The OSI Seven-Layer Model

Introduction

One of the most important networking concepts to understand is the Open Systems Interconnect (OSI) reference model. This conceptual model, created by the International Organization for Standardization (ISO) in 1978 and revised in 1984, describes a network architecture that allows data to be passed between computer systems.

This chapter looks at the OSI model and describes how it relates to real-world networking. It also examines how common network devices relate to the OSI model. Even though the OSI model is conceptual, an appreciation of its purpose and function can help you better understand how protocol suites and network architectures work in practical  applications.

The OSI Seven-Layer Model

As shown in Figure 4.1, the OSI reference model is built, bottom to top, in the following order: physical, data link, network, transport, session, presentation, and application. The physical layer is classified as Layer 1, and the top layer of the model, the application layer, is Layer 7.

7 - Application

6 - Presentation

5 - Session

4 - Transport

3 - Network

2 - Data-link

1 - Physical

Physical Layer (Layer 1)

The physical layer of the OSI model identifies the network’s physical character- istics, including the following specifications:

. Hardware: The type of media used on the network, such as type of cable, type of connector, and pinout format for cables.

. Topology: The physical layer identifies the topology to be used in the network. Common topologies include ring, mesh, star, and bus.

In addition to these characteristics, the physical layer defines the voltage used on a given medium and the frequency at which the signals that carry the data oper- ate. These characteristics dictate the speed and bandwidth of a given medium, as well as the maximum distance over which a certain media type can be used.

Data Link Layer (Layer 2)

The data link layer is responsible for getting data to the physical layer so that it can be transmitted over the network. The data link layer is also responsible for error detection, error correction, and hardware addressing. The term frame is used to describe the logical grouping of data at the data link layer.

The data link layer has two distinct sublayers:

. Media Access Control (MAC) layer: The MAC address is defined at this layer. The MAC address is the physical or hardware address burned into each network interface card (NIC). The MAC sublayer also controls access to network media. The MAC layer specification is included in the IEEE 802.1 standard.

. Logical Link Control (LLC) layer: The LLC layer is responsible for the error and flow-control mechanisms of the data link layer. The LLC layer is specified in the IEEE 802.2 standard.

Network Layer (Layer 3)

The primary responsibility of the network layer is routing—providing mecha- nisms by which data can be passed from one network system to another. The network layer does not specify how the data is passed, but rather provides the mechanisms to do so. Functionality at the network layer is provided through routing protocols, which are software components.

Protocols at the network layer are also responsible for route selection, which refers to determining the best path for the data to take throughout the network. In contrast to the data link layer, which uses MAC addresses to communicate on the LAN, network layer protocols use software configured addresses and special routing protocols to communicate on the network. The term packet is used to describe the logical grouping of data at the network  layer.

Transport Layer (Layer 4)

The basic function of the transport layer is to provide mechanisms to transport data between network devices. Primarily it does this in three ways:

. Error checking: Protocols at the transport layer ensure that data is sent or received correctly.

. Service addressing: Protocols such as TCP/IP support many network services. The transport layer makes sure that data is passed to the right service at the upper layers of the OSI model.

. Segmentation: To traverse the network, blocks of data need to be bro- ken into packets that are of a manageable size for the lower layers to handle. This process, called segmentation, is the responsibility of the transport layer.

Protocols at the Transport Layer

Protocols that operate at the transport layer can either be connectionless, such as User Datagram Protocol (UDP) , or connection-oriented, such as Transmission Control Protocol (TCP). For a further discussion of these protocols, and of the dif- ference between connection-oriented and connectionless protocols, refer to the later section “Connectionless and Connection-Oriented   Protocols.”

Flow Control

The transport layer is also responsible for data flow control, which refers to how the receiving device can accept data transmissions. Two common methods of flow control are used:

. Buffering: When buffering flow control is used, data is temporarily stored and waits for the destination device to become available. Buffering can cause a problem if the sending device transmits data much faster than the receiving device can manage it.

. Windowing: In a windowing environment, data is sent in groups of seg- ments that require only one acknowledgment. The size of the window (that is, how many segments fit into one acknowledgment) is defined when the session between the two devices is established. As you can imagine, the need to have only one acknowledgment for every, say, five segments can greatly reduce overhead.

Session Layer (Layer 5)

The session layer is responsible for managing and controlling the synchroniza- tion of data between applications on two devices. It does this by establishing, maintaining, and breaking sessions. Whereas the transport layer is responsible for setting up and maintaining the connection between the two nodes, the ses- sion layer performs the same function on behalf of the application.

Presentation Layer (Layer 6)

The presentation layer’s basic function is to convert the data intended for or received from the application layer into another format. Such conversion is nec- essary because of how data is formatted so that it can be transported across the network. Applications cannot necessarily read this conversion. Some common data formats handled by the presentation layer include the following:

. Graphics files: JPEG, TIFF, GIF, and so on are graphics file formats that require the data to be formatted in a certain way.

. Text and data: The presentation layer can translate data into different formats, such as American Standard Code for Information Interchange (ASCII) and Extended Binary Coded Decimal Interchange Code (EBCDIC).

. Sound/video: MPEG, MP3, and MIDI files all have their own data for- mats to and from which data must be converted.

Another very important function of the presentation layer is encryption, which is the scrambling of data so that it can’t be read by anyone other than the intend- ed recipient. Given the basic role of the presentation layer—that of data-format translator—it is the obvious place for encryption and decryption to take place.

Application Layer (Layer 7)

In simple terms, the function of the application layer is to take requests and data from the users and pass them to the lower layers of the OSI model. Incoming information is passed to the application layer, which then displays the informa- tion to the users. Some of the most basic application-layer services include file and print capabilities.

The most common misconception about the application layer is that it repre- sents applications that are used on a system such as a web browser, word proces- sor, or spreadsheet. Instead, the application layer defines the processes that

enable applications to use network services. For example, if an application needs to open a file from a network drive, the functionality is provided by components that reside at the application layer.

Physical (Layer 1)                         Defines the physical structure of the network and the topology.

Data link (Layer 2)                  Provides error detection and correction. Uses two distinct

sublayers: the Media Access Control (MAC) and Logical Link Control (LLC) layers. Identifies the method by which media are accessed. Defines hardware addressing through the MAC sublayer.

Network (Layer 3)                  Handles the discovery of destination systems and address-

ing. Provides the mechanism by which data can be passed and routed from one network system to another.

Transport (Layer 4)                 Provides connection services between the sending and

receiving devices and ensures reliable data delivery. Manages flow control through buffering or windowing. Provides segmentation, error checking, and service identifi- cation.

Session (Layer 5)                   Synchronizes the data exchange between applications on

separate devices.

Presentation (Layer 6)                   Translates data from the format used by applications into

one that can be transmitted across the network. Handles encryption and decryption of data. Provides compression and decompression functionality. Formats data from the application layer into a format that can be sent over the net- work.

Application (Layer 7)              Provides access to the network for applications.