Transmission Media – Telecommunication Systems
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Transmission Media – Telecommunication Systems

The telecommunications objective is to produce high-quality voice, video, and data communication between any pair of desired locations, whether the distance between locations is 1 or 10,000 km. The distance between the two locations determines the type of transmission equipment used for setting up the connection. First, communication over a distance on the order of a few meters, such as within a building, is done using metallic wires, optical fibers, or very small cell radios. Any routing of information within the building is done by a switch on the premises, a switch known as a private branch exchange (PBX).

When the distance is extended to a neighboring building or to span a distance within a village, town, or city, the local telephone network is usually used. This entails making a connection to the nearest switching exchange by a pair of copper wires or radio, routing the initiating party to the desired 
receiving party, and completing the connection on the recipient’s pair of copper wires, or radio, which are also connected to the nearest exchange. The switching exchange is also known as the central office, or CO, and the terms are used interchangeably in the rest of this text. The connection between the CO and the customer is called the local loop, while the term subscriber is also used for a customer they are also used interchangeably. If the connection is within the same neighborhood, the two parties are connected via the same CO, but if the connection is across town, routing from one CO to another is necessary It is at this stage that the choice of technology becomes important in the overall cost of the network. In the early days of telecommunications, all interexchange traffic was done using numerous pairs of copper wires (one pair for each interconnection). This was very cumbersome because interexchange cables were required, and such connections required hundreds or thousands of copper pairs. A technique known as multiplexing was subsequently devised for passing multiple simultaneous telephone calls (referred to as traffic) down one pair of copper wires. More recently, optical fibers have been introduced to fill this role. Future networks will connect COs to customers using optical fibers in the local loop, but the manner in which this should be done is still being debated in many parts of the world. The mobile telephone also comes into the local loop category, and over the past few years, the deployment of cellular mobile systems has experienced an explosive growth on a global scale. A cellular radio that is nonmobile (sometimes called fixed wireless), when used instead of a cable pair to the customer premises, is known as a wireless local loop (WLL). 

The next stage of interconnection is intercity, or long-distance, connections. The contenders to fill this role are microwave radio, optical fiber, and satellite. Microwave and satellite communications are far more mature technologies, but fiber optics technology has recently caught up and in many aspects has overtaken the other two. The rapid progress made by fiber optics over the past 10 years indicates that it is in a good position. 

There are several advantages of geostationary satellite links for long-distance telecommunications media. First, the broadcast nature of satellites is very attractive, especially for television transmission. The information transmitted from a satellite can be received over a very large area, enabling it to serve a whole continent simultaneously. Also, the cost of satellite communications is independent of the distance between the source and the destination (e.g., transmission over 1 or 5000 km costs the same). However, the satellite system only becomes cost competitive with microwave radio and optical fiber systems when the distance is large (e.g., greater than 500 km). There are some situations that are ideally suited to satellite communications. 

The technology used by satellite communications overlaps terrestrial microwave radio technology to a large extent. The radio nature and operating frequencies are the same. The main differences lie in the scale of the components. Because the satellite link is over 36,000 km long, high-power transmitters and very low-noise receivers are necessary. Also, the size and weight of the satellite electronics must be kept to an absolute minimum to minimize launch costs. Considerable attention has recently been devoted to very small aperture terminal (VSAT) satellite technology. As the definition of VSAT implies, these systems have earth station terminals that use antennas of only 1 to 4 m in diameter. This is a significant reduction from the 30-m-diameter antennas used in the original earth station designs of the 1970s. The use of such small diameter antennas enables business organizations to use satellite communications cost-effectively because a complete earth station can be placed on company premises. Again, long distance and broadcast-type transmission produce the highest cost-effectiveness. 

Perhaps one of the main disadvantages of satellite communications is the propagation delay. It takes approximately a quarter of a second for the signal to travel from the earth up to the satellite and back down again. This is not a problem in two-way speech communication, provided echoes are removed from the system by sophisticated electronic circuits. If satellites are used for intercontinental communications, three geostationary satellites are needed for complete global coverage. In order to speak to someone at a place on the earth diametrically opposite, or outside the “vision” of one satellite, a double satellite hop is required, which produces a propagation delay of about half a second. Some user discipline is required in this situation because of interruption of the speaker, as occurs in normal conversation, results in a very disjointed dialogue. This delay is totally unsatisfactory for many people. However, data communications and data over voice channels, such as Telefax, etc., are not adversely affected by this delay time. The double hop delay can be improved a little by satellite-to-satellite transmission, particularly if there are more than three satellites in the global system. The cost of this type of transmission is considerably less, as one of the earth stations is eliminated from the connection. 

A major new application of satellite communications is the much-publicized global mobile telephone system. Many telecommunications organizations are already catering to the demands of the urban populations of many countries by offering a mobile telephone handset that uses UHF or microwave radio technology to make the interconnection between the CO and the customer. Already the enormous demand for these systems warrants the operation of a global mobile telephone network. Such satellite systems have been proposed by several consortia, the first of which was a project called Iridium. This project was proposed by Motorola and comprises 66 satellites operating in low earth polar orbit to provide a global cellular network structure. The initial estimated cost of this project in 1990 was in excess of 2.5 billion U.S. dollars. Whether all of the proposed projects literally get off the ground remains to be seen. One major obstacle to the use of these systems in urban areas is the fact that penetration of signals from satellites through buildings to individual handsets is poor, particularly in high-rise buildings. Satellite communications and microwave mobile communications are an inherently narrow band in nature compared to optical systems. 

Although the long-term future of satellite systems might be uncertain, voice, broadcast TV, and low-bit-rate data traffic (with occasional high-bitrate traffic for alternative routing during fiber restoration) should keep satellite technology alive for many years to come. Microwave mobile cellular radio systems are excellent for voice transmission, but presently rather limited for data transmission. Optical fiber for the vast majority of wideband home services appears to be imminent in the foreseeable future for major cities. In rural areas and developing countries, it is considerably further in the future. Wideband services can be provided by satellite and microwave radio, but the cost of this limited bandwidth resource is comparatively high.

The bulk of long- and medium-distance telephone traffic is currently transmitted over terrestrial-style microwave radio and optical fiber links, which at present are primarily digital electronics technologies. The process of digital multiplexing, which is a means of combining voice, video, and data channels into one composite signal ready for transmission over the satellite, microwave radio, or optical fiber link.

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