Issue link: http://www.epageflip.net/i/1043933
November/December 2018 I 33 FEC (forward error correction). Introduced in the 1960s, RS-FEC is still one of the most prevalent channel coding techniques used today. It is a digital mechanism 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. Direct Attach Copper Cables Shown in Figure 3, DAC cables are an alternative when the cable itself is made of copper instead of optical fiber. A DAC may be passive to provide a direct electrical connection or active when signal processing circuitry is integrated in the DAC built-in connectors. 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, AOC cables cost more. When comparing AOC cables to traditional fiber optic cables connected to pluggable optics, AOCs provide simplicity of installation without the need to consider intercon- nection loss, and they eliminate 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 explained earlier. ANSI/TIA-942-B addresses the use of direct attach cabling in clause 7.3.4 and recommends the following: • Cable lengths for direct attach cabling between equipment in the EDA should be no greater than 7 m and should be between equipment in immediately (i.e., not multiple) adjacent racks or cabinets in the same row. • Direct attach cabling within distributors (MDs, IDs, HDs) and entrance spaces should be constrained within the distributor or entrance space and within a contiguous row. The corresponding ISO/IEC data center standard, ISO/ IEC 11801-5, restricts the use of direct-attach cords within a distributor, within a single cabinet, frame or rack, and be- tween adjacent cabinets, frames, or racks in the same row. Operational Challenges Because AOC and DAC cables do not provide test access to the actual fiber or copper cabling, traditional media test and certification tools cannot be used to certify the cable. Experience indicates a fairly high rate of failure for AOC and DAC cables to the extent that if a user suspects that a faulty cable has been installed, it is thrown away without the technician knowing whether it was really the source of the problem. At up to $1000 per cable, the cumulative cost to the user or data center can potentially be budget draining. Therefore, a test tool that can accept dual SFP/ QSFP transceivers and generate and analyze traffic must be used. Testing AOC and DAC is a critical step to ensure that any issues with network performance are not due to the AOC/DAC cable or its installation. Consider that it is costlier to troubleshoot a faulty cable once installed than to test it upfront (e.g., it is necessary to trace and locate the far end). Causes of AOC/DAC cable failure include simple FIGURE 3: DAC cable. A massive amount of information is exchanged within data centers and a key requirement is the capacity to scale.