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Figure 1
Traditional access to customers.

The switching at Central was originally accomplished by human operators who manually interconnected the customers' calls. The transmission medium was copper wires either cabled or open. Control was provided by the customers' verbal instructions and the operators' manual actions. This division in function (i.e., transmission, switching, control, and terminal equipment) still exists in today's telecommunications networks, albeit in radically different forms.

Transmission Media and Multiplexing

Transmission equipment for telephony has evolved from simple open wire carrying a single conversation to optical fibers that can carry many thousands of conversations.

Signals from a home or business are carried over a twisted pair of copper wires, called the local loop, to a centrally located local office. Hundreds and even thousands of wire pairs are carried together in a single large cable, either buried underground in a conduit or fastened above ground to telephone poles. At the central office, each pair of wires is connected to the local switching machine. The transmission quality of the early installations was highly variable. Today, however, the plant that is being installed has the capability to transmit at least basic rate ISDN (144 kbps). This is true even for long loops (greater than 12,000 feet) that require loop extension equipment or lower-resistance wire. (Future plans will reduce the number of those long loops.)

In order to reduce costs, methods were developed to combine (multiplex) a number of subscribers on a single transmission medium from the central office, with the individual wire pairs split off nearer to the subscribers (Figure 2). As advances in technology progressed, multiplexing kept pace by increasing the number of conversations carried over a single path. Only a few years ago, multiplexing provided tens of conversation paths over a pair of wires. Initially this was accomplished by shifting each telephone signal to its own unique frequency band.

A major advance in multiplexing was accomplished when normal voice signals were converted into a coded digital form. In this form, the digital signals could be regenerated repeatedly without loss of the voice or other information content.

With today's time division multiplexing, each telephone signal is converted to a digital representation. That representation is inserted into fixed time slots in a stream of bits carrying many digitized telephone signals, with the overall stream operating at a high bit-rate. (An uncompressed voice signal requires 64,000 bits per second [bps] in digital form.)

The multiplexed signals can be transmitted over a variety of transmission media. The most common multiplexing system, called T1, operates over two pairs of copper wires carrying 24 telephone signals, at an overall bit-rate of 1.544 million bits per second (Mbps). First installed in 1962, the system is still widely used today.

With optical fiber, a beam of light is transmitted through a very thin, highly pure glass fiber. The light travels in parallel rays along the axis of the fiber. Many telephone signals are multiplexed together, and the light

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