QSFP28 optical modules with the same 100G rate, SR4, LR4, and ER4 are quite different in price, fiber usage, and transmission distance. Choosing the wrong model will waste fiber resources at best, or the link will not light up at worst. In fact, you only need to look at three things when selecting a model: link distance, optical fiber resources, and light and receive margin.

Check the distance first, then look at the number of fiber cores

SR4 uses multi-mode, and OM4 is typically within 100 meters. It uses 8-core MPO for parallel transmission, which is suitable for short-distance interconnection in rows and equipment rooms; LR4 uses two-core single-mode, four-wavelength division multiplexing, and the typical distance is 10 kilometers; ER4 also has two-core single-mode, and can generally reach 40 kilometers. Prioritize SR4 or directly connect copper cables in the computer room, and then consider LR4/ER4 between buildings and across campuses. Don't spend the budget of long-distance models on links of tens of meters.

Calculate the attenuation margin first for single mode long distance

  • The attenuation budget is equal to the fiber attenuation plus the splice point plus the connector loss. It is usually recommended to leave a margin of more than 3dB in engineering;
  • The attenuation of G.652 single mode in the 1310nm window is generally estimated to be about 0.35dB per kilometer;
  • ER4 has high luminous power, and an optical attenuator is required for direct connection in short links to prevent overloading of the receiving end;
  • Before choosing ER4 for old optical fibers, you should first use an OTDR to measure the total channel loss. Don't just guess based on distance.

Compatibility is as important as writing code

The 100G module is more sensitive to the switch model. Before placing an order, it is recommended to write the code in advance according to the device brand and check the firmware version. The entire series of EB-LINK optical modules supports free coding and customization according to brands such as Cisco, Huawei, H3C, ZTE, Ruijie, etc., and is tested one by one before shipment. For inter-building 100G links, the method of testing the channel first and then determining the model can be referred to100G optical fiber interconnection upgrade between buildings in a large data center in East China and Data center high-speed interconnection solution.

To sum up in one sentence: compare DAC and AOC within 100 meters, choose SR4 for short-distance multi-mode scenarios, directly use single-mode LR4 for more than 300 meters, and only use ER4 for urban scenarios of 40 kilometers. According to these three steps, you will basically not choose the wrong 100G model.