| Telecommunications - Telephone Networks From this page you should learn: The architecture of telecommunications systems How telecom networks use fiber optics How telecom evolved into the Internet When we say telecommunications or telecom, we are talking about a subset of communications that has evolved out of the telephone systems that provided voice communications to practically everyone around the world since the late 19th century. Telephone networks were the first major users of fiber optics. Fiber optic links were used to replace copper or digital radio links between telephone switches, beginning with long distance links, called long lines, where fiber's distance and bandwidth capabilities made fiber significantly more cost effective. Telcos use fiber to connect all their central offices and long distance switches because it has thousands of times the bandwidth of copper wire and can carry signals hundreds of times further before needing a repeater - making the cost of a phone connection over fiber only a few percent of the cost of the same connection on copper. The development of fiber optics was all focused on the replacement of the copper cables, wireless and satellite communications for the worldwide telephone network. Thus, much of the research and development of fiber optics was done by the telephone companies like AT&T, GTE and ITT in the US and British Telecom, Nippon Telephone and Telegraph in Japan and ITT subsidiaries worldwide. Each of these technologies had significant drawbacks at the time, including the distances that could be covered and the bandwidth needed for digital telephone transmission which was growing rapidly. Satellites also had the delay caused by the time of flight of radio signals from the ground to geostationary satellites which made many overseas calls very annoying.  Architecture of the telephone system The phone system consisted of three distinct applications. First was long distance, the backbone networks connecting cities, then came metropolitan connections between phone central offices and finally the so-called “last mile,” the local loop or connection from the central office switch to the subscriber - the home. Later, connections to wireless antenna sites was added to support cellular services. The long distance networks included about 10% of all the route miles of the phone system. Metropolitan connections between local central offices was about another 10% of route miles. The connection to the subscriber represented about 80% of all the route miles. Long Distance Networks Came First Since the long distance links aggregated all the traffic between cities, they not only represented the longest individual links but also had the most traffic. By the mid-1960s, the first electronic long distance switches were developed using transistors to replace old mechanical switches. Traffic between these switches was the highest priority for the telcos to expand and upgrade, thus becoming the first application for fiber optics. Research groups at telco labs like Bell Labs had reached a point that copper wires were no longer considered for future uses. Microwave wireless links were used in many locations. More bandwidth was needed. Scientists and engineers tried two approaches, millimeter waves in waveguides buried underground and fiber optics. By the early 1970s, fiber was determined to be the best/only choice. The first applications in telephone networks were connecting switches over short links, since the early fiber optic links could only go short distances and at relatively slow speeds, but those still were much greater than other available technologies. The development of fiber optics and integrated circuit technology led to rapid increases in the speed of fiber optic networks. The first fiber optic links installed in 1977 operated at 1.544 megabits/s, the standard T1 rate, using 850 nm lasers over multimode fiber. Technology was being developed rapidly, so links quickly moved to 145 Mb/s), facilitated by faster electronics, faster 1310 nm lasers and singlemode fiber. The same singlemode fiber introduced in the early 1980s and the same 1310 nm lasers are still used today for most links, a tribute to the scientists and engineers who developed it then. While the same singlemode fiber may be used, the speed of telecom networks has grown a million times faster, from megabits/second in 1980 to gigabits/second in 2000 to approaching terabits/second for a single signal and more than 10 terabits/second over a single fiber using multiple wavelengths today. The same singlemode fiber introduced in the early 1980s and the same 1310 nm lasers are still used today for most links, a tribute to the scientists and engineers who developed it then. While the same singlemode fiber may be used, the speed of telecom networks has grown a million times faster, from megabits/second in 1980 to gigabits/secondi in 2000 to approaching terabits/second for a single signal and more than 10 terabits/second over a single fiber using multiple wavelengths today.  Increase of telecom fiber optic link speeds over time Several technologies contributed to the speed increases in fiber optic networks. In the mid 1990s, a new type of laser, the distributed feedback or DFB laser, was developed that allowed faster speeds of as well as facilitating dense wavelength division multiplexing (DWDM.) Coherent transmission technology developed in the early 2000s overcomes the inherent dispersion of direct modulation links, allowing signal rates of terabits/second over link lengths of more than 1,000 km. And that speed is only for one signal. These links operating over singlemode fiber can carry many signals operating at different wavelengths called wavelength division multiplexing (WDM.) WDM works because different wavelengths of light do not mix in the fiber. Combining the speed of each signal with wavelength division multiplexing means that total bit rates above 10 terabits/second are possible on a single fiber. Service upgrades can be easily made by adding wavelengths to a link which is usually easier than increasing bit rates. The “long” in long distance refers to the length of links needed to connect distant cities. When the telephone network was on copper wires, the typical link was only a few miles. Early fiber optic links on multimode fiber at 850 nm were limited to about 15 miles (25 km) due to the high attenuation of the fiber, about 3 dB/km. Moving to lower loss singlemode fiber at 1310 nm and then to 1550 nm took advantage of the lower attenuation of glass fiber at longer wavelengths, below 0.5dB/km at first. Today the manufacture of fiber has improved so the attenuation is much lower still, below 0.4 dB/km at 1310 nm and 0.2 dB/km at1550 nm. Links of 100km are normal. Coherent transmission increases the possible length to over 1,000 km. When the signal gets attenuated too much, the link needs a repeater. The early repeaters essentially consisted of a detector receiver followed by a laser transmitter which built the signal level back up to its original level. These devices were complicated and, as with more complicated electronic devices, needed considerable electrical power and were not reliable. A repeater on a ground link was not hard to repair, but one in a submarine cable was very difficult to fix. The solution for repeaters was the invention of the fiber amplifier which could regenerate the signal without converting it back to an electronic signal. It had been known for many years that an optical fiber could be used to create a laser by stimulated emission in the fiber itself. The addition of erbium doping in the fiber and a pump laser of the correct wavelength caused stimulated emission in the fiber itself. The building of fiber optic networks did not wait for the development of new technologies. Parallel to the development of technology was the construction of network cable plants. Beginning with the phone companies and expanding with private carriers, fiber optic networks were installed at a fast pace starting in the last half of the 1980s. By the mid 1990s, most long distance communications in countries like the US was on fiber optics, perfect timing for the explosive growth of the Internet. But long distance networks represented only a fraction of the total telephone system - about 10%. Another 10% is in local or metropolitan networks connecting central offices and that work was done next. The remaining 80% is the "last mile" connecting individual subscribers to the telephone network. Fiber to the home (FTTH) was not cost effective until passive optical networks (PONs) and lower cost components were used starting around 2005. Today FTTH is the goal of practically every telco and Internet service provider. Submarine Cables Today, with the exception of some rugged or remote locations, the entire telephone backbone is fiber optics. Cables on the land are run underground, direct buried or aerial, depending on the geography and local regulations. Continents are also connected on fiber over hundreds of submarine cables. AT&T developed and installed the first transoceanic fiber optic cable across the Atlantic in 1988, only a few years after the first terrestrial links, thus beginning the revolution in international communications. The research and development that led to the first transatlantic fiber optic cable, TAT-8, took less than a decade because it could use much of the R&D done for prior copper cables. TAT-8 was more than 10 times faster than the fastest copper cables. It was still in service in 2002 when the bandwidth of new cables made it obsolete. The fiber amplifiers were first used on submarine cables in 1996 and coherent communications began to be used around 2010. With coherent systems, distances of thousands of kilometers are possible, making long submarine cables easier to design, install and operate with higher reliability.  Completing The Telecom Network After long distance links were converted to fiber, telcos began replacing shorter links between switches with fiber, for example between switches in the same metropolitan area. Today, practically all the telephone networks have been converted to fiber. Telcos and other groups are now running fiber right to the home, (FTTH) using low cost passive optical network (PON) systems that use splitters to share the cost of some fiber optic components among as many as 32 subscribers. More on FTTH, FTTH PON types and FTTH network architecture.  Fiber Optics And Wireless Communications Wireless networks are not completely wireless. In fact only the final link between the customer’s device – cell phone, tablet or laptop computer – is wireless. All wireless antennas – cellular or WiFi – are connected into the communications system using fiber optic cables. That is another application of fiber optics that is important enough to have its own chapter in this book. Cell phone towers with many antennas will have large cable trays or pedestals where fiber cables connect to the antenna electronics. More on wireless. The Internet The Internet evolved from the telephone network and has always been based on a fiber optic backbone. It started as part of the telephone network when it was primarily voice traffic and data traffic was mixed into the total traffic. The Internet’s existence as we know it today depends on the development of the fiber optic communications backbone. In fact, one can say the Internet depended on the development of the digital phone system and fiber optics for its existence as we know it today. But as the Internet developed, its needs diverged from the telephone communications system. The Internet started as a US government funded defense network to connect government computers. It had to run on the telephone network since that was the only nationwide network that existed in the late 1960s and early 1970s. The Internet was developed as the telephone network was being converted from analog to digital. The digital phone network was optimized for voice communications not data. Voice was digitized in a way that was adequate for voice with a limited audio frequency range of under 4,000 Hz and compressed in amplitude to reduce the digital bandwidth requirements of a voice call to only 64 kilobits/second. Digital voice was transmitted in very short data packets. Switching was similar to analog phone connections where a connection was established and maintained as long as the call lasted. The phone system had to be synchronized to maintain phone connections. As data communications between computers increased in volume, different methods of data transfer were developed to optimize transmission of large amounts of data. Typical data volumes were much higher than with voice, so packets of data were much longer and mixed on network’s as they were sent, asynchronously, so each packet included address information for the sender and recipient. As data communications between computers developed, a completely different set of protocols from the phone system developed. This led to some interesting conflicts. Should the computer networks use phone system protocols since the data was eventually formatted to be transmitted over phone networks? That was soundly rejected by those developing computer networks because the short packets of phone networks were too inefficient for the large amounts of data being transmitted and the constant connection of the phone system – often sending no data when speakers paused during voice conversations - was unnecessary and inefficient compared to the routing methods used for data. But since around the year 2000 when data traffic exceeded voice traffic, the communications networks started migrating from telco protocols to Internet protocols (IP) and routing instead of switching, leading to what was called VoIP – voice over IP networks. Now the telcos are moving their voice communications to Internet protocol (IP) for lower costs. Table of Contents: The FOA Reference Guide To Fiber Optics |
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