The OSI model is a layered model and a conceptual standard used for defining standards to promote multi-vendor integration as well as maintain constant interfaces and isolate changes of implementation to a single layer. It is NOT application or protocol specific. In order to pass any Cisco exam, you need to know the OSI model inside and out.
the model is made up of 7 layers, namely the application layer, presentation layer, session layer, transport layer, network layer, data link layer and the physical layer. An easy way to remember the order of these layers is, "All people seem to need data processing", where the first letter of every word represents the respective OSI model layer. These seven layers can be divided into two sets as application set and transport set. The application set consists of application layer, presentation layer and session layer, while the transport set comprises the remaining layers. All these layers together work in synchronization for transmitting data from one computer to another. Given below is a brief information on functions of these layers.
| Layer | Description | Device | Protocol |
| Application | Provides network access for applications, flow control and error recovery. Provides communications services to applications by identifying and establishing the availability of other computers as well as to determine if sufficient resources exist for communication purposes. | Gateway | NCP, SMB, SMTP, FTP, SNMP, Telnet, Appletalk |
| Presentation | Performs protocol conversion, encryption and data compression | Gateway and redirectors | NCP, AFP, TDI |
| Session | Allows 2 applications to communicate over a network by opening a session and synchronizing the involved computers. Handles connection establishment, data transfer and connection release | Gateway | NetBios |
| Transport | Repackages messages into smaller formats, provides error free delivery and error handling functions | Gateway | NetBEUI, TCP, SPX, and NWLink |
| Network | Handles addressing, translates logical addresses and names to physical addresses, routing and traffic management. | Router and brouter | IP, IPX, NWLink, NetBEUI |
| **Data Link | Packages raw bits into frames making it transmitable across a network link and includes a cyclical redundancy check(CRC). It consists of the LLC sublayer and the MAC sublayer. The MAC sublayer is important to remember, as it is responsible for appending the MAC address of the next hop to the frame header. On the contrary, LLC sublayer uses Destination Service Access Points and Source Service Access Points to create links for the MAC sublayers. | Switch, bridge and brouter | None |
| Physical | Physical layer works with the physical media for transmitting and receiving data bits via certain encoding schemes. It also includes specifications for certain mechanical connection features, such as the adaptor connector. | Multiplexer and repeater | None |
Here is an easy way to memorize the order of the layers:
All People Seem To Need Data Processing. The first letter of each word corresponds to the first letter of one of the layers. It is a little corny, but it works.
Application Layer: The application layer is said to be the closest layer to the user. It is through this layer that a user can interact with the software application that will aid in the data transfer. The main functions of the application layer are identifying the user who wants to communicate, determine whether the data and networks sources are available and lastly synchronizing the communication between the two users. File Transfer Protocol (FTP), Telnet, Simple Mail Transfer Protocol (SMTP), etc., are some of the actual implementations of application layer.
Presentation Layer: The main function of presentation layer is to convert the data into a format that could be easily recognized by the application layers of other end users. Compression/decompression, conversion, encryption/decryption, coding/decoding, etc., are some of the tasks undertaken by this layer. Thus this layer converts the data obtained from the application layer into a format that can be easily identified by other network layers.
Session Layer: Session layer plays an important role in establishing, maintaining and terminating the connection between two users. This layer controls the communication between the source user and the destination user and also decides the time of communication. Any error report related to application layer, presentation layer and session layer, are provided by this layer.
Transport Layer: The transport layer is responsible for delivering the data or the messages between the two users. The first task that the transport layers carries is to divide the data in packets. This data is further re-assembled at the side of the receiver user. The transport layer is also responsible for error free data transmission and it also guarantees that the data will be received in correct sequence at the receiver end. UDP, SPX, TCP, etc., are some of the protocols that operate on this layer.
Network Layer: The main function of the network layer is to provide switching technologies, routing technologies and transmitting data from one node to another. This layer creates logical paths for the data transmission, which are known as virtual circuits. Network layer is also responsible for handling errors, packet sequencing, controlling network congestion and addressing. Thus this layer is responsible for the setting up the required network for transferring data from one user to other.
Data Link Layer: Providing reliable transit of the data through a physical network, is one of the main functions of the data link layer. This layer decides the physical configuration of various devices. The data link layer is responsible for synchronization of various physical devices that will transmit the data. It makes sure that the frames are transferred in correct order and asks for retransmission in case of error. Basically, the data link layer is a layer that provides reliable transmission services to the network layer.
Physical Layer: As the name suggests, the physical layer is the layer that deals with the physical components of a network. It is responsible for activation, maintenance and deactivation of various physical links that act in data transmission. Electrical signals, voltage levels, data transmission rates, etc., are some of the major elements defined by the physical layer. It is also responsible for passing and receiving bytes from the physically connected medium.
Presentation Layer: The main function of presentation layer is to convert the data into a format that could be easily recognized by the application layers of other end users. Compression/decompression, conversion, encryption/decryption, coding/decoding, etc., are some of the tasks undertaken by this layer. Thus this layer converts the data obtained from the application layer into a format that can be easily identified by other network layers.
Session Layer: Session layer plays an important role in establishing, maintaining and terminating the connection between two users. This layer controls the communication between the source user and the destination user and also decides the time of communication. Any error report related to application layer, presentation layer and session layer, are provided by this layer.
Transport Layer: The transport layer is responsible for delivering the data or the messages between the two users. The first task that the transport layers carries is to divide the data in packets. This data is further re-assembled at the side of the receiver user. The transport layer is also responsible for error free data transmission and it also guarantees that the data will be received in correct sequence at the receiver end. UDP, SPX, TCP, etc., are some of the protocols that operate on this layer.
Network Layer: The main function of the network layer is to provide switching technologies, routing technologies and transmitting data from one node to another. This layer creates logical paths for the data transmission, which are known as virtual circuits. Network layer is also responsible for handling errors, packet sequencing, controlling network congestion and addressing. Thus this layer is responsible for the setting up the required network for transferring data from one user to other.
Data Link Layer: Providing reliable transit of the data through a physical network, is one of the main functions of the data link layer. This layer decides the physical configuration of various devices. The data link layer is responsible for synchronization of various physical devices that will transmit the data. It makes sure that the frames are transferred in correct order and asks for retransmission in case of error. Basically, the data link layer is a layer that provides reliable transmission services to the network layer.
Physical Layer: As the name suggests, the physical layer is the layer that deals with the physical components of a network. It is responsible for activation, maintenance and deactivation of various physical links that act in data transmission. Electrical signals, voltage levels, data transmission rates, etc., are some of the major elements defined by the physical layer. It is also responsible for passing and receiving bytes from the physically connected medium.
