What’s the Difference between Active and Passive Direct Attach Cables (DAC)?

What are Direct Attach Cables, and Why Are They Important?

Back in the past, when internet speeds were never more than 10 GBPS, networking equipment consisted of many SFP transceiver plug-in slots and a lot of RJ45 ports that had the capacity to run at 10/100/1000Mbps. The SFP slots that were used were mainly reserved for transceivers that supported optical transmission.

But in many applications such as that between server and Top-of-Rack (ToR) switches, the fiber-optic transceivers and cables used did not really make sense from the cost point of view. It was much cheaper to make use of CAT5 Ethernet cables at speeds of 1Gbps. Also, SFP transceivers that supported 1GbE over RJ45 ports could be used only when all of the allocated RJ45 ports were accounted for.  

In 2009, 10Gbps SFP+ transceivers were introduced. At that time the whole concept of copper interconnection underwent a major change. The main issue back then was the unavailability of proper 10GBASE-T SFP+ pluggable optical transceivers. The technologies that were used back then were not capable of packaging this interface in accordance with the power, cost and size restrictions of the SFP+ form factor.

To account for this problem, the SFP+ Multi-Source Agreement came with an APPENDIX E which defined the SFP+ based-Direct Attach Copper (DAC) cables specifications. The support for the DAC cables was classified as “10GSFP+CU”, and was listed in the MSA document as OPTIONAL.

It was only in 2017, over 8 years after the introduction of the SFP+ MSA, 10GBASE-T RJ45 SFP+ modules that it was possible to match the size and power requirements as specified in the MSA document. Keeping this in mind, let’s talk about the 10GSFP+CU Direct Attach Copper (DAC) cables.

DAC TECHNOLOGY

The MSA document defines DAC as follows:

“10GSFP+Cu cable assemblies are effectively constructed out of a pair of SFP+ modules with the OE components replaced with copper cabling”

The transmit and receive pairs on the passive AC are directly connected from the SFP+ edge connector to the wires of the cable. The difference is that the DC blocks are added in order to offer protection to the host switch from all sorts of harmful voltages which could have been coupled into the cable.

But the technology is more complex than simply connecting the wires over the copper cable and getting a data speed of 10 Gbps. For one, the transmit and receive signals that cross the SFP+ interface are capable of travelling just a few inches before they are converted to a signal capable of long-distance transmission. Now, if you want to enable successful transmission beyond just a few inches, you will need to undertake a couple of signal processing steps such as:

1. Signal Amplification: When the signal travels through many meters over the copper capable, it will be considerably attenuated. That is why it is important to increase the signal power much higher from the less powerful machine level signals. It is done through Signal Amplification.

1. Signal Equalisation: The 10Gbps signals need a high bandwidth that runs into hundreds of MHz to as much as a GHz. Hence it is important to account for issues such as time delay and phase delay. Signal Equalisation involves the pre-emphasis of a portion of the signal that gets attenuated during the transmission and de-emphasising it upon reception. 

The main difference between Passive DAC and Active DAC, which is what this article is about, is whether the signal processing is done or not.

Active DAC vs. Passive DAC

The main difference between Active DAC and Passive DAC is that Active DAC makes use of electronics for signal conditioning, while Passive DAC does not make use of electronics for signal conditioning.

Passive DAC is used when signal conditioning integrated into a port is provided by a switch. It costs less than Active DAC, but has a higher upfront cost.

Active DAC is used when a signal conditioning integrated into a port is not provided by a switch. Active DAC generally costs more than Passive DAC, but the switch that comes with Active DAC costs less.

Over 80 percent of SFP+ equipped switches that are available for sale support Passive DACs. The rest of the 20% work on no signal conditioning and hence are only compatible with the costly Active DACs along with copper interconnection. The switches that don’t have signal conditioning on each SFP+ port cost much less.

Which Switch is Better for You – One That Supports Active DAC or Passive DAC?

Whether you choose a switch that supports Passive DACs or Active DACs will depend on two factors…

Fiber Connectivity – If the application for which the switch is required makes use of SFP+ transceivers only, then you may not need the switch to support Passive DACs.  However, if the SFP+ ports are compatible with the DACs, then you are going to save more over the long-term with low-cost Passive DACs compared to the more expensive Active DACs, although the switches may be priced higher.

Length of DAC Spans – What is the primary application for which the DACs are used? If it is limited to within racks or between adjacent racks, which means a short distance of 7 meters or less, then you are better off using switches that support Passive DACs. However, if the DACs are more than 7 meters in length, then Active DACs would make for a better choice. In case the switch supports signal conditioning identifies an Active DAC, then it will not make use of its internal signal conditioning circuitry. That application is left to the Active DAC.

We provide a wide range of DACs, which includes both Active and Passive DACs for all brands of switches, not just the famous brands, but also those that are new or not as well known. Our DACs are available in different lengths ranging from 0.5m to 15m. They are available at a range of speeds from 10Gbps to 40Gbps. Contact us to know more.