Network Basics

The Open Systems Interconnection (OSI) Network model is a seven-layer abstracted view of how network communication occurs. The OSI definition is used to teach students about networking and for commercial applications. The seven layers of the OSI model are: 1) Application, 2) Presentation, 3) Session, 4) Transport, 5) Network, 6) Data Link, and 7) Physical. As networking has become more mature and implemented in industry, the abstractions of the OSI model have been implemented in four physical layers in the TCP/IP networking model: 1) Link Layer, 2) Internet Layer, 3) Transport Layer, and 4) Application Layer.

History

1. The OSI model was created to abstractly define network communications and protocols. It has become the primary tool by which networking fundamentals are taught. The original intention of the model was to provide network protocol definitions, but the work became too complex and expensive. As a result, the model has not been used beyond the abstract level for protocol definition and implementation. The TCP/IP model was successively proposed by DARPA (Defense Advanced Research Projects Agency) as a less abstract definition of how electronic information should be formatted, addressed, shipped, routed and delivered to the correct electronic address.

Network Link Layer

2. The link layer is the lowest layer of the protocols in the TCP/ (Transmission Control Protocol) IP (Internet Protocol) model. This layer is where packets of information are moved between the Internet layer interfaces of any two entities on the same connection or link. The actual process of sending and receiving information packets on the link is controlled by either networking firmware or device drivers. These controls perform the housekeeping tasks of adding packet header information required for transmission of a data packet, then actually sending and receiving the data.

Network Internet Layer

3. The Internet layer is where packets of data are transported across a network or groups of networks. The IP performs the function of getting packets of data from the sender to the desired recipient by routing the information across interconnected networks. The IP carries data packets for multiple layers of protocols. The protocol type of a data packet is determined by uniquely identified protocol numbers. ICMP and IGMP are two examples of different protocols used at the Internet layer. ICMP is used to send diagnostic information regarding IP stack transmissions, whereas IGMP is for management of multi-cast information.

Network Transport Layer

4. A number of network responsibilities occur at the transport layer, including end-end message transfer capabilities, error control, flow control and fragmentation. There are two primary means of providing connections at the transport layer: 1) connection-based, TCP (Transmission Control Protocol); and 2) connectionless-based, UDP (User Datagram Protocol). On top of the Internet Protocol, which is a "best effort" delivery service, the transport layer adds a reliability check for communications. TCP provides this reliability by doing the following: 1) ensuring data arrives in the correct order; 2) ensuring there is little error in the data; 3) ensuring duplicate information is not delivered; 4 ) ensuring lost information is resent; and 5) implementing traffic congestion control measures. UDP is referred to as connection-less because is does not guarantee delivery. Reliability for UDP is provided through error detection algorithms. Typical applications of the UDP protocol are grounded in streaming media (audio, voice and video), which emphasize speed of delivery over reliability.

Network Application Layer

5. Application layer protocols are used for network communications. Some examples of the protocols at the application layer are the simple mail transfer protocol (SMTP) and file transfer protocol (FTP). Information at this layer is sent via one of the transport layer protocols (TCP or UDP), which are then wrapped in lower layer protocols for delivery. Protocols at this layer are commonly used with client-server programs using common ports.