45

4.Прочитайте текст и найдите ответы на вопросы:

1.What is a communications channel?

2.Does information originate from one or many sources?

3.What is a simplex channel?

4.What are data bits encapsulated into multibit messages for?

5.What are examples of simplex, half-duplex and fullduplex communications channels?

1.A communications channel is a pathway over which information can be conveyed. It may be defined by a physical wire that connects communicating devices, or by a radio, laser, or other radiated energy source that has no obvious physical presence. Information sent through a communications channel has a source from which the information originates, and a destination to which the information is delivered. Although information originates from a single source, there may be more than one destination, depending upon how many receive stations are linked to the channel and how much energy the transmitted signal possesses.

2.In a digital communications channel, the information is represented by individual data bits, which may be encapsulated into multibit message units. A byte, which consists of eight bits, is an example of a message unit that may be conveyed through

32

a digital communications channel. A collection of bytes may itself be grouped into a frame or other higher-level message unit. Such multiple levels of encapsulation facilitate the handling of messages in a complex data communications network.

3.The message source is the transmitter, and the destination is the receiver. A channel whose direction of transmission is unchanging is referred to as a simplex channel. For example, a radio station is a simplex channel because it always transmits the signal to its listeners and never allows them to transmit back.

4.A half-duplex channel is a single physical channel in which the direction may be reversed. Messages may flow in two directions, but never at the same time, in a half-duplex system. In a telephone call, one party speaks while the other listens. After a pause, the other party speaks and the first party listens. Speaking simultaneously results in garbled sound that cannot be understood.

5.A full-duplex channel allows simultaneous message exchange in both directions. It really consists of two simplex channels, a forward channel and a reverse channel, linking the same points. The transmission rate of the reverse channel may be slower if it is used only for flow control of the forward channel.

33

5.Заполните пропуски подходящими по смыслу словами.

connected, has.

1.A communications channel is a pathway over which information can be ….

2.Communicating devices can be … by a wire or by a radio, laser, or other radiated energy source.

3.The transmitted signal … low energy.

4.In a half-duplex system messages … in two direct ions, but never at the same time.

5.Message encapsulation … the handling of messages in a complex data communications network.

6.A byte … of eight bits.

7.Garbled sound that cannot be … .

6.Перескажите текст, опираясь на слова (или составьте свой план пересказа текста):

Pathway, wire and radiated energy, source and destination, encapsulation, radio, telephone line, full-duplex channel.

В разделе «Тексты для чтения» самостоятельно прочитайте и переведите

«Serial Communications», «Asynchronous vs. Synchronous Transmission».

34

Parity and Checksums

1. Слова к тексту.

along with вместе

alter ['ɔ:ltə] v изменять(ся); менять(ся); вносить изменения, переделывать append [ə'pend]v прибавлять; прилагать

cause [kɔ:z] n причина; v быть причиной, причинять, вызывать

checksum ['tʃeksʌm] n контрольная сумма

consequence ['kɒnsɪkwəns] n (по)следствие;

constitute ['kɒnstɪtju:t] v составлять

convention [kən'venʃn] n обычай, обыкновение; конвенция

corrupt [kə'rʌpt] v портить

data error ['deɪtə 'erə] ошибка в данных

defense [dɪ'fens]n защита

disturbance [dɪ'stɜ:bƽns] n нарушение

drop [drɒp] n снижение

efficiency [ɪ'fɪʃnsɪ] n действенность, эффективность

even ['i:vn]a чётный

fail [feɪl] v потерпеть неудачу; не иметь успеха

faulty packet ['fɔ:ltɪ 'pæk ɪt] поврежденный пакет

flawed [flɔ: d] data поврежденные данные

in this case [keɪs] в таком случае

35

match [mæt ʃ] v подходить (под пару), соответствовать noticeable ['nəυtɪsəbl] a заметный, приметный

odd [ɒd] a лишний; добавочный; нечётный

parity bit ['pær ətɪ bɪt] бит чётности, разряд чётности prevent [prɪ'vent]v мешать, препятствовать purpose ['pɜ:pəs] n намерение, цель

reach [ri:tʃ] v достигать, доходить receipt [rɪ'si:t]n получение

redundant [rɪ'dʌndənt] a излишний, чрезмерный

request [rɪ'kwest]n просьба; требование

result [rɪ'zʌlt] n результат

sequence ['si:kwəns] n последовательность

sufficient [sə'fɪʃnt] a достаточный

throughput ['θru:pυt] n пропускная способность; производительность value ['vælju:] n ценность

worth [wɜ:θ] a стоящий

36

2. Соотнесите слова.

3.Прочитайте текст и ответьте на вопрос: What are parity bits and checksums used for?

1.Noise and momentary electrical disturbances may cause data to be changed as it passes through a communications channel. If the receiver fails to detect this, the received message will be incorrect, resulting in possibly serious consequences. As a first line of defense against data errors, they must be detected. If an error can be flagged, it

37

might be possible to request that the faulty packet be resent, or to at least prevent the flawed data from being taken as correct. If sufficient redundant information is sent, oneor two-bit errors may be corrected by hardware within the receiver before the corrupted data ever reaches its destination.

2. A parity bit is added to a data packet for the purpose of error detection. In the even-parity convention, the value of the parity bit is chosen so that the total number of '1' digits in the combined data plus parity packet is an even number. Upon receipt of the packet, the parity needed for the data is recomputed by local hardware and compared to the parity bit received with the data. If any bit has changed state, the parity will not match, and an error will have been detected. In fact, if an odd number of bits (not just one) have been altered, the parity will not match. If an even number of bits have been reversed, the parity will match even though an error has occurred.

3. Another approach to error detection involves the computation of a checksum. In this case, the packets that constitute a message are added arithmetically. A checksum number is appended to the packet sequence so that the sum of data plus checksum is zero. When received, the packet sequence may be added, along with the checksum, by a local

38

microprocessor. If the sum is nonzero, an error has occurred. As long as the sum is zero, it is highly unlikely (but not impossible) that any data has been corrupted during transmission.

4. Errors may not only be detected, but also corrected if additional code is added to a packet sequence. If the error probability is high or if it is not possible to request retransmission, this may be worth doing. However, including error-correcting code in a transmission lowers channel efficiency, and results in a noticeable drop in channel throughput.

4.Продолжите предложения.

1.Noise and momentary electrical disturbances may … .

2.If an error can be flagged, it might be possible to … .

3.A parity bit is added to a … .

4.The parity needed for the data is recomputed by … .

5.A checksum number is appended to … .

6.If the error probability is high or … .

7.If the sum is nonzero, an … .

8.If an even number of bits have been reversed, the parity will match … .

5.Ответьте на вопросы к тексту.

1.What can disturb data communication?

2.How can data corruption be prevented?

3.Tell about parity and checksum methods.

Вразделе «Тексты для самостоятельного чтения» прочитайте «Data Compression», «Data Encryption», «Noise and Electrical Distortion».

39

Тексты для самостоятельного чтения

Для работы с текстами рекомендованы онлайн-словари

Multitran (www.multitran.ru) и ABBYY Lingvo (www.lingvo.ru).

What is a Modem?

1.A modem (a modulator/demodulator) lets you connect your computer to a standard telephone line so you can transmit and receive electronically transmitted data. It is the key that unlocks the world of the Internet and its World Wide Web, commercial online services, electronic mail (E-mail), and bulletin board systems (BBSs).

2.Depending upon how your computer is configured and your preferences, you can have an external, internal or PC modem card. All three types work the same way, but each has its advantages and disadvantages.

3.An external modem is the simplest type of modem to install because you don't have to open the computer. External modems have their own power supply and connect with a cable to a computer's serial port. The telephone line plugs into a socket on the rear panel of the modem. Because external modems have their own power supply, you can turn off the modem to break an online connection quickly without powering down the computer. Another advantage over an internal modem is that an external modem's separate power supply does not drain any power from the computer. You also can monitor your modem's connection activity by watching the status lights.

40

3.Most internal modems come installed in the computer you buy. Internal modems are more directly integrated into the computer system and, therefore, do not need any special attention. Internal modems are activated when you run a communications program and are turned off when you exit the program. This convenience is especially useful for novice users. Internal modems usually cost less than external modems, but the price difference is usually small. The major disadvantage with internal modems is their location: inside the computer. When you want to replace an internal modem you have to go inside the computer case to make the switch.

4.PC Card modems, designed for portable computers, are the size of a credit card and fit into the PC Card slot on notebook and handheld computers. These modems are removed when the modem is not needed. Except for their size, PC Card modems are like a combination of external and internal modems. These devices are plugged directly into an external slot in the portable computer, so no cable is required other than the telephone line connection. The cards are powered by the computer, which is fine unless the computer is batteryoperated. Running a PC Card modem while the portable computer is operating on battery power drastically decreases the life of your

batteries. (From http://www.crm.mb.ca)

Different Internal Modems

A modem is used to connect a computer to the Internet. There are internal and external modems. Internal modems are connected internally to the computer's motherboard and may not be removable. Two main types of internal modems exist: WiFi (wireless modems) and dial-up

41