User:Uossef.72/ Information transmission networks

When forming networks consisting of a set of computers and other devices we take care of converting it from the branching form of the transmitted computer to the serial form To be sent via telephone lines to the receiving computer And then we need to convert these data To the branching form at the receiving tip .We also take care of converting data from digital to analog so that we can send it over telephone lines, which is what modems do .The data transfer organization should be responsible for the detection and analysis of errors and the audit of data Networks are also concerned with the need to direct the data to the desired destination, meaning that there must be nominated (keys) that can elect the data that must be passed to this or that computer To prevent inertness during the transfer process, the analog signal must be amplified because it is important to know the frequency of this signal and not the shape of it. So a set of functions must be achieved in order to transfer data within networks.

Computer networks
A computer network is the infrastructure through which data is transmitted between parties.

The data transfer process is between a data sender (Source) and a receiving party (Destination). The sender must add other information to the transmitted data such as add sender and receiver addresses، Data type, data size, service source, and a lot more. This additional information is inherently diverse، Layers had to be classified so that each (layers)would add information to suit its function, making it easier to develop networks and track technical malfunctions. This is the advantage of having layers in computer networks where functions and problems are easily distributed to the respective layer.

For example, if a computer appears unable to connect to the network, we limit the causes of the problem to Layer 1 or Layer 2.

layers
The functions were divided into seven sections and each of these functions was called a layer – and these layers are, of course, fake - For easy memorization of their order, the first words of the famous saying can be used: *(Please Do not Trust sales people Always)* The first three layers (bottom) depends on the hardware, while the top three layers on software، the Middle is used as a link between application-related services and network services. To build a network, key parts such as peripherals (workstation), network Devices and transmission Media must be provided. The OSI Model consists of seven layers and is shown in the picture.

The seven layers of networks

The first layer
The first layer (physical Layer) and its function is to connect the device to the transmission media Whatever the type of Conveyor medium, whether it is Optical fiber, UTP, Coaxial cable, or even the conveyor medium is vacuum. Function (Physical Layer) converts the information to be transmitted (Transmitted Data) into electrical, optical or wireless signals suitable for the conveyor medium and the signal form (Digital Encoding) suitable for the conveyor medium type and vice versa.

The second layer
The second layer (Data Link) has the function of giving the physical address of the device connected to the network and this address is a constant that does not change by changing the location of the device on the network and can be likened to the serial number of the cellular device (Serial no.), Which remains constant regardless of the cellular network that works on the device. This layer also checks for errors of received information (Cyclic Redundancy Check “CRC”). This layer is also evaluated with an important function (media access control) in the sense of setting up a mechanism to control network selection by parties' It is understood that the network has several users (parties) communicating with each other through the network, so it is necessary to find ways to ensure that the shared network is used by all parties without affecting the other.

the third layer
The third layer (network layer) has the function of selecting the best path that the transmitted information can take in order to reach the intended target (destination) with the best path. This layer also specifies a variable address (Logical Address) for the parties, which is similar to the usual cellular number and this number changes depending on the network on which it operates.

The fourth layer
The fourth layer (Transmission Layer) is classified into two types (Transmission Control Protocol “TCP”) and User Datagram Protocol “UDP. The transmission control protocol adjusts the work of the transmission of information where the parties agree on the size of the transmissions during the data transfer process (Window Size) as well as the use of the retransmission principle in the event of an error in the information received، This protocol also initializes the connection before data transfer (Connection-oriented). As for (user data protocol), unlike (transmission tuning facility), it does not use any means to adjust the data transfer process and the connection is not initialized before the transfer process (Connection-less). This layer specifies the mechanism of sending information, whether it is (Port Control Protocol) or (user data protocol), and also gives port numbers for each of the transmitted and received information.

The fifth layer
The fifth layer (session layer) has the function of opening, closing and monitoring sessions between sender and receiver. For example, when writing the title of Yahoo! In the web browsing program, the site opens only after you click on the word " GO " or (enter) here the (session Layer) opens a  (session) with Yahoo and monitors the session or closes it as required.

The sixth layer
The sixth layer (presentation Layer) acts as a translator between several languages, it is able to determine the type of information received and sent (text, flash, wave, format of transmitted documents, ....) and to identify the program that deals with each type, the debug program often opens the application/program that acts on behalf of the information, for example running acrobat reader inside internet explorer. In order to display the received information (pdf) type. It also recognizes the type (text enconding).

The seventh layer
The seventh layer (application layer) is a program that works through the computer network and examples of IT (Internet Explorer), (Outlook Express) and others.

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