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:

• Routers

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:

• Switches
• Network bridges

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

Network protocols categorized on this layer include:

• Raw voltages that represent bits
• Repeaters