Passive and Active Direct Attach Cables - What is the Difference?

Introduction of Direct Attach Cables

Direct attach cables (DAC) are an alternative to the fiber optic transceivers, they eliminate the need of using the transceivers by permanently attaching both ends of the cable with transceivers that can be terminated in the SFP+ slot in the communication equipment such as switches, routers, storage and servers. Figure 1 shows a typical direct attach cable.

 

Direct attach cables are used for smaller distance links, normally direct attach cables are available in lengths of 1 meter, 3 meters, 5 meters, 7 meters, 10 meters and 15 meters. General uses of direct attach cables are connecting the equipment in the same rack, connecting the equipment installed in adjacent racks or connecting the equipment within a mid-sized datacenter. Direct attach cables use both copper and fiber cable assemblies. The decision to use copper or fiber cable is dependent on various factors such as electro-magnetic interference and space availability.

 

Types of Direct Attach Cables

There are two main types of direct attach cables:

 

Copper/Twinax Direct Attach Cable

Fiber Optic Direct Attach Cable

These are further classified as Passive Copper/Twinax Direct Attach Cable and Active Copper/Twinax Direct Attach Cable. Fiber optic direct attach cable is available as Active Fiber Optic Direct Attach Cable only. In the next section, we will compare the active and passive types of direct attach cables and look at their specific uses.

 

Passive Direct Attach Cables

Passive direct attach cables are copper cables with fixed transceivers at both ends, these cables terminate on the communication equipment and provide connectivity between devices. Passive direct attach cables are usually available in lengths of up to 5-7 meters. Passive direct attach cables are thicker and consume more space. They are difficult to manage if a large number of cables are terminating on a single equipment. Passive direct attach cables have length limitations because copper cable cannot be used for longer distance 10G connectivity.

 

Active Direct Attach Cables

Active direct attach cables can be constructed of either copper or fiber with transceivers fixed at both ends. Active direct attach cables are available in lengths of up to 15 meters. Active direct attach cables have lesser thickness than passive direct attach cables primarily due to lesser thickness of the fiber optic as compared to the copper cable used in passive direct attach cables.

 

The primary difference between active and passive direct attach cables is the additional components of active transmitter and active receiver present in the active direct attach cable. On the contrary, passive direct attach cables do not have any active component in them rather they rely on the signals provided to them by the communication equipment.

 

Conclusion:

The choice of using passive direct attach cable or active direct attach cable is purely circumstantial. Below are some pros and cons of using either type of cable.

 

Passive Direct Attach Cables

Pros:

 

Cost effective

Flexible to bend

Cons:

 

Thick – difficult to manage/harness

Shorter length

Electro-magnetic interference can cause packet loss and other issues

Active Direct Attach Cables

Pros:

 

Cost effective

Longer lengths than passive direct attach cables

No electro-magnetic interference in active optical direct attach cables

Thinner – easy to manage and consume less rack space

Cons:

 

Fiber cable cannot bend beyond a certain limit

Higher chance of failure than passive cable due to presence of active component