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.