Network Communications

1. The application layer is the only part of a communications process that a user sees, and even then, the user doesn't see most of the work that the application does to prepare a message for sending over a network. The layer converts a message's data from human-readable form into bits and attaches a header identifying the sending and receiving computers.

2. The presentation layer ensures that the message is transmitted in a language that the receiving computer can interpret (often ASCII). This layer translates the language, if necessary, and then compresses and perhaps encrypts the data. It adds another header specifying the language as well as the compression and encryption schemes.

3. The session layer opens communications and has the job of keeping straight the communications among all nodes on the network. It sets boundaries (called bracketing) for the beginning and end of the message, and establishes whether the messages will be sent half-duplex, with each computer taking turns sending and receiving, or full-duplex, with both computers sending and receiving at the same time. The details of these decisions are placed into a session header.

4. The transport layer protects the data being sent. It subdivides the data into segments, creates checksum tests - mathematical sums based on the contents of data - that can be used later to determine if the data was scrambled. It can also make backup copies of the data. The transport header identifies each segment's checksum and its position in the message.

5. The network layer selects a route for the message. It forms data into packets, counts them, and adds a header containing the sequence of packets and the address of the receiving computer.

6. The data-link layer supervises the transmission. It confirms the checksum, then addresses and duplicates the packets. This layer keeps a copy of each packet until it receives confirmation from the next point along the route that the packet has arrived undamaged.

7. The physical layer encodes the packets into the medium that will carry them - such as an analogue signal, if the message is going across a telephone line - and sends the packets along that medium.

8. An intermediate node calculates and verifies the checksum for each packet. It may also reroute the message to avoid congestion on the network.

9. At the receiving node, the layered process that sent the message on its way is reversed. The physical layer reconverts the message into bits. The data-link layer recalculates the checksum, confirms arrival, and logs in the packets. The network layer recounts incoming packets for security and billing purposes. The transport layer recalculates the checksum and reassembles the message segments. The session layer holds the parts of the message until the message is complete and sends it to the next layer. The presentation layer expands and decrypts the message. The application layer converts the bits into readable characters, and directs the data to the correct application.

Questions:

1. What is the only part of a communications process that a user sees? 2. Which layer ensures that the message is transmitted in a language that the receiving computer can interpret? 3. Which layer protects the data being sent? 4. What does the network layer form data into? 5. Why may an intermediate node reroute the message?

II. Перепишите и письменно переведите 3 и 4 абзацы текста.

III. Определите тип условных предложений и письменно переведите их, обращая внимание на разницу в переводе форм сослагательного наклонения во II, III типах условных предложений и форм изъявительного наклонения в предложениях I типа.

1. It would be difficult to illustrate this method on homogeneous systems. 2. The graph will be symmetrical about the axis of x if the equation contains only even powers of y. 3. If he clarified his statement, we could have a better understanding of the situation. 4. If we discussed the major problems facing us, instead of the details, we could realize our plan more successfully. 5. You would have understood his work in case you knew the work of his predecessors in this area. 6. Were they ready, we might begin the research. 7. But for her help, they would have come across a lot of difficulties.

IV. Определите тип условных предложений. Выберите правильный вариант формы глагола. Письменно переведите предложения.

1. She will not attend the conference tomorrow if she (is not ready/will not be ready/be ready) with her paper. 2. If you (know/knew/will know) the length of the radius, you could easily find the diameter. 3. If Cantor (didn’t develop/shouldn’t develop/hadn’t developed) his theory of infinite classes, Zeno’s and Galileo’s paradoxes would not have been resolved. 4. I (wouldn’t have agreed/won’t agree/ didn’t agree) to write this article unless I knew the subject. 5. (Were/Has been/Will be) the set concept so simple as it may seem, the mathematicians would have applied it much earlier. 6. If it were not for his help, we (shouldn’t design/wouldn’t have designed/ won’t design) this machine in due time.

V. Выберите правильный вариант формы глагола в сослагательном наклонении в конструкциях, содержащих wish/if only. Письменно переведите предложения.

1. I wish I (could/can/shall) prove ‘continuum hypothesis’. 2. The students wish scientists (will develop/had developed/develop) an intelligent tutoring system with capabilities for adaptation to a particular student. 3. We wish we (don’t use/don’t have to use/didn’t have to use) similar notations for the set of elements of any finite or infinite sequence. 4. The government wishes hackers (would not penetrate/shouldn’t penetrate/don’t penetrate) computer systems in control of missiles. 5. If only he (learn/had learned/have learned) to program! 6. If only I (found/find/should find) the solution to the problem!

VI. Переведите письменно предложения, учитывая способы перевода придаточных предложений и оборотов, содержащих формы сослагательного наклонения.

1. I’d rather establish this relation by assuming e to be any preassigned positive number as small as desired. 2. You’d better use consistent notations. 3. It’s time we started the experiment. 4. It is essential that one should make a distinction between finite and infinite sets. 5. He advised that the students should read this book. 6. In axiomatic set theory Zermelo regards a set as if it were simply an undefined object satisfying a given list of axioms.

VII. Выберите правильный вариант и письменно переведите предложения, содержащие формы сослагательного наклонения.

1. I (would better/would rather/had rather) represent zero or any positive integer by using just these digits. 2. You (had better/would better/would rather) take care to distinguish this subset from the subset of X. 3. It’s time you (solve/solved/would solve) the problem. 4. It’s necessary that the students (should give/would give/gave) suitable examples to prove the theory. 5. Cantor insisted that the set of transcendental numbers (had/should have/will have) the power of the continuum.

VIII. Переведите письменно предложения, обращая внимание на то, как переводятся модальные глаголы в сочетании с разными формами инфинитива.

1. They must have shown that this property is true. 2. The function must have been represented geometrically. 3. He may have defined the function by means of a formula. 4. The values may have been selected quite small. 5. You might have encountered this symbol in the previous chapter. 6. You should have fulfilled your promise.