Using 40G/100G Twinax Cables for Data Center Interconnection

High-speed direct attach cable (DAC) assemblies, or Twinax cable assemblies, are now very common in data centers. The Twinax cable solutions give engineers another option to build a high-speed Ethernet interconnect, which is a lower-cost, denser alternative to fiber optic transmission. This article will give you a better understanding of the characteristics of DAC and high-speed data center interconnects, so let's get started!

What Is a Direct Attach Cable (DAC)?

Direct Attach Cable (DAC) is constructed of Twinax copper and terminated at the factory with a module to supply the electrical connectivity to the active devices. These modules are not removable off the cable, therefore all direct attach cables are a fixed length. Compared to using a combined fiber optic cable and an optical transceiver, which is one of the shortcomings of employing a DAC cabling. The Twinax cables are available in varying gauges from 24 to 30AWG. It can provide interconnections up to a maximum distance of 15m over Twinax copper cables, while active DAC cables can reach a maximum distance of 100m and their transmission rates can reach 10Gbps, 40Gbps or higher.

Four different types of high-speed DAC cables are commonly used including direct and breakout, passive and active. Direct attached DAC cables could link one port/line card to the other. Breakout DAC cables provide a connection from one port to up to four different ports. The active DAC cable will allow a slightly extended transmission distance than the passive DAC cable because it utilizes electronics in the modules at both ends to enhance the signal. Click here to know more about DAC cable.

Figure 1: DAC Cable

Different Types of DAC Cable: Passive vs Active DAC Cables

There are two different types of high-speed direct attach cables, including direct attach copper cables and direct active optical cables. The difference between these two types of cables is the different transmission media, which use copper transmission or fiber optic transmission, respectively. The following will introduce the two different materials of direct attach cable.

Direct attach copper cable (DACC) is divided into active and passive, passive DAC is designed without any electrical components, therefore, its power consumption is only 0.15w, its maximum transmission distance is 7m, active DAC contains electrical components inside the connector, the electrical components inside the connector can improve its signal level to achieve higher signal transmission quality, and its transmission distance is also farther, its transmission distance is generally 5m to 10m, because with electrical components, its power consumption is also higher, and the price is also more expensive.

The direct active optical cable (DAOC) connects the fiber optic connections inside the transceiver to form a completed fiber optic assembly, it is similar to a direct attach copper cable, but it can reach a maximum transmission distance of 15m, which is two times higher than the limit of a passive DAC. In addition, the additional electronics of the ACC bring an increase in power consumption, averaging about 0.5-1.0W.

Figure 2: High Speed Cable Connection

The Difference Between Direct Attach Cable vs. Optical Transceiver Module

Compared to the DAC and AOC cables and optical transceiver modules, they are both used to connect switches to routers or servers to build a transmission link. Optical modules insert fiber optic cables into transceiver modules for data transmission, while DAC and AOC are fixed cables from the factory, and both ends of it are already terminated as different form factors of the transceiver. The next section is a detailed comparison table of high-speed direct attach cable and optical transceiver modules.

This table shows that DAC cable has a shorter transmission distance and lower power consumption than fiber optic module, but its price is also lower. So we can easily see that when the transmission distance is within 7m, we recommend using a passive DAC cable solution, when the transmission distance is between 7-15m, we recommend using an active DAC cable solution, when the distance does not exceed 100m, we recommend using AOC cable. When the transmission distance exceeds 100m, we can only choose a combination of optical modules and fiber.

How to Build a Data Center Interconnection by 40G/100G Twinax Cables

The high speed direct attach cable contains various configurations, all of which can be applied to connect old and new devices, using the high speed direct attach cable to minimize the cost per connection point. The Twinax cables can typically be connected to 32 to 56 port top-of-rack switches, the maximum amount of connection up to 128 links (4x25G times 32 ports). In general, higher data rate DAC cables are backward compatible with lower rate DAC cables. For example, 100G QSFP28 DAC cables are backward compatible with 40 Gb/s QSFP DAC cables.

The use of high-speed DAC cables also brings several advantages.

Maintenance-free: The combination of optical module and fiber requires frequent maintenance by engineers, and its connectors are easily dirty, while the high-speed direct link cable does not have this problem.

Without laser: The optical module has a laser inside, which makes it more expensive, and the laser is easily damaged in subsequent use, which will bring a series of after-sales problems.

Easy to install: Optical fiber installation requires engineers to fuse and splice the optical fiber on-site, while DAC cable termination is already processed at the factory, requiring no additional operations by engineers. It helps Data Centers save on Installation costs

Excellent Performance: It has great exchangeability of integration scheme for the data center to build a short-distance interconnection.

Energy-saving: Its power consumption is so low that the passive cable is low power supply requirements for the port., and it uses copper material for transmission, a material that allows it to have good heat dissipation performance, which can further save energy.

Low Price: The price of high speed direct attach copper cable is much lower than the combination of optical module plus fiber, using Twinax cable can greatly reduce the cost of the whole cabling system, refer to the price of QSFPTEK, 1M 40G QSFP+ passive direct attach copper cable is only $14.

Conclusion

DAC products have the advantages of low cost and low power consumption, but the transmission distance is short, so its application scenario is generally from 1m to 5m. DAC products also have the advantages of simple structure, few parts, and high reliability. It is usually to be used for building short distance Data Center Interconnection, which requires low cost and low power consumption. If you want to know more about DAC Twinax cable, please contact QSFPTEK Technical Support via sales@qsfptek.com. Welcome to get a quote.