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IMPORTANTA TCP/IP - Grafica protocoalelor TCP/IP

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Although the OSI reference model is universally recognized, the historical and technical open standard of the Internet is Transmission Control Protocol/Internet Protocol (TCP/IP). The TCP/IP reference model and the TCP/IP protocol stack make data communication possible between any two computers, anywhere in the world, at nearly the speed of light. The TCP/IP model has historical importance, just like the standards that allowed the telephone, electrical power, railroad, television, and videotape industries to flourish. To get up-to-date information on networking models and standards, visit the following websites:

The U.S. Department of Defense (DoD) created the TCP/IP reference model because it wanted a network that could survive any conditions, even a nuclear war. To illustrate further, imagine a world at war, criss-crossed by different kinds of connections - wires, microwaves, optical fibers, and satellite links. Then imagine that you need information/data (in the form of packets) to flow, regardless of the condition of any particular node or network on the internetwork (which in this case may have been destroyed by the war). The DoD wants its packets to get through every time, under any conditions, from any one point to any other point. It was this very difficult design problem that brought about the creation of the TCP/IP model, and which has since become the standard on which the Internet has grown.

As you read about the TCP/IP model layers, keep in mind the original intent of the Internet; it will help explain why certain things are as they are. The TCP/IP model has four layers: the application layer, the transport layer, the Internet layer, and the network access layer. It is important to note that some of the layers in the TCP/IP model have the same name as layers in the OSI model. Do not confuse the layers of the two models, because the application layer has different functions in each model.

Application Layer
The designers of TCP/IP felt that the higher level protocols should include the session and presentation layer details. They simply created an application layer that handles high-level protocols, issues of representation, encoding, and dialog control. The TCP/IP combines all application-related issues into one layer, and assures this data is properly packaged for the next layer.

Transport Layer
The transport layer deals with the quality-of-service issues of reliability, flow control, and error correction. One of its protocols, the transmission control protocol (TCP), provides excellent and flexible ways to create reliable, well-flowing, low-error network communications. TCP is a connection-oriented protocol. It dialogues between source and destination while packaging application layer information into units called segments. Connection-oriented does not mean that a circuit exists between the communicating computers (that would be circuit switching). It does mean that Layer 4 segments travel back and forth between two hosts to acknowledge the connection exists logically for some period. This is known as packet switching.

Internet Layer
The purpose of the Internet layer is to send source packets from any network on the internetwork and have them arrive at the destination independent of the path and networks they took to get there. The specific protocol that governs this layer is called the Internet protocol (IP). Best path determination and packet switching occur at this layer. Think of it in terms of the postal system. When you mail a letter, you do not know how it gets there (there are various possible routes), but you do care that it arrives.

Network Access Layer

The name of this layer is very broad and somewhat confusing. It is also called the host-to-network layer. It is the layer that is concerned with all of the issues that an IP packet requires to actually make a physical link, and then to make another physical link. It includes the LAN and WAN technology details, and all the details in the OSI physical and data link layers.

Grafica protocoalelor TCP/IP

  • FTP - File Transfer Protocol
  • HTTP - Hypertext Transfer Protocol
  • SMTP - Simple Mail Transfer protocol
  • DNS - Domain Name System
  • TFTP - Trivial File Transfer Protocol

The TCP/IP model emphasizes maximum flexibility, at the application layer, for developers of software. The transport layer involves two protocols - transmission control protocol (TCP) and user datagram protocol (UDP). You will examine these, in detail, later in the CCNA curriculum. The lowest layer, the network access layer, refers to the particular LAN or WAN technology that is being used.

In the TCP/IP model, regardless of which application requests network services, and regardless of which transport protocol is used, there is only one network protocol - internet protocol, or IP. This is a deliberate design decision. IP serves as a universal protocol that allows any computer, anywhere, to communicate at any time.

Comparatia intre OSI si TCP/IP ( protocoale)


  • both have layers
  • both have application layers, though they include very different services
  • both have comparable transport and network layers
  • packet-switched (not circuit-switched) technology is assumed
  • networking professionals need to know both


  • TCP/IP combines the presentation and session layer issues into its application layer
  • TCP/IP combines the OSI data link and physical layers into one layer
  • TCP/IP appears simpler because it has fewer layers TCP/IP protocols are the standards around which the Internet developed, so the TCP/IP model gains credibility just because of its protocols. In contrast, typically networks aren't built on the OSI protocol, even though the OSI model is used as a guide. Otherwise it is wrong and poor grammar.


You know that all communications on a network originate at a source, and are sent to a destination, and that the information that is sent on a network is referred to as data or data packets. If one computer (host A) wants to send data to another computer (host B), the data must first be packaged by a process called encapsulation.

Encapsulation wraps data with the necessary protocol information before network transit. Therefore, as the data packet moves down through the layers of the OSI model, it receives headers, trailers, and other information.


The word 'header' means that address information has been added.

To see how encapsulation occurs, lets examine the manner in which data travels through the layers as illustrated in the Figure . Once the data is sent from the source, as depicted in the Figure, it travels through the application layer down through the other layers. As you can see, the packaging and flow of the data that is exchanged goes through changes as the networks perform their services for end-users. As illustrated in the Figures, networks must perform the following five conversion steps in order to encapsulate data:

Figure :

  1. Build the data.

As a user sends an e-mail message, its alphanumeric characters are converted to data that can travel across the internetwork.

  1. Package the data for end-to-end transport.

The data is packaged for internetwork transport. By using segments, the transport function ensures that the message hosts at both ends of the e-mail system can reliably communicate.

  1. Append (add) the network address to the header.

The data is put into a packet or datagram that contains a network header with source and destination logical addresses. These addresses help network devices send the packets across the network along a chosen path.

  1. Append (add) the local address to the data link header.

Each network device must put the packet into a frame. The frame allows connection to the next directly-connected network device on the link. Each device in the chosen network path requires framing in order for it to connect to the next device.

  1. Convert to bits for transmission.

The frame must be converted into a pattern of 1s and 0s (bits) for transmission on the medium (usually a wire). A clocking function enables the devices to distinguish these bits as they travel across the medium. The medium on the physical internetwork can vary along the path used. For example, the e-mail message can originate on a LAN, cross a campus backbone, and go out a WAN link until it reaches its destination on another remote LAN. Headers and trailers are added as data moves down through the layers of the OSI model.

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