The OSI Reference Model - A Clear and Concise Illustration !
The Open System Interconnection (OSI) model is a set of protocols that attempt to define and standardize the data communications process. The OSI model is set by the International Standards Organization (ISO). The OSI model has the support of most major computer and network vendors, many large customers, and most governments, including the United States.
The OSI model is a concept that describes how data communications should take place. It divides the process into seven groups, called layers. Into these layers are fitted the protocol standards developed by the ISO and other standards bodies, including the Institute of Electrical and Electronic Engineers (IEEE), American National Standards Institute (ANSI), and the International Telecommunications Union (ITU), formerly known as the CCITT (Comite Consultatif Internationale de Telegraphique et Telephone).
The OSI model is not a single definition of how data communications actually takes place in the real world. Numerous protocols may exist at each layer. The OSI model states how the process should be divided and what protocols should be used at each layer. If a network vendor implements one of the protocols at each layer, its network components should work with other vendors’ offerings.
The OSI model is modular. Each successive layer of the OSI model works with the one above and below it. At least in theory, you may substitute one protocol for another at the same layer without affecting the operation of layers above or below. For example, Token Ring or Ethernet hardware should operate with multiple upper-layer services, including the transport protocols, network operating system, internetwork protocols, and applications interfaces. However, for this interoperability to work, vendors must create products to meet the OSI model’s specifications.
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The OSI model is not a single definition of how data communications takes place. It states how the processes should be divided and offers several options. In addition to the OSI protocols, as defined by ISO, networks can use the TCP/IP protocol suite, the IBM Systems Network Architecture (SNA) suite, and others. TCP/IP and SNA roughly follow the OSI structure.
Although each layer of the OSI model provides its own set of functions, it is possible to group the layers into two distinct categories. The first four layers— physical, data link, network, and transport—provide the end-to-end services necessary for the transfer of data between two systems. These layers provide the protocols associated with the communications network used to link two computers together.
The top three layers—the application, presentation, and session layers — provide the application services required for the exchange of information. That is, they allow two applications, each running on a different node of the network, to interact with each other through the services provided by their respective operating systems. The following is a description of just what each layer does.
1. The Physical layer provides the electrical and mechanical interface to the network medium (the cable). This layer gives the data-link layer (layer 2) its ability to transport a stream of serial data bits between two communicating systems it conveys the bits that move along the cable. It is responsible for making sure that the raw bits get from one place to another, no matter what shape they are in, and deals with the mechanical and electrical characteristics of the cable.
2. The Data-Link layer handles the physical transfer, framing (the assembly of data into a single unit or block), flow control and error-control functions (and retransmission in the event of an error) over a single transmission link it is responsible for getting the data packaged and onto the network cable. The data link layer provides the network layer (layer 3) reliable information-transfer capabilities. The data-link layer is often subdivided into two parts—Logical Link Control (LLC) and Medium Access Control (MAC)—depending on the implementation.
3. The Network layer establishes, maintains, and terminates logical and/or physical connections. The network layer is responsible for translating logical addresses, or names, into physical addresses. It provides network routing and flow-control functions across the computer-network interface.
4. The transport layer ensures data is successfully sent and received between the two computers. If data is sent incorrectly, this layer has the responsibility to ask for retransmission of the data. Specifically, it pro-vides a network-independent, reliable message-independent, reliable message-interchange service to the top three application-oriented layers. This layer acts as an interface between the bottom and top three layers. By providing the session layer (layer 5) with a reliable message-transfer service, it hides the detailed operation of the underlying network from the session layer.
5. The Session layer decides when to turn communication on and off between two computers—it provides the mechanisms that control the data-exchange process and coordinates the interaction between them. It sets up and clears communication channels between two communicating components. Unlike the network layer (layer 3), it deals with the programs running in each machine to establish conversations between them.
6. The Presentation layer performs code conversion and data reformatting (syntax translation). It is the translator of the network, making sure the data is in the correct form for the receiving application. Of course, both the sending and receiving applications must be able to use data sub-scribing to one of the available abstract data syntax forms.
7. The Application layer provides the user interface between the software running in the computer and the network. It provides functions to the user’s software, including file transfer access and management (FTAM) and electronic mail.
Unfortunately, protocols in the real world do not conform precisely to these neat definitions. Some network products and architectures combine layers. Others leave layers out. Still others break the layers apart. But no matter how they do it, all working network products achieve the same result—getting data from here to there. The question is, do they do it in a way that is compatible with networks in the rest of the world?
Protocols & The OSI Model
A LAN protocol is a set of rules for communicating between computers. Protocols govern format, timing, sequencing, and error control. Without these rules, the computer cannot make sense of the stream of incoming bits.
But there is more than just basic communication. Suppose you plan to send a file from one computer to another. You could simply send it all in one single string of data. Unfortunately, that would stop others from using the LAN for the entire time it takes to send the message. This would not be appreciated by the other users. Additionally, if an error occurred during the transmission, the entire file would have to be sent again. To resolve both of these problems, the file is broken into small pieces called packets and the packets are grouped in a certain fashion. This means that information must be added to tell the receiver where each group belongs in relation to others, but this is a minor issue. To further improve transmission reliability, timing information and error correcting information are added. Because of this complexity, computer communication is broken down into steps. Each step has its own rules of operation and, consequently, its own protocol. These steps must be executed in a certain order, from the top down on transmission and from the bottom up on reception. Because of this hierarchical arrangement, the term protocol stack is often used to describe these steps. A protocol stack, therefore, is a set of rules for communication, and each step in the sequence has its own subset of rules.
What is a protocol, really? It is software that resides either in a computer’s memory or in the memory of a transmission device, like a network interface card. When data is ready for transmission, this software is executed. The software prepares data for transmission and sets the transmission in motion. At the receiving end, the software takes the data off the wire and prepares it for the computer by taking off all the information added by the transmitting end. There are a lot of protocols, and this often leads to confusion. A Novell network communicates through its own set of rules (its own protocol called IPX/SPX), Microsoft does it another way (NetBEUI). DEC does it a third way (DECnet), and IBM does it yet a fourth (NetBIOS). Since the transmitter and the receiver have to “speak†|
