Direct attach cables, or DAC, are cable assemblies that are terminated with connector modules (most commonly SFP+, XFP or QSFP+) at both ends. As such, DAC plug into the same ports as ordinary transceivers to support Ethernet and Fibre Channel (FC).
DAC are available in various lengths to suit customer need, but are often used in short reach interconnection applications where high bit rates are required such as within the data centre between servers, switches and storage. DAC can be found in inter-rack and intra-rack applications because they use the twinax format, which is the same as coax, but with two conductors instead of one.
DAC remain popular and are widely used in data centres globally because they offer cost and power savings, compared to alternatives.
What’s the difference between active and passive DAC?
Passive and active DACs appear visually the same. However, there are a number of important differences:
- Active DAC contain electronics for signal conditioning, whereas passive DAC do not contain electrical components.
- Active DAC can perform certain active functions that previously took place inside the switch. In contrast, passive DAC do not consume power and are seen by the switch or device as a transparent transmission medium.
- Generally speaking, passive DAC operate over short distances (5m or under) and active DOC operate over longer distances (5m and over). This is due to the chip generally performing signal boosting and equalisation which means that the active cable can be longer.