ISO/OSI, IEEE 802.2, and TCP/IP

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

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

The IEEE divided the data link layer into two sublayers, the Logical LinkControl (LLC) and Medium Access Control (MAC) sublayers. The LLC sublayer, knownas the IEEE 802.2 standard, is responsible for data link functions that areindependent of the underlying medium. The MAC sublayer is responsible for datalink functions that depend on the medium of the LAN implementation. The LANimplementation includes ARCnet, Ethernet, Fast Ethernet, Token Bus, Token Ring,and FDDI, which conform to IEEE 802.2. Microsoft's Network Device InterfaceSpecification (NDIS) and Novell's Open Data-Link Interface (ODI) are two goodimplementations of the LLC and MAC sublayers. The physical layer in the IEEE LANstandards is a physical NIC, such as an Ethernet adapter. Every NIC has a unique48-bit or 16-bit address, known as a hardware or MAC address, to identify itselfor to be identified for data transmission in the two lowest layers.

TCP/IP is a suite of protocols that the US Department of Defense developedon 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 networkprotocol today.

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

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

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