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The largest cause of attenuation is scattering. Scattering occurs when light collides with individual atoms in the glass and is anisotropic. Light that is scattered at angles outside the numerical aperture of the fiber will be absorbed into the cladding or transmitted back toward the source Scattering is also a function of wavelength, proportional to the inverse fourth power of the wavelength of the light. Thus if you double the wavelength of the light, you reduce the scattering losses by 2 to the 4th power or 16 times.
For example, the loss of multimode fiber is much higher at 850 nm ( called short wavelength) at 3 dB/km, while at 1300 nm (called long wavelength) it is only 1 dB/km. That means at 850 nm, half the light is lost in 1 km, while only 20% is lost at 1300 nm.
Therefore , for long distance transmission, it is advantageous to use the longest practical wavelength for minimal attenuation and maximum distance between repeaters. Together, absorption and scattering produce the attenuation curve for a typical glass optical fiber shown above.
Fiber optic systems transmit in the "windows" created between the absorption bands at 850 nm, 1300 nm and 1550 nm, where physics also allows one to fabricate lasers and detectors easily. Plastic fiber has a more limited wavelength band, that limits practical use to 660 nm LED sources.
More: Wavelength Bands Used For Fiber Optic Transmission
Multimode fiber's information transmission capacity is limited by two separate components of dispersion: modal and chromatic. Modal dispersion comes from the fact that the index profile of the multimode fiber isn't perfect. The graded index profile was chosen to theoretically allow all modes to have the same group velocity or transit speed along the length of the fiber. By making the outer parts of the core a lower index of refraction than the inner parts of the core, the higher order modes speed up as they go away from the center of the core, compensating for their longer path lengths.
The switches that switch the traffic over the network of a provider and between networks of providers are built to not hinder the traffic as it is switched. Modern non-blocking architectures switch packets in less than one millisecond from one switch to another18 and can switch over 1 Terabit per second. At one of the busiest IXPs, the AMS-IX in Amsterdam, traffic in April 2007 reached up to 260 Gbit/s and averaged 165 Gbit/s. In 2008 they expect to exceed 500 Gbit/s, with some customers needing multiple 10 Gbit/s ports to exchange traffic. It is this continuing growth in traffic that has made 10 Gbit/s networks more and more standard and called for the standardisation of 100 Gbit/s Ethernet, which is now ongoing.
In many OECD countries backhaul networks have seen an enormous overbuild of capacity in the late 1990s and the start of the 21st century. This overbuild was evident on the main routes, such as in the ëgolden triangleí covering London, Paris, Frankfurt, Amsterdam. Between 10 and 20 networks have laid infrastructure on this route. In general companies would lay 12 ducts on these routes, filling 2 of these ducts and leaving 10 empty. On some routes there were thousands of available fibres. The customers that bought some fibres on these networks would often use WDM (wavelength division multiplexing) equipment to increase capacity on the lines and resell that capacity to their own customers in direct competition with the network that they had bought connectivity from in the first place. Similar situations occured on a smaller scale in metropolitan networks, also, leaving a number of larger cities in OECD countries with multiple networks and many empty ducts. In commercially less-attractive areas there may be only two or less networks with fibre connections in the first mile from a particular location, though even here there are often empty ducts available.