Even if you don’t work in a data center environment, you’re likely familiar with the two dominant interconnect media: copper and fiber. Copper cable runs work great up to approximately 10G speeds, at which point losses become significant and the cabling will limit interconnect lengths. At higher speeds, fiber has been a predominant part of the data center interconnect architecture and will continue to be for the foreseeable future. Fiber optic cabling is expensive, but there is an alternative that is being used more often as a replacement for copper in moderate-length cable runs.
The alternative to traditional copper cabling is known as active copper, a form of cabling that uses a transceiver to provide signal transmission across the copper cable medium. These cable runs are an acceptable alternative to fiber in certain situations, including at very fast data rates where losses accumulate quickly.
Active Copper Cabling Options
The three standard interconnect media on a PCB, in assemblies, and between servers/routers in a data center environment are outlined in the table below. All three options have similar topologies for routing high-speed bitstreams between components, both inside a rack-mount unit or between separate units.
If you are working in a data center environment and are looking for a reliable, lower cost architecture for shorter cable runs between rack-mount servers, consider using active copper cable and transceivers. They can support very high data rates on shorter cable runs between neighboring servers using transceivers, and active copper transceivers have the same form factor as fiber transceivers.
Why Active Copper
Active copper cabling will have similar loss per unit length as passive copper cabling. The advantage in active copper cabling is the ability to boost the signal strength so that it can be sent over longer distances and still be registered at the receiver end of the cable link. In this way, the active copper transceiver pair increases the available loss budget so that a longer cabling run can be used; this increase in signal strength and known copper losses (insertion loss) can be used to determine the allowed cable length for active copper.
Loss Budget vs. Length
To determine whether an active copper strategy is more useful than an optical fiber, three pieces of information need to be known when building the interconnect:
- Active copper cable loss per unit length
- Gain provided by the active copper transceiver pairs
- Total loss on the interconnect before and after the transceivers
Some cables will be packaged with a transceiver pair, such as the twinax active copper cabling example product shown below. This type of cabling hooks into a standard SFP connector with a cage along the PCB edge.
Once the signal reaches the end of the active cable run, it may then be routed over a PCB into the receiving chip’s PHY interface. This receiver could be a CPU, GPU, TPU, or specialized compute accelerator. This completes the link between two components on each end of the interconnect.
Looking to the Future of Data Centers
The predominant data center architecture involving active copper and fiber interconnects between servers, routers, and accelerators isn’t likely to change soon. However, the use of active copper and fiber will continue to increase as interconnect data rates also increase. Larger data rate means greater losses at the top end of a channel’s bandwidth, which requires shorter copper cable runs or some other mechanism to reduce loss.
At the next data rate limit (448G), it is still unknown whether the industry will adopt a doubling of data rate through modulation, or through pushing bandwidths to 112 GHz. The current 56 GHz bandwidth limit for 224G PAM-4 has already pushed copper and PCBs to their limit, and even very short runs across a board are being implemented with flying active copper directly into a processor module.
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