Direct Attach Vs. Active Optical Cable – Differences, Applications, and Benefits

A direct-connect copper cable is a cable with fixed connectors at both ends. In the networking industry, these cables are usually referred to as DACs. Active copper cables and passive copper cables are the two types of direct attach cables. In general, the connectors of a DAC cannot be separated from the copper cables they connect. DACs transmit data in the form of electrical signals because they do not support the conversion of light to electricity or electricity to light. In other words, these cables have nothing to do with the optical transmission of data and fiber optic cables. Now let's talk more about the different types of DACs.

Passive copper cable

It is a directly connected cable that contains a shielded high-speed differential copper cable for data transmission. A passive copper cable does not contain any chips or ICs on the circuit boards on either end and therefore does not perform any signal processing during data transmission. In data centers, passive copper cables are often used in short-distance applications, e.g. to connect network components located in the same cabinet or installed in adjacent cabinets. Extremely low power consumption, high reliability and low cost are some of the most notable features of passive copper cables.

Most common uses of DAC cables

10G Ethernet connections

A large number of DACs in any data center are used just for connecting the 10G ports on switches, routers, servers or other devices. In the current scenario, 10G ports are perhaps the most common, and DAC cables provide a fast, easy and reliable connection between these ports. An SFP+ to SFP+ Plus Direct Attach Cable is all you need to instantly establish a 10G Ethernet connection. However, this solution is only suitable for shorter, 1 to 2 meter long connections. For a longer connection, you need an SFP+ optical transceiver with a suitable fiber optic cable type.

40G Ethernet connections

Most devices have QSFP+ size ports for data transmission over 40G Ethernet connections. The ratio between 10G and 40G ports in data centers and networks is rapidly decreasing. DAC cables provide a direct connection between 40G ports to establish direct Ethernet connections. In addition, these cables can also be used to break up a 40G port into four 10G Ethernet ports. We offer DAC breakout cables for such applications.

100G Ethernet connections

DAC cables provide direct connectivity between 100G and 40G ports. In addition, DAC breakout cables are also available, which can be used to break up a 100G port into four 25G ports. Both active and passive cables are suitable for such a connection. However, this solution is only suitable for short distances (

Active optical cables

Active optical cables or AOCs are fitted with an electrical connector at both ends. The electrical connectors of these cables contain all the necessary components required to convert electrical data into optical data. These cables receive electrical input and convert it into light signals that are transmitted over a fiber optic cable. The main purpose behind the invention of AOCs was to provide the industry with an alternative to direct-connect copper cables to support high-performance computing applications. AOCs offer many advantages, such as;

- These cables offer a higher bandwidth and a greater range
- AOCs ensure reliable and secure data transmission
- These cables are better in terms of EMI/RFI exposure
- Active optical cables weigh less than the DACs

InfiniBand, LAN, proprietary interconnect networks and OmniPath are some typical applications for active optical cables. AOCs can enable connection distances of up to 100 meters, but are more common in data centers for server-to-ToR type applications where connection distances are typically

Available types of active optical cables

Active optical cables are becoming increasingly popular for 10, 40 and 100G applications in the data center. AOCs are differentiated by connector type, cable type and cable length. Some manufacturers even offer customized active optical cables. On the market, these cables are available in a variety of connector configurations, such as

- 10G SFP+ AOC
- 25G SFP28 AOC
- Breakout 40G QSFP+ to 8x LC AOC
- 100G QSFP28 AOC
- Breakout 100G QSFP28 to 4x SFP28 AOC
- 120G CXP AOC
- 40G QSFP+ AOC
- Breakout 40G QSFP+ to 4x SFP+ AOC
- 56G QSFP+ AOC

Increasing use of AOCs in data centers

Active optical cables are far more powerful than direct-attach cables. Many data center operators have already switched from copper to AOC cables to ensure an optimized switching fabric between various 25G/40G devices such as routers, switches and servers. Active optical cables are seen as a viable solution for connecting ToR switches to aggregation switches. In addition, these cables are also used to connect ToR switches to storage subsystems as they have a longer reach than DACs.
Below are the three most common network configurations where system operators prefer AOCs over other options;

Scenario 1: An AOC cable can establish a direct connection between two switches, as shown in the following figure;

Scenario 2: An active optical breakout cable can be a highly cost-effective way to achieve connectivity between neighboring racks and within individual racks. In the illustration below you can see how an active optical breakout cable 40GbE QSFP+ to 4 x SFP+ is used to connect 1 x 40G QSFP+ port to 4 x 10G SFP+ ports.

Scenario 3: A combination of OEO converters, single-mode patch cables and AOC cables can be used to create a suitable long-distance communication link between two switches that are far apart. The ability to mix and match different types of fiber optic cables remains.

How to select AOC and DAC

Direct-attach cables are less expensive but cannot be used for connections longer than a few meters. However, due to their reliability and versatility, DACs are ideal for short distance applications where the length of the connections is between 1 and 5 meters. Direct Attach Cables are also known for being extremely power efficient.

AOCs, on the other hand, are characterized by a long transmission distance. Connections of up to one meter are possible with these cables. Customer-specific AOC cables for connections of up to 300 meters are also possible. However, such long active optical cables are more expensive and consume more power.
Active optical cables are less bulky, susceptible to electromagnetic interference and can be bent at steeper angles. For this reason, active optical cables are preferred in data centers.