Direct Attach Cables Definition

In today’s data center, more bandwidth is needed to support the ever-increasing networking applications that require high data-rate transmission at growing amount. Direct attach cables have been designed as a cost-effective and low power consumption solution for such high-density networking connectivity. This article puts its focus on some basic information of direct attach cables, such as the definition, classification and the great use in 40GbE with QSFP modules.

Direct Attach Cables Definition

Direct attach cable (DAC), a kind of optical transceiver assembly, is a form of a high-speed cable with “transceivers” on either end used to connect switches to routers or servers. DACs are much cheaper than the regular optics since the “transceivers” on both ends of DACs are not real optics and their components are without optical lasers. In storage area network, data center, and high-performance computing connectivity, they are a preferable choice for their low cost, low power consumption, and high performances.

Direct Attach Cables Classification

Different standards exist when classifying DACs.

According to the material of cables used, DACs fall on two kinds: direct attach copper cables and active optical cables. First look at the figure below to know about the outlook information about them.

  • Direct Attach Copper Cables—They are designed in either active or passive versions. Both are widely used in the telecommunication market. For instance, Cisco 40G QSFP+modules deploy both active copper cables and passive copper cables. Like QSFP-H40G-ACU10M, FOCC compatible Cisco QSFP-H40G-ACU10M works over active copper cables for the 40-gigabit link and reaches a short distance.
  • Active Optical Cables—AOC is one form of DAC. It uses electrical-to-optical conversion on the cable ends to improve speed and distance performance of the cable while mating with electrical interface standard. Compared with direct attached copper cable, its smaller size, electromagnetic interference immunity, lower interconnection loss and longer transmission distance make it popular among consumers. Cisco QSFP-H40G-AOC1M operates over active optical cables for superior performance in 40GbE (Gigabit Ethernet).

By Ethernet transmission rate standard, 10G SFP+ cables, 40G QSFP+ cables, and 120G CXP cables are available. Take QSFP-H40G-CU1M for example, FOCC compatible Cisco QSFP-H40G-CU1M runs on passive copper cable, and it can support a high data rate of 40Gb/s for a relatively short distance.

QSFP+ DACs Solutions in 40GbE

QSFP+ DACs, as a really low latency solution, is widely used in 40GbE. They have 4 channels in one interface, providing a 3-4X density of XFP, SFP+. Besides, the use of QSFP+ DACs modules can save a lot of money, since they leave out the expensive optical lasers and some electronic components. Becoming increasingly popular in short transmission distances, QSFP+ DACs modules can also provide enhanced scalability and flexibility. QSFP+ DACs is a pretty suitable solution for 40GbE.


Direct attach cables, through aggregation, also provides a low-cost upgrade path for higher speed (100G) interconnects. FOCC offers high-performance and high-quality DAC assembly solutions. Besides 40G QSFP+ cables products (QSFP-H40G-ACU10M, QSFP-H40G-CU1M, and QSFP-H40G-AOC1M) mentioned above, other 10G SFP+ cables and 120G CXP cables products can also be found in FOCC.

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