Janea Taylor – Ancient Tech Blog

The OSI Model

The OSI model is a set of standards that is used to describe how data is transmitted over a network. The model is divided into seven parts, also known as layers. Each layer serves a different purpose and function in regard to data transmission (Wikipedia – OSI Model definition, 2006). The following describes each layer and their basic functions.

Layer 1 – Physical

The Physical layer typically describes networking hardware equipment and devices such as modems, hubs, and cabling as well as network card connector types. The physical layer interacts directly with the media access control sub-layer of the Data Link layer (Wikipedia – OSI Model definition, 2006).

Layer 2 – Data Link

The Data Link layer consists of two sub-layers: Logical Link Control (LLC) and Media Access Control (MAC). The LLC sub-layer is responsible for flow control as well as error detection and recovery. The MAC sub-layer is responsible for controlling access to media (Bird & Harwood, 2003, pp. 154-155). The Data Link layer typically operates through the use of software, such as a device driver for a network adapter card (OSI Model Layers, 2006). Devices that operate at the Data Link layer include network cards, switches as well as bridges. The Data Link layer is also responsible for physical addressing (Wikipedia – OSI Model definition, 2006).

Layer 3 – Network

The Network layer defines protocol types, logical addressing and is responsible for routing data packets (Bird & Harwood, 2003, pp. 155-157). A router is a type of device that operates at this layer. Some network protocols include: IP, IPX, and RIP (Wikipedia – OSI Model definition, 2006).

Layer 4 – Transport

Like the Data Link layer, the Transport layer handles flow control and error detection (Bird & Harwood, 2003, p. 157). Some transport protocols include: TCP, SPX, and ARP (Wikipedia – OSI Model definition, 2006).

Layer 5 – Session

The Session layer is responsible for maintaining end-to-end communications through the use of protocols such as: Named Pipes and RPC (Bird & Harwood, 2003, p. 159).

Layer 6 – Presentation

Data conversion, encryption/decryption and translation occurs at the Presentation layer (Bird & Harwood, 2003, pp. 159-160).

Layer 7 – Application

The Application layer is capable of handling flow control, error recovery, and network access (Bird & Harwood, 2003, p. 160). There are several types of applications and protocols that operate at this layer, including HTTP, FTP, SMTP, and Telnet (OSI Model Layers, 2006).

When a website is accessed, the data that is transmitted in the process will go through every layer of the OSI model. The process is as follows:

A user opens a browser window and attempts to connect to a website. The browser then attempts to communicate via the HTTP protocol over the application layer with the web server. If the website requires that the data be encrypted, the browser will encrypt or decrypt the data that is being transmitted. This encryption and decryption happens at the presentation layer. The browser then attempts creates a session between the client computer and the server computer so that the data can actually be transmitted between the computers on the session layer. The next layer is the transport layer and this is where the transmission of the data actually begins (OSI Reference Model, 2006). As the browser attempts to submit and request data to and from the web server, it will need to be routed to the proper location. The data will be routed over the Internet on the network layer based on the logical address of the server. Once the browser is aware of where to actually go to communicate with the web server, the data transmission will begin. The network card will prepare the data on the data link layer and then it will begin actually physically transmitting the data over the physical layer via a network medium, such as a cable or wireless frequency. The data will travel over the physical layer to and from the web server. When the browser request gets to the web server, it goes from the physical layer, to the data link layer where the network card begins receiving the data. The data then travels back up the OSI model through each layer. When data is sent from the web server back to the client system, the same process is followed (Bird & Harwood, 2003, pp. 152-153).

Network architects and engineers should be familiar with the OSI model because it will help them troubleshoot problems on a network (Network Troubleshooting Guide, 2006). Knowing how to identify which layer the problem is occurring at will help to determine what the actual cause of the problem is. Network architects should also be familiar with the standards that are defined when designing and building networks.

References:

Bird, D. & Harwood, M. (2003). Network+, Exam N10-002. Que Publishing

Wikipedia – OSI Model definition. Retrieved on January 4, 2006 from http://en.wikipedia.org/wiki/OSI_model

OSI Model Layers. Retrieved on January 4, 2006 from http://www.geocities.com/SiliconValley/Monitor/3131/ne/osimodel.html

OSI Reference Model. Retrieved on January 13, 2006 from http://www.how2pass.com/CCNA/study_material/osilayers.htm

Network Troubleshooting Guide. Retrieved on January 13, 2006 from http://support.3com.com/infodeli/tools/netmgt/tncsunix/product/091500/c1ovrvw.htm

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