Networking, The OSI Layer Network Model,Application Layer

In an overall effort to identify and standardize all the levels of communication needed in networking, ISO developed a networking model called the Open Systems Interconnect (OSI) reference model. This model includes all the logical levels of communication needed for one user or application to communicate with another over a network.

To accomplish this overall or complete communication, seven layers, or levels, were identified. This model is developed and understood in much the same way as the model for communications over phone lines, except that the OSI model covers strictly software and firmware, not hardware. Communication between adjacent layers is considered direct, but communication between matching layers is considered logical or virtual.

When studying the OSI model, keep in mind that not all networks have a separate software or firmware layer that matches each of the seven layers. In fact, there is probably no network in use today that perfectly follows the model, but the model does serve the networking industry as a reference point for discussing different levels or layers in a network.

For example, firmware on a network card operates in the physical layer and the datalink layer in the model.

The data-link layer is responsible for disassembling data into segments to be assigned to separate packets and later reassembling packets into contiguous data. The physical layer is responsible for passing packets to and receiving packets from the network media or cabling. In the context of this course, it would be unproductive to try to distinguish which portion of firmware on a Token Ring or Ethernet network interface card (NIC) is the data-link layer and which portion is the physical layer because both are contained on the NIC.

However, it is useful to talk about Token Ring and Ethernet covering these two layers and then to look for other software or firmware on the network that is managing the layers higher up in the model.

For example, once you know that an Ethernet network handles the bottom two layers of data transmission, you can ask the question, “What software on the network determines the best possible route to send a packet so it can arrive at its destination?”

This question is addressed by the network layer. The answer will not be found on the firmware of the Ethernet card because Ethernet does not encompass the network layer of the OSI model.

By referring to the OSI model, manufacturers have a structure from which to work as

they develop and enhance new networking software, protocols, and designs.

Application Layer

The Application layer of the OSI model is responsible for interfacing with the application software using the network.

For example, suppose you are using a word processor such as Microsoft Word. Word can open a document file that is stored on your hard drive, and it can just as easily open a document file stored on a file server connected to the LAN. The file server is known to the word processor as just another drive, such as Drive F. You can open C:\Data\MyFile.doc or F:\Data\MyOtherFile.doc. When the word processor attempts to open the file on Drive F, which is the file server, it communicates the request to the Application layer of the network software. The communication over the network is transparent to the application from that point forward.

The file is retrieved over the network and presented to the word processor by the OSI Application layer. An example of software that handles the Application layer is NFS for Chameleon by NetManage (NFS stands for Network File Service).