Computer networks

There are three major categories of networks distinguished primarily by ge­ography: wide-area networks, metro­politan-area networks, and local-area networks.

Wide-area networks (WANs) provide communications across a country or the entire world. The telephone network is the most pervasive example. It is often viewed as a single entity, even though it is made up of a variety of separate com­panies. Using the telephone network, for example, it is possible for any two computers, equipped with the appropri­ate hardware and software, to communi­cate with one another. It is also possible for a traveller to withdraw cash from his or her own bank account from an automatic teller machine (ATM) in Tokyo or London. More and more banks, as well as other businesses, are linking together in a worldwide network to make automated transactions around the world as easy as they are around the corner.

Metropolitan-area networks (MANs) provide communications across and among major metropolitan areas. Pub­lic-safety agencies such as police and fire departments, companies, oper­ate and maintain their own private net­works. They use MANs to allow their mobile “out-of-office” workers to talk to, and to pass data back and forth to one another.

Local-area networks (LANs) provide communications within specific build­ings or facilities. This type of network could be used, for example, in a com­pany where computers are located in the sales, accounting, production, and pur­chasing departments. Each computer would handle the applications unique to its department. When communication among the computers is required, the computers pass data to one another over the network. For example, purchasing information must be passed to the ac­counting department for inclusion in the accounts payable system, or production-scheduling information must be passed to the purchasing department for use by the purchasing and inventory systems.

Local-area networks have many differ­ent topologies, or ways in which com­puters can be connected to the network. Throughout the history of networks, the star topology, in which a central controller connected terminals or personal computers, was the dominant form of connectivity. Today, the two most com­mon topologies are called ring and busnetworks.

Ring Networks. Computers on a ring network are hooked together one after the other to form one continuous ring. To access each computer, a piece of software called a token is passed around the ring. A token is the electronic equivalent of an envelope. It contains a destination ad­dress and a fixed amount of information. The most popular token method is called token passing, in which one token at a time passes from computer to com­puter and carries messages around the network.

Bus networks. On a bus network computers are simply connected by a cable routed along a path in the vicinity of each device that must be connected to it. The most popular bus network is called Ethernet. Bus networks allow multiple messages to be sent simultaneously. This is often accomplished by “listening” to the net­work to see if any computer is transmit­ting. If the network is free, the computer can send its data. If two computers acci­dentally send data at the same time, a collision occurs, and after detecting the collision, the computers wait a random amount of time to transmit again. This access control method goes by the name of Carrier Sense Multiple Access with Collision Detection (CSMA/CD ).

There are two means of relaying infor­mation over networks: circuit switching and packet switching. In the case of mak­ing a telephone connection between Kyiv and Mykolaiv, the phone com­pany uses circuit switching to find a free circuit, or physical link, among the many possible alternatives between the two cities, and keeps that circuit open for the duration of the call. If the call consists of two computers talking to one another, they may only be exchanging short bursts of data and spending the majority of their time waiting for addi­tional data. Regardless of whether trans­mitting or waiting, the circuit is still con­nected, and the user is paying for the connect time, which results in a very inefficient use of bandwidth.

Packet switchingbreaks messages into smaller units called packets. Each packet contains the address of its desti­nation and the data to be sent. An anal­ogy can be found in the postal system, where letters (data) are placed in ad­dressed envelopes, sent by truck or plane through the mail (channel), and delivered to the proper receiving ad­dress. With packet switching, messages are broken into packets that are trans­mitted in short bursts over a communi­cation network to the specified address, where they are reassembled into the original message.

Whereas circuit switching makes a physical connection between two points, packet switching makes a virtual connection. By sending data in small packets, each with its own address, time and space on a network can be allocated more effectively. Many small packets can be routed over many different paths on the network instead of large messages routed over a single path. If errors occur, usually only a single packet of data is lost instead of the entire message. If different paths in the network go down, packets can easily be rerouted because no physi­cal connection has been established, only a virtual connection.

1.4 Translate the following words and words-combinations into English so that you could form compound nouns.

Телефонна мережа, пристрої (засоби) зв‘язку, комп‘ютерні технології, перемикання каналів, комутація пакетів, мережа зв‘язку, зіркова топологія, магістральна мережа, метод керування доступом, базова станція, виявлення колізій(конфліктів), кільцева мережа, телефонний зв‘язок, телефонна промисловість, мережа даних;

1.5 Give English equivalents of the following words and word-combinations.

Забезпечувати зв‘язок, відповідне апаратне і програмне забезпечення, сфера зв’язку, стирати (згладжувати) відмінності, сходитися в мережу, сотовий телефон, усувати необхідність, обробляти(керувати) програму, взаємовиключний, передавати інформацію по мережі, фізичний зв‘язок, віртуальний зв‘язок, смуга частот, ефективно розміщувати, встановити фізичний зв‘язок, передавати через шлюз, обмінюватись пакетами даних.

1.6 Fill in the blanks with the words from the text.

1. A data communications network is a group of devices ____________ interconnected by communications circuits so that users can share data, software programs and hardware resources.

2. _____________ is a network contained in a relatively small area.

3. Dissimilar networks can be interconnected by ____________, devices that help manage communications, control traffic on a large networks and translate protocols.

4. The Internet is the example of a _____________network.

5. _____________is an international standard for transmitting digital text, sound, voice and video data over telephone lines.

6. For long-distance or world-wide communications, computers and LANs are usually connected into a ____________ network to form a single integrated network.

1.7 Translate into English.

1.Для забезпечення зв‘язку між локальними мережами застосовуються засоби міжмережної взаємодії, які називаються мостами і маршрутизаторами.

2.Мости використовують для зв‘язку мереж з однаковими комунікаційними системами, наприклад для зв‘язку двох мереж Ethernet.

3.Маршрутизатори сполучають мережі з різними комунікаційними системами, оскільки вони мають засоби перетворення пакетів одного формату на інший.

4.Існують мости-маршрутизатори, що об‘єднують функції двох систем.

5.Для забезпечення зв‘язку з різними комп‘ютерними системами призначені шлюзи.

1.8 Substitute the underlined word or phrase with the most suitable word from the box.

Connect join many at the same time protocols widespread send via with common architecture configuration numerous transmit structure dissimilar different

1. Bus networks allow multiple messages to be sent simultaneously.

2. Dissimilar networks can be linked through gateways.

3. The physical distribution of the devices and their interconnecting communication circuits is known as network “topology”.

4. The software consists of the rules which determine the formats by which information can be exchanged between different networks.

5. Internet and Arpanet transfer data and e-mail for university researchers and academicians, commercial groups and ordinary people.

6. The telephone network is the most pervasive example.

1.9 Answer the questions.

1. What does the convergence of data communications and telecommunications mean for society?

2. What is the difference between analog and digital networks?

3. What are the main categories of networks? In what way are they distinguished?

4. What are the two most common topologies of local-area networks?

5. What are two basic means of relaying information over networks? Which of them is more effective? Why?

6. What are the benefits of connecting computers and peripherals in a network?

7. What are the advantages of packet switching over circuit switching?

2.1 Read and translate the following words and word-combinations.

Carry conversations, a flexible metallic shielding, a filament of glass, through open space, around the world, wide-area wireless communications, infrared technology, a popular wireless medium, cordless telephones, communication satellites, geosynchronous satellite-based communication, communication application.

2.2 Learn the key words and word-combinations.

amplify(v) – підсилювати

bandwidth(n) – ширина смуги частот

coaxial cable – коаксіальний кабель

communication level – канал зв‘язку

compression(n) – ущільнення

convert(v) – перетворювати

convey(v) – передавати

dongle (n) –електронний захисний ключ-заглушка, механізм захисту від несанкціонованого доступу

FDDI (Fiber Distributed Data Interface) – протокол обміну розподіленими даними через світлопровід

fiber-optic cable – волоконно-оптичний кабель

frequency(n) – частота

frequency-division multiplexing – частотне ущільнювання

geosynchronous(adj) – геосинхронний

hertz(n) – герц

iRDA (Infrared Data Association) Асоціація по засобам передачі даних в інфрачервоному діапазоні

microwave(n) – мікрохвиля

OBEX (OBject EXchange) технологія OBEX, технологія обмiну об’єктами всередині робочої групи

per unit of time – за одиницю часу

remote-control device – пристрій дистанційного контролю

repeater antenna – ретрансляторна антенна

satellite channel – супутниковий канал

shrink(v) – ущільнювати

spread-spectrum radio – радіозвя’зок з передачею сигналів у широкому діапазоні

time slot – інтервал часу

time-division multiplexing – ущільнювання у часі

time-sharing service – послуги в режимі розподілу часу

twisted-pair cable – вита пара

Wi-Fi (Wireless Fidelity) стандарт Wi-Fi на бездротовий звязок

Read and translate the text.

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