Introduction
A variety of radio, cable and optic-optic technologies are used to provide the backbone long-distance links for telecommunications carriers. Understanding the characteristics of these systems enables a better understanding of the entire telecommunications industry. Some of these technologies - such as microwave links - are also suitable for operation by an organization, as a cost-effective alternative to purchasing equivalent services from a carrier.
Historical Perspective
The most important technologies for long-distance and international telecommunications links are optic-optic cable, geostationary satellites and microwave links. Before discussing these, a number of older technologies will be mentioned - because they are still in use in some parts of the world.
Before the development of optics-optics in the early 1980s, long distance terrestrial and submarine cable communication was achieved with steel armored cable, with multiple copper conductors - for signal and power - and amplifiers or regenerators at regular intervals to compensate for the losses of sending high frequency signals through dozens of kilometers of cable. These systems were generally designed for carrying analogue audio signals with some kind of multiplexing so that a single signal of several hundred kHz on a single conductor could carry, for instance, 64 independent voice circuits. These techniques and cables are far more expensive and tricky to maintain than the optical fiber cables, which have largely replaced them.
These systems were capable of carrying digital information if it was transformed into an audio signal, which the cables could carry. Other than the digital data of telex, there was little call for transmission of digital information.
Similarly to these cables, long-distance microwave links for multiplexed voice circuits were installed by carriers - for instance with a chain of towers every 50 km or so, with microwave dishes, and repeater electronics to regenerate the signals and send them to the next tower.
International communications via microwave links to geostationary satellites have been commercially important since the 1960s, but again the systems were primarily designed for handling analogue signals - either multiplexed voice circuits or video signals, which involve frequencies of several MHz.
Now, all new long-distance and international communication links carry digital information exclusively. This is partly because of the demand for carriage of data, but primarily because digital transmission techniques are more efficient and reliable than analogue techniques.
Optical Fiber Links
Optic-optic communication links are almost always digital in nature. An important exception is their use in Hybrid Fiber Coaxial (HFC) cable systems - a broadband local access technology for data and video . In principle it is easy: turn a semiconductor laser-diode on and off according to the ones and zeroes of the data to be sent, shine its light along a hair-thin and very transparent fiber for a few dozen kilometers, detect the light that emerges with a photo-diode (a small photosensitive silicon device) amplify the signal and detect the flashes of lights in a synchronized way so as to recover the original ones and zeroes.
When fibers of silica (Silicon Dioxide - also known as quartz) were first used for communications purposes, the light waves bounced from the inside edges of a relatively thick fiber - leading to the light taking many different paths as it traveled. This fiber is known as multi-mode - since there are multiple paths for the light to take towards its destination. The trouble with this is that some paths are longer than other, so the light arrives with different time delays - and this smears the timing of short pulses and so sets a severe limit on how fast the pulses - and therefore the transmitted data - can be.
Long distance, high capacity, optic-optic links were first widely installed in the late 1980s. These systems, many of which remain in active service, use a combination of technologies known as the Plesiochronous Digital Hierarchy, or PDH. This technology was superseded in the mid 1990s by a more advanced system known as Synchronous Digital Hierarchy (SDH), and the capacity of SDH systems is now being dramatically extended with Wavelength Division Multiplexing. All three technologies are worth examining in some detail because they are so important in today's telecommunications networks.
PDH Fiber-Optic Links
Plesiochronous Digital Hierarchy (PDH) links use single-mode fiber. In contrast to the scattered light-paths of multi-mode fiber, single-mode fiber directs the infrared light straight down a single path in the center of the fiber. This is achieved by altering the composition of the silica so that light travels slightly slower in the center of the fiber - causing any diverging light wave to be 'dragged' back onto the right course. This ensures that all light takes the same path and arrives at the same time, enabling short pulses of light to arrive intact - and so enabling the carriage of a greater number of Megabits per second.
Silica is most transparent to light in the infrared wavelengths around 1310 nanometres (1.31 microns or 0.0013 millimetres) and around 1550 nm. For compactness, and reliability, the light source must be a tiny semiconductor laser-diode - a device only a few millimetres long - rather than a bulky gas laser. Laser-diodes with sufficient power were initially available only at the 1310 nm wavelength, and these are used in PDH systems.
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