functional parts.

1. The “High-density QSFP+ connector”. This is the SFF-8436 electronic connector which can be plugged into a router or switch(Instead of QSFP+ transceivers SFP+ transceivers also available in the market).
2. The “4-Channel full-duplex active optical cable transceiver”. This optical transceiver is embedded inside the shell so you cannot see it. This optical transceiver part is responsible for Opto-electro (O-E) and electro-Opto (E-O) conversion.
3. The MPO optical connector (the black part). This connector is permanently attached to the shell and fiber. This permanent attachment shields the optical interface from the end-user and also from environmental contaminants.
4. The ribbon optical fiber cable (this picture shows a yellow jacket single-mode fiber, but multimode fiber models are also available).

The above product has four single-mode fiber optic transceivers per end, each operating at data rates from 1 to 10.3125 Gb/s and supporting a reach of up to 4,000 meters. This active optical cable is electrically compliant with the SFP+ interface and supports InfiniBand, Ethernet, Fiber Channel and other protocols.

Commonly used AOC cables

AOC’s are becoming one of the most popular cabling solutions in the data center. In the modern market, a variety of AOC cables have been launched for 10G/40G/100G applications, including 10G SFP+ AOC, 25G SFP28 AOC, 40G QSFP+ AOC, 40G QSFP+ to QSFP+ AOC, 40G QSFP+ to 4 SFP+ breakout AOCs, 56G QSFP+ AOC, 40G QSFP+ to 4x SFP+ breakout AOC, 40G QSFP+ to 8xLC breakout AOC, 100G QSFP28 AOC, 100G QSFP28 to 4x SFP28 breakout AOC and 120G CXP AOC, etc. These AOC optics are commonly used for short-range multi-lane data communication and interconnect applications between two devices, such as rack-to-rack, shelf-to-shelf interconnect, storage, hubs, switches, routers, servers, etc.

Why Active Optical Cables than Direct attached Copper cables

Copper cable is heavy and bulky, making it difficult to physically manage the datacenter. And because of the nature of electrical signals, electromagnetic interference (EMI) limits copper’s performance and reliability. If you have a scaled-out high-performance cluster, these problems really add up. So that was why Intel and Luxtera invented Active Optical Cable(AOC) assemblies to fill the gap.

Understanding the advantages of AOC cables can answer why AOC than DAC cables,

1. Although both cables are used for short-range data communication, the active optical cable is able to provide a longer reach than direct attach copper cable among devices.
2. Active optical cable has a higher bandwidth because its signal transmits through optical fiber as an optical signal which transmits faster than an electrical signal in copper cable. The maximum throughput of AOC cable is up to 40 Gbps with QSFP+.
3. The weight of active optical cable is lighter than copper cable due to the optical fiber material. It is possible for AOC cable to achieve simpler cable management with a lower weight.
4. EMI (electromagnetic interference) immunity is another benefit of active optical fiber. EMI is a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrostatic coupling or conduction. Since the optical fiber is a kind of dielectric which is unable to conduct electric current, the active optical cable will not be affected by the electromagnetic energy.

Let us make a comparison between DAC cables Vs AOC Vs using the traditional way of fiber optics cabling(2 Transceivers +Structured cabling)

From the above table, we can conclude that:

1. The lower weight and smaller bend radius of AOCs enable simpler cable management in high-density deployments.
2. The thinner AOC cables free up a lot of space for increased airflow that is better helping to balance generated heat in crowded systems.
3. AOCs have great advantages over DACs especially when transmission distance reaches above 15 meters.
4. Optical fiber in AOC has considerable dielectricity, thus its EMI resistance level is considerably high.
5. AOC is a cost-effective and flexible short-reach (≤100m) direct-attach option in the interconnect system.
6. Since the connectors of AOC are factory pre-terminated, it is less affected by the repeating plug during daily use. It has also been proved that AOC has better reliability than that of transceivers.

Conclusion

DAC cable and AOC cable are widely applied in storage area networks, data centers, and high-performance computing connectivity, etc. AOC vs. DAC: the correct choice can be made when you are clear about the advantages and disadvantages of each. I hope both of my articles can help you enough to make the decision of yours on this.

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