The Application Layer is the one which interacts with the user. A Software application that implements a communicating component, uses the OSI Application Layer to establish the application’s communication. For example a word processor that does not have communications capabilities, would not be concerned with the OSI Application Layer. On the other hand, an application with communication capabilities (e.g a Web Browser), has to implement the OSI Application Layer standards.
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The main function of this layer is to define data formats, coding, and conversion functions that are applied to application layer data. These functions ensure that information sent from the application layer of one system would be readable by the application layer of another system. Encryption and compression of Data are also defined as Presentation layer functions. [Read more…]

The Session Layer starts, controls, and manages communication conversations (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. The session layer creates ways to imply which flows are part of the same session and which flows must complete before any are considered complete. [Read more…]

The Transport Layer (e.g TCP) provides reliable delivery of Data by using error checking, acknowledgments, flow control, and sequence checking. Multiplexing of incoming data for different flows to applications on the same host (for example, TCP ports) is also performed. Flow control in this layer ensures that the transmitting device does not send more data than the receiving device can process.

Examples of Layer 4 Specifications and Protocols
• TCP
• UDP
• SPX
• Port Numbers (e.g 80 for HTTP)

This Layer defines logical addressing (e.g IP address) in order to identify each endpoint in a network and provide end-to-end delivery of packets. Think of the Network Layer as the postal service in a country. When you want to send a letter to a friend, you just need to know the destination postal address (e.g destination IP address) and you send the letter. The postal service (e.g the Network Layer) will determine the path to the destination (e.g IP Path Determination) and take care of the delivery (e.g using IP Routing). The two endpoints do not need to know the exact path taken for the delivery of the letter.

This Layer also encapsulates data into Packets, and defines how to fragment a packet into smaller pieces to accommodate media with smaller Maximum Transmission Units. An IP Router works in this Layer, since it can use the IP address information to route packets.

Examples of Layer 3 Specifications and Protocols
• IP
• IPX
• AppleTalk

The Data Link Layer provides error detection and error recovery of the data flowing across the physical network link. The Specifications in this OSI Layer define network and protocol characteristics such as Physical addressing, network topology, error notification, flow control etc. The physical addressing defined in this layer determines how devices are addressed on the data link layer. An example of physical addressing is the MAC address which is hard-coded in all Ethernet Network Interface Cards (NIC). Each Ethernet NIC that exist in the world has a unique MAC address. The Data Link Layer also combines bits into Frames. Each Frame has a Layer 2 Header which includes the Source and Destination MAC addresses among other fields.

Examples of Layer 2 Specifications and Protocols
• IEEE 802.3 Ethernet
• HDLC
• Frame Relay
• ATM
• PPP

The Physical Layer defines the Electrical, Mechanical, and functional specifications of the physical link used to connect the communication device on the network. This layer defines characteristics like voltage levels, data rates, maximum transmission distances, connectors etc.

The specifications in this Layer can be categorized as LAN or WAN: [Read more…]