Understanding the OSI Seven Layer Networking Model

A computer network is made up of many parts and the longest-used method to conceptualize computer networks is known as the Open Systems Interconnection (OSI) Seven Layer Model. For Network Technicians the OSI Seven Layer Model provides a practical tool for diagnosing network problems as well as a common language for describing networks.

Layer 1 – Physical: Includes the cable used to move bits of data between computer systems. Unshielded Twisted Pair (UTP) with a CAT 5E or CAT6 rating is common in most networks. Fiber Optic cable which uses light waves as to transmit data packets as opposed to electrical waves has significant speed and distance advantages over UTP, but lacks the durability. Hubs, which operate much like old-time telephone switchboards moving data to connected systems are also included in Layer 1 of the OSI Model, as well as radio waves transmitted through the air to a Wireless Access Point (WAP).

Layer 2: Data Link: Includes the Network Interface Card (NIC), which serves as the interface between the computer and the network. The NIC is usually built into the motherboard and is assigned a Media Access Control (MAC) address that gives each system connected to a network a unique ID. The MAC address is used to ensure that data is delivered to the right computer, and NICs use electricity to send and receive packets of binary data as pulses of electricity, light, or radio waves.

Layer 3: Network: Includes software designed to use logical addressing called protocol that is used to efficiently move data packets along the physical and data link layers of the OSI Model. The most common protocol is TCP/IP which works in tandem with Routers at layer 3 to ensure that data is sent and received at the desired computer systems. Other protocols that use logical addressing such as POP and SMTP for email also reside at the network layer.

Layer 4: Transport: Includes protocol that is specifically designed for assembly and disassembly of data packets. Data must be sent in chunks that will fit into specific packet sizes, which is then reassembled into readable data by the receiving computer system. The transport layer network protocol breaks up the data and assigns sequence numbers which allows receiving systems to process the data.

Layer 5: Session: Includes protocol that initiates data transfer sessions between computer systems, accepts incoming sessions, and opens and closes existing sessions. The session layer’s network protocol also keeps track of computer naming conventions to facilitate data transfer sessions between computer systems.

Layer 6: Presentation: Includes software that converts data into standardized file formats which allow computers to receive data and convert it into a format that is readable by the computer systems. PDF and Adobe Acrobat are an example of standardized file format software that allow computer systems to convert data into readable files.

Layer 7: Application: Includes software applications that allow users to access and utilize data files. Web browsers such as Internet Explorer and Mozilla Firefox, as well a email programs like Outlook Express are found at the Application layer. Layer 7 also includes code built into operating systems that enable network-aware applications and Application Programming Interfaces (APIs).

Please Do Not Throw Sweet Potato Away: A useful mnemonic for memorizing the seven layers of the OSI Seven Layer Networking Model.