2.2. Identifying the Seven Layers of the OSI Model and Their Functions

To understand anything and then develop it you need a way to relate to it, and computer communication is no exception. As computer communication became a reality, we needed a common language that we could use to communicate about computer communication. In the early 1980s representatives from over 60 countries, collectively known as the International Standards Organization (ISO), decided on and developed a model of communication upon which hardware and software could be developed and connected. They named the model the Open System Interconnection (OSI) model and then began to use it to create new hardware and software.

Initially, the OSI model was supposed to include an OSI protocol as well, but the OSI protocol was never fully developed. Every protocol that has been developed, however, can be loosely mapped to the OSI model. Since the OSI model is the basis for all computer communication and for all protocols, you should understand the structure, purpose, and function of the model. In this section, we will discuss how each of the layers of the OSI model function.

2.2.1. Critical Information

You should understand that the OSI model is composed of seven layers. You should be able to describe the function of each layer and how each of the adjacent layers relate to each other. Finally, you should understand that the layers that are not adjacent to each other are independent of one another.

2.2.1.1. The Seven Layers of the OSI Model

The OSI model is composed of seven layers (see Figure 2.2). Each of the layers has a defined purpose. The layers are numbered from the bottom up. In the sections that follow, we will discuss the name, number, and function of each layer of the model, beginning with the top layer.

Figure 2.2. The OSI model

2.2.1.1.1. Application Layer

The Application layer, also called Layer 7, is the highest layer in the OSI model. It contains applications that facilitate network communication. These are not applications like Microsoft Word or Excel, but rather application protocols such as Hypertext Transfer Protocol (HTTP) for browsing the World Wide Web or File Transfer Protocol (FTP) for transferring files on net-works and over the Web. At the Application layer, the data still resembles something that people can read and interpret.

2.2.1.1.2. Presentation Layer

At the Presentation layer (Layer 6) data is first converted into a form that can be sent over a network. At this layer data is compressed and decompressed and encrypted or decrypted, depending on which direction it's traveling. You can think of the Presentation layer as the "translation layer."

2.2.1.1.3. Session Layer

The Session layer (Layer 5) is responsible for establishing, synchronizing, maintaining, and then terminating the sessions between computers. It also handles error detection and notification. You can think of the Session layer as the "traffic cop" that directs the network traffic and lets the appropriate traffic flow at the appropriate time.

2.2.1.1.4. Transport Layer

The Transport layer (Layer 4) handles the actual processing of data between devices. This layer is responsible for resending any packets that do not receive an acknowledgment from the destination address. It's also responsible for any problems that are associated with fragmentation of packets.

2.2.1.1.5. Network Layer

The Network layer (Layer 3) is responsible for providing the mechanism by which data can be moved from computer to computer or from network to network. The Network layer does not actually move the data; instead it provides the addressing information and route discovery that are necessary to move the data to the appropriate location. The Network layer contains many protocols that facilitate these services, including Internet Protocol (IP), Internet Control Message Protocol (ICMP), Internet Group Management Protocol (IGMP), and Address Resolution Protocol (ARP).

2.2.1.1.6. Data Link Layer

The Data Link layer (Layer 2) is responsible for sending data to the Physical layer so that it can be put onto the "wire" or network media. The Data Link layer is subdivided into two other layers: the Logical Link Control (LLC) and the Media Access Control (MAC) layers. The LLC connects the Data Link layer to the higher-level protocols such as IP at the Network layer. The MAC layer connects the Data Link layer to the physical connection and provides the MAC address. The Data Link layer also defines the technology that is used for the network. This layer can also perform checksums, which are calculations that the system uses to make sure that packets are not damaged in transit.

2.2.1.1.7. Physical Layer

The Physical layer (Layer 1) defines the physical characteristics of the network such as the type of cable that must be used as well as the voltage that will be used to transmit data through the network. Since the Physical layer defines these characteristics, it also establishes the topology of the network. Many standards are defined at this layer, such as the IEEE 802.3 standard for Ethernet as well as the IEEE 802.5 standard for Token Ring networks.

2.2.2. Exam Essentials

Be able to list the names and the order of each of the layers of the OSI model. The OSI model has seven layers as follows:

7. Application

6. Presentation

5. Session

4. Transport

3. Network

2. Data Link

1. Physical

Understand the purpose of each layer of the OSI model. You should understand the main purpose and functions of each layer of the OSI model. In addition, you should be familiar with the types of protocols that are found in each layer.