What is Direct Attached Cables(DAC)

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Direct
Attach Cables(DAC) are a form of shielded high-speed cable with SFP connectors
on either end (SFP, SFP+, QSFP, QSFP+ modules). These are generally used to
connect switches to routers, storage or servers within the same rack or
adjacent racks. DAC cables are becoming increasingly popular, not just within
data centers but within wider areas of the industry, as they offer a very cost-effective
(almost half than using transceiver and fiber cables separately), efficient and
easy way of making these connections. 
The modules cannot be removed from the cable and so all DAC cables are a
fixed length. This is one of the drawbacks of using DAC cables compared to
using a combination of fiber cables and optical transceivers. Mainly there are
two types of DAC cables and those are

 

·Direct
Attached Copper Cables(DAC cables)

·Active
Optical Cables(AOC cables)

 

Direct
Attached Copper Cables (DAC cables)

 

DAC
cables are made from Twinax copper cables and are factory terminated with
modules that provide an electrical connection directly into active equipment.
Since high-speed cables are sensitive to electromagnetic fields, the shielding
is an important quality factor for the copper cables. This is written on the
cables in the form of AWG ratings (American Wire Gauge (AWG) system which
defines wire size in diameters and it’s showing the quality of shielding) like
28AWG and 30AWG. When AWG values increases it will increase the protection
against electromagnetic waves. The longer the distance, the higher the AWG
rating should be.

 

There
are different types of high-speed DAC cables; direct and breakout, passive and
active. A direct connect DAC cable will connect one port/line card to another (overall
single cable). In fact, direct connect DAC cables are the most commonly seen in
data centers. A breakout DAC cable will allow one port to be connected up to 4
other ports (DAC cable where there is a QSFP+ transceiver with 40Gbps transfer
speed can be terminate into 4 x SFP+ cables where each terminal will provide
10Gbps transfer speed).

 

The
DAC cable’s termination can be either SFP transceiver or QSFP transceivers. So
depending upon the characteristics of these transceivers the DAC cables
properties will also change.

 

Even
though we have classification direct and breakout cables, the main
differentiation is based on the classification Passive and Active cable
structure which is a vital part in defining the cables to be used.

 

Passive
DAC cables

 

The
most basic version of this cable is the ‘Passive Assembly’ where the data
cables are directly connected to the Transmitting and Receiving signal. Hence,
it has a minimal power consumption of no more than 0.15w and generally for
short reach up to 7meters. This is the most commonly used ones also due to the
reason that DAC cables are used mainly for the interconnection between devices
in the same rack or adjacent racks.

 

 

Active
DAC cables

 

The
active cable version contains more sophisticated electronics where the
transmitting power is amplified to give a better data-pulse. You can see this
in the product naming like ACU and not CU (e.g. SFP-H10GB-ACU5M vs
SFP-H10GB-CU5M). An active DAC cable will offer slightly longer transmission
distance than a passive DAC cable as it uses electronics within the modules at
each end to boost the signals. It allows a little longer transmission distance
up to 15m and ensures better transmission quality. whereas it has a higher
price and consumes more power around 0.5-1w.

 

As
a summary of DAC cables, we can say that it’s used to establish the connection
between devices when the speed is at 10G we will choose SFP+ to direct DAC cables
(passive or active depends on distance). When the speed rate is up to 40G we
will choose QSFP+ DAC cables (passive or active depends on distance).