ISO/OSI, IEEE 802.2, and TCP/IP

In 1983, the International Standards Organization (ISO) developed a network model called Open Systems Interconnection (OSI) Reference Model, which defined a framework of computer communications. The ISO/OSI Reference Model (ISO/OSI model) has seven layers, each of which has a different level of abstraction and performs a well-defined function. The ISO/OSI model requires that the function of each layer define the international standardized network protocols. The seven layers are physical, data link, network, transport, session, presentation, and application layers.

• The physical layer physically transmits signals across a communication medium.
• The data link layer transforms a stream of raw bits (0s and 1s) from the physical layer into an error-free data frame for the network layer.
• The network layer controls the operation of a packet transmitted from one network to another, such as how to route a packet.
• The transport layer splits data from the session layer into smaller packets for delivery on the network layer and ensures that the packets arrive correctly at the other end.
• The session layer establishes and manages sessions, conversions, or dialogues between two computers.
• The presentation layer manages the syntax and semantics of the information transmitted between two computers.
• The application layer, the highest layer, contains a variety of commonly used protocols, such as file transfer, virtual terminal, and email.

The Institute of Electrical and Electronic Engineers (IEEE) developed a set of LAN standards, known as IEEE Project 802, which the ISO accepted as international standards. The IEEE LAN standards addressed only the lowest two layers, the physical and data link layers, of the ISO/OSI model.

The IEEE divided the data link layer into two sublayers, the Logical Link Control (LLC) and Medium Access Control (MAC) sublayers. The LLC sublayer, known as the IEEE 802.2 standard, is responsible for data link functions that are independent of the underlying medium. The MAC sublayer is responsible for data link functions that depend on the medium of the LAN implementation. The LAN implementation includes ARCnet, Ethernet, Fast Ethernet, Token Bus, Token Ring, and FDDI, which conform to IEEE 802.2. Microsoft's Network Device Interface Specification (NDIS) and Novell's Open Data-Link Interface (ODI) are two good implementations of the LLC and MAC sublayers. The physical layer in the IEEE LAN standards is a physical NIC, such as an Ethernet adapter. Every NIC has a unique 48-bit or 16-bit address, known as a hardware or MAC address, to identify itself or to be identified for data transmission in the two lowest layers.

TCP/IP is a suite of protocols that the US Department of Defense developed on ARPANET beginning in 1969. TCP/IP has grown far beyond the initial project. It is the standard protocol on the Internet and is the most widely used network protocol today.

IP implements the function of the network layer. Major protocols in IP include Address Resolution Protocol (ARP), Reverse Address Resolution Protocol (RARP), Internet Control Message Protocol (ICMP), and Internet Group Management Protocol (IGMP). ARP defines how to resolve an IP address of a host to a hardware address; RARP defines how to obtain an IP address using a host's hardware address; ICMP defines how to communicate among routers and hosts; and IGMP defines how to implement multicasting.

TCP is one of the two implementations of the transport layer in a TCP/IP network. The other implementation is User Datagram Protocol (UDP). TCP provides reliable, warranted delivery of data from one computer to another, such as a fax, and UDP provides only best-effort delivery similar to regular mail.

A TCP/IP application fits into the top three layers of the ISO/OSI model, the session, presentation, and application layers. Commonly used TCP/IP applications include Telnet and ftp.