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is responsible for a specific part of network communication. These layers interact with the layer above and below them only. This interaction very narrowly defines a layer’s purpose. The two common network models that use layers are the Open System Inter-connection (OSI) reference model and the TCP/IP reference model.
The OSI Reference Model
The early development of LANs, MANs, and WANs was chaotic in many ways. The early 1980s saw tremendous increases in the number and size of networks. As compa-nies realized the money they could save and the productivity they could gain by using networking technology, they added networks and expanded existing networks almost as rapidly as new network technologies and products were introduced.
By the mid-1980s, these companies began to experience difficulties from all the imple-mented expansions. It became more difficult for networks that used different specifica-tions and implementations to communicate with each other. These companies realized that they needed to move away from proprietary networking systems. Proprietary sys-tems are privately developed, owned, and controlled. In the computer industry, propri-etary is the opposite of open. Proprietary means that one company or a small group of companies controls all usage of the technology. Open means that free usage of the technology is available to the public.
To address the problem of network incompatibility and the inability to communicate with one another, the International Organization for Standardization (ISO) researched different network schemes, such as DECnet, Systems Network Architecture (SNA), and TCP/IP, to find a set of rules. As a result of this research, the ISO created a net-work model that would help vendors create networks that would be compatible and operate with other networks.
The process of breaking down complex communications into smaller discrete tasks can be compared to the process of building an automobile. When taken as a whole, the design, manufacture, and assembly of an automobile is a highly complex process. It
is unlikely that a single person would know how to perform all the required tasks to build a car from scratch. This is why mechanical engineers design the car, manufactur-ing engineers design the molds to make the parts, and assembly technicians each assemble a part of the car.
The OSI reference model, released in 1984, was the descriptive scheme that the ISO created. This reference model provided vendors with a set of standards that ensured greater compatibility and interoperability among the various types of network technol-ogies that were produced by many companies around the world.
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The OSI reference model is the primary model used as a guideline for network commu-nications. Although other models exist, most network vendors today relate their prod-ucts to the OSI reference model, especially when they want to educate users on the use of their products. The OSI reference model is considered the best tool available for teaching people about sending and receiving data on a network.
The OSI reference model defines the network functions that occur at each layer. More importantly, it is a framework that facilitates an understanding of how information travels throughout a network. In addition, the OSI reference model describes how information, or data packets, travels from application programs (such as spreadsheets and documents) through a network medium (such as wires) to another application program that is located in another computer on a network, even if the sender and receiver have different types of network media.
The OSI reference model has seven numbered layers, each of which illustrates a partic-ular network function:
Layer 7—Application layer Layer 6—Presentation layer Layer 5—Session layer Layer 4—Transport layer Layer 3—Network layer Layer 2—Data link layer Layer 1—Physical layer
This separation of networking functions is called layering. Dividing the network into seven layers provides the following advantages:
It breaks network communication into smaller, simpler parts.
It standardizes network components to allow multiple-vendor development and support.
It allows different types of network hardware and software to communicate.
It prevents changes in one layer from affecting the other layers so that they can be developed more quickly.
It breaks network communication into smaller components to make learning easier.
By working through the layers of the OSI reference model, you will understand how data packets travel through a network and what devices operate at each layer. As a result, you will understand how to troubleshoot network problems if they occur dur-ing data packet flow.
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OSI Layers and Functions
Each OSI layer has a set of functions that it must perform for data packets to travel from a source to a destination on a network. The following sections briefly describe each layer in the OSI reference model.
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; instead, it provides services only to applica-tions outside the OSI model. Examples of such applications are spreadsheet programs and word-processing programs. The application layer establishes the availability of intended communication partners and also synchronizes and establishes agreement on procedures for error recovery and control of data integrity. Examples of the Layer 7 applications include Telnet and HTTP.
Layer 6: The Presentation Layer
The presentation layer ensures that the information that the application layer of one system sends out can be read by the application layer of another system. If necessary, the presentation layer translates among multiple data formats by using a common for-mat. One of the more important tasks of this layer is encryption and decryption. The common Layer 6 graphic standards are PICT, TIFF, and JPEG. Examples of Layer 6 standards that guide the presentation of sound and movies are MIDI and MPEG.
Layer 5: The Session Layer
As its name implies, the session layer establishes, manages, and terminates sessions between two communicating hosts. The session layer provides its services to the pre-sentation layer. It also synchronizes dialogue between the two hosts’ presentation lay-ers and manages their data exchange. In addition to handling session regulation, the session layer offers provisions for efficient data transfer, class of service, and exception reporting of session layer, presentation layer, and application layer problems. Exam-ples of Layer 5 protocols are the Network File System (NFS), X-Window System, and AppleTalk Session Protocol (ASP).
Layer 4: The Transport Layer
The transport layer segments data from the sending host’s system and reassembles it into a data stream on the receiving host’s system. The boundary between the transport layer and the session layer can be thought of as the boundary between application pro-tocols and data-flow protocols. Whereas the application, presentation, and session
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layers are concerned with application issues, the lowest four 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 reliability of transport between two hosts are the concern of the transport layer. In providing com-munication service, the transport layer establishes, maintains, and properly terminates virtual circuits. Transport error detection and recovery and information flow control are used to provide reliable service. Examples of Layer 4 protocols are Transmission Control Protocol (TCP), User Datagram Protocol (UDP), and Sequenced Packet Exchange (SPX).
Layer 3: The Network Layer
The network layer is a complex layer that provides connectivity and path selection between two host systems that might be located on geographically separated networks. Additionally, the network layer is concerned with logical addressing. Examples of Layer 3 protocols are Internet Protocol (IP), Internetwork Packet Exchange (IPX), and AppleTalk.
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 logical) addressing, net-work topology, network access, error notification, ordered delivery of frames, and flow control.
Layer 1: The Physical Layer
The physical layer defines the electrical, mechanical, procedural, and functional speci-fications 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.
Peer-to-Peer Communications
For data packets to travel from the source to the destination, each layer of the OSI model at the source must communicate with its peer layer at the destination. This form of communication is called peer-to-peer communication. During this process, the pro-tocols at each layer exchange information, called protocol data units (PDUs), between peer layers. Each layer of communication on the source computer communicates with a layer-specific PDU and with its peer layer on the destination computer, as shown in Figure 2-17.
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Figure 2-17 Peer-to-Peer Communication
Data packets on a network originate at a source and then travel 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. Each layer then adds whatever headers it needs to perform its function. As the data moves through the layers of the OSI model, additional headers are added. The group-ing of data at the Layer 4 PDU is called a segment.
The network layer provides a service to the transport layer. The network layer moves the data through the internetwork by encapsulating the data and attaching a header to create a packet (the Layer 3 PDU). The header contains information required to com-plete the transfer, such as source and destination logical addresses.
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 the physical addresses required to complete the data link functions, and the frame trailer contains the frame check sequence (FCS), which is used by the receiver to detect whether the data is in error. This then becomes the data that is passed down to the physical layer.
The physical layer 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.
Network devices such as hubs, switches, and routers work at the lowest three layers. Hubs operate at Layer 1, switches operate at Layer 2, and routers at Layer 3. The first layer that deals with the end-to-end transport between end users is the transport layer (Layer 4).
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