Networking Guide : The OSI Reference Model

Even though most computer network technologies do not follow strictly to the OSI Model in that they combine several OSI layers functions in one protocol, the OSI Model is still used as a reference and a guideline in network design. Understanding the OSI Model will help you understand how a network works. The OSI Model protocol stack is explained in the following table:

Layer Layer Name Functions Examples
Layer 7 Application Layer application support HTTP, FTP, Telnet, SMTP, SNMP, POP3, IMAP4
Layer 6 Presentation Layer data format conversion, data compression, and encryption  
Layer 5 Session Layer user identification; establishing, maintaining, and terminating a session SIP
Layer 4 Transport Layer end-to-end transport TCP, UDP, RTP, RTCP
Layer 3 Network Layer addressing, routing IP, IPSec, IPX, NetBEUI, AppleTalk, ICMP
Layer 2 Data Link Layer medium access control, error detection, retransmission Ethernet, Wi-Fi, HomePNA, HomePlug, PPP, PPTP, L2TP, ATM, Frame RelayToken Ring, FDDI
Layer 1 Physical Layer electrical/optical signaling, cabling, connector pin assignment RF, UTP, STP, coax, fiber optic, connectors, signaling, voltages

Due to its complex functions, the Data Link Layer is divided into two sublayers, that is Media Access Control (MAC) sublayer and Logical Link Control (LLC) sublayer. MAC sublayer is the lower part, closer to the Physical Layer. MAC sublayer controls access to the physical medium. LLC sublayer is the upper part that interfaces with the Network Layer.

Session Layer, Presentation Layer, and Application Layer are often referred to as Upper Layers. These layers basically handle user connection and data formatting. In most network technologies, such as TCP/IP, the differences between the three layers are blurred and their functions are often handled by one protocol.

Physical Layer, Data Link Layer, Network Layer, and Transport Layer are referred to as Lower Layers. The lower layers generally concern with how data is transported across the network.

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