DAC vs AOC

In data center networking, performance is of high importance. Structured network infrastructures in which all components of the infrastructure function as separate entities are able to offer high speeds (up to 400Gbps). However, they are expensive in terms of procurement, installation, and maintenance. This increased the need for cheaper and high-performance solutions. As a result, Direct Attached Copper Cables (DAC) and Active Optical Cables (AOC) were developed.

 

DAC and AOC assemblies consist of a cable with a pluggable transceiver attached at both ends. These cables come in a variety of lengths and are available with various transceiver form factors (SFP; SFP +, QSFP). Since the cables already have a transceiver on both sides, installation is easy. DAC is available as both active and passive, while AOC is only available as an active solution.

 

Passive DACs are the most basic form of direct cables. The cable used for a passive DAC is a twinax copper cable. The thickness of this cable can vary, the longer the distance to be bridged, the thicker the cable will be. Passive refers to the fact that the signal that is sent is not electronically amplified by the transceivers. The transceivers contain Electrically Erasable Programmable Read-Only Memory (EEPROM), which contain specific information about the transceiver. They don’t however amplify the electronical signal, causing power consumption to be low (0.15W or less). The maximum cable length for a passive DAC is approximately 5-7 meters.

 

Just like a passive DAC, an active DAC uses a twinax coper cable to connect transceivers at both ends. However, in an active DAC the electronic signal is amplified by integrated circuits within the transceiver. This amplification in active DACs makes it possible for the signal to bridge a distance that is approximately 2 times longer than that of passive DACs. This means that an Active DAC cable can be used for distances up to around 15 meters. Since components within the transceivers now have an active role, the power consumption of an active DAC is higher and lies between 0.5 to 1W.

An Active Optical Cable (AOC) is only available as an active assembly and not as passive. In an AOC the twinax copper cable, which is used in DACs, is replaced by a full duplex optical cable. These optical cables are available as either single or multimode. An AOC still uses optical transceivers on both ends of the cable. Since AOCs use optical cables rather than twinax copper cables, they allow for higher bandwidths to be transmitted across longer distances. As AOC uses optical transceivers instead of regular transceivers the power usage of the AOC is higher (around 1 - 2W) and will be more expensive in general.         

DAC and AOC cables provide a lot of advantages such as the simplicity of their “plug and play”, lower costs, and lower power consumption. However, at this point they are limited by the maximum distance they are able to bridge (100m for AOC). Because of this distance limitation, they will not yet replace structured cabling entirely.

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