Telecommunications Processors

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Telecommunications processors such as modems, multiplexers, bridges, front-end processors, and other devices perform a variety of support functions between the terminals and computers in a telecommunications network. Let's take a look at some of these devices and their functions.

Modems. Modems are the most common type of communications processor. They convert the digital signals from a computer or transmission terminal at one end of a communications link into analog frequencies, which can be transmitted over ordinary telephone lines.

A modem at the other end of the communications line converts the transmitted data back into digital form at a receiving terminal. This process is known as modulation and demodulation, and the word modem is a combined abbreviation of those two words.

Modems come in several forms, including small stand-alone units, plug-in circuit boards, and micro electric modem chips. Many modems also support a variety of telecommunications interface functions, such as transmission error control, automatic dialing and answering, and a faxing capability.

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... Modems are used because ordinary telephone networks were primarily designed to handle continuous analog signals (electromagnetic frequencies), such as those generated by the human, voice over the telephone.

Since data from computers are in digital form (voltage pulses), devices are necessary to convert digital signals into appropriate analog transmission frequencies and vice versa.

However, digital communications networks that transmit only digital signals and do not need analog/digital conversion are becoming commonplace. Since most modems also perform a variety of telecommunications support functions, modems may still be needed in digital networks.

Multiplexers. A multiplexer is a communications processor that allows a single communications channel to carry simultaneous data transmissions from many terminals. Thus, a single communications line can be shared by several terminals.

Typically, a multiplexer merges the transmissions of several terminals at one end of a communications channel, while a similar unit separates the individual transmissions at the receiving end.

This is accomplished in two basic ways. In frequency division multiplexing (FDM), a multiplexer effectively divides a high-speed channel into multiple slow speed channels. In time division multiplexing (TDM), the multiplexer divides the time each terminal can use high-speed line into very short time slots, or time frames.

The most advanced and popular type of multiplexer is the statistical time division multiplexer, most commonly referred to as a statistical multiplexer. Instead of giving all terminals equal time slots, it dynamically allocates time slots only to active terminals according to priorities assigned by a telecommunications manager.

Internetwork Processors. As we have previously mentioned, many local area networks are interconnected by internetwork processors such as bridges, routers, hubs, or gateways to other LANs or wide area networks. A bridge is a communications processor that connects two similar LANs, that is, LANs based on the same network standards or protocols.

A router is a communications processor that connects LANs to networks based on different protocols. A hub is a port switching communications processor. Advanced versions of hubs provide automatic switching among connections called ports for shares access to a network's resources.

LAN workstations, servers, printers, and other LAN resources are connected to ports, as are bridges and routers provided by the hub to other LANs and WANs. Networks that use different communications architectures are interconnected by using a communications processor called a gateway.

All these devices are essential to providing connectivity and easy access between the multiple LANs within an organization and the wide area networks communications channel. In many cases, star networks take the form of hierarchical networks.

In hierarchical networks, a large headquarters computer at the top of the company's hierarchy is connected to medium-size computers at the divisional level, which are connected to small computers at the departmental or work group level. A variation of the ring network is the mesh network.

This uses direct communications lines to connect some or all of the computers in the ring to each other. Another variation is the tree network, which joins several bus networks together.

In most cases, distributed processing systems use a combination of star, ring, and bus approaches. Obviously, the star network is more centralized, while ring and bus networks have a more decentralized approach. However, this is not always the case.

For example, the central computer in a star configuration may be acting only as a switch, or message-switching computer, that handles the data communications between autonomous local computers.

Star, ring, and bus networks differ in their performances, reliabilities, and costs. A pure star network is considered less reliable than a ring network, since the other computers in the star are heavily dependent on the central host computer.

If it fails, there is no backup processing and communications capability, and the local computers will be cut off from the corporate headquarters and from each other. Therefore, it is essential that the host computer be highly reliable. Having some type of multiprocessor architecture to provide a fault tolerant capability is a common solution.

Star network variations are common because they can support the chain-of-command and hierarchical structures of most organizations. Ring and bus networks are most common in local area networks. Ring networks are considered more reliable and less costly for the type of communications in such networks. If one computer in the ring goes down, the other computers can continue to process their own work as well as to communicate with each other.

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