The OSI Model

The Open System Interconnection (OSI) model defines a networking framework for implementing protocols in seven layers.
- the OSI model is based on work done at IBM in 1974.

Application (Layer 7)	This layer supports application and end user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. Application services such as FTP, Telent, and electronic mail, exist entirely at the application level.
Presentation (Layer 6)	This layer provides independence from differences in data representation by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.
Session (Layer 5)	This layer establishes, coordinates, and terminates communications between applications at each end. It deals with session and connection coordination.
Transport (Layer 4)	This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures data transfer. The best known layer 4 protocol is TCP.
Network (Layer 3)	This layer provides switching and routing; creating virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing. The best known layer 3 protocol is the Internet Protocol.
Data Link (Layer 2)	At this layer, data packets are encoded and decoded into bits. Bridges and switches operate at this layer. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sublayers: Media Access Control (MAC) layer and Logical Link Control (LLC) layer. The MAC sublayer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.
Physical (Layer 1)	This layer conveys the bit stream through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier. Ethernet and ATM are protocols with physical layer components.

Where is SSL in the OSI model?

© 2006 John Michael Pierobon

Notes