Text 1. The OSI Seven-layer Model

The logical structure of the ISO reference model is made up of seven protocol layers. The three lowest layers (1-3) are network dependent and are concerned with the protocols associated with the data communication network being used to link the two communicating computers. In contrast, the three upper layers (5-7) are application oriented and are concerned with the protocols that allow two end user application processes to interact with each other, normally through a range of services offered by the local operating system. The intermediate transport layer (4) masks the upper application-oriented layers from the detailed operation of the lower network-dependent layers.

The function of each layer is specified formally as a protocol that defines the set of rules and conventions used by the layer to communicate with a similar peer layer in another (remote) system. Each layer provides a defined set of services to the layer immediately above. It also uses the services provided by the layer immediately below it to transport the message units associated with the protocol to the remote peer layer. For example, the transport layer provides a network-independent message transport service to the session layer above it and uses the service provided by the network layer below it to transfer the set of message units associated with the transport protocol to a peer transport layer in another system.

Conceptually, therefore, each layer communicates with a similar peer layer in a remote system according to a defined protocol.

The application layer provides the user interface, normally an application program/process, to a range of network wide distributed information services. These include file transfer access and management, as well as general document and message interchange services such as electronic mail.

The presentation layer is concerned with the representation (syntax) of data during transfer between two communicating application processes. It negotiates and selects the appropriate transfer syntax to be used during a transaction so that the syntax (structure) of the messages being exchanged between two application entities is maintained.

The session layer provides the means that enables two application layer protocol entities to organize and synchronize their dialogue and manage their data exchange.

The transport layer acts as the interface between the higher application oriented layers and the underlying network-dependent protocol layers.

Vocabulary

7.2 Compile the vocabulary (term) log as shown in the preface (part of speech, definition, translation, synonyms and antonyms if possible, example of use). Words (concepts) are given below.

Layer; ISO; OSI; link layer; session layer; transport layer; transaction; presentation layer; transfer; user interface; protocol; intermediate; convention.

Grammar

7.4 Translate the following sentences and state the function of the Present Participle Passive (attribute or adverbial modifier?).

1) Being written in pencil, the text was difficult to read. 2) The service being announced will be provided by the end of this month. 3) The three lowest layers are concerned with the protocols associated with the data communication network being used to link the two communicating computers. 4) The presentation layer selects the appropriate transfer syntax so that the syntax of the messages being exchanged between two application entities is maintained. 5) Being instructed a computer stores the code number 01000001.

Comprehension Check

7.5 Answer the following questions.

1) How many protocol layers is the logical structure of the ISO reference model made up of? 2) What are the three lowest layers concerned with? 3) What is the function of the intermediate transport layer? 4) What is the operation principle of each layer? 5) What does the application layer provide? 6) What is the presentation layer used for? 7) Is there any difference between the presentation layer and the session layer? 8) What can you say about the transport layer?

7.6 Check how well you understand the following words and word combinations (explain or give definitions).

To be made up of, associated with, to interact, upper, the set of rules and conventions, to communicate with, immediately, message units, provided by, to transfer, similar, a defined protocol, to include, electronic mail, the representation of data, during a transaction, to manage data exchange.

Developing language skills

7.7 Complete these sentences with these verbs:

Masks selects specified provides offers

1) The function of each layer is …. as a protocol.

2) The transport layer …. a number of classes of service.

3) The intermediate transport layer …. the upper application-oriented layers from the detailed operation of the lower network-dependent layers.

4) Each layer …. a defined set of services to the layer immediately above.

5) The presentation layer …. the appropriate transfer syntax to be used during a transaction.

7.8 Use these linking words to complete the sentences:

in order to, however, and, although, as long as.

1) The new robot worked well .... Donovan watched it. 2) .... watch Dave in an emergency they had to create their own emergency. 3) …. the hole at the end of the tunnel was too small for a man to go through, they could look through it. 4) Powell lifted the detonator ...threw it down the tunnel. 5) The result, ... , was not what they expected.

7.9 Read the following text, guess or suggest a title to it, and write a summary (approximately 7 sentences).

Text 2. ________________________

For the propagation of radio waves, a transmitter and receiver are used. A radio wave acts as a carrier of information-bearing signals; the information may be encoded directly on the wave by periodically interrupting its transmission (as in dot-and-dash telegraphy) or impressed on it by a process called modulation. The actual information in a modulated signal is contained in its sidebands, or frequencies added to the carrier wave, rather than in the carrier wave itself. The two most common types of modulation used in radio are amplitude modulation (AM) and frequency modulation (FM). Frequency modulation minimizes noise and provides greater fidelity than amplitude modulation, which is the older method of broadcasting. Both AM and FM are analog transmission systems, that is, they process sounds into continuously varying patterns of electrical signals which resemble sound waves. Digital radio uses a transmission system in which the signals propagate as discrete voltage pulses, that is, as patterns of numbers; before transmission, an analog audio signal is converted into a digital signal, which may be transmitted in the AM or FM frequency range. A digital radio broadcast offers compact-disc-quality reception and reproduction on the FM band and FM-quality reception and reproduction on the AM band.

Receiving antennas intercept part of this radiation, change it back to the form of electrical signals, and feed it to a receiver. The most efficient and most common circuit for radio-frequency selection and amplification used in radio receivers is the superheterodyne. In that system, incoming signals are mixed with a signal from a local oscillator to produce intermediate frequencies (IF) that are equal to the arithmetical sum and difference of the incoming and local frequencies. One of those frequencies is applied to an amplifier. Because the IF amplifier operates at a single frequency, namely the intermediate frequency, it can be built for optimum selectivity and gain. The tuning control on a radio receiver adjusts the local oscillator frequency. If the incoming signals are above the threshold of sensitivity of the receiver and if the receiver is tuned to the frequency of the signal, it will amplify the signal and feed it to circuits that demodulate it, i.e., separate the signal wave itself from the carrier wave.

There are certain differences between AM and FM receivers. In an AM transmission the carrier wave is constant in frequency and varies in amplitude (strength) according to the sounds present at the microphone; in FM the carrier is constant in amplitude and varies in frequency. Because the noise that affects radio signals is partly, but not completely, manifested in amplitude variations, wideband FM receivers are inherently less sensitive to noise. In an FM receiver, the limiter and discriminator stages are circuits that respond solely to changes in frequency. The other stages of the FM receiver are similar to those of the AM receiver but require more care in design and assembly to make full use of FM's advantages. FM is also used in television sound systems. In both radio and television receivers, once the basic signals have been separated from the carrier wave they are fed to a loudspeaker or a display device (usually a cathode-ray tube), where they are converted into sound and visual images, respectively.

7.10 Make up 10 questions of your own using the information in text 2.

Unit 8

8.1 Read and translate the text. Use a dictionary to help you.