The data center is where a massive amount of information is being exchanged and where a key requirement is the capacity to scale. Beyond the need for power, cooling, storage and switching inside the data center is the necessity for practical and efficient cabling. Data centers can roughly be divided between hyper scale, multi-tenant, and private. The use of cables as described in this paper, namely Active Optical Cables (AOC) and Direct Attach Copper (DAC), applies to all three categories, although this topic is especially applicable to hyper scale data centers. This application note covers practical operational considerations such as the validation of AOC and DAC to save time and reduce costs in data centers.
Data Center Architectures
Figure 1 provides an example of a data center and its interconnectivity to the external world. Within the data center, there are a few potential architectures:
Top of Rack (TOR) architecture is where the cabling between switch and server stays within a rack. This has the benefit of reducing the overall amount of cabling with the downside of reduced efficiency in the usage of Ethernet switch ports which are limited to within a rack.
End of Row/Middle of Row (EOR/MOR) configuration is where switch ports are grouped together leading to longer cables. There are two EOR/MOR examples where in one case, cables connect directly between servers and switch ports. In the second case, physical connectivity goes
through a patch panel which provides the benefit of greater connection flexibility with the disadvantage of a higher number of cables.
Figure 1. Data Center Architecture
Active Optical Cables
Active Optical Cables as shown in figure 2 are used in a limited range of interconnect applications in data centers. For high-speed links at 40GE and 100GE, this means using multiple lanes of data over ribbon cables. At 10GE or 25GE, a single lane or fiber per direction is sufficient. An AOC is often based on multi-mode fibers while some (like PSM4) is on single-mode fiber. A key attribute is that AOC employs the same connectors as pluggable optics and performs electrical-to-optical conversions at each cable end. In practice, this means QSFP terminations for 40GE and 100GE and SFP terminations for 10GE and 25GE. The AOC is therefore active and includes transceivers, control chips and modules, in addition to the fiber optics cable. AOC cables are of fixed length, starting at just a few meters with possibilities up to 100 meters or more. Technically, an AOC does not have to comply with an Ethernet interface type although many advertise a certain type in their coded information; table 1 provides a list of potential Ethernet interface types. From the table, RS-FEC stands for Reed-Solomon Forward Error Correction; it is a digital machine designed to extend transmission distance by adding redundancy to a signal which enables code word self-correction at the far-end. The RS-FEC algorithm, when specified for use with a cable, runs on the Ethernet switches and servers found at each end of the physical connection.
Figure 2. AOC Cable
Direct Attach Copper Cables
Direct Attach Copper (DAC) cables as shown in Figure 3 are an alternative when the cable itself is made of copper instead of an optical fiber. A DAC may be passive to provide a direct electrical connection or active when signal processing circuitry is integrated into the DAC built-in connectors. Just as with an AOC, a DAC will be terminated by SFP or QSFP depending on the line rate. As a comparison, AOC cables support longer transmission distances, use less power, and are more lightweight than DAC cables. However, they cost more and optical fibers can be more easily damaged than copper cables.
When comparing AOC cables to traditional fiber optic cables connected to pluggable optics, AOCs provide the simplicity of installation without the need to consider interconnection loss and eliminating the need to clean and inspect fiber end-faces before making a connection. However, AOC cables cannot be used in EOR/MOR configurations
that use patch panels as described earlier.
Figure 3. DAC Cable