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.
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.
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.
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.
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.
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.
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.
An intermediate node calculates and verifies the checksum for each packet. It may also reroute the message to avoid congestion on the network.
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.
Task 11. Fill in the blanks with the proper word from the text 11B.
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1. The message is ____________into bits by the ________________layer.
2. The ___________ layer confirms the arrival of the packets, logs them in, and calculates the ____________ for each packet.
3. The incoming ___________ are recounted by the network layer for security and billing purposes.
4. The checksum is re-____________by the transport layer which also reassembles the message segments.
5. The parts of the message are held _______________the session layer _____________the message is complete. Then it sends the message to the next _______________________.
6. The message is compressed and ___________________ by the presentation layer.
7. The application layer converts the bits into __________________ characters, and ____________ the data to the correct application.
Task 12. Mark the following statements true or false.
1. Most of the work that an application does to prepare a message for sending over a network is not seen by the user.
2. ASCII is always used to transmit data.
3. The encryption layer compresses the message.
4. The network layer keeps track of how many packets are in each message.
5. The network layer keeps a copy of each packet until it arrives at the next node undamaged.
6. Analogue signals are used on ordinary telephone lines.
7. When a message arrives at its destination, it passes through the same seven network communications layers as when it was sent, but in reverse order.
Task 13. Find the answers to these questions in the text 11B.
1. Into what units is data subdivided by the following layers?
a. transport layer
b. network layer
2. What is the purpose of a transmission checksum test?
3. How long does the data-link layer keep a copy of each packet?
4. What processes can be carried out at intermediate nodes?
5. Which network communications layer is described by each of the following statements?
Task 14. Translate the sentences into English.
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2. , , , , .
3. , "", , , .
4. , , , , .
5. 璺 , .
6. .
7. , .
8. , , , , .
9. .
10. 1 . .
11. 㳿 璺: Ethernet .
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12. , , .
13. .
14. .
15. 璺 , "".
16. "" (to convert) , .
17. .
18. , , .
19. ᒺ .
20. .
21. , -, .
22. .
23. , .
24. , 璺 .
25. 璺 .
26. , .
27. , , , , .
GRAMMAR REVIEW
QUESTIONS
General questions
They begin with an auxiliary verb (Yes / No questions)
| Tense | Question |
| Present Simple | Do you live in London? / Does he live in London? we she I it they Are you (a) student(s)? / Is he a student? we she they it |
| Present Continuous | Are you working now? / Is he working now? we she they it Am I working now? |
| Present Perfect | Have you been to London? / Has he been to London? we she they it I |
| Present Perfect Continuous | Have we been waiting here long? / Has he been waiting here long? you she I they it |
| Past Simple | Did I see Tom yesterday? Were you at home yesterday? We we you they they Was he he she she it it |
| Past Continuous | Were you watching TV at 7 oclock yesterday? we they Was he watching TV at 7 oclock yesterday? she it I |
| Past Perfect | Had Sally done the work by the time the boss came? |
| Future Simple | Will Sally stay at home tomorrow? |
| Future Continuous | Will Sally be working all day tomorrow? |
| Modal Verbs | Can you help me? Must he send you the documents? |
Special questions
They begin with a question-word (why, who, what, where, when, how, whose, which) or word-combinations: how + adjective / adverb; what + noun
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| Tense | Questions |
| Present Simple |  Why do you like abstract art?
does he
 Why are they busy?
is he
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| Present Continuous | What are you doing now?
is he
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| Present Perfect |   How many letters have you sent yet?
has he
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| Present Perfect Continuous |  How long have they been studying English?
has she
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| Past Simple |  you
Who did he see at the meeting yesterday?
they etc.
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| Past Continuous | What were you, they, doing when we arrived?
was he, she, it, I
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| Past Perfect | he
Where had she lived before he (she, they) moved to Paris?
they etc.
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| Future Simple |  you
What will he do in summer?
they etc.
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| Future Continuous | you
Where will they be staying while in Paris?
he etc.
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| Modal Verbs | Where can I leave my bags? When should they contact you? |
Note! If a question has a preposition it is usually put at the end of the sentence.
E.g.: She was talking to an old friend.
Who was she talking to?
He is looking at his girl-friend.
Who is he looking at?
Alternative questions
They begin with an auxiliary verb and have or + an alternative
E.g.: Do you like classical or pop music?
Have you bought five or six cakes?
Did they stay in London or in Brighton?
Tag-questions
They are formed with the auxiliary verb used to form general questions in each grammar tense. If the sentence is positive, the tag question is negative and if it is negative, the tag-question is positive.
E.g.: He likes cats, doesnt he?
They are not our students, are they?
She has been working here for five years, hasnt she?
There are no armchairs here, are there?
But! I am right, arent I?
We put the tag-question will you at the end of a request to make it more polite.
E.g.: Close the door, will you?
We put the tag-question shall we at the end of a question if it is a suggestion to do something together.
E.g.: Lets go to the garden to have tea, shall we?
