The Open System Interconnection (OSI) Model
The Open System Interconnection (OSI) reference model describes how information from a software application in one computer moves through a network medium to a software application in another computer. The OSI reference model is a conceptual model composed of seven layers, each specifying particular network functions.
The model was developed by the International Organization for Standardization (ISO) in 1984, and it is now considered the primary architectural model for intercomputer communications. The OSI model divides the tasks involved with moving information between networked computers into seven smaller, more manageable task groups. A task or group of tasks is then assigned to each of the seven OSI layers. Each layer is reasonably self-contained so that the tasks assigned to each layer can be implemented independently. This enables the solutions offered by one layer to be updated without adversely affecting the other layers.
The seven layers of the OSI model are:
- Layer 7 - Application
- Layer 6 - Presentation
- Layer 5 - Session
- Layer 4 - Transport
- Layer 3 - Network
- Layer 2 - Data link
- Layer 1 - Physical
Layer 7 - Application Layer
This is the layer that interacts with the operating system and/or application whenever the user chooses to transfer files, e-mail, create terminal sessions or other network software services or related activities. Telnet and FTP are applications that exist entirely in the application level.
Layer 6 - Presentation Layer
Layer 6 takes the data provided by the Application layer and converts it into a standard format that the other layers can understand. Protocols at the presentation layer are part of the operating system and application the user runs on a workstation. Information is formatted for display or printing in this layer. Codes within the data, such as tabs or special graphics sequences, are interpreted. This layer also encrypts data to be sent across a network, as well as providing translation of other character sets.
Layer 5 - Session Layer
The session layer establishes, manages, and terminates communication sessions. Communication sessions consist of service requests and service responses that occur between applications located in different network devices. These requests and responses are coordinated by protocols implemented at the session layer. Some examples of session-layer implementations include Zone Information Protocol (ZIP), the AppleTalk protocol that coordinates the name binding process; and Session Control Protocol (SCP), the DECnet Phase IV session layer protocol.
Layer 4 - Transport Layer
The transport layer provides a high level of control for moving information between systems, including more sophisticated error handling, prioritization, and security features. The transport layer provides quality service and accurate delivery by providing connection oriented services between two end systems. It controls the sequence of packets, regulates traffic flow, and recognizes duplicate packets. The transport layer assigns packetized information a traffic number that is checked at the destination. If data is missing from the packet, the transport layer protocol at the receiving end arranges with the transport layer of the sending system to have packets re-transmitted. This layer ensures that all data is received and in the proper order.
Layer 3 - Network Layer
The network layer defines the network address, which differs from the MAC address. Some network layer implementations, such as the Internet Protocol (IP), define network addresses in a way that route selection can be determined systematically by comparing the source network address with the destination network address and applying the subnet mask. Because this layer defines the logical network layout, routers can use this layer to determine how to forward packets. Because of this, much of the design and configuration work for internetworks happens at Layer 3, the network layer.
Layer 2 - Data-Link Layer
The data link layer defines the rules for sending and receiving information across the physical connection between two systems. This layer encodes and frames data for transmission, in addition to providing error detection and control. Because the data link layer can provide error control, higher layers may not need to handle such services. However, when reliable media is used, there is a performance advantage by not handling error control in this layer, but in higher layers. Bridges operate at this layer in the protocol stack.
Layer 1 - Physical Layer
Physical layer specifications define characteristics such as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and physical connectors for activating, maintaining, and deactivating the physical link between communicating network systems. Well-know physical layer interfaces for local area networks (LANs) include Ethernet, Token-Ring, and Fiber Distributed Data Interface (FDDI).
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