Principles of Internet working, OSI - TCP / IP model

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Centralized computer systems - the concept of 'computer center' where a large computer serves the entire organization has been replaced by 'computer network' - a large number of independent computers that are linked together in a network that can exchange information. The network allowed the following:

In essence it is about connecting more small local area networks (LAN) to a larger group (MAN) and their integration into a unique set of state, world (WAN). LAN is a network of privately owned within one or several buildings whose distance is not greater than a few kilometers. Uses bus or star topology with data transfer speeds 10-1000 Mbps (1 megabit/sec = 1'000'000 bit/sec). MAN is a larger version of the LAN that uses similar technology, connecting multiple companies or institutions or city and can be private or public. Can support both data and voice and to be associated with the local cable TV network. Commonly used DQDB (Distributed Queue Dual Bus), consists of two unidirectional bus (cable) that all computers are connected, which simplifies design. WAN connects geographically large area, country or continent. The terminology used to comprehend its functionality is based on the following basic concepts:

LAN, MAN and WAN are terms that describe the computer network and geographical distribution. Characteristics outside the terms stated as PAN (Personal Area Network), SAN (Storage Area Network) and WLAN (Wireless Local Area Network); personal network, data storage and wireless networks. Technology development will certainly define or define another kind of '-AN'. Most networks are organized as a series of layers or levels, each layer is built over those below him, without which is not functional. Number of levels, names, contents and functions are different layers of different types of networks, i.e. different models of network operations occurred during the historical development of the various manufacturers of computing and networking equipment. Inability to communicate different network models has led to a lot of problems in the alignment of their mutual communication. But one model 'survivor' is up to date and is the basis for the functioning of the Internet. This is the TCP / IP model.

The following figure shows the organization of the TCP / IP model by layers in relation to the OSI model.

 OSI - TCP / IP model  

Figure 7.4.3 OSI and TCP / IP reference model.

In all networks, the task of each level is to offer some services to the highest levels that do not necessarily take into account the details of the implementation of these services. Layer 'n' on one computer communicates with a layer of 'n' on the other computer, with a set of rules called a protocol. Thus, the communication between two or more computers on the same layer (peer-to-peer) according the network model.

There is no real data transfer between layers - data and control information is sent to the layer below until it reaches the lowest layer below which the physical medium of communication is really going. The interface between adjacent layers, which defines the basic operations and services the lower layer offers the top. Set of layers and protocols makes network architecture. A list of protocols, one for each layer, makes the protocol stack. Data transfer through the layers of the present model better illustrates the animated Figure 7.4.6.

Internet from the viewpoint of the network layer can be viewed as a set of interconnected subnetworks or autonomous systems. There is no real structure, but there are several major backbone to which regional networks are connected, and on them LAN universities, companies, Internet service providers and others. What holds it together is the network protocol level IP (Internet Protocol), whose task in the best possible way to transmit an IP DATAGRAM (IP PACKET) from source to destination, regardless of whether they are on the same network or not, and whether there are or no network among them.

IP packet (DATAGRAM) consists of two basic parts:

Information contained within the entire TCP services which are in turn composed of its parts. In a way around the IP header TCP header in which the data is.

The header contains fields that describe fifteen protocol version, header length, total length of the datagram (up to 64 kB), type of service, a lifetime package, the source and destination address (32 bits) and others.

An IP address consists of two parts: network number (network address) to which is connected a computer and the number of computer (address of the computer) in the network. There are several different formats-class IP address (A, B, C, D), which differ in the number of bits allocated to the network address and the address of the computer and that the total does not exceed 32 bits.

TCP (Transmission Control Protocol) is designed to provide a reliable stream of bytes from one end to the other to unreliable networks, which can be distinguished by topology, bandwidth, delay, packet size, or something else. Every computer that supports TCP has a TCP transmission entity (a piece of software) as a user process or as part of the core that manages TCP flows. TCP entity accepts from local processes data stream users, breaks it into pieces that do not exceed 64 kB and sends each piece as a separate IP datagram that when he gets to the other end, handing the TCP entity restores the original stream of bytes.

TCP service is provided so that both the sender and the receiver create end points (end points) or connection point (SOCKET) each of which is defined by the IP address of the host and port number (16 bit number for host and service ID). To ensure TCP service, it must establish a link between the socket on the host that sends and socket on the host that receives the data. Port numbers below 256 (the well-known ports) are reserved for standard services, and each host can decide for itself how it will allocate its other port's. Sending and receiving TCP entities exchange data in a data SEGMENT. Segment consists of a 20 byte TCP header (with optional part) followed by a 0 or more bytes of data, and the resulting collection of data from several data entering or breaking of one subscription. Each segment including the TCP header must fit into 65'535 byte IP packets. If the segment is too large for the network through which they must pass, the router performs fragmentation into smaller segments, each of which gets its IP header.

TCP segment format consists of:

The header contains ten fields that begin with a source port (16 bits) and a destination port (16 bits), and follows the sequence number of TCP connections and others.

TCP connection is a byte stream, not the flow of messages, i.e., the TCP software does not know what they mean bytes transferred to and does not preserve message boundaries, but sends them as segments which consist of groups of bytes. The basic protocol used by TCP entities working on the principle that after sending segment sender starts its timer. When sent to the segment arrives at the destination recipient sends a confirmation to the segment - a number that matches the number of the next segment, where he expects to receive. If the timer expires before the confirmation is received, the segment is sent again.

UDP (User Data Protocol) to send IP datagrams with short header added without the necessary certificate to establish a connection. Use it, for example, many client-server applications with a single request and a single response, meaning very simple communication.

Packet transmission of the medium takes place through the lowest level of the OSI - TCP / IP model. Transmission paths are different, but almost all have in common that the package adds the MAC address of the network card or some other type of interface (Media Access Control) of source and destination, which together forms the FRAME of data stream exchanged between the source and destination.

Frame is a basic group of data that is in the Ethernet world transformed into digital electrical signal to transfer data between computers. 1500 B most is how much Frame can once convey in the world of Ethernet, the size of an MTU (Maximum Transmission Unit), when in reality it is the maximum packet length in bytes to which some of the interface device can handle. Therefore, the data is to transfer 'chopped' into smaller units that can be sent in a row, in sequence. As the entire Internet is based on TCP / IP model, which are continually being developed and upgraded, given the subject of a separate Chapter 7.4.3. OSI model is a theoretical model that serves as a guide manufacturers in the development of computer hardware and software for maintaining the mutual compatibility. Through him to explain and develop all real models.


Thus, the data transfer between the layers of the TCP / IP - OSI model is carried out according to the rules (protocols) which must be respected, otherwise there is no communication between computers and / or network devices. While not satisfy the rule of a lower layer of the presented model can not be used the rule of the higher layer. Let us imagine that there are four different sizes of boxes, so that the smaller box is in a larger box. The first biggest box is labeled 'ETHERNET' and it is a rule that applies to the method of communication between computers and / or devices via wires ie. physical medium that connects them. When the first box is open inside box is another smaller second box labeled 'IP/ARP' in it is a rule that allows the interlocutors mutually identify and establish communication. When second box is open inside is smaller third box lebeled 'TCP/UDP' and that describes how the identified users to exchange information and what to do if an error occurs. After opening the third box, inside will be fourth smallest box labeled 'HTTP/DNS/SMTP' that will allow the end user to read the contents of a news portal in a web browser or view incoming e-mail. The smallest box can not be opened if it's not properly open larger boxes. During communication, all the boxes have to be opened. If during communication any of boxes for any reason is closed, communication will be interrupted.

Example with boxes leads to the conclusion that each box has in it one or more smaller boxes with the same name, precisely a group of boxes of the same size and the label on the box determines which of them to be open. In the group of same boxes label of the each box may be supplemented, relational using the additional rules (protocols) in order to obtain the desired result. A set of protocols used by Internet best displays 'Scheme of the TCP / IP protocol stack' in a separate Chapter of protocols.


Citing of this page:
Radic, Drago. " IT - Informatics Alphabet " Split-Croatia.
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