Open Systems Interconnection Reference Model
Note: this is a detailed, technical description of the seven layers of the OSI model.
There are seven layers in the OSI Model (in order, from highest to lowest):
Layer 7, The Application Layer
The Application Layer defines the programs themselves, as seen by the user. Web browsers such as Firefox or Internet Explorer, or command-line utilities such as telnet and FTP (not to be confused with the protocols telnet and ftp), reside on the Application layer.
Layer 6, The Presentation Layer
Such functions as encoding and decoding, encryption, converting to/from various character sets, and the transformation of data structures into and out of XML are examples of functions performed at the Presentation layer.
Layer 5, The Session Layer
Establishing and managing "sessions" between computers happens at the Session layer. An example is whether full or half duplex is used between two machines.
note: layers 5 through 7 traditionally consist of only protocols, not devices
Layer 4, The Transport Layer
Reliable transfer of data is the responsibility of protocols that reside on the Transport layer.
Physical devices on this layer include:
Network protocols categorized on this layer include:
- The Transmission Control Protocol (the 'TCP' in TCP/IP, the protocol in use on the Internet)
Layer 3, The Network Layer
Correct routing of packets happens on the Network layer.
Physical devices on this layer include:
Network protocols categorized on this layer include:
- The Internet Protocol (the 'IP' in TCP/IP)
Layer 2, The Data Link Layer
The Data Link layer has the "lowest" protocols (the next layer, the Physical layer, deals with actual bits and voltages).
Physical devices on this layer include:
Network protocols categorized on this layer include:
- Ethernet MAC (Media Access Control) addresses
- LLC (Logical Link Control)
Layer 1, The Physical Layer
The Physical layer deals with the raw "bits" or voltages "on the wire."
Physical devices on this layer include:
- Cabling such as Category 5 (CAT 5) cable
- Network cards (NIC cards)
- Hubs
- Repeaters