Vocabulary. LAN (Local Area Network) – локальная вычислительная сеть еstate - имущество campus - территория variety - множество
Text Local Area Networks (LANs) may be defined as transmission and switching systems that provide high- speed communication between devices located on a single site. This could be an office complex, an industrial estate, a college campus or any closely linked group of buildings in which a variety of workstations need to communicate with each other. A typical LAN may cover a distance ranging from a few meters to around ten kilometers. As much of our daily work in the office involves using different kinds of communication media, LANs have been seen to be particularly useful in the fields of office automation (OA) and distributed data processing (DDP). Several studies have shown that there are four fundamental levels of communication in office systems:
1. Communication within a group, which could be a particular department in a company. The distance of transmission ranges from a few meters to about one hundred meters. 2. Communication with other internal groups or departments in the same company. The transmission distance ranges from a few hundred metres to perhaps two or three kilometers. 3. Communication with branch offices of the same company. The distance here ranges from a few dozen kilometers to hundreds of kilometers. 4. Communication with the outside world. The transmission distance ranges from a few kilometers to thousands of kilometers.
Typically we find that about 50% of all office communication circulates within one geographic site and that 70% of all communication circulates within the same company. Only 30% ever goes beyond the company into outside world, to customers and suppliers, for example. Many large companies, or groups working on the same site, are being faced with the choice of continuing with their own PABXs, which may be electromechanical or electronic, or of installing a LAN. Installing a LAN is certainly very expensive but it offers a great variety of advantages over a PABX. Suppliers of LAN have been offering systems based on two major classes of architecture, the ring and bus topologies. A third solution that is also sometimes suggested is based on the classic star network, in which a central processor controls all other nodes in a master/ slave manner. The central processing unit (CPU) in ring and bus topologies can be located anywhere in the network, making for truly decentralized processing, whereas it controls every operation in a star network. In contrast to LAN suppliers, the effort of PABX suppliers has been placed, for the main part, on replacing electromechanical systems with modern, electronic PABXs where the customer’s requirements have been almost exclusively for voice- only systems. The late entry of PABX suppliers into the OA market has been seen by many as the chasing of a new expanding market with “second best” technology. So which is the better for the office: a PABX or a LAN system? A lot obviously depends on the size and specific needs of each company, but LAN topology and architecture seem to be more suitable for levels one and two because of the very high data rates, high occupancy and transaction levels that are involved. PABX, on the other hand, seem to be more suitable for levels three and four, where communication over longer distances is required. Other arguments in favour of the PABX are that most people are already familiar with it and know how to use all its facilities. A PABX can normally be easily upgraded through software modifications to provide new facilities for the office of the future. It also offers full access to all national telecommunication services and an electronic PABX gives the user features such as call detail recording. The PABX has single wire connectivity and cabling probably already runs from the PABX to every workstation in the company. A final argument is that most users have limited budgets and prefer to continue with a technology that has been tried and tested, especially as they consider that voice traffic, rather than data, will remain the dominant form of communication. The principal argument in favour of LANs is their ability to handle large amounts of data at high speed. Also their networks, either ring or bus, require less cabling than the star networks of PABXs, and LANs offer distributed control rather than the very centralized systems provided by PABXs. This gives LANs more power and flexibility. It is also easier to share specialized resources with a LAN and different terminals can be connected more economically than on a PABX. Finally, the LAN frees the PABX for other functions. Against the LAN, we can argue that it is costly to install; it is limited in communication distance; there is lack of privacy and a relatively small bandwidth; and it can only accommodate a limited number of terminals. For some operations a LAN may also be less reliable than a PABX. Since LANs appear to be particularly well- suited to the electronic office, they will certainly continue to develop in different forms using transmission media (coaxial cable and/ or optical fibres) which meet the specific requirements and technical possibilities of individual companies in terms of architecture and investment.
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