The international standards organization for the Open Systems Interconnection (or OSI) has defined the following seven layers of networks:
• Physical Layer—Defining the electrical and mechanical interfaces to the network, it determines the upper limit of the transmission speed needed for audio and video information.
• Data Link Layer—Comprising the access protocol to the physical layer, it deals with error correction, flow control, frame synchronization, and the transmission of data frames.
• Network Layer—Containing switches and router packets, it establishes logical associations of remote stations and provides services such as addressing, congestion control, error handling, internetworking, and packet sequencing.
• Transport Layer—Provides a program-to-program connection.
• Session Layer—Coordinates interactions between user application processes on different hosts, including multi-cast (defined as one to many, multi-drop), many-to-one sessions, and point-to-point.
• Presentation Layer—Manages abstract data structures and converts different data formats and codes.
• Application Layer—Contains protocols such as ftp, SMTP, telnet, and email.
The TCP/IP protocol used on the Internet collapses layers 5, 6, and 7 of the above OSI Model to a single application layer, thus forming a five-layer protocol.
Tanenbaum, A. Computer Networks, 4th ed. Upper Saddle River, NJ: Prentice Hall, 2003.