The OSI and TCP/IP Models -- Explained.

Network modeling is an important way to visualize and conceptualize how networks function with the many various interconnected pieces of hardware, software, infrastructure, protocols and applications. Modeling is a fantastic way to imagine how complex interconnected systems function as a whole, and its imperative for troubleshooting network problems when they arise. These models are tools for communication between professionals as well.

First, let's explore the Open Systems Interconnection reference model, or quite simply, the OSI model.

The OSI Model at a glance:

LAYER 7: APPLICATION LAYER

The Application layer refers to the code built into operating systems that enable network-aware applications. Developers use an Application Programming Interface (API) to implement networking functions into their applications. All applications built for the TCP/IP model are by definition, network aware. A prime example is an online computer game. The developers have implemented the ability for a game to communicate over a network like the Internet, allowing one computer to communicate to a game server, which in turn then communicates to another computer over the Internet, so that players can play online games with their friends!

LAYER 6: PRESENTATION LAYER

The Presentation layer is an older layer that doesn't make as much sense in today's computer networking world as it used to. The Presentation layer translates data from the lower layers into a format usable by the Application layer. This layer performs bi-directional translation, so it works in either way; when an application sends and receives data. Many applications now have this functionality built directly into them in today's world.

LAYER 5: SESSION LAYER

The Session handles all active sessions for a system, initiates sessions, accepts the incoming sessions and opens and closes existing sessions. This layer connects devices together using port numbers so that one system can send a document to another device which could then print that document for the user, as an example.

LAYER 4: TRANSPORT LAYER

The Transport layer is the 'Assembly/Disassembly' layer. The transport protocol breaks apart large chunks of data into Internet Protocol (IP) packets and then are put into frames or, TCP segments and UDP datagrams, depending on the connection type. The computer then sends these frames to its Network Interface Card (NIC) for sending. The Transmission Control Protocol (TCP) is a connection-oriented protocol which sends the data in an ordered and organized fashion, requires a two-way connection between two systems over a port to port connection, and transmission is guaranteed! If a packet is lost along the way, the receiving end will know, and that packet will be resent. This is a slower system than UDP, but is used for connections that require 100% data transfer without fail; such as a a client browsing the web and downloading files. The User Datagram Protocol (UDP) is a bit of the opposite. UDP is connection-less and therefore its far faster than TCP. UDP allows an application or service to send data very quickly without first establishing a connection. UDP has many uses, but a major use we all know and love is media streaming. Streaming services for music or video use UDP because they can afford to lose a few packets here and there. This results in a much faster service for the client. This is the same reason why sometimes video streams have artifacts or look strange when bandwidth is low. Devices operating within the Transport layer include Gateways and Firewalls.

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