Getting The Most Out of 400G AOC/DAC Cables
In today’s data-driven world, high-speed data transfers and reliable connectivity are critical to the success of every data center. As data rates continue to rise, it is vital to ensure the integrity of data transmission across networks. Essential components in that data transfer within a rack, from rack-to-rack, or from network-to-network are AOC/DAC/AEC cables (Active Optical Cables, Direct Attach Cable, Active Electrical Cable). Although they are critical elements, 400G cable assemblies have been untestable – until now. Bit Error Rate (BER) testing of up to 400G AOC/AEC/DAC cables is an indispensable way to increase the uptime of the network and make troubleshooting easier.
BER testing is a measure of the error rate in digital communication systems. It is the number of erroneous bits that are transmitted and received per total number of transmitted bits. BER testing is crucial for verifying the performance and reliability of high-speed data transmission systems, including AOC/AEC/DAC cable assemblies. These cables are designed to transmit data at high speeds. Normally, even a small number of bit errors can have a significant impact on the integrity of the data. However, at 400G, errors are expected (see below).
The main goal of BER testing an AOC/DAC/AEC cable, at the most basic level, is to verify that it’s working. If it’s not, then it can be replaced with another cable (that has been tested) with confidence, rather than replacing it without knowing if it is truly the root cause of the issue. If it is working, then a BER test can ensure that the data transmitted across the cables is accurate and reliable. By measuring the BER of the cables, engineers can identify any potential errors or issues that may be present in the system. BER testing can also help detect and prevent data loss or corruption, which can lead to costly downtime and potentially devastating consequences for businesses.
An important thing to note is the distinction between direct straight cables and breakout cables. A breakout cable is when a common endpoint on one pluggable splits, or “breaks out,” into multiple endpoints in what feels like a one-to-many configuration. An example of this is a single physical connector on an Ethernet switch that gets broken out to several different servers. This concept exists for both active (AOC) and passive (AEC/DAC) cables. A practical example is a 4x100G cable with 4 different QSFP endpoints and a common QSFP-DD. Testing such cables implies testing each individual endpoint, which means multiple tests. It is possible to create a single test report containing the results for each breakout using a tool such as Job Manager by VIAVI.
One factor to be aware of with higher rate cables, especially at 400GigE, is that a certain amount of bit errors is normal, and even acceptable. That said, it is essential to understand how many errors are acceptable. The algorithm to compensate for transmission errors is Forward Error Correction (FEC). Although very beneficial, there are error limits beyond which FEC cannot address. As such, BER testing remains critical and where the primary result becomes the pre-FEC BER and where the post-BER result must be error free. Hence, any high-speed BER test needs to incorporate the FEC algorithm element.
A benefit of BER testing in general is that it can help optimize the performance of the system. By measuring the BER at various points in the network, engineers can identify any bottlenecks or weak points in the system. For example, a routine testing methodology can identify if the problem is the server, top of rack (TOR) switch, or the AOC/DAC/AEC cable itself. Engineers can then make the necessary adjustments including replacing the cable, upgrading the hardware, or changing the network topology.
When it comes to 400G AOCs/DACs/AECs, BER testing is particularly important due to the high data transfer rates and customer traffic consolidation. High-speed data links are a double-edged sword. They offer valuable scalability, density, and cost benefits but with more production traffic riding those high-capacity links, the business risk also rises. For example, a problem on a well-used 400G AOC cable could create a lot more angry customers than if a 1G link went down. BER testing is a relatively easy way to identify any potential issues that may arise due to the high data transfer rates, such as signal degradation or noise interference.
In conclusion, BER testing with its pre-FEC and post-FEC components is a critical part of ensuring the performance and reliability of 400G AOCs and AEC/DACs in data centers. By measuring the BER of these cables, engineers can identify any potential issues, optimize performance, and prevent data loss or corruption. BER testing is a crucial step in the cable deployment process, and neglecting it can lead to costly downtime, network disruptions, and potentially devastating consequences for businesses. To learn more, download our application note on AOC/AEC/DAC testing.
