The Open System Interconnection (OSI) model defines a networking framework to implement protocols in seven layers. Use this handy guide to compare the 7 layers of the OSI model and understand how they interact with each other.
This article has been reviewed and updated by Amy Yang.
In this definition...
What does the OSI model do?
The Open System Interconnection (OSI) model defines a networking framework to implement protocols in seven layers. There is really nothing to the OSI model. In fact, it’s not even tangible. The seven OSI layers don’t perform any functions in the networking process. Unlike the TCP/IP (transmission control protocol/internet protocol) model, the OSI model is a conceptual framework that helps to better understand the complex interactions that are happening in the network framework.
Who developed the OSI Model?
The International Standards Organization (ISO) developed the Open Systems Interconnection (OSI) model. It divides different network communication and data transmission into seven layers, which contain their own network devices. In this model, OSI layers one to four are considered the lower layers and mostly concern themselves with moving data around. OSI layers five to seven, called the upper layers, contain application-level data. Networks operate on one basic principle: “Pass it on.” Each upper and lower layer takes care of a very specific job and then passes the data on to the next layer.
The 7 layers of the OSI model
In the OSI model, control is passed from one layer to the next, starting at the application layer (Layer 7) in one station and proceeding to the bottom layer. It is then passed over the channel to the next station and back up the hierarchy. The OSI model takes the task of internetworking and divides that up into what is referred to as a vertical stack that consists of the following seven layers:
Note: Click each hyperlink in the list below to read detailed information and examples of each layer or continue scrolling to read the full article:
- Layer 7—Application
- Layer 6—Presentation
- Layer 5—Session
- Layer 4—Transport
- Layer 3—Network
- Layer 2—Data Link
- Layer 1—Physical
Did You Know…? Most of the functionality in the OSI model exists in all communications systems, although two or three OSI layers may be incorporated into one. OSI is also referred to as the OSI Reference Model or just the OSI Model.
OSI Layer 7: Application
OSI Model Layer 7, or the application layer, supports application and end-user processes. Communication partners, quality of service, and any constraints on data syntax are identified and user authentication and privacy are considered. Moreover, everything at this layer is application-specific. This layer provides application services for file transfers, email, and other network software services. Telnet and FTP are applications that exist entirely at the application level. Tiered application architectures are part of this layer.
Layer 7 Application examples include WWW browsers, NFS, SNMP, Telnet, HTTP, FTP
OSI Layer 6: Presentation
The presentation layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format and vice versa. This layer works to transform data into the form that the application layer can accept. For example, the presentation layer could convert a text computer file that is EBCDIC-coded into an ASCII-coded file.
The presentation layer also formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.
Layer 6 Presentation examples include encryption, ASCII, EBCDIC, TIFF, GIF, PICT, JPEG, MPEG, MIDI.
OSI Layer 5: Session
This layer establishes, manages, and terminates connections and communication between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It also deals with session and connection coordination.
An example of a session layer protocol is the ISO 8327 protocol. This protocol can be used to recover a lost connection. This session layer protocol also may close and reopen connections that have not been used for a long period of time.
Layer 5 Session examples include NFS, NetBios names, RPC, SQL.
OSI Layer 4: Transport
OSI Model Layer 4, or the transport layer, provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error detection and recovery and flow control. It ensures complete data transfer, which treats data as a stream of bytes, as performed by the TCP or transport layer protocol.
The transport layer also provides layers 5 through 7, which are application-oriented, standardized access, so they do not need to consider the characteristics of the communication network itself. The TCP and the user datagram protocol (UDP) are the only protocols that are implemented in every major operating system. These protocols also use a port number to determine which one incoming traffic should be directed to.
Layer 4 Transport examples include SPX, TCP, UDP.
OSI Layer 3: Network
After the transport layer is layer 3, or the network layer, which provides switching and routing technologies. It also uses network layer protocols, like routing protocols, to create logical paths, known as virtual circuits, that transmit data from node to node at an assigned destination and source IP address. Routing and packet forwarding are functions of the network layer as well as addressing, internetworking, error control, congestion control, and packet sequencing.
A packet in the network layer is a data unit that contains source and destination IP addresses, a protocol specification field, data, and a trailer field, which includes information about error connections. Each packet makes it easier to retransmit interrupted or lost pieces of data.
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Layer 3 Network examples include AppleTalk DDP, IP, IPX.
OSI Layer 2: Data Link
At OSI Model Layer 2, or the data link layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sub layers: The media access control (MAC) layer and the logical link control (LLC) layer. The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control, and error checking.
The data link layer also sends data frames from the network layer to the physical layer. A data frame is the protocol data unit, or PDU of the data link layer and represents a group of information. This layer also needs to create and recognize frame boundaries since the physical layer just accepts and transmits data without analyzing the meaning of its structure.
In this layer is where the MAC address operates, which is a hard coded 48-bit (6 byte) address. This lets computers uniquely identify themselves in the network. This layer is also where logical addresses are created, which allows users to assign a custom address to their computers and other devices. This is important because MAC addresses cannot be changed, and the logical address can better represent said devices.
Layer 2 Data Link examples include PPP, FDDI, ATM, IEEE 802.5/ 802.2, IEEE 802.3/802.2, HDLC, Frame Relay.
OSI Layer 1: Physical
OSI Model Layer 1, or the physical layer, conveys the bit stream—electrical impulse, light, or radio signal—through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards, and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.
Layer 1 Physical examples include Ethernet, FDDI, B8ZS, V.35, V.24, RJ45.
DID YOU KNOW….? Two similar projects from the late 1970s were merged in 1983 to form the Basic Reference Model for Open Systems Interconnection standard (the OSI model). It was published in 1984 as standard ISO 7498.
