Where and when to use AOC cables
February 16, 2023
Where and when to use AOC cables
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.
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.