Communication Network.

Communication Network.

1 - The application layer is the only part of the communication process that the user sees, and even then, he does not see the bigger part of the work performed by the application to prepare the message to be sent over the network. The layer converts the data of a message (which can be read by any person) in bits, and appends a header identifying the sending computer and receptor.

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


3 - The session layer and opens the communication task is to keep communication flowing between all nodes of the network. She determines boundaries (called bracketing) for the beginning and end of the message and determines whether the message is sent in half-duplex, with each computer sending and receiving alternately, or full-duplex, with both sending and receiving simultaneously. The details of these options are placed in the header of the session.

4 - The Transport Layer protects data being sent. She divided them into segments and creates checksum tests
- Mathematical sums content-based data
- it can be used later to determine whether the data were truncated. You can also generate backup copies of data. The header of this layer identifies the checksum of each segment and its position in the message.

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

6 - The data link layer oversees the transmission. It confirms the checksum, addresses and duplicate packets. This layer keeps a copy of each packet until receiving confirmation from the next point of the way in which the packet arrived unchanged.

7 - The physical layer encodes the packets through a medium that can take them - as an analog signal, if the message is being transmitted via telephone line - and sends packets through that medium.

8 - An intermediate node calculates and verifies the checksum of each package. You can also redirect the message to avoid network congestion.

9 - At the receiving node, the proceeso layer that sent the message in this way takes the reverse direction. The physical layer reconverted the message in bits. The data link layer recalculates the checksum, and confirms the arrival have access to packages. The network layer recounts the packets coming for security and verification. The transport layer recalculates the checksum and rearranges the segments of the message.

The session layer maintains the message parts until it is complete, sending it to the next layer. The presentation layer expands and decrypts the message. Layer application converts the bits into characters that can be read and directs the data to the correct application.

Source: Evolution of Computers