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OSI Model of Networking
The Open Systems Interconnection Basic Reference Model (OSI Model) is a layered, abstract description for communications and computer network protocol design, developed as part of the Open Systems Interconnection initiative. Internet and network communication is not as easy as binary 1’s and 0’s. The International Organization for Standardization created the OSI model, it is the most common network model used by network professionals today. Knowing this model will help in understanding and troubleshooting networks and some components. The OSI model looks like this:
- Physical layer
- Data-Link Layer
- LLC sub layer
- MAC sub layer
- Network Layer
- Transport Layer
- Session Layer
- Presentation Layer
- Application Layer
The Physical layer or layer 1, of the OSI model provides the transmission process of various physical components. Layer 1 is responsible for the sending and receiving of data on networks. Ethernet cabling or SCSI function at the physical layer as well as hubs and repeaters.
The Data-Link layer or layer 2, of the OSI model is responsible for the physical and logical addressing. This layer transfers data between network nodes on wide and local area networks. Examples of data link protocols are Ethernet for local area networks and PPP, HDLC and ADCCP for point-to-point connections. Depending on the technology in use this is one of the three layers that can perform error recovery. In some networks, such as IEEE 802 local area networks, the data link layer is split into Media Access Control (MAC) and Logical Link Control (LLC) sub layers.
The Network layer or layer 3, or the OSI model deals with the logical addresses of hosts and performing network routing functions. The two address protocols in this layer are Internet Protocol (IP) and Internetwork Packet Exchange (IPX) protocols. The network layer also carries out network routing and reports delivery errors. Routers perform at this level of the OSI model. The next 4 layers are not required; they are only needed at user end points. The network layer also performs network-wide error recovery, where as the data-link layer only checks the data involved. Some have suggested that there be a layer 3.5 called the Internetwork layer, which involves two separate networks that are interconnected.
The Transport layer or layer 4 of the OSI model controls the data transfer between end users. It is also in charge of connection management and flow control. The two protocols that are most widely known are Transport Control Protocol (TCP) and User Datagram Protocol (UDP). The transport layer converts messages into TCP and UDP packets. TCP as well as some other protocols keep track of packets and can resend them if something happens during transmission.
The Session Layer or layer 5 of the OSI model manages and controls the connections between points, such as computers or controllers. The session layer is responsible for establishing and negotiating connections, releasing connections when completed, dialogue, synchronize connections. These services are required for users. It can operate in either half or full duplex mode. SQL, NetBIOS, SAP, ASP, and DNA SCP run in the session layer.
The Presentation Layer or layer 6 of the OSI model formats, transforms, compresses, and encrypts. It is also concerned with data syntax. It also responds to requests from the application layer and issues requests to the session layer. The presentation layer is about the formatting and presentation of data. Some of the formats used to today for encoding include TIFF, JPEG, MPEG, and MIDI.
The Application Layer—or layer 7 of the OSI model—makes network accessibility available to application processes such as email and file transfer. The processes are not located in this layer; it is more of a portal for the application to gain access to the Internet or network. These applications such as Word, Outlook, or CBAS are not part of the OSI model. Some the protocols used in this layer include HTTP, HTTPS, FTP, Telnet, POP3, SMTP, DNS, and DHCP. Error correction occurs at this level by resending data during the transfer process that was lost.
