Communications

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

2. The presentation layer ensures that the message is transmitted in а 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 аll 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 а 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 а route for the message. It forms data into packets, counts them, and adds а 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 асору 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 medium that will carry them – such as analogue signal, if the message is going across telephone line – and sends the packets along that medium.

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

9. At the receiving node, the layered process sent the message on its way is reversed. The physical layer reconverts the message into 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 sport 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.

15. Fill in the missing words in the following sentences then put the sentences in the correct order:

a) The checksum is recalculated by the _______________ layer which also reassembles the message _______________.

b) The message is _______________ and _______________ by the presentation layer.

c) The message is reconverted into _______________ by the physical layer.

d) The session layer then sends the message to the next _______________.

е)The application layer converts the bits into _______________ characters, and directs the data to the correct.....

f)The incoming _______________ are recounted by the network layer for _______________ and billing purposes.

g) The______________ layer confirms the arrival of the packets, ____________them in, and calculates the ______________ for each packet.

h) The parts of the message are _______________ by the _______________ layer until the message is _______________.

16. Using information from the text, mark the following as True or False:

		True	False
	Most of the work that an application does to prepare а message for sending over а network is not seen by the user.
	АSСII is always used to transmit data.
	The encryption layer compresses the message.
	The network layer keeps track of how many packets are in each message.
	The network layer keeps а сору of each packet until it arrives at the next node undamaged.
	Analogue signals are used on ordinary telephone lines.
	When а message arrives at its destination, it passes through the same seven network communications layers as when it was sent, but in reverse order.