Accounting - to the masses! Imposing optical budget

The expression “optical budget” causes deep sadness among technical translators - they say that the English “optical budget” is more correctly translated as the “energy potential of the line” or, at least, as the “optical balance”. Most likely, they are right: tracing paper has never been the best way to translate. However, in this case, they, apparently, will have to accept: “the optical budget” has gone so deeply into the daily life of the people of the telecom, that you cannot replace it with any balance ...

But what is this optical budget and how is it eaten?

Making ends meet

Any oscillation, spreading in a real physical environment with resistance, gradually comes to naught. Upon reaching a certain boundary, our signal dissolves into a sea of ​​noise, and no most sensitive receiver decodes it anymore - this boundary determines the range of this type of communication. Some theoretical, ideal long-range indicators are given in the description of the standards, but it should be understood that in most cases this is nothing more than a “spherical horse in a vacuum,” which can be considered with caution with the hardly achievable maximum.

In order for the calculation of signal propagation to be at least approximately consistent with reality, it must take into account the real characteristics of the transmitter, receiver, and all that is between them. For fiber-optic communication lines (FOCL), this equation is called the optical budget. Let's see how it is composed.





Optical budget calculation scheme



A signal source is characterized by its power, which varies within certain limits; take its maximum value. On the path of optical wave propagation, it will be weakened at the points of cable joint (connectors, optocrosses, welding) and gradually along the entire length of the cable. At the end point, the luminosity of the stream must not be less than the limit threshold of sensitivity of the receiver - otherwise the network will not work. The budget should converge in both directions, if the traffic is duplex in nature. All the parameters related to the line can be considered constants for a certain type of optical radiation - here they are.

Parameter	Multimode, 850 nm	Singlemode, 1310 nm
Line attenuation	3 dB / km	0.4 dB / km
Connector attenuation	0.5 dB	0.2 dB
Attenuation in the weld	0.1 dB	0.1 dB

Calculation examples

Consider the budget of a specific optical line. We consider that a single-mode communication line between a server with an Intel network card and a Cisco Catalyst switch has a length of 10 km, and there are 2 optical crosses on it. Let's agree that hereinafter we will talk about 10 Gigabit Ethernet - from the point of view of optics, the speed of the data does not matter, but the parameters of network interfaces differ:

	Luminosity, dBm		Sensitivity, dBm
	Min	Max	Min	Max
Intel Ethernet Server Adapter	-8,2	0.5	-12,6	0.5
Cisco X2 LR	-8,2	0.5	-14,4	0.5

Of the pair of boundary values, we need only one, the second is given to complete the picture. Note that the parameters of optical modules are very similar, but this is not always the case. One more note: in order not to embrace the immense, we will lose here the optical subtleties associated with the characteristics of the interfaces and their measurements and limit ourselves to the totals - this is quite enough for us. These characteristics for a specific interface can be found in the product description on the manufacturer's website.

So, we will carry out the calculation (in both directions it will be the same):

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As you can see, the budget converges with a large margin - this is generally typical for single-mode lines, as, indeed, for multimode, if you use them in the normal mode. Consider the same problem with SR optics and a line length of 300 meters:

	Luminosity, dBm		Sensitivity, dBm
	Min	Max	Min	Max
Intel Ethernet Server Adapter	-5.0	-1.0	-11,1	-1.0
Cisco X2 SR	-7,3	-1,2	-9.9	-1.0

Optical budget from Intel to Cisco and vice versa:



-1.0 - 0.5 - 0.5 - 0.5 - 0.5 - 0.5 - 0.5 - 0.3 * 3 = -4.9

-1.2 - 0.5 - 0.5 - 0.5 - 0.5 - 0.5 - 0.5 - 0.3 * 3 = -5.1



The stock is a little smaller (given the lower sensitivity), but still sufficient. The question is, why then consider the budget, if everything always fits together? The answer is: not all optical lines work in a comfortable mode. The main purpose of the budget is to come to the rescue when something goes wrong. Let's say there is a line that behaves not the way it should be. What is the problem - in equipment malfunction or in principle it cannot work stably in this configuration? This is where our tsifirki can help out.

So, we will make a conclusion: in the case when the line is made “according to the mind”, with the observance of standards and according to common sense, it is not necessary to consider the optical budget, you can do this only when problems arise. However, there are cases where the calculation is simply necessary - and we proceed to them.

Special cases

Imagine the opposite situation: you need to connect the equipment via an optical line of very short length, literally “back to back”. Is this mode comfortable for network interfaces? By no means. Overloading (glare) can lead to errors on the port, and even in general to its physical failure. In order to bring the port to life, you need to reduce the luminous flux - put the attenuator. Attenuators are fixed or variable attenuation; how to substitute it into the equation above - I think everyone already guessed it.



Optical attenuator



It is known that when selecting attenuators they often use the “spear method”, but the right word is not to engage in fortune telling, when in a few minutes it is possible by calculation to get optimal indicators for this particular case.



Here the case is even more complicated. Sometimes passive couplers - a splitter - are placed on the optical lines. A splitter is a translucent glass prism that passes part of the light through itself and reflects part to the other side. Thus, literally “free of charge”, without any active devices, you get two copies of one signal. The main part of the light remains in the line, and the second copy is fed to the traffic analyzer, anomaly detector, corporate security device, etc.





Optical splitter



The peculiarity of the situation is that in the case of splitters, the budget must be considered for two lines - the main and forked. In addition, the splitter introduces a significant attenuation in the line, it also makes life difficult. Let's add to our optocross also the splitter and see what happens.







Calculate the budget for the splitter 70/30. For a single-mode line, the attenuation introduced by them is as follows: 2 dB in the main line and 6.1 in the branch line. To the previous equations it is only necessary to add this damping (or rather, to subtract).



Primary: 0.5 - 0.2 - 0.2 - 0.2 - 0.2 - 0.2 - 0.2 - 10 * 0.4 - 2 = -6.7

Forked: 0.5 - 0.2 - 0.2 - 0.2 - 0.2 - 0.2 - 0.2 - 10 * 0.4 - 6.1 = -10.8



The branched line works at the limit of its sensitivity, it is better to change the type of the splitter to 60/40 - this will give us an extra decibel in a dangerous area.

Now the multimode (2.4 / 6.3 dB attenuation), for simplicity, we calculate one direction:



The main: -1.0 - 0.5 - 0.5 - 0.5 - 0.5 - 0.5 - 0.5 - 0.3 * 3 - 2.4 = -7.3

Forked: -1.0 - 0.5 - 0.5 - 0.5 - 0.5 - 0.5 - 0.5 - 0.3 * 3 - 6.3 = -11.2



As you can see, the branch line went out of the budget, and even replacing the splitter with 60/40 will not solve the problem. In general, it can be said that installing splitters on multimode lines is a serious problem and it is better without a strong need not to do this.



I would like to complete the topic with the following words. Probably, the ability to calculate the optical budget of lines is not one of the most essential skills of a network administrator. However, if you are in charge of optical networks or you intend to try yourself in this field, we advise you at least to have an idea about this subject in order to know in the future where to look and what to do.



If the topic is habra interest, next time we will talk about practical ways to control optical lines, typical optical problems and the influence of the curvature of hands on the trajectory of the light beam.