AOC Cables: AOC vs DAC and Application Examples

In a similar way to Direct Attach Copper (DAC) cables, Active Optical Cables (AOC) offer a low-power solution for high-speed, fixed distance fiber optic links in-rack.

Let’s take a look at AOC cables in a bit more depth, understand the differences between AOC and DAC and where you might want to consider using this low cost, quick to install cabling solution.

Covered in this article:

+ What is AOC
+ Differences between DAC and AOC
+ Where to use AOC cables
+ Advantages of AOCs

What is AOC?

An Active Optical Cable is an all-in-one solution consisting of multimode optical fiber cable of a fixed length with transceivers attached permanently to both ends

Typically AOC cables range in length from 1 metre to 100 metres, and can be provided with SFP+, SFP28, QSFP+ and QSFP-DD transceivers (as well as break-out cables) with speeds of up to 800G.

Differences between DAC and AOC

Whilst both DAC and AOC are plug and play solutions which make them simple and straightforward to use in the field, there are a few differences which mean they tend to be used in slightly different situations within the data center.

DAC cables, as the name suggests, consist of copper wire connecting the two fixed transceiver modules at each end, whilst AOC consists of multimode fiber with transceivers permanently attached to each end. The table below shows the key differences.

Feature	DAC	AOC
Plug and Play	√	√
Cable Type	Twinax copper	Multimode OM3/OM4 fiber
Distance	Limited range 0.5 to 15m (recommendation of max 10m)	Reaches up to 100m (recommendation of max 30m)
Data Rate	Speeds are up to 800G	Speeds are up to 800G
Power Consumption	Virtually zero. Passive cables <0.3W	~50% of power consumption versus 2 transceivers
Cost	Lowest cost point to point solution	Much reduced CAPEX versus 2 transceivers and fiber patch cable
Form Factors Supported	SFP+/SFP28/QSFP+/QSFP28/QSFP-DD	SFP+/SFP28/QSFP+/QSFP28/QSFP-DD
Passive/Active	Supports active and passive switch ports	Supports active switch ports
EMI	Vulnerable to EMI	EMI has no impact
Bend Radius	AWG 24 to 30 (higher ratings for thinner cable and greater bendability)	Thinner and shorter bend radius. Lightweight compared to DACs

Where to use AOC cables

AOC cables, like DAC provide data rates of up to 800G with a similar range of transceivers available ranging from SFP+ to QSFP28. AOC cables are also available to support QSFP-DD.

Whereas DACs are used for linking servers and storage to Top of Rack (ToR) switches, AOCs tend to be used for connecting End of Rack (EoR) switches.

DAC cables are less costly than AOC, but are limited in range up to 10 meters and their bulk makes them harder to install in tight spaces and corners, so in data centers where there is limited space for cabling AOCs are ideal. AOCs are available up to a fixed length of 100 metres, however, because they are thinner there can be a risk of damage and breaking at longer lengths, so AOCs tend to be used for distances in the 10s of meters, rather than 100. AOCs can also be used in environments with higher Electromagnetic Interference (EMI), as the signal is transmitted via light versus electrically, as in DACs.

Advantages of AOCs

Compared to buying transceivers and cables separately investing in plug-and-play AOCs can save on time spent splicing fiber to the correct length and joining transceivers, and, as the transceivers are already connected there is no requirement to the clean fiber ends.

AOC cables are ideal in situations where high-bandwidth transmission is needed over longer distances within the data center. They are simple to install in the field with the flexibility needed to adapt to tight spaces.