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Home , OSI model

The OSI Model and You

By David Neier, with Stephen Turbek 29 January 2001 Razorfish Report 048

The Open System Interconnection ticular protocol. Typically these pro- such as text and images, translating (OSI) model describes the way in tocols add control, encapsulation, between the computer and which information travels across a conversion functions and/or the network formats. This includes network, such as how a user’s information to the front or back of character set conversion (between request for a web page, is com- the original message. This allows languages), and municated from the user’s browser seemingly transparent delivery of . Common examples of program over the network to the the message to the destination. this format are HTML and JPEG. web server and how that page is Application Application sent back. Using a model like this Presentation Presentation allows programmers to standardize their software design and break Session Upper OSI Layers Session the program into modular pieces Transport Lower OSI Layers Transport which can be independently devel- Network Network Network Network oped and revised. Data Link Data Link Data Link Data Link

The Physical Physical Physical Physical The collection of communication Client Computer Router Router Server layers is commonly referred to as The Open Systems Interconnection model visualized the ‘protocol stack’, visualized as a Separation Between Layers Layer 5: Session stack of seven interconnecting sec- The OSI model is separated into The manages com- tions. Each layer accomplishes its two distinct levels. The upper ‘appli- munications between computer own task and then hands the infor- cation’ level includes the appli- systems. It establishes the connec- mation on to the next layer, using cation, presentation and session tion ID numbers, the coordination a variety of protocols (communi- layers while the lower ‘dataflow’ of acknowledgement numbering cation standards) to interface with levels include the transport, net- and retransmission procedures, the the user, for operating system func- work, data link and physical layers. release of a connection when fin- tions, information conversion and ished, and re-establishing a session the delivery of this information to the Layer 7: Application between computers should it fail. destination device. The top layer of the OSI model is The Session layer would either the . Applications resume the interrupted dialog, or ini- Communication Between Layers (another term for interactive com- tiates a new session to the desti- The various layers pass network puter program) allow the end-user nation device. A common Session requests to each other using a par- to browse the web, email friends protocol is RCP. or colleagues, and initiate various network related tasks and opera- Layer 4: Transport tions. One of the best known pro- This layer provides reliable end- tocols used in the application layer to-end communications by pro- is the Hyper Text Transfer Protocol viding service addressing, flow (HTTP), the standard used to send control, multiple connections, data- Web requests and pages between gram segmentation, and raw data browsers and servers. error checking. The breaks data into segments so it can David Neier Layer 6: Presentation be sent over the network and reas- is an Infrastructure and Security Engineer This layer defines data formats, sembles the segments at the other at Razorfish New York. ©2001 RAZORFISH, INC. RAZORFISH IS A REGISTERED TRADEMARK OF RAZORFISH, INC. AND THE RAZORFISH LOGO IS A SERVICE MARK OF RAZORFISH, INC. ALL RIGHTS RESERVED. The OSI Model and You end, and ensures that the data is The has multiple The data packet travels over the received at the appropriate device. specifications that work together. A and arrives at the Bank’s An example Transport protocol is common example of the Physical web server. It then travels in reverse Transport Control Protocol (TCP), layer is the cables we use order up the OSI stack (Physical used on the Internet. to connect computers to the net- layer first) to be processed by the work at the office. The cables web server’s Application layer. The Layer 3: Network themselves are a standard called process is then reversed and data The has many func- “Category 5”. “RJ45” is the stan- is sent back down the OSI stack tions. One of the most important of dard for the size and shape of the on the bank’s server towards the these functions includes the assign- jack at the end of the cable, and the end user’s machine. Once the data ment of logical IP addressing (the Ethernet standard defines what the is received, it travels up the OSI names of the network computers) to pins on the plug are used for. stack on the end user’s machine network devices. Other functions until it reaches the browser (Appli- include providing for network rout- A Simplified Walk Through cation layer where the initial com- ing, of the connections, Let’s say that you, the end-user are munication began) with a response, and sequencing of the constructed using your PC to do some online completing the process. packets. A common Network Pro- banking with your bank’s web site tocol is (IP) used over a secure () connection. Behind each click of your mouse on the Internet. there is a vast and complex system When the user clicks on a link, the of packets, protocols and pro- Layer 2: Data Link web page request enters the OSI cesses that comes to life. It works The Data provides the stack at the Application layer, using even if you don’t understand it, but physical addressing –known as the browser as an entrance point. you no longer have that problem. MAC addresses– to the device on The requested data is passed to the the network and manages flow , where the data control. The orga- packet is encrypted and formatted. nizes data into a rudimentary The Session layer will establish and structure known as a . The maintain the HTTP session. Data frame contains information about continues on to the Transport layer, the physical source and destination where the end-to-end communica- address and fields that are respon- tion between user and destination sible for synchronization, flow con- device is established. The data trol, and error checking. An passes down to the Network layer for more information example protocol is PPP, used by where logical source and des- home computer modems to call tination addressing is attached basics their ISPs. and encapsulated within the data http://www.ieng.com/cpress/ packet. The data passes to cc/td/cpress/fund/ith/ith01gb.htm Layer 1: Physical the Data Link layer, where logical The foundation of communications (IP) addressing is converted into ISO’s OSI reference model between any computer systems, the physical source and destination http://www.cstp.umkc.edu/personal/ Physical layer, defines the physical addresses (MAC), and information kkev/osi/osi. path through which the information is added for flow control. Finally, flows. It includes the transmission the data reaches the Physical layer media (the wires) and the actual where a transportation medium Razorfish Reports are published for our data signals (the current). such as Ethernet cabling is utilized. colleagues and the interested public.

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