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15 novembre 2011

The losses of copper and long distances may reduce the audio to a very low level and, as a result, making the audio more sensitive to noises and to external interferences. Fiber Optic is the best solution in those installations in which we have very long distances.


  1. Introduction: What the optic fiber is?

Sometimes in our installations, we might face long distances due to the length of our condominium. Although in MDS we can have up to 1200m, thanks to the RS-485 protocol, this feature is related to the data transmission, therefore we can face audio issues since it is transmitted analogically in MDS, as well as in conventional 4+N systems, by the wires 2 (downlink) and 6 (uplink). In this case, the losses of copper and long distances may reduce the audio to a very low level and, as a result, making the audio more sensitive to noises and to external interferences.

Fiber Optic is a transmission line based on light reflections instead of electron flow. It is commonly used in telecommunications for many interesting reasons related to data transmission, although the main features and the most important ones in our case are:

  • Very low losses.
  • Immunity to noises and electromagnetic inductions, since it is based on light theory and not in electromagnetic principles.
  • Reduced size and lower cost than copper.
  • Waterproof and weatherproof.
  • High transmission speed which give us a very high bandwidth.

With all these features, we can say that Fiber Optic is the best solution in those installations in which we have very long distances.


  1. Principles of Operation: How does it work?

Transmission in Fiber Optic cable is based on light transmission through silica fibers. Instead of electrons flow and voltage levels, in this case the data is codified by “light pulses” and these pulses contain the digital information. Now the question is, how these light pulses are transmitted?

Snell’s Law:

This phenomenon of light pulses transmission by reflections is based on a physics theorem called “SNELL’S LAW”. Snell’s law is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water and glass.

  • N1 & N2:  are the refraction index, a constant which depends on the material.
  • ϴ1: is the angle of incidence.
  • ϴ2: is the angle of refraction.

In this picture there is another angle: ϴ3. This angle corresponds to the reflection angle (not refraction) and it always has the same value as ϴ1.

Critic angle:

One of the most important concepts of this law is that we can get the whole ray of light reflected (not refracted). Therefore, the critic angle is defined as the value of  when ϴ2 = pi/2. Take notice that always ϴ2 <= pi/2. This critic angle will depend on the materials used (reflection index: n1, n2).

In the same way, if ϴ1 >= ϴc, the whole ray would be reflected and not refracted, since ϴ2 cannot be more than 90º (see again the picture). In other words, the reflected ray disappears and the whole ray (100%) is reflected without any losses.


Practical case:  

After this short explanation about Snell’s Law, we can explain why this physics theorem is so important in our case: Fiber Optic transmission.

The fiber optic cables are made basically by two different types of silica: Core and Cladding. Each type of silica has its own different refraction index (nx, as we can see on the formula). Therefore, following the Snell’s Law, the behavior of the fiber optic would be as follows:

In this picture, the most interesting concept is the acceptance cone. The acceptance cone establishes the angle of entrance that the ray of light needs in order to be transmitted through the core of the fiber optic cable (complying Snell’s Law). This acceptance cone depends on the materials used, following this formula:


Where  establishes the maximum angle of the acceptance cone.

Once we have explained the operation principles, it is easy to know why fiber optics is so recommendable for very long distances. To sum up:

Losses in Fiber Optic depend fundamentally on the light refractions. Following Snell’s Law and complying with the acceptance cone, regarding our case, we will have data/audio/video information transmitted by light pulses through the fiber optic cable practically without any losses and that is the reason why we will be able to reach very long distances (kilometers).


  1. Types of Fiber Optic:

There are two main types of Fiber Optic cables: Mono-mode and Multi-mode. The difference between these types is easy:

  • Multi-mode allows the transmission of several rays of light through fiber optic cable, and this fact gives to us the following features:
    • Installation: Easy to install. As several rays of light can be transmitted (multiple ways), the connectors, devices and light supplies are more economic since we do not need very sensitive components.
    • Price: The devices are more economic, however the cable used is more expensive since the production cost is higher.
    • Bandwidth and distance: Since the rays of light have multiple ways, we will face interferences known as “intermodal interferences”. This will limit the bandwith of our fiber optic system; therefore we can reach lower distances, 1km or 2km maximum.


  • Mono-mode, odd-mode or single-mode allows the transmission of only one ray of light through the cable. Related to this, we will have these features:
    • Installation: Difficult to install. Since we have just one way for the light, the devices, connectors, repeaters and light supplies must be very sensitive. This implies that we have to be very careful to connect any of these devices in our system.
    • Price: Due to this fact, the devices are quite expensive. Nevertheless, the fiber optic cable is quite more economical because it is much easier to produce.
    • Bandwidth and distance: As we have only one ray of light, we will face less dispersion than in multi-mode. In this case we will be able to reach distances close to 400km if we use a good light supply (LASER).


  1. FERMAX Fiber Optic device:

Until now, we have been talking about the main features of the optic fiber and how it works. At this point, it is easy to know that this type of cable can help us when we face very long distances (kilometers) to have a good audio level and less external interferences with a very small, resistant cable.

In FERMAX, we provide the device Ref.1345 – MDS Fiber Optic Converter. This device will allow us to have a fiber optic section to ensure our MDS projects when we face very long distances. Although this device is only available for MDS systems, MDS is in fact our hugest and reliable system which can be combined with any other such as VDS or BUS2.


This MDS Fiber Optic Converter is actually a kit which contains transmitter + receiver. These devices transform the electric signal to optic and vice-versa, in both ways. In fact we can consider them as “transceivers”. In detail these devices look like as follows:



  • V1: Coaxial Cable for video signals
  • D0: RS-485 Data input and output, bidirectional. Prepared to connect the shielded twisted pair D1&D2 or Sa&Sb, data in MDS systems
  • D1: No use
  • D2: Connection for audio
    • 1 – OUTPUT (wire 2, audio downlink)
    • 2 – NEGATIVE
    • 3 – INPUT (wire 6, audio uplink)
    • 4 – NEGATIVE
  • D3: Potential free contact
    • 1,2 – open contact: 1-input, 2-output
    • 3,4 – Incoming activation

(short-circuit between these terminals will close the “open contact” In the receiver)



  • V1: Coaxial Cable for video signals
  • D0: RS-485 Data input and output, bidirectional. Prepared to connect the shielded twisted pair D1&D2 or Sa&Sb, data in MDS systems
  • D1: No use
  • D2: Connection for audio
    • 1 – INPUT (wire 6, audio downlink)
    • 2 – NEGATIVE
    • 3 – OUTPUT (wire 2, audio uplink)
    • 4 – NEGATIVE
  • D3: Potential free contact
    • 1,2 – open contact: 1-input, 2-output
    • 3,4 – Incoming activation                                          

(short-circuit between these terminals will close the “open contact” in the transmitter)

NOTE: D2 is different in the transceiver and in the receiver. As you can see, the rest is the same.

This device uses MONO-MODE (odd-mode) fiber optic cables. As we have mentioned, this cable is the most economical one, which can get longer distances. Although mono-mode cable is difficult to install, in our case this is not a problem since in the module explained above we will connect the cable in a Plug&Play way, the rest is transparent to the user.

In other words, we will get the advantages of mono-mode fiber optic cables, avoiding disadvantages like installation difficulties.

On the following link, you can download the datasheet of this device to get more specific information, including schematics and wiring examples:


This device is available and 100% recommendable when we face long distances (>700m).



  1. Practical Case of study:

We will see the following practical case of study of a hypothetic installation.

In this case we will have good audio level from each panel to the apartments in our internal blocks, but from the Guard Unit, the audio will have low level due to the distance and the attenuation of copper. Additionally, it may appear interferences in audio due to the distance of the copper cable used for the FXL.

In this case, a Fiber Optic link would solve the problem, placed in the FXL our system will behave as if the Guard Unit were connected in the internal block A. Therefore, for the next FXL segment we could use a copper cable with a good section without any problem, and in the end we will have avoided low audio problems as well as audio interferences over the 800m section.

Now imagine that our final customer wants to add an additional Guard Unit in the parking entrance, 1km far from the General Entrance:

With two fiber optic links we would have solved the problem of audio level as above. In fact each fiber optic link reduces the distance-effect as it was 0m, so in this case, the maximum distance in the FXL-Bus we would face is in theory 300m:



Our FERMAX fiber optic kit Ref.1345 includes internal resistors to adapt the transmission line for audio and video signals; it means that these resistors adapt the MDS audio wires and the coaxial to the internal circuit of the transceiver. Nevertheless, this is the reason why we do not recommend using more than 2 fiber optic links in cascade, just in order not to face any negative effect in the MDS audio/video signals due to this fact.

For any special requirement or any further information, please get in touch with the international technical department. We will be pleased to give support.


  1. Conclusions:

Sometimes we can face projects where the distances are too long to transmit video/audio signals over common coaxial or copper cable. In most of them, having a good section or a good quality cable is enough but when we speak about distances over 500m, the audio level and video can be disturbed due to the copper features.

For those cases, FERMAX provides a fiber optic solution, based on optic technologies, easy to install and reliable, with all the advantages that the technology gives us nowadays. Very low losses, immunity against external interferences, low cost of the cable and more protection against atmospheric agents, are some of the advantages that we can get in this way.

Although fiber optic has also some disadvantages such as the price of optic devices or complex in installation, in our case we avoid all of these disadvantages since we provide a complete kit which makes fiber optic very easy to use.


FERMAX is working together with the new technologies in telecommunications and adapting its products to the international market. Fortunately, technologies and new researches give us the chance to improve day after day, therefore, it is a good moment to take the next step.