DAC vs AOC – differences and use in Ethernet networks
Network cabling has been evolving towards optical fiber to enable higher bandwidths. In some scenarios, however, copper Twinax cable still plays a key role. A good example is the SFP+ Twinax copper cable, also known as a DAC cable, which is an optimized solution for 10 GbE, 25 GbE, 40 GbE and 100 GbE applications over short distances. It is an interesting alternative to optical transceivers combined with patch cords over a few meters.
What is a DAC cable?
DAC (Passive Direct Attach Cable), also known as Twinax, is a passive or active copper cable factory-terminated (with modules without lasers) in SFP+, SFP28, QSFP+ or QSFP28 form factors. This solution is used mainly for short, multi-meter connections between switches, servers and network devices located in the same rack or within a short distance of each other.
This solution is characterized by:
- low power consumption,
- very low transmission latency,
- simple installation,
- lower cost compared to optical links.
Most common use cases
DAC is definitely the best option for short-range links, for example in data centers and server rooms, where it is commonly used for in-rack and rack-to-rack connections: including connecting top-of-rack switches to servers and storage devices in the rack or in adjacent racks, for link speeds such as:
- 10G SFP+ DAC,
- 25G SFP28 DAC,
- 40G QSFP+ DAC,
- 100G QSFP28 DAC.
DAC/Twinax is also recommended in applications with a larger number of access devices or in devices with high port-density within a single rack (for example between network devices such as switches or routers). In such cases you get clear and simple connections.
With densely packed cables inside and between racks, you must not exceed the bend radius and you must protect the cable from deformation (the permanent bend radius for most 1 m and 3 m Twinax cables is 23 mm; for a 5 m cable it is 38 mm).
DAC cables are heavy and have a large bend radius. To suspend them inside the rack, you sometimes need to use hooks or similar accessories so that the modules do not fall out of the ports. Unfortunately, this makes it harder to access active devices inside the rack.
What is an AOC cable?
AOC (Active Optical Cable) is an active optical cable equipped with integrated optical modules. Unlike DAC, it uses optical fiber transmission, which enables links over significantly longer distances.
AOC is used wherever standard DAC connections are insufficient due to link length or the operating environment.
The solution provides:
- longer transmission distance,
- immunity to EMI,
- lower cable weight,
- the ability to run connections between racks and across data center zones.
When should you choose DAC?
Direct Attach Copper cables are best suited for short connections:
- switch–server,
- switch–switch.
The solution is most often chosen where:
- devices are located close to each other,
- low link cost is important,
- you want to reduce power consumption,
- link length does not exceed a few meters.
When is AOC the better choice?
This solution is worth using in environments that require a longer transmission distance or higher immunity to electromagnetic interference.
AOC is used, among others:
- between rack cabinets,
- in large data centers,
- in industrial environments,
- for 40G and 100G connections over longer distances.
DAC or AOC – which solution to choose?
The choice of the right solution depends primarily on the operating environment and the required link length.
DAC will be the best solution for short connections within a single rack or between devices located close to each other. AOC, on the other hand, is suitable for more complex environments that require longer transmission distance and resistance to electromagnetic interference.
- DAC cables do not contain lasers, which makes them cheaper than their active counterparts (AOC), which consume more power and are used over longer distances such as 7 m and 10 m; for distances not exceeding 7 m it is recommended to use passive DAC cables.
- DAC cables dissipate heat well.
- DAC and AOC can be installed and replaced during operation (hot-pluggable).
- DAC cables transmit data over only a few meters, while AOC can carry data over longer distances, for example 100 m.
- For connections within a single rack, DAC is usually chosen, while AOC is used for rack-to-rack connections.
- In DAC, transmission takes place over copper; in AOC, a laser is responsible for transmission and the signal is carried over optical fiber.
- DAC cables are very inexpensive, much cheaper than their active AOC counterparts, where the price is significantly affected by the cost of the lasers in the modules at both ends of the cable.
- DAC cables are susceptible to electromagnetic interference; AOC is not susceptible to such interference.
- The DDM diagnostic module is available only in AOC, not in DAC.
DAC vs AOC – key differences
Transmission medium
DAC: copper
AOC: optical fiber
Maximum distance
DAC: a few meters
AOC: up to several dozen meters
Power consumption
DAC: low
AOC: higher
Cost
DAC: lower
AOC: higher
EMI immunity
DAC: lower
AOC: high
Cable weight
DAC: higher
AOC: lower
Vendor coding
SFP+, QSFP+ and QSFP28 cables, just like transceivers (for example SFP, SFP+ or QSFP+), are vendor coded, meaning they are software-configured to work with devices from a given manufacturer. When you choose DAC cables from LightOptics you get a more universal solution, because Twinax cables can be compatible on both sides with devices from different vendors and you can adapt them to your needs. This way, connecting switches from multiple vendors is no longer an obstacle. LightOptics offers passive Twinax 10G and 40G cables in lengths of 1 m, 3 m, 5 m and 7 m compliant with ROHS, MSA and EMI/EMC.
Interested in buying or want to check the technical specification? See the available DAC cables in our store. In this article, however, we explain how these cables work in more detail.
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