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The Telephone – Part 2. The Telephone System. One of the first problems in building today’s telephone network was how to connect one telephone to any one of a very large number of telephones.
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The Telephone System • One of the first problems in building today’s telephone network was how to connect one telephone to any one of a very large number of telephones. • Without a central switch, you would need a wire going from each telephone to every other telephone in the network. • Clearly it was not possible to build a telephone network that could connect any two of millions of phones using this method of wiring.
This problem is overcome by the use of a central switch. • The earliest telephone systems were built with a central patch board attended by an operator. • Each telephone had one line between it and the operators patch board. • A person wanting to make a call first called the operator and told her the name of the person to be connected to. • Almost all of the operators were women for over fifty years.
The operator would then make a connection by plugging a cord into the correct socket, joining the two wires from the two telephones, and making the connection between them. • This system needed only as many wires as there were phones, but needed an operator to operate the switchboard. • Early switchboards did not operate all day and all night, but maintained business hours. No calls were made if the operator was not at the switchboard.
Several attempts were made to design an automatic switch. • The most successful was invented by Almon Strowger and first put into commercial service in 1892. • Strowger switches are still used today in a few places. • The Strowger Switch used electromagnetic stepper switches to count the pulses generated by a telephone dial. • This automatically made a connection based on the dialed number.
Modern telephone switches are all automatic. • They are actually special purpose computers built to do nothing but make and control telephone connections. • Operators are only needed to help customers who are having a problem or to give information. • The information operator uses a computer to locate numbers and then an automatic artificial voice announces the numbers in most modern systems.
Telephone switches are used in two basic ways. • One kind of switch is used to connect individual telephones to each other and to the telephone network. This is called a private branch exchange or PBX. • When the switch is located at the telephone company, it is called a central office switch. • The second kind of switch is used to connect other switches to each other to pass calls from one local area to another. This is called a tandem switch.
Connecting Central Offices • The telephone in your home is connected to the nearest central office by a pair of copper wires. • It is common for two central offices that are close together to be linked by large bundles of copper pairs called trunk cables. • The wires may be run on overhead telephone poles or underground.
Each pair of wires in the cable carries one conversation at a time. • Therefore, the total number of conversations that can be carried is set by the number of pairs in the cable. • Fiber-optic cable is replacing the large bundles of copper wires between central offices. • Fiber-optic cable carries information on beams of light that are guided through a flexible glass strand that is small and lightweight.
One strand of fiber-optic cable can carry thousands of simultaneous conversations through a technique called multiplexing. • A multiplexer is an electronic device that combines many telephone or computer signals onto one wire or fiber-optic cable. • At the other end of the wire, or fiber-optic cable, a demultiplexer is needed to separate the signals.
Central Offices can also be connected by microwave radio links. • Microwave radio links also take advantage of multiplexing to combine many conversations onto one radio link. • It is not uncommon for more than one thousand conversations to pass over one radio link. • Telephone offices on two continents are often connected through a satellite link. • Communication satellites act as relay stations in the sky for microwave radio links.
Modern Telephone Services • The adoption of new technologies to provide telephone service has produced new forms of telephone services. • The integration of different technologies brings about new solutions to communication problems.
One device that has resulted from combining technologies is the cordless telephone. • A cordless phone includes a base unit and a hand held unit. • Each has a low power radio transmitter and receiver so that they can communicate with each other. • Cellular phones have a wider range than cordless phones.
The base station in one cell is connected by phone lines to the base stations in adjacent cells. • When a cell phone moves out of the range of one base station, its call is handed off to the base station in the next cell. • This enables continuous service over long distances.
The introduction of digital and computer technology to the phone system has made extra services such as call waiting, call forwarding, three way calling, visual call display etc. • Other services will be offered in the future as new technology becomes available.
