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The OSI Model


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2:  The OSI Model

Select Objective:

2.1.   Understand a General Model of Communication in Terms of Layers.

2.2.   Understand the OSI Model.

2.3.   Understand How the OSI Model Compares and Contrasts With the TCP/IP Model.

2:  The OSI Model    Describe how to use the concept of layers to analyze problems in the flow of materials and ideas.
You can use the concept of layers to help you understand the action that occurs during the flow of information from one computer to another. Shown in the graphic are various items that involve a flow of elements(vezi imaginea ANALYZING NETWORK IN LAYERS-pag.3 ). There are four layers (arbitrary number) that help describe the details. You can imagine the analogies as representations of the networking systems of their larger counterparts- the water system, the highway system, the postal system, and the telephone system.

Now examine the 'Comparing Networks' chart.  (vezi tabelul)What network are you examining? What is flowing? What are the different forms of the object that is flowing? What are the rules for flow? Where does the flow occur? The networks listed in this chart, specifically those that refer to communication in layers, give you more analogies to help you understand computer networks.

Another example of how you might use the concept of layers to analyze an everyday subject is to examine human conversation. When you create an idea that you wish to communicate to another person, the first thing you do is choose (often subconsciously) how you want to express that idea, then you decide how to properly communicate it, and finally, you actually deliver the idea. 

Imagine a young person seated at one end of a very long dinner table. On the other end of the table, quite a distance away, sits the young person's grandmother. The youngster speaks English. The grandmother prefers to speak Spanish. The table has been set with a wonderful meal that the grandmother has prepared. Suddenly the young person shouts at the top of his lungs, 'Hey, you! Give me the rice!' and reaches across the table to grab it. In many cultures, this action is considered quite rude. What should the young person have done to communicate his/her wishes in an acceptable manner?

To help you find the solution to this question, analyze the communication process in layers. First you have the idea – the young person wants rice; then you have the representation – spoken English; next is the method of conveyance/transport – 'Hey, you'; and finally, the medium – shouting (sound) and grabbing (physical action) across the table for the rice. 

From this group of four layers, you can see that three of them prevent the young person from communicating his/her idea in an appropriate/acceptable manner. The first layer – the idea – is acceptable. The second layer (representation), of spoken English instead of Spanish, and the third layer (transport), of a loud demand instead of a polite request, most definitely do not follow acceptable social protocol. The fourth layer (medium), shouting and grabbing from the table rather than politely requesting assistance from another person seated nearby, is unacceptable behavior in most any social situation.

By analyzing this interaction in terms of layers you can understand more clearly some of the problems of communication (human or computer), and how you might solve them.    Describe source, destination, and data packet.
All communications on a network originate at a source, then travel to a destination. The information that travels on a network is referred to as data, packet, or data packet. The source address in a packet specifies the identity of the computer that sends the packet. The destination address specifies the identity of the computer that finally receives the packet.

Data are logically grouped units of information that move between computer systems. They include the original user information along with other elements necessary to make communication of the information possible and reliable. At its most basic level, computer data consist of binary digits, or bits (0s and 1s). Computers that send one or two bits of information, however, are not very useful, so other groupings - bytes, kilobytes, megabytes, and gigabytes - are necessary.    Define medium.
During your study of networking, you will hear references to the word 'medium'. (Note: The plural form of medium is media.) In networking, a medium is a material through which data travel. It could be any of the following materials:

  • telephone wires
  • Category 5 UTP (used for 10Base-T Ethernet)
  • coaxial cables (used for cable TV)
  • optical fibers (thin glass fibers that carry light)
  • other types of sophisticated copper wiring

There are two more types of media that are less obvious, but should nonetheless be taken into account in network communications. First, is the atmosphere (mostly oxygen, nitrogen, and water) that carries radio waves, microwaves, and light.

The second medium is not a medium in the physical sense, but are patterns of electromagnetic waves that travel in the vacuum of outer space, where there is virtually no matter, no molecules, and no atoms along which they can travel. Communications without some type of wire or cable are called wireless or free-space communications.    Define protocol.
Protocol is the set of rules that make communication more efficient. Some common examples are as follows:

  • In Congress, a form of Roberts Rules of Order makes it possible for hundreds of Representatives, who all like to talk, to take turns, and to communicate their ideas an orderly manner. 
  • While driving a car, other cars (should!) signal when they wish to make a left turn; if they do not, then the roads would be chaos. 
  • While flying an airplane, pilots obey very specific rules for communication between airplanes and between airplanes and air traffic control. 
  • When answering the telephone, someone says, 'Hello,' then the person calling says, 'Hello. This is. ', and so it goes back and forth. 
  • At a drive-thru restaurant, if someone fails to observe the customary protocols, they usually receive the wrong order. 

One technical definition of data communications protocol is: a set of rules, or an agreement, that determines the format and transmission of data. Layer n on one computer communicates with layer n on another computer. The rules and conventions used in this communication are collectively known as the layer n protocol.'    Describe the evolution of ISO networking standards.
The early development of LANs, MANs, and WANs was chaotic in many ways. The early 1980s saw tremendous expansion in the area of network deployment. As companies realized the money they could save and the amount they could gain in productivity by using network technology, they added networks and expanded existing networks almost as rapidly as new network technologies and products were introduced. By the mid-1980s, growing pains from this expansion were felt. It became harder for networks that used different specifications and implementations to communicate with each other. The only way they could move away from proprietary networking was for network vendors and suppliers to agree on a set of networking standards.

The International Organization for Standardization (ISO) researched networks schemes like DECNET, SNA, and TCP/IP in order to find a set of rules. As a result of this research, the ISO created a network model that would help vendors create networks that would be compatible with, and operate with, other networks. The OSI reference model (Note: Do not confuse with ISO.), released in 1984, was the descriptive scheme they created. It provided vendors with a set of standards that ensured greater compatibility and interoperability between the various types of network technologies that were produced by the many businesses around the world.

To check on the current status of the continually evolving networking standards), view this website address:    State the purpose of the OSI reference model.
The OSI reference model is the primary model for network communications. Although there are other models in existence, most network vendors, today, relate their products to the OSI reference model, especially when they want to educate users on the use of their products. They consider it the best tool available for teaching people about sending and receiving data on a network.

The OSI reference model allows users to view the network functions that occur at each layer. It is a method of illustrating how information travels throughout a network. It explains, visually, how information, or data, travels from application programs (e.g. spreadsheets, documents, etc.), through a network medium (e.g. wires, etc.), to another application program that is located in another computer on a network, even if the sender and receiver have different types of networks.

In the OSI reference model, there are seven numbered layers, each of which illustrates a particular network function. This separation of networking functions is called layering. Dividing the network into these seven layers provides the following advantages:

  • divides the interrelated aspects of network operation into less complex elements
  • defines standard interfaces for plug-and-play compatibility and multi-vendor integration
  • enables engineers to specialize design and promote symmetry in the different internetwork modular functions so that they interoperate
  • prevents changes in one area from affecting other areas, so that each area can evolve more quickly
  • divides the complexity of internetworking into discrete, more easily learned, operation subsets    Identify each of the seven layers of an OSI model.
The problem of moving information between computers is divided into seven smaller and more manageable problems in the OSI reference model. Each of the seven smaller problems is represented by its own layer in the model. The seven layers of the OSI reference model are:

Layer 1: The Physical Layer
Layer 2: The Data Link Layer
Layer 3: The Network Layer
Layer 4: The Transport Layer
Layer 5: The Session Layer
Layer 6: The Presentation Layer
Layer 7: The Application Layer

An easy way to help you memorize the names of the layers might be to use a mnemonic device such as All People Seem To Need Data Processing. During the course of this semester you will start with Layer 1 and work your way through the OSI model, layer by layer.

For more information about the OSI model, visit the following sites:    Describe each of the seven layers of an OSI model.
Each individual OSI layer has a predetermined set of functions that it must perform in order for communication to occur. Note the brief description of each OSI layer in the OSI 7-layer diagram.

The seven layers and their functions can be described, in more detail, as follows:

Layer 7: The application layer

The application layer is the OSI layer that is closest to the user; it provides network services to the user’s applications. It differs from the other layers in that it does not provide services to any other OSI layer, but rather only to application processes outside the scope of the OSI model. Examples of such application processes are spreadsheet programs, word processing programs, and banking terminal programs.

The application layer identifies and establishes the availability of intended communication partners, synchronizes cooperating applications, and establishes agreement on procedures for error recovery and control of data integrity. It also determines whether sufficient resources for the intended communication exist. If you want to remember Layer 7 in as few words as possible, think of browsers.

Layer 6: The presentation layer

The presentation layer ensures that the information that the application layer of one system sends out is readable by the application layer of another system. If necessary, the presentation layer translates between multiple data representation formats by using a common data representation format. If you want to think of Layer 6 in as few words as possible, think of encoding, representation of data, ASCII.

Layer 5: The session layer

As its name implies, the session layer establishes, manages, and terminates sessions between applications. Sessions consist of dialogues between two or more presentation entities. The session layer provides its services to the presentation layer. Also, the session layer synchronizes dialogue between presentation layer entities and manages their data exchange. In addition to basic regulation of conversations (sessions), the session layer offers provisions for data expedition, class of service, and exception reporting of session layer, presentation layer, and application layer problems. If you want to remember Layer 5 in as few words as possible, think of dialogues and conversations.

Layer 4: The transport layer

The transport layer segments and reassembles data into a data stream. The boundary between the session layer and the transport layer can be thought of as the boundary between media-layer protocols and host-layer protocols. Whereas the application, presentation, and session layers are concerned with application issues, the lower three layers are concerned with data transport issues.

The transport layer attempts to provide a data transport service that shields the upper layers from transport implementation details. Specifically, issues such as how reliable transport over an internetwork is accomplished are the concern of the transport layer. In providing reliable service, the transport layer provides mechanisms for the establishment, maintenance, and orderly termination of virtual circuits, transport fault detection and recovery, and information flow control, to prevent one system from overrunning another with data. If you want to remember Layer 4 in as few words as possible, think of quality of service, and reliability.

Layer 3: The network layer

The network layer is a complex layer that provides connectivity and path selection between two end systems that may be located on geographically diverse networks. You'll learn more about Layer 3 in Chapter 3, 'Networking Devices.' If you want to remember Layer 3 in as few words as possible, think of path selection, switching, addressing, and routing.

Layer 2: The data link layer

The data link layer provides reliable transit of data across a physical link. In so doing, the data link layer is concerned with physical (as opposed to network, or logical) addressing, network topology, line discipline (how end systems use the network link), error notification, ordered delivery of frames, and flow control. If you want to remember Layer 2 in as few words as possible, think of naming, framing, and of media access control.

Layer 1: The physical layer

The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. Such characteristics as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other, similar, attributes are defined by physical layer specifications. If you want to remember Layer 1 in as few words as possible, think of signals and media.    Define encapsulation.
Encapsulation allows computers to communicate data. 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. Then, as the data moves down through the layers of the OSI model, it receives headers, footers, and other information. Examine the layer diagrams to see how encapsulation occurs. (Note: The word 'header' means that address information has been added.

As networks perform their services for users, the packaging and flow of the information that is exchanged goes through changes. The following example of encapsulation illustrates the five conversion steps that networks must perform.

  1. Build the data. As a user sends an e-mail message, its alphanumeric characters are converted to data that can traverse 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 ones and zeros (bits) for transmission on the medium (usually a wire). A clocking function enables the devices to distinguish these bits as they traverse 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.    Identify data names at each layer (segment/packet/frame/bits) of the OSI model.
The OSI model defines layers, interfaces between layers, protocol data units (PDUs) for each layer, and protocols for each layer. Each layer of communication, on the source computer, communicates with a layer-specific PDU, and with its peer layer on the destination computer.

Data packets on a network originate at a source and are then sent to a destination. Each layer depends on the service function of the OSI layer below it. To provide this service, the lower layer uses encapsulation to put the PDU from the upper layer into its data field; then it adds whatever headers and trailers the layer needs to perform its function. Next, as the data moves down through the layers of the OSI model, headers and trailers are added. After Layers 7, 6, and 5 have added their information, Layer 4 adds more information. This grouping of data, the Layer 4 PDU, is called a segment.

The network layer, for example, provides a service to the transport layer, and the transport layer presents data to the internetwork subsystem. The network layer has the task of moving the data through the internetwork. It accomplishes this task by encapsulating the data within a header. This header contains information required to complete the transfer, such as source and destination logical addresses. The network layer provides a service to the transport layer by encapsulating the data within a header and creating a packet (the Layer 3 PDU).

The data link layer provides a service to the network layer. It encapsulates the network layer information in a frame (the Layer 2 PDU); the frame header contains information (e.g. physical addresses) required to complete the data link functions. The data link layer provides a service to the network layer by encapsulating the network layer information in a frame.

The physical layer also provides a service to the data link layer. The physical layer encodes the data link frame into a pattern of 1s and 0s (bits) for transmission on the medium (usually a wire) at Layer 1.    Describe the importance of the TCP/IP model.
Although the OSI model is universally recognized, the historical and technical open standard of the Internet is the TCP/IP reference model and the TCP/IP protocol stack. TCP/IP makes data communication between any two computers, anywhere in (or outside of) the world, at nearly the speed of light ,and has historical importance - such as the standards that allowed the telephone, electric, railroad, television, and videotape industries to flourish. To get up-to-date information on networking models and standards, visit the following websites:    Name, and describe, the layers of the TCP/IP model.
The U.S. Department of Defense (DoD) created the TCP/IP 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 Department of Defense 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 then become the standard on which the Internet has grown.

As you read about layers, keep in mind the original intent of the Internet ; it will it 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 layer.

Application Layer
The designers of TCP/IP felt that the higher level protocols should include session and presentation layer details, so 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 assumes 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 could be circuit switching) It does mean that Layer 4 segments travel back and forth for some period.

Internet Layer
The purpose of the Internet layer is to send source packets from any network on the nternetwork 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 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.

For more TCP/IP information, visit the following website    Explain the protocol graph for TCP/IP.
The diagram shown is called a protocol graph. It illustrates the some of the common protocols that are specified by the TCP/IP reference model. At the application layer, you will see different network tasks you may not recognize, but as a user of the Internet, probably use every day. You will examine all of these during the course of the CCNA curriculum. These applications include the following:

  • file transport protocol (FTP)
  • hypertext transfer protocol (HTTP)
  • simple mail transport protocol (SMTP)
  • domain name service (DNS)
  • trivial file transport protocol (TFTP).

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 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, any where, to communicate at any time.    Compare the OSI model with the TCP/IP model, layer by layer.
If you compare the OSI model and the TCP/IP model, you will notice that they have similarities and differences.


  • 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, no networks are built around specific OSI-related protocols, even though everyone uses the OSI model to guide their thinking.    Explain why and how the curriculum will use the OSI model as its foundation, but will also utilize TCP/IP model.
Although TCP/IP protocols are the standards with which the Internet has grown, this curriculum will use the OSI model for the following reasons:

  • It is a worldwide, generic, protocol-independent standard.
  • It has more details, which makes it more helpful for teaching and learning.
  • It has more details, which can be helpful when troubleshooting.

Many networking professionals have different opinions on which model to use. You should become familiar with both. You will use the OSI model as the microscope through which to analyze networks, but you will also use the TCP/IP protocols throughout the CNAP curriculum. Remember that there is a difference between a model (i.e. layers, interfaces, and protocol specifications) and an actual protocol that is used in networking. You will use the OSI model but the TCP/IP protocols.

You will focus on TCP as an OSI Layer 4 protocol, IP as an OSI Layer 3 protocol, and Ethernet as a Layer 2 and Layer 1 technology. The diagram shows that later in the course you will examine one particular data link and physical layer technology out of the many choices available. That will be Ethernet. If you want a preview of Ethernet, visit

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