16.2 Networking Protocols
16.2.1 The OSI model
There are many types of networking protocols. We will focus on the OSI model and some of the more common networking protocols. When two nodes on a network wish to communicate, they must use networking protocols.

The OSI model was developed by ISO (International Organization for Standardization) to describe the functions that occur as two nodes on a network communicate. OSI is short for Open Systems Interconnection and is an internationally recognized, structured, and standardized architecture for network design. The 7-layer OSI reference model or just OSI model is one of the most useful tools we have for analyzing and comparing various technologies and networking products from different vendors. The model or "protocol stack" itself is rarely adhered to completely by any one vendor but it serves as a frame of reference for everyone to compare to.

The OSI model helps to define the process of communication between two devices or nodes on a network. Various protocols operate at each layer to pass the data from one device or node to another. These "nodes" can be user devices such as workstations and servers or network devices such as routers and switches. The important thing to realize is that the OSI model does not really come into play unless we want to communicate over a network between two devices. If you open a local document file on your hard disk with a word processor on your standalone PC at home, the OSI model is not involved. If you open a file from your workstation, which is actually in your home folder on a network server in another building, the OSI model is involved. When your word processor attempts to open the file on the server, network communication must take place from the users level down through the protocol stack on your workstation and out the Network Interface Card (NIC) on your computer, through the network media (cable) to the server and up its protocol stack to retrieve the file.

By splitting the process of communicating from one device to another over a network into smaller digestible chunks we can study it more easily. We also provide a structured blueprint from which network equipment engineers can create their products and create modular network designs. Thus, the OSI model facilitates communication and education relating to networks as well as the development of networking products and services. Figure shows the seven layers of the OSI model with a brief description of the function of each layer. These are divided into two main groups: the Applications layers and the Data Flow layers as shown in Figures and .

Application layer (Layer 7) - Network Processes to applications.

Provides network communication services to user applications for file transfers, WWW access, Email and remote terminal access. For example, a word processing application is serviced by file transfer services at this layer.

Presentation layer (Layer 6) - Data Representation

Provides translation for character sets, data compression, encryption and support for various file types (graphics, video, sound).

Session layer (Layer 5) - Inter-host Communications

Keeps track of which application is doing what and where. It establishes, manages and terminates the relationships between multiple applications such as client and server. Acts as a funnel to the lower 4 layers. This layer establishes, maintains, and manages sessions between applications.

Transport layer (Layer 4) - End-to-End Connections

This is the shipping department of OSI model. Packages can be shipped via guaranteed services or non-guaranteed. Segments information into smaller Packets for processing by the network layer. This layer segments and reassembles data into a data stream. The transport layer has the potential to guarantee a connection and offer reliable transport.

Network layer (Layer 3) - Addressing and Best Path

Packet are addressed and directed through an internetwork (between networks) at this layer. This layer determines the best way to move data from one place to another. The router operates at this layer. This layer uses logical addressing schemes that can be managed by an administrator. The Internet Protocol (IP), IPX and AppleTalk are examples of addressing schemes.

Data link layer (Layer 2) - Access to Media

These are the shipping methods and drivers. The drivers of the shipping vehicles (Truck, Train, Boat, Plane) and how they get on to the pathways. This layer provides physical transmission across the medium. It handles error notification, network topology, and flow control. This layer uses Media Access Control (MAC) addresses, which also are referred to as physical or hardware addresses.

Physical layer (Layer 1) - Binary Transmission

The physical cabling and connections (wired and wireless) that make up the pathways for electricity, light and radio waves (the highways, railways, shipping lanes and airways). This layer provides the electrical, mechanical, procedural, and functional means for activating and maintaining the physical link between systems. This layer uses such physical media as twisted-pair, coaxial, and fiber-optic cable.

Interactive Media Activity (Flash, 95 kB)
  OSI Layer Exercise

In this drag-and-drop media activity, you are to drag the term on the left of the graphic to match the appropriate definition listed on the right.