The dark secrets of the "bright budget"

    How to calculate the budget of optics (light) and choose the best optical tap (coupler) for work.



    Using IXIA passive optical tapes , engineers often ask to check the optics budget in order to select the tapas with the most suitable division factor, since the introduced attenuation also depends on the division coefficient.





    So what is this “budget of light”, and how is it considered?


    At either end of the optical link is a transceiver. A transceiver both transmits data using light at a given power level and receives light. When receiving, the light should be at a sufficient level to support the Bit Error Rate (BER) link. If the light level is too low, the error rate will increase. Most transceiver documentation indicates the required light level (photodetector sensitivity - Receiver Sensitivity) for this BER. For example, a BER better than 10 ^ -12 (ten at minus twelfth) should have a power value better than -24 dBm. If the transceiver has an emitter power of 4 dBm, then this means that the total attenuation along the transmission path should be less than 28 dB. This is the Maximum Attenuation Value.


    The actual attenuation is the sum of the losses caused by the fiber itself, any connectors on the path (including patch panels) and any passive optical tapes that are present in the path. In addition, there are chromatic dispersion effects that can occur during fiber deflections and further degrade performance — they should also be included in any attenuation calculations. The summation of the losses is called the attenuation budget, and the attenuation budget should always be less than the Maximum Attenuation Value.


    Let's look at all the parameters that make up the calculations of the light budget (optics).


    1. Transceiver attenuation values — emitter power and receiver sensitivity levels vary depending on the technology and manufacturer. Below are the emitter power and receiver sensitivity of one of the leading manufacturers of 1G and 10G transceivers.



    Please note that the attenuation value must be less than the difference between the worst average transmitter power and receiver sensitivity. As can be seen from the table, the characteristics of the transmitted power can vary widely (often more than 6 dB). How much the real power value is closer to the best theoretical value depends on the manufacturer and the quality of the transceivers.


    2. Cable attenuation - cable losses depend on both wavelength and light transmission mode. The difference between a high-quality fiber supplier and a low-quality cable supplier can also be significant. Losses will also vary depending on the distance traveled.


    Typical Losses:


    Multimode - (850 nm) 3dB / km - OM4 cable closer to 2.5dB / m, and OM3 - closer to 3.5dB / km;
    Single mode - (1310 nm) 0.4dB / km (type OS 2);
    Single mode - (1550 nm) 0.3dB / km (type OS 2).


    Thus, in data centers using a 100 m fiber optic cable, losses for a multimode are significant, but for a single mode this is not so important. Conclusion - if in the data center there are problems with the budget of optics (attenuation), use single mode. Also apply higher quality (OM4) to minimize attenuation.


    3. Connector attenuationConnector loss depends on how and where the fiber is connected. The smallest attenuation occurs in the case of high-quality connection of the connector with the optical cable. Typical Losses:


    Multimode - (850 nm) 0.3 dB;
    Single mode - (1310/1550 nm) 0.2 dB.


    “Field” connections, which are made in haste, introduce more attenuation, possibly up to 1.0 dB. Conclusion - if possible, avoid poor-quality connections in a hurry and minimize the number of connections throughout the tract.


    4. Splicing loss - sometimes it is more advantageous to spliced ​​fiber than to use connectors. In this case, the multimode attenuation is similar to mechanical connectors (about 0.3 dB), although splicing by welding, which is often used for a single-mode cable, can be better than 0.1 dB. "Welding" is also preferable in extreme conditions of use.


    5. Fiber cleanliness - a single speck of dust can seriously degrade the performance of optics. Therefore, it is important to have protection and plugs on the connectors, transceivers and tapas.


    6. Other attenuation - for single-mode fiber optic lines at long distances, chromatic dispersion is allowed due to the fact that not one wavelength is emitted, but a spectrum of slightly different wavelengths that propagate at different speeds. This effect can usually be ignored in data centers.


    Once the total budget of light (optics) is calculated, the correct division factor in tapas can be selected based on the insertion loss.


    Examples:


    Consider two working examples in a data center.


    Case 1: A 5 meter line inside the rack using 1G multimode fiber.


    A. The value of light attenuation is 9.00dB.
    B. The attenuation of the cable is 0.03dB.
    C. Attenuation of the connectors 2 * 0.3dB = 0.6dB.
    D. Total Attenuation ABC 8.37dB.


    In this case, there is 8.37 dB “light budget” available for use with any of the tapes. The attenuation introduced by Ixia Flex Taps is indicated on the datasheets on the manufacturer’s website. In this case, the insertion attenuation of multimode tapes 1G with a division ratio of 50/50 is 4.5 dB (excluding connectors). So, in this case, no problems are expected when using the standard 50/50 tapas.


    Case 2: 100 meter line using a 10G single-mode cable.


    A. The value of the attenuation of light 4.40 dB.
    B. Cable attenuation 0.04 dB.
    C. Attenuation of the connectors 4 * 0.2 dB = 0.8 dB.
    D. Total Attenuation 3.56 dB.


    In this case, when using optical tapas, there can be only 3.5 dB of insertion attenuation. 10G single-mode tap attenuation of 3.7 dB, which is higher than the desired limit. Therefore, in this case, it is advisable to use tap with less attenuation. The single-mode Ixia Flex Tap with a 60/40 division factor contributes 2.8 dB to the link, so using this particular tap reduces the risks of future problems.



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