OSI REFERENCE MODEL


As networking became more popular in the world, a well-organized, logical framework for connecting networks and developing applications was needed. In the late 1970s, the International Standards Organization (ISO) developed the OSI networking reference model.

The OSI reference model is a seven-layer logical approach to network communication that includes specifications for the actual hardware connection to the network at the bottom layers, and rules for applications and more complicated functions at the higher layers. Networking rules for communication, also known as protocols, exist at almost every layer of the OSI model. The more complicated the protocol, the higher up on the model it resides. Network transmission, security, session connection information, and hardware are each associated with a particular layer.

Picture yourself sitting at your computer, working on a Microsoft Word document. You are actually utilizing functions that reside at the top layer of the OSI reference model, known as the application layer, or layer 7. You decide to attach the Word document to an e-mail message and send it to a co-worker. The message, or signal that you are sending is directed from the application layer (layer 7) down to the physical layer (layer 1), where it is placed in converted format (0s and 1s) on a network medium, such as a wire, and transmitted to your co-worker. Your co-worker on the receiving end accepts the message through his or her physical layer (layer 1). The message is converted back to a readable format from 0s and 1s, and is presented to your co-worker’s application layer (layer 7).

Here are the seven layers of the OSI reference model, starting with layer 7.

  1. Application Layer: Applications, e-mail, FTP, user authentication, and any other major services that the end user directly interacts with are associated with this high-level layer. Network access and forms of error recovery are handled at this layer. High-level devices, such as gateways, are present at this layer. Application-specific protocols, such as X.500, SMTP, SNMP, Telnet, and SMB, reside at this layer, as well as the presentation and session layers.

  2. Presentation Layer: Data on the sending computer is converted to a format that can be transmitted over media to another computer. On the receiving end, data is converted into a format that the end user or application layer can understand. Encryption and data translation occur at this level. The network redirector operates at this level.

  3. Session Layer: The session layer establishes, holds, and controls sessions or connections between two applications. It provides checkpoint and synchronization service between two communication sessions. Security is handled at this layer.

  4. Transport Layer: The transport layer’s primary concern is flow control and data handling. Large forms of data are broken down into manageable packets that can be presented to the higher layers on the receiving end. The successful transmission of data is acknowledged at this layer. If the transfer of information is incomplete or interrupted, this layer is responsible for requesting the information to be retransmitted by the sending application or session. Transfer protocols such as TCP, NetBEUI, NWLink, and SPX reside at this layer.

  5. Network Layer: The network layer is responsible for the routing of information to the correct network, device, or computer. Logical names are converted to physical names at this layer. In other words, computer Internet Protocol (IP) addresses are converted to their Media Access Control (MAC) equivalents. Priority of connection and quality of service are also handled at this layer. A network router and switch reside at this layer, as do network protocols, such as IP and IPX.

  6. Data Link Layer: Data frames received from the network layer are converted into bits (0s and 1s) at the data link layer in preparation for the physical layer. On the receiving end, bits are packaged together into frames that can be understood by the higher layers. Frame synchronization, flow control, and error handling are addressed at this level. The data link layer has two sublayers, known as the Logical Link Control (LLC) and MAC layers. The LLC sublayer is associated with IEEE standards 802.1 and 802.2, and is responsible for the implementation and placement of Service Access Points (SAPs). The MAC sublayer is associated with IEEE standards 802.3, 802.4, 802.5, and 802.12. The MAC sublayer communicates directly with an NIC. It is responsible for error-free communication between network interfaces. Devices called bridges, which segment network traffic, operate at this layer.

  7. Physical Layer: The physical layer is the physical adapter or connection to the network wire or medium. It is where bits of information are prepared to go across the network medium. Incoming bits of information are organized and prepared to move through the higher layers.

If you are interested in pursuing a career in networking, it is very important that you understand the theory behind the OSI reference model. You will also need a solid understanding of the OSI reference model and the IEEE specifications if you are interested in passing CompTIA’s Network+ certification examination. However, that is another story altogether—or is it? This book has been designed as a springboard to your computer certification future. You will already have a great start toward CompTIA’s Network+, Security+, and Server+ certification examinations when you complete this book.

If you are interested in viewing one of the very best charts available regarding the OSI Reference Model, please visit the following Geocities Web site: http://www.geocities.com/SiliconValley/Monitor/3131/ne/osimodel.html.




The A+ Certification & PC Repair Handbook
The A+ Certification & PC Repair Handbook (Charles River Media Networking/Security)
ISBN: 1584503726
EAN: 2147483647
Year: 2003
Pages: 390

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