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Use High Speed Direct Attach Cable for Data Center Interconnection

May 25, 2016

High speed direct attach cable (DAC) assemblies, or twinax cable assembly used in data centers. It provides a lower cost, higher density alternative cabling solution for high speed 10G-400G interconnect to fiber optic transceivers. To further comprehend their features, let us walk you through some insights in respect to what DAC cables are and power high speed data center interconnect.

What Is DAC Cable?

What is DAC cable? A direct attach cable, short for DAC cable, is a type of factory terminated twinax cable that connects directly into “transceivers” and can be brought in several lengths for short distances of up to 15 meters. These cables consist of fixed lengths of shielded copper coaxial or fiber optic cable with fixed pluggable transceivers on either end with varying gauges from 24 to 30AWG. DAC cable can achieve interconnection up to 15m via twinaxial copper cable, and reach 100m over active optical cable at 10Gbps, 40Gbps or beyond.

There are three common DAC cable Types:

Passive vs Active DAC Cables

High speed direct attach cable can be classified into direct attach copper cable(DACC) and direct active optical cable (DAOC). Read AOC cables for the detailed information.

DACC cables can be either passive or active. Passive DAC cable contains no electrical components, so it has minimal power consumption of <0.15w, but the linking distance is limited to 7m. While active DAC cable contains electrical components in the connectors that can boost signal levels, allowing reaching greater distances (5m or more) via copper media and ensure better transmission quality. This makes active copper cables slightly more expensive and consumes more power than direct attach passive copper cables.

high speed dac cables

Figure 1: Passive or active DAC cables

Direct active optical cable bonds the fiber connection inside the transceiver ends, creating a complete fiber cable assembly much like a direct attach copper cable, but with a 15 meter reach capability, 2x improvement over a passive DACs limit. Also, the additional circuitry of the ACC does increase its power consumption to around 0.5-1.0 W, on average.

Direct Attach Cable vs. Optical Transceiver Module

DAC, AOC cables and optical transceiver modules are used to connected switches with one another when creating a stack or switches to routers or servers. Optical transceiver modules need fiber cables to be plugged into the transceiver module, while DAC and AOC cables are the fixed cable assemblies with different form factor connectors on both ends. Read DAC AOC cables comparison for detailed information.

Reach Cable Type Power Consumption Weight Bend Radius Application Price
High Speed Direct Attach Cable Around 1m-100m Twinax copper cable or fiber optic cable 0.15w-1w DAC:Heaviest AOC: Lightest 24 AWG-30 AWG ToR, Adjacent racks, Middle of Row, End of Row, Zone to Zone Low
Optical Transceiver Module Per TIA/IEEE standard (Up to 160km) Twinax copper cable or fiber optic cable Up to 4.5w Depend on transceivers & cabling used Depend on cabling used ToR, Middle of Row, End of Row, Zone to Zone High

According to the table, it’s obvious that high speed direct attach cable costs less, consume less power. But it usually has a limited distance compared to transceiver attached with cabling. Thus for any short range connection measuring from 5m to 100m, a better performing DAC cables offer easier and more affordable solution. But when the distance range is beyond 100m, optical transceivers are recommended.

Power High Speed Data Center Interconnect

High speed direct attach cables comes in many different configurations linking both new and older equipment, which optimize costs at every connection point. DAC cables are used in 32-to-56 port top-of-rack switches supporting up to 128 links (4x25G times 32 ports). Usually higher data rate DAC cable is backwards compatible to lower data rate DAC twinax cables. For example, 10G SFP+ DAC cables is backward compatible to 1 Gb/s.

DAC cables in different form factors are as follows:

Shop Passive or active dac cables at FS.com.

FS direct attach cables

Figure 2: FS all series of Direct Attach Cables

What's new - 400G QSFP-DD DAC Cables

400G direct attach cables for short-distance DCI (Data Center Interconnect) have been mass-produced and put into market, which includes 400G DAC and 400G AOC. For more tech information about 400G DAC cables.

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