Optimal navigation in optical networks

Optimal navigation in optical networks

Optical fibres transmit data in the form of light or light signals over long distances. While electrical signals migrate in electrons from one end to the other, the photons (light particles) take over this task in optical fibres.

Through optical fibres, optical signals can bridge large distances without amplifiers. Despite high distances, a high bandwidth is possible. The bandwidth of a single optical fibre is around 60 THz.

Telecommunication networks with optical fibre

In order to achieve high speeds in telecommunications networks, one usually uses optical connections between the nodes. In the switching centers the transmitted light signals are most times converted into electrical signals, evaluated and further processed. For further transmission, they are then converted back into light signals. At this point, the disadvantages of optical transmission systems become visible. For processing, optical signals must first be converted into electrical signals.

Optical fibres made of plastic have a diameter of approximately 0.1 mm. They are extremely flexible but also sensitive. The fibre core (core glass) is the central area of an optical fibre, which serves to guide the light wave. The core consists of a material with a higher refractive index than the overlying jacket. A reflection takes place on the walls in the interior of the fibre, so that the light beam is guided almost without loss around each corner. The cladding glass is the optically transparent material of a fibre at which the reflection takes place. The cladding glass is also a dielectric material with a lower refractive index than the core. The dielectric material is non-metallic and non-conductive. It therefore contains no metallic components. The coating is the plastic coating, which is applied as mechanical protection on the surface of the jacket glass. Buffering is called the protective material, which is extruded on the coating. It protects the obtical fibre against environmental influences. Buffering is also available as a tube that isolates the fibre from stress in the cable when the cable is moved.

Advantages of optical fibres against copper cables

  • Fibre optic cables can be installed in parallel with other supply lines. There are no electromagnetic interferences.
  • Because of the optical transmission, no spurious radiation or mass problems exist.
  • Distance-related losses due to inductances, capacitances and resistances do not occur.
  • Virtually frequency-independent line attenuation of the signals.
  • Transmission rates can be expanded almost indefinitely by several carrier waves with different wavelengths (color spectrum).

However, optical fibres are more expensive than copper cables. The cost of materials and the effort involved in assembly are higher. However, optical fibres have a considerably lower attenuation and are thus more suitable for long distances.


Multimode & Singlemode fibres

Both fibre optic cables have the same basic structure. Around the fibre lie two layers of fabric and / or plastic for the insulation and protection of the fibre. This is coated with a PVC or LSZH (Low Smoke Zero Halogen) layer. The difference in the fibre types is already in the design phase. This is inside the cable and consists of a core of pure glass, surrounded by a cladding layer of reflective glass, which focuses the light beams inside the core into a single coherent one-way beam.

The singlemode fibre core

Singlemode fibres have a very small glass fiber core diameter of 9μm, which allows only one type of light. As a result, the number of reflections resulting from the light transmitted through the core, as opposed to multimode fibres, is drastically reduced. This in turn lowers the transmission attenuation and allows the signal to move faster and farther.

What are singlemode connections used for?

Single-mode fibres are often used over long distances to transmit a large bandwidth reliably from point A to point B with an absolute minimum of interference or data errors, which is possible over many kilometers. Singlemode fibre patch cables are usually characterized by a yellow sheath and are currently manufactured according to OS2 ISO / IEC 24702 standard (with 0.4 dB/km) in a 9/125 μm ratio (core diameter / sheath diameter). It is important to pay attention to the use of G.652.D Low Waterpeak fibres as they offer an increased bending radius and increase the attenuation in the wavelength range between the 2nd and 3rd optical window. G.652.D singlemode fibres should be used especially when implementing CWDM / DWDM installations. However, a trend for shorter single-mode data links is already apparent, since the fibre is more complicated by its thin nature, but is now cheaper than a multimode fibre. Users also have the advantage of increasing bandwidth not shortening the link length. In this case, however, the overall damping of the connection has to be taken into account.

The multimode fibre core

Multimode fibre patch cables have a larger core diameter of 50μm, which transmits several modes of light. Because of the larger diameter, more data can be transmitted. However, far more refraction takes place and a greater attenuation is produced. As a result, multimode fibres are more likely to be used in backbones and local area networks (LANs), at far shorter distances than single-mode fibres, since the greater the bandwidth, the shorter is the possible connection length.

What are multimode connections used for?

Multimode fibre patch cables are available in different versions due to their historical development. Current designs in the glass fibre data transmission technology are the variants OM2 and OM3 according to ISO 11801 standard and OM4 according to TIA-492-AAAD standard. The core/sheath diameter ratio is 62.5/125 μm for OM1 fiber types. The diameter ratio is somewhat lower for OM2 (500 MHz/km, jacket color: orange), and for laser-optimized multimode fibres (LOMMF). It is 50/125 μm for OM3 (1500MHz/km, coat color: Aqua/Turquoise) 3500 MHz/km, jacket color: magenta/violet) fibers. OM2 fibres are designed for bandwidths of up to 10G. OM3 and OM4 fibres can also be used for higher bandwidths (currently up to 100G).

Source: Elektronik Kompendium / CBO

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