An Ideal Open Systems Interconnection (OSI) model

The Open Systems Interconnection (OSI) model is an architectural model that represents networking communications. It was introduced in 1978 by the International Organization for Standardization (ISO) to standardize the levels of services and types of interactions for computers communicating over a network.

What does the OSI model do?

A way of representing the complexities of computer networking in a 7-layer model, ranging from the physical hardware of networking all the way up to how application programs talk to the network. The 7 layers are: physical, data link, network, transport, session, presentation, and application. The 7-layer OSI model can be used to help diagnose network problems. It is also used as a measurement of how well people know their networking. If you’re looking for a job in networking, you should familiarize yourself with the OSI model.

How to use the OSI model?

The OSI model is used as a common reference point when comparing the function of different protocols and types of network hardware. Understanding the OSI model is important for comparing different products.

The architecture of the OSI model:

The OSI model divides network communication into seven layers. Each layer has a defined networking function, as described in below

1. Application = the application layer provides the interface between the networking protocol stack and the software running on the computer. For example, this layer provides the interface for e-mail, file transfers, Telnet and File Transfer Protocol (FTP) applications. Applications use the services provided by application layer protocols, which in turn use the services provided by the other layers beneath them.

2. Presentation layer = the presentation layer (sometimes referred to as the syntax layer) is responsible for translating each computer’s native syntax into a common transfer syntax readable by the other computers on the network. In some cases, the transfer syntax can provide functions such as data compression and encryption.

3. Session Layer = the session layer provides many functions involved in the regulation of the dialog between two computers communicating over the network. For example, the session layer sets up, regulates, and terminates exchanges between the applications at each end of the communication.

4. Transport Layer = the transport layer provides functions that complement those of the network layer, including guaranteed delivery (which uses packet acknowledgments to ensure data is received), flow control (which regulates transmission speed to avoid dropped packets), and end-to-end error detection (which enables the receiving system to detect damaged packets).

5. Network Layer = the network layer defines the functions that provide end-to-end communications between computers on different networks. Chief among these functions is routing, which enables computers to relay traffic through intermediate networks to a destination on a remote network any distance away. Other functions include packet sequencing, end-to-end error detection from the sender to the recipient, congestion control, and addressing. While the data-link layer is responsible for local traffic on a single network, the network layer is responsible for directing traffic to its ultimate destination.

6. Data-Link Layer = the data-link layer defines the interface between the network medium and the software running on the computer. Among the data-link layer functions are packet addressing (which allows computers to direct traffic to specific destinations on the local network); media access control (which allows multiple computers to share a single network medium without conflicting); and formatting the frame used to encapsulate data for transmission over the network. The data-link layer divides into two sub layers. The logical link control (LLC) sub layer controls elements such as error checking from node to node on the same LAN, frame synchronization, and flow control. The media access control (MAC) sub layer controls the movement of data packets to and from one network interface card (NIC) to another across a shared channel.

7. Physical Layer = the physical layer defines the nature of the network medium—the actual fabric of the network that joins all the computers together—and the nature of the signals transmitted over the medium. In most cases, the network medium is a form of copper cable that uses electric currents for signaling, but fiber-optic and wireless media are becoming increasingly prevalent.

OSI Model Reference Table

Layer	Function	Protocols	Network Components
Application User Interface	Used for applications specifically written to run over the network Allows access to network services that support applications; Directly represents the services that directly support user applications Handles network access, flow control and error recovery Example apps are file transfer,e-mail, NetBIOS-based  applications	DNS; FTP; TFTP; BOOTP; SNMP;RLOGIN; SMTP; MIME; NFS; FINGER; TELNET; NCP; APPC; AFP; SMB	Gateway
Presentation Translation	Translates from application to network format and vice-versa All different formats from all sources are made into a common uniform format that the rest of the OSI model can understand Responsible for protocol conversion, character conversion,data encryption / decryption, expanding graphics commands, data compression Sets standards for different systems to provide seamless communication from multiple protocol stacks Not always implemented in a network protocol		Gateway Redirector
SessionSyncs and Sessions	Establishes, maintains and ends sessions across the network Responsible for name recognition (identification) so only the designated parties can participate in the session Provides synchronization services by planning check points in the data stream => if session fails, only data after the most recent checkpoint need be transmitted Manages who can transmit data at a certain time and for how long Examples are interactive login and file transfer connections, the session would connect and re-connect if there was an interruption; recognize names in sessions and register names in history	NetBIOSNames PipesMail Slots RPC	Gateway
Transport Packets; Flow control & Error-handling	Additional connection below the session layer Manages the flow control of data between parties across the network Divides streams of data into chunks or packets; the transport layer of the receiving computer reassembles the message from packets A train is a good analogy => the data is divided into identical units Provides error-checking to guarantee error-free data delivery, with on losses or duplications Provides acknowledgment of successful transmissions; requests retransmission if some packets don’t arrive error-free Provides flow control and error-handling	TCP, ARP, RARP;SPXNWLink NetBIOS / NetBEUI ATP	Gateway Advanced Cable Tester Brouter
NetworkAddressing; Routing	Translates logical network address and names to their physical address (e.g. computername ==> MAC address) Responsible for addressing determining routes for sending managing network problems such as packet switching, data congestion and routing If router can’t send data frame as large as the source computer sends, the network layer compensates by breaking the data into smaller units. At the receiving end, the network layer reassembles the data Think of this layer stamping the addresses on each train car	IP; ARP; RARP, ICMP; RIP; OSFP;IGMP;IPX NWLink NetBEUI OSI DDP DECnet	Brouter RouterFrame Relay Device ATM Switch Advanced Cable Tester
Data LinkData frames to bits	Turns packets into raw bits 100101 and at the receiving end turns bits into packets. Handles data frames between the Network and Physical layers The receiving end packages raw data from the Physical layer into data frames for delivery to the Network layer Responsible for error-free transfer of frames to other computer via the Physical Layer This layer defines the methods used to transmit and receive data on the network. It consists of the wiring, the devices use to connect the NIC to the wiring, the signaling involved to transmit / receive data and the ability to detect signaling errors on the network media	Logical Link Control error correction and flow control manages link control and defines SAPs 802.1 OSI Model 802.2 Logical Link Control	Bridge SwitchISDN Router Intelligent Hub NIC Advanced Cable Tester
		Media Access Control communicates with the adapter card controls the type of media being used: 802.3 CSMA/CD (Ethernet) 802.4 Token Bus (ARCnet) 802.5 Token Ring 802.12 Demand Priority
Physical Hardware; Raw bit stream	Transmits raw bit stream over physical cable Defines cables, cards, and physical aspects Defines NIC attachments to hardware, how cable is attached to NIC Defines techniques to transfer bit stream to cable	IEEE 802IEEE 802.2ISO 2110 ISDN	Repeater Multiplexer Hubs Passive Active TDR Oscilloscope Amplifier