OSI (Open System Interconnection) model...
The seven layers of the OSI Basic Reference Model are (from bottom to top):
- The Physical Layer describes the physical properties of the various communications media, as well as the electrical properties and interpretation of the exchanged signals. Ex: this layer defines the size of Ethernet coaxial cable, the type of BNC connector used, and the termination method.
- The Data Link Layer describes the logical organization of data bits transmitted on a particular medium. Ex: this layer defines the framing, addressing and checksumming of Ethernet packets.
- The Network Layer describes how a series of exchanges over various data links can deliver data between any two nodes in a network. Ex: this layer defines the addressing and routing structure of the Internet using Internet Protocol (IP).
- The Transport Layer describes the quality and nature of the data delivery. Ex: this layer defines if and how retransmissions will be used to ensure data delivery. This is the use of Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) is located.
- The Session Layer describes the organization of data sequences larger than the packets handled by lower layers. Ex: this layer describes how request and reply packets are paired in a remote procedure call.
- The Presentation Layer describes the syntax of data being transferred. Ex: this layer describes how floating-point numbers can be exchanged between hosts with different math formats.
- The Application Layer describes how real work actually gets done. Ex: this layer would implement file system operations.
Adapted from a student (Kirk Lee in the JHU IT program, 2001)