About the OSI network model...

The Open Systems Interconnect (OSI) Reference Model was developed by the International Standards Organization and adopted in 1983 as a common reference for the development of data communication standards. The OSI model divides the functions of data communications into seven "layers" as follows:

Physical Layer defines the electrical, mechanical, functional and procedural characteristics used to access and send a stream of bits over a physical medium. Physical layer would include the definition of the RJ-45 connector and CAT5 cable commonly used with Ethernet.

Data Link Layer is responsible for reliable delivery at the lowest levels, including framing of data and error detection and correction. Ethernet (IEEE 802.2) and the MAC are defined at this level.

Network Layer establishes and maintains connections over a network and provides addressing, routing, and delivery of packets to hosts. IP, PPP, IPX, and X.25 are found at this layer.

Transport Layer provides error detection, correction, end-to-end error recovery, and software flow control of data between networks. TCP and UDP are found at this layer.

Session Layer is responsible for establishing and managing sessions (connections) between applications and the network. One of the more common session layer managers is the Windows WinSock socket API.

Presentation Layer performs data and protocol negotiation and conversion such that data is exchangeable between hosts while transportable across the network. Data compression and encryption are also performed at this layer.

Application Layer is used by software applications to prepare data for use by the other six OSI layers. This layer is the only interface an application has to the rest of the network. HTTP, FTP, email (SMTP & POP3), SNMP, etc., are all found at this layer.