Active vs. Passive Direct Attach Cables (DAC)

Direct Attach Cable (DAC) | FluxLight

SYNOPSIS (…if you don’t have time to read the whole thing)

  • An Active DAC contains electronics for signal conditioning.
  • A Passive DAC does not contain electronics for signal conditioning.

If a switch provides signal conditioning integrated into a port, then a Passive DAC is used. A Passive DAC costs less than an Active DAC, but the upfront cost of the switch is more.

If a switch does not provide signal conditioning integrated into a port, then an Active DAC is used. An Active DAC costs more than a Passive DAC, but the upfront cost of the switch is less.

Why choose one implementation over the other? Keep reading …

INTRODUCTION

In the “less-than-10Gbps” era, networking equipment was typically designed with a number of SFP transceiver plug-in slots and a bunch of RJ45 ports capable of running at 10/100/1000Mbps. The SFP slots were usually reserved for transceivers supporting optical transmission. However, in many applications, such as between server and Top-of-Rack (ToR) switches, the cost of fiber-optic transceivers and cables wasn’t cost effective. At 1Gbps, an extremely low-cost alternative is to use a very inexpensive CAT5 Ethernet cable to achieve full-rate 1Gbps interconnect in such cases. Furthermore, SFP transceivers supporting 1GbE over an RJ45 were available (see: GLC-T) when all the built-in RJ45 ports were spoken for.  

As 10Gbps SFP+ transceivers were being defined (circa 2009), the whole equation of copper interconnection changed. The biggest problem was the lack of viable 10GBASE-T SFP+ pluggable optical transceivers. At that time, the technology simply didn’t exist to package this interface within the cost, size and power constraints of the SFP+ form factor. Recognizing this issue, the SFP+ Multi-Source Agreement (SFP+ MSA, see: SFF-8431) included an appendix, APPENDIX E, defining the specifications for SFP+ based Direct Attach Copper (DAC) cables. Support for these cables, referred to as “10GSFP+CU”, is listed as OPTIONAL in the MSA document.

It should be mentioned that in 2017, almost 8 years since the release of the SFP+ MSA, 10GBASE-T RJ45 SFP+ modules that meet the size and power requirements of the MSA, and are cost effective, are finally available (see: SFP-10G-T). The focus of this article, however, is 10GSFP+CU Direct Attach Copper (DAC) cables.

DAC TECHNOLOGY

Background

Appendix E of the SFP+ MSA provides specifications for DAC cables in the SFP+ form factor. As stated in the MSA:

“10GSFP+Cu cable assemblies are effectively constructed out of a pair of SFP+ modules with the OE components replaced with copper cabling as shown in the Figure (below).”

DAC Diagram | FluxLight

This diagram shows the minimal implementation required for a passive DAC. The transmit and receive pairs are simply connected directly from the edge connector of the SFP+ to the wires within the cable. The only thing added are the DC blocks to protect the host switch from any harmful voltages that may have been coupled onto the cable.

One might ask, “is that all there is to it…just connect up the wires and you have 10Gbps over a copper cable?” The answer is…No, it’s not that simple. The transmit and receive signals crossing the SFP+ interface are designed only to travel a few inches before they are converted to a signal designed for long-distance transmission (typically optical). To achieve successful transmission beyond that few inches, a number of signal processing steps must be taken including:

  1. Signal Amplification: Knowing the signal will travel some meters over a copper cable will attenuate the signal considerably, the signal power must be raised substantially from the low-power “machine level” signal.
  2. Signal Equalization: Since 10Gbps signals require hundreds of MHz or even GHz of signal bandwidth, compensation for issues of time delay and/or phase delay must be introduced. This basically involves the pre-emphasis of that portion of the signal that will be skewed during transmission and, reciprocally de-emphasized upon reception. This process is referred to as equalization.

Clearly, these functions are not included in the diagram above from the SFP+ MSA. So, if this signal processing is necessary, where is it done? Where the signal processing is done is the difference between a Passive DAC and an Active DAC.

Active vs. Passive DACs

If a DAC contains the signal conditioning circuitry, it is an “Active DAC”. If the DAC does not contain this circuitry, it is a “Passive DAC”. For a Passive DAC to work, the host networking equipment must support the signal processing functions. When an SFP+ is inserted, networking gear compatible with Passive DACs reads the module type, and the signal conditioning is activated only when a Passive DAC is detected. In all other cases, standard “machine level” signals are sent across the SFP+ edge connector.

More than 80% of all SFP+ equipped switches on the market today support Passive DACs. The remaining less than 20% perform no signal conditioning and, therefore, must be equipped with substantially more expensive Active DACs if copper interconnection is desired. However, these switches, since they do not contain the signal conditioning on each SFP+ port, are considerably less expensive.

Which Way To Go….

The decision regarding which way to go, a switch supporting Passive DACs or one requiring Active DACs, depends on several factors including:

  • Fiber Connectivity – If the application of a given switch is expected to utilize primarily/exclusively fiber SFP+ transceivers, the added cost of a switch that supports Passive DACs may not be warranted. On the other hand, if SFP+ ports are expected to be equipped primarily with DACs, the higher cost of the switch will be quickly recovered by the savings using low-cost Passive DACs versus higher priced Actives.
  • Length of DAC Spans – If the primary application of the use of DACs is limited to about 7 meters or less (e.g., within a rack or between adjacent racks), a solution supporting Passive DACs is indicated. However, if most of the DACs deployed with the switch are greater than 7 meters in length (e.g., ToR to EoR), Active DACs would be required regardless of the switch, so a low-priced Active-DAC-only switch may be the right choice. In fact, if a switch supporting signal conditioning detects an Active DAC inserted, it will not use its internal signal conditioning circuitry but will leave that function to the Active DAC itself.

FLUXLIGHT 10G DAC SOLUTIONS

FluxLight provides a full range of DACs, both passive and active, for all major and many minor brands of switches. DACs are available in lengths from 0.5m to 15m and are available at speeds of 10Gbps and 40Gbps. The following is a sample of the popular DACs offered by FluxLight:

PART NUMBER BRAND COMP. LENGTH SPEED ACTIVE/

PASSIVE

SFP-H10GB-CU1M Cisco 1m 10G PASSIVE
SFP-H10GB-ACU15M Cisco 15m 10G ACTIVE
QSFP-H40G-CU5M Cisco 5m 40G PASSIVE
QSFP-H40G-ACU10M Cisco 5m 40G ACTIVE
QFX-SFP-DAC-5M Juniper 5m 10G PASSIVE
J9283B HP 3m 10G PASSIVE
CAB-Q-S-5M Arista 5m 40G-4x10G PASSIVE

SFP+ and QSFP pre-terminated cables that use fiber optic rather than copper cables are also available from FluxLight. These assemblies are referred to as Active Optical Cables or AOCs.

FOR MORE INFORMATION

For more information about FluxLight’s 100% OEM-compatible DAC cables, please visit:

FLUXLIGHT DACS

If you would like to contact one of our experts about DACS, AOCs or our complete line of Optical Transceivers, please call 888-874-7574 or email: sales@fluxlight.com or quotes@fluxlight.com

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