The Scholar's Approach to the Internet
02/09/2001Welcome back. This is the second installment to the series, Networking as a Second Language. If you missed the first installment, Fundamentals of an Internetwork, feel free to brush up. In this column we'll examine networking from a high-level view using an academic model that is an industry standard. This should provide you with a good basis for topics to be presented in future articles.
The OSI 7-layer model
The Open Systems Interconnect (OSI) Reference Model divides commonalties of network operations into modules, called layers. The OSI network model has seven layers (modules) and each layer serves a specific purpose. The reference model exists to provide continuity across multi-vendor hardware and software applications. This means that the manner in which computers and devices communicate on a network must adhere to this standard.
The 7 layers in a nutshell
In my last column, I discussed a simple LAN network consisting of two computers (hosts) connected on a single wire. Letšs take another look at this network using the standardized reference model. For the sake of argument, host A is an Intel PC architecture and host B is a Macintosh. The hosts represent two distinct hardware vendors, and the network interface cards may have Intel chipsets, 3Com, or countless other proprietary components. Yet these two devices can communicate over the same wire as well as exchange data.
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The Macintosh is running an e-mail application and the mail server is on the PC. Communication would be peer-level to peer-level between the two devices. Conceptually, the application layer on the Macintosh is communicating to the application layer in the PC. This peer level communication is a language or a set of rules on how the devices must talk to one another. The special set of rules for peer level communication is called a protocol.
You will notice that the application layer is at the top of the model and the physical layer is at the bottom. In order for the e-mail application on the Macintosh to request mail from the server on the PC, the Macintosh request must logically transverse its way down the seven layers on the local device and work back up the seven layers in the PC mail server. The design of the layers is modular, therefore, each layer has its own set of protocols.
The upper layers
Keep in mind that this is an abstract model. Professionals often refer to the upper layers as the application layer. Although, the upper three layers of the reference model include the application layer, presentation layer, and session layer.
The application layer is the user layer, i.e., what the user sees: the browser, the mail reader, or the spreadsheet. It is merely that, the application. The application layer provides the windowing or interface between the user applications and the lower layers of the model.
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The presentation layer is concerned with data formatting. The Intel PC and the Macintosh have different internal byte formats. The presentation layer is responsible for syntax and formatting in the application layer. Types of formats could include ASCII for telnet, JPEG for graphics, and QuickTime formats for movies.
The session layer is responsible for authentication and authorization. The Linux operating system uses X-Windows as a graphical interface. X-Windows allows virtual connections to different hosts. The sessions layer manages and terminates these connections.
The transport layer
The transport layer provides the functionality for applications to communicate across platforms. The purpose of the transport layer is to guarantee correctness of data before it is passed up the layers to the application. The common protocols operating at this layer are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).
The TCP protocol is said to be connection-oriented. That is, setup is required on both systems that are communicating with each other. A virtual circuit is established between the communicating applications. With this virtual circuit, the TCP protocol can check to determine whether data made it to its destination properly. If not, the protocol will attempt to re-send the data. For this reason the TCP protocol is called reliable.
The UDP protocol does not establish a virtual circuit, as does TCP. UDP is called a connectionless protocol since it does not check to see if a datagram was delivered properly.
 | Figure 3. The transport layer uses specific port numbers to identify the type of service requested by the application. TCP (or UDP) connect the application layer to the network layer. |
Another function of the transport layer that you should be aware of is the use of port numbers. Servers can run multiple services, such as e-mail and FTP. These services are used commonly and have reserved 16-bit values called port numbers. TCP and UDP use port numbers to identify the type of service requested by the application. Ports from 1 to 1023 are reserved ports. The most common "well-known" ports are 80 for HTTP, 23 for telnet, 25, for e-mail, and 21 for FTP.
The lower layers
Now we can roll up our sleeves and start playing in the internals of the network. The lower layers of the OSI model are responsible for the physical connectivity of the host and the routing of data in the network. The layers include 1 through 3, the physical layer, the datalink layer, and the network layer.
Layer 3 is the network layer and its predominant protocol is Internet Protocol (IP). You are probably familiar by now with the protocols TCP/IP. We are working with layer 3 and 4 protocols in our example. The network layer is not limited to IP because there is also the Internet Control Message Protocol (ICMP) for pinging another host on the network. The Internet Group Management Protocol (IGMP) for multicast data is also included in this group.
 | Figure 4. The bottom sections of the model -- network, datalink, and physical -- are sometimes referred to as the network layer, and provide delivery of data up to the transport layer, which in turn relays the information to the application layer. |
Layer 3 could also be considered the routing layer. The term router and layer 3 switching can be used interchangeably. Routers are typically high-end, network-specific hardware components that manage the segmentation of networks. However, routers can also be optimized software applications running on servers for small networks.
The network layer is the interface layer of the OSI model. The purpose of the network layer is to provide delivery of data up to the transport layer.
Layer 2 is the datalink layer in the OSI model. This layer may encompass the operating system on your computer, the network interface card, and its device driver. These are the components required for your computer to get a signal out onto the network. Hardware devices at this layer include, as mentioned, the network interface card, hubs, and layer 2 switches to name a few. Every piece of hardware manufactured has a special identification called a Media Access Control (MAC) address.
Finally, there is the physical layer, layer 1. This is the actual wire or cable coming out of your computer. Several protocols exist at this layer, but the predominant layer 1 protocol for networks is Ethernet.
The 7-layer cake
In these explanations I did exercise some creative liberties to get the point across in the minimal amount of words. Volumes and dissertations have been written on the OSI Reference Model, and this certainly isnšt my objective. For the moment I just want to acquaint you with the architecture and philosophy behind the reference model since it is used by industry and is a standard.
Hopefully I didnšt dump too many details on you too fast. There are a lot of new concepts presented here. Donšt get overwhelmed. Just consider the model to be a giant, 7-layer cake. In the next couple of weeks, I will be slicing out pieces of this cake and feeding it to you appetizing servings. Donšt miss the next installment where wešll take a look at the physical layer and the Ethernet protocol. Until next time.
Michael J. Norton is a software engineer at Cisco Systems.
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