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's passive optical tapes , engineers often ask to check the optics budget in order to select tapes with the most suitable division ratio, since the attenuation is also dependent on the division ratio.

So what is this “light budget” and how is it counted?

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

The actual attenuation is the sum of the losses made by the fiber itself, any connectors on the way (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 losses is called the attenuation budget, and the attenuation budget must always be less than the Maximum Attenuation Value.

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

1. Attenuation values ​​of transceivers — emitter power levels and receiver sensitivity 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 should 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 actual 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 both on the wavelength and on the light transmission mode. The difference between a high-quality fiber supplier and a poor 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 is closer to 2.5dB / m, and OM3 is closer to 3.5dB / km;
Single mode - (1310 nm) 0.4dB / km (type OS 2);
Single mode - (1550 nm) 0.3 dB / km (type OS 2).

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

3. Attenuation of connectors - the loss of connectors depends on how and where the optical fiber is connected. The least attenuation occurs in the case of high-quality connection of the connector with an optical cable. Typical losses:

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

"Field" connections, which are made in a hurry, contribute more attenuation, possibly up to 1.0 dB. Conclusion - one should, if possible, avoid low-quality connections “in haste” and minimize the number of connections throughout the path.

4. Splicing losses - it is sometimes more beneficial to splice fiber than to use connectors. In this case, the multimodal attenuation is similar with mechanical connectors (about 0.3 dB), although splicing by “welding”, which is often used for single-mode cable, can be better than 0.1 dB. "Welding" is also preferable in extreme conditions of use.

5. Fiber purity - one speck of dust can seriously degrade the characteristics of optics. Therefore, it is important to have protection and plugs on connectors, transceivers and tapes.

6. Other attenuation - for single-mode fiber-optic lines at long distances chromatic dispersion is allowed because 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 light budget (optics) has been calculated, the correct division factor in tapas can be chosen based on the insertion loss.

Examples:

Consider two working examples in the data center.

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

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

In this case, there is an 8.37 dB "light budget" available for use by any of the tapes. The attenuation introduced by Ixia Flex Taps is specified in datasheets on the manufacturer's website. In this case, the insertion attenuation of a multimode with a 1G tap with a 50/50 division ratio is 4.5 dB (excluding connectors). Hence, in this case, no problems are foreseen when using the standard 50/50 tapa.

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

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

In this case, when using optical tapes, there can be only 3.5 dB of insertion loss. 10G single-mode tap introduces a damping of 3.7 dB, which is above the desired limit. Therefore, in this case, it is advisable to use tap with less attenuation. A single-mode Ixia Flex Tap with a 60/40 division ratio puts 2.8 dB into the link, so using this particular tap reduces the risk of future problems.