Seven Layers of OSI Model and functions of seven layers of OSI model

InSeven Layers of Open Systems Interconnection (OSI) Modellesson, you will learn about the seven layers of OSI model and their functionsIf network communications need to happen without any trouble, many problems must be solved. Coordinating all these problems are so complex and not easy to manage. To make these tasks smooth, in 1977 the International Standards Organization (ISO) proposed the Open Systems Interconnection (OSI) network model. The Open Systems Interconnection (OSI) model breaks down the problems involved in moving data from one computer to another computer. Open Systems Interconnection (OSI) model categorizes these hundreds of problems to Seven Layers. A layer in Open Systems Interconnection (OSI) model is a portion that is used to categorize specific problems.Open Systems Interconnection (OSI) Seven Layered reference model is only just a reference model. All the problems which are related to the communications are answered by specific protocols operating at different layers. The following image shows the seven layers described in Open Systems Interconnection (OSI) model.

Seven Layers of Open Systems Interconnection (OSI) Model

Layer 1. Physical LayerThe first layer of the seven layers of Open Systems Interconnection (OSI) network model is called the Physical layer. Physical circuits are created on the physical layer of Open Systems Interconnection (OSI) model. Physical layers describe the electrical or optical signals used for communication. Physical layer of the Open Systems Interconnection (OSI) model is only concerned with the physical characteristics of electrical or optical signaling techniques which includes the voltage of the electrical current used to transport the signal, the media type (Twisted Pair,Coaxial Cable,Optical Fiberetc), impedance characteristics, physical shape of the connector, Synchronization etc. The Physical Layer is limited to the processes needed to place the communication signals over the media, and to receive signals coming from that media. The lower boundary of the physical layer of the Open Systems Interconnection (OSI) model is the physical connector attached to the transmission media. Physical layer of the Open Systems Interconnection (OSI) model does not include the transmission media. Transmission media stays outside the scope of the Physical Layer and are also referred to as Layer 0 of the Open Systems Interconnection (OSI) Model.- Layer 1 Physical examples include Ethernet, FDDI, B8ZS, V.35, V.24, RJ45.The physical layer, the lowest layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers. It provides: Data encoding: modifies the simple digital signal pattern (1s and 0s) used by the PC to better accommodate the characteristics of the physical medium, and to aid in bit and frame synchronization. It determines:

What signal state represents a binary 1 How the receiving station knows when a "bit-time" starts How the receiving station delimits a frame Physical medium attachment, accommodating various possibilities in the medium:

Will an external transceiver (MAU) be used to connect to the medium? How many pins do the connectors have and what is each pin used for? Transmission technique: determines whether the encoded bits will be transmitted by baseband (digital) or broadband (analog) signaling. Physical medium transmission: transmits bits as electrical or optical signals appropriate for the physical medium, and determines:

What physical medium options can be used How many volts/db should be used to represent a given signal state, using a given physical mediumLayer 2. Datalink LayerThe second layer of the seven layers of Open Systems Interconnection (OSI) network model is called the Datalink layer. The Data Link layer resides above the Physical layer and below the Network layer. Datalink layer is responsible for providing end-to-end validity of the data being transmitted. The Data Link Layer is logically divided into two sublayers, The Media Access Control (MAC) Sublayer and the Logical Link Control (LLC) Sublayer.Media Access Control (MAC) Sublayer determines the physical addressing of the hosts. The MAC sub-layer maintainsMAC addresses (physical device addresses)for communicating with other devices on the network.MAC addressesare burned into the network cards and constitute the low-level address used to determine the source and destination of network traffic.MAC Addresses are also known as Physical addresses, Layer 2 addresses, or Hardware addresses.The Logical Link Control sublayer is responsible for synchronizing frames, error checking, and flow control.- Layer 2 Data Link examples include PPP, FDDI, ATM, IEEE 802.5/ 802.2, IEEE 802.3/802.2, HDLC, Frame Relay, The data link layer provides error-free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume virtually error-free transmission over the link. To do this, the data link layer provides:

Link establishment and termination: establishes and terminates the logical link between two nodes. Frame traffic control: tells the transmitting node to "back-off" when no frame buffers are available. Frame sequencing: transmits/receives frames sequentially. Frame acknowledgment: provides/expects frame acknowledgments. Detects and recovers from errors that occur in the physical layer by retransmitting non-acknowledged frames and handling duplicate frame receipt. Frame delimiting: creates and recognizes frame boundaries. Frame error checking: checks received frames for integrity. Media access management: determines when the node "has the right" to use the physical medium.

Layer 3. Network LayerThe third layer of the seven layers of Open Systems Interconnection (OSI) network model is the Network layer. The Network layer of the OSI model is responsible for managinglogical addressinginformation in the packets and the delivery of those packets to the correct destination. Routers, which are special computers used to build the network, direct the data packet generated by Network Layer using information stored in a table known as routing table. The routing table is a list of available destinations that are stored in memory on the routers. The network layer is responsible for working with logical addresses. The logical addresses are used to uniquely identify a computer on the network, but at the same time identify the network that system resides on. The logical address is used by network layer protocols to deliver the packets to the correct network. The Logical addressing system used in Network Layer is known asIP address.IP addresses are also known as Logical addresses or Layer 3 addresses.- Layer 3 Network examples include AppleTalk DDP, IP, IPX.The network layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors. It provides:

Routing: routes frames among networks. Subnet traffic control: routers (network layer intermediate systems) can instruct a sending station to "throttle back" its frame transmission when the router's buffer fills up. Frame fragmentation: if it determines that a downstream router's maximum transmission unit (MTU) size is less than the frame size, a router can fragment a frame for transmission and re-assembly at the destination station. Logical-physical address mapping: translates logical addresses, or names, into physical addresses. Subnet usage accounting: has accounting functions to keep track of frames forwarded by subnet intermediate systems, to produce billing information.Communications SubnetThe network layer software must build headers so that the network layer software residing in the subnet intermediate systems can recognize them and use them to route data to the destination address.

This layer relieves the upper layers of the need to know anything about the data transmission and intermediate switching technologies used to connect systems. It establishes, maintains and terminates connections across the intervening communications facility (one or several intermediate systems in the communication subnet).In the network layer and the layers below, peer protocols exist between a node and its immediate neighbor, but the neighbor may be a node through which data is routed, not the destination station. The source and destination stations may be separated by many intermediate systems.Layer 4. Transport LayerThe fourth layer of the seven layers of Open Systems Interconnection (OSI) network mode is the Transport layer. The Transport layer handles transport functions such as reliable or unreliable delivery of the data to the destination. On the sending computer, the transport layer is responsible for breaking the data into smaller packets, so that if any packet is lost during transmission, the missing packets will be sent again. Missing packets are determined by acknowledgments (ACKs) from the remote device, when the remote device receives the packets. At the receiving system, the transport layer will be responsible for opening all of the packets and reconstructing the original message.Another function of the transport layer is TCP segment sequencing. Sequencing is a connection-oriented service that takes TCP segments that are received out of order and place them in the right order.The transport layer also enables the option of specifying a "service address" for the services or application on the source and the destination computer to specify what application the request came from and what application the request is going to.Many network applications can run on a computer simultaneously and there should be some mechanism to identify which application should receive the incoming data. To make this work correctly, incoming data from different applications are multiplexed at the Transport layer and sent to the bottom layers. On the other side of the communication, the data received from the bottom layers are de-multiplexed at the Transport layer and delivered to the correct application. This is achieved by using "Port Numbers".The protocols operating at the Transport Layer, TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) uses a mechanism known as "Port Number" to enable multiplexing and de-multiplexing. Port numbers identify the originating network application on the source computer and destination network application on the receiving computer.- Layer 4 Transport examples include SPX, TCP, UDP.The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers.

The size and complexity of a transport protocol depends on the type of service it can get from the network layer. For a reliable network layer with virtual circuit capability, a minimal transport layer is required. If the network layer is unreliable and/or only supports datagrams, the transport protocol should include extensive error detection and recovery.

The transport layer provides: Message segmentation: accepts a message from the (session) layer above it, splits the message into smaller units (if not already small enough), and passes the smaller units down to the network layer. The transport layer at the destination station reassembles the message. Message acknowledgment: provides reliable end-to-end message delivery with acknowledgments. Message traffic control: tells the transmitting station to "back-off" when no message buffers are available. Session multiplexing: multiplexes several message streams, or sessions onto one logical link and keeps track of which messages belong to which sessions (see session layer).Typically, the transport layer can accept relatively large messages, but there are strict message size limits imposed by the network (or lower) layer. Consequently, the transport layer must break up the messages into smaller units, or frames, prepending a header to each frame.The transport layer header information must then include control information, such as message start and message end flags, to enable the transport layer on the other end to recognize message boundaries. In addition, if the lower layers do not maintain sequence, the transport header must contain sequence information to enable the transport layer on the receiving end to get the pieces back together in the right order before handing the received message up to the layer above.End-to-end layersUnlike the lower "subnet" layers whose protocol is between immediately adjacent nodes, the transport layer and the layers above are true "source to destination" or end-to-end layers, and are not concerned with the details of the underlying communications facility. Transport layer software (and software above it) on the source station carries on a conversation with similar software on the destination station by using message headers and control messages.Layer 5. Session LayerThe position of Session Layer of the Seven Layered Open Systems Interconnection (OSI) model is between Transport Layer and the Presentation Layer. Session layer is the fifth layer of seven layered Open Systems Interconnection (OSI) Model. The session layer is responsible for establishing, managing, and terminating connections between applications at each end of the communication.In the connection establishment phase, the service and the rules (who transmits and when, how much data can be sent at a time etc.) for communication between the two devices are proposed. The participating devices must agree on the rules. Once the rules are established, the data transfer phase begins. Connection termination occurs when the session is complete, and communication ends gracefully.In practice, Session Layer is often combined with the Transport Layer.- Layer 5 Session examples include NFS, NetBios names, RPC, SQL.The session layer allows session establishment between processes running on different stations. It provides:

Session establishment, maintenance and termination: allows two application processes on different machines to establish, use and terminate a connection, called a session. Session support: performs the functions that allow these processes to communicate over the network, performing security, name recognition, logging, and so on.

Layer 6. Presentation LayerThe position of Presentation Layer in seven layered Open Systems Interconnection (OSI) model is just below the Application Layer. When the presentation layer receives data from the application layer, to be sent over the network, it makes sure that the data is in the proper format. If it is not, the presentation layer converts the data to the proper format. On the other side of communication, when the presentation layer receives network data from the session layer, it makes sure that the data is in the proper format and once again converts it if it is not.Formatting functions at the presentation layer may include compression, encryption, and ensuring that the character code set (ASCII, Unicode, EBCDIC (Extended Binary Coded Decimal Interchange Code, which is used in IBM servers) etc) can be interpreted on the other side.For example, if we select to compress the data from a network application that we are using, the Application Layer will pass that request to the Presentation Layer, but it will be the Presentation Layer that does the compression.- Layer 6 Presentation examples include encryption, ASCII, EBCDIC, TIFF, GIF, PICT, JPEG, MPEG, MIDI.The presentation layer formats the data to be presented to the application layer. It can be viewed as the translator for the network. This layer may translate data from a format used by the application layer into a common format at the sending station, then translate the common format to a format known to the application layer at the receiving station.

The presentation layer provides:

Character code translation: for example, ASCII to EBCDIC. Data conversion: bit order, CR-CR/LF, integer-floating point, and so on. Data compression: reduces the number of bits that need to be transmitted on the network. Data encryption: encrypt data for security purposes. For example, password encryption.

Layer 7. Application LayerThe Application Layer the seventh layer in OSI network model. Application Layer is the top-most layer of the seven layered Open Systems Interconnection (OSI) network model. Real traffic data will be often generated from the Application Layer. This may be a web request generated from HTTP protocol, a command from telnet protocol, a file download request from FTP protocol etc.In this lesson(Seven Layers of Open Systems Interconnection (OSI) Model), you have learned what are the Seven Layers of Open Systems Interconnection (OSI) Model and the functions of these seven layers. The top-most layer of the Seven Layers of Open Systems Interconnection (OSI) Model is the Application Layer and the bottom-most layer of the Seven Layers of Open Systems Interconnection (OSI) Model is Physical Layer.- Layer 7 Application examples include WWW browsers, NFS, SNMP, Telnet, HTTP, FTP

The application layer serves as the window for users and application processes to access network services. This layer contains a variety of commonly needed functions:

Resource sharing and device redirection Remote file access Remote printer access Inter-process communication Network management Directory services Electronic messaging (such as mail) Network virtual terminals