alt="NA"/>FIGURE 1.8 The lower layers
The following network devices operate at all seven layers of the OSI model:
■ Network management stations (NMSs)
■ Web and application servers
■ Gateways (not default gateways)
■ Servers
■ Network hosts
Basically, the ISO is pretty much the Emily Post of the network protocol world. Just as Ms. Post wrote the book setting the standards – or protocols – for human social interaction, the ISO developed the OSI reference model as the precedent and guide for an open network protocol set. Defining the etiquette of communication models, it remains the most popular means of comparison for protocol suites today.
The OSI reference model has the following seven layers:
■ Application layer (layer 7)
■ Presentation layer (layer 6)
■ Session layer (layer 5)
■ Transport layer (layer 4)
■ Network layer (layer 3)
■ Data Link layer (layer 2)
■ Physical layer (layer 1)
Some people like to use a mnemonic to remember the seven layers, such as All People Seem To Need Data Processing. Figure 1.9 shows a summary of the functions defined at each layer of the OSI model.
FIGURE 1.9 OSI layer functions
I’ve separated the seven-layer model into three different functions: the upper layers, the middle layers, and the bottom layers. The upper layers communicate with the user interface and application, the middle layers do reliable communication and routing to a remote network, and the bottom layers communicate to the local network.
With this in hand, you’re now ready to explore each layer’s function in detail!
The Application Layer
The Application layer of the OSI model marks the spot where users actually communicate to the computer and comes into play only when it’s clear that access to the network will be needed soon. Take the case of Internet Explorer (IE). You could actually uninstall every trace of networking components like TCP/IP, the NIC card, and so on and still use IE to view a local HTML document. But things would get ugly if you tried to do things like view a remote HTML document that must be retrieved because IE and other browsers act on these types of requests by attempting to access the Application layer. So basically, the Application layer is working as the interface between the actual application program and the next layer down by providing ways for the application to send information down through the protocol stack. This isn’t actually part of the layered structure, because browsers don’t live in the Application layer, but they interface with it as well as the relevant protocols when asked to access remote resources.
Identifying and confirming the communication partner’s availability and verifying the required resources to permit the specified type of communication to take place also occurs at the Application layer. This is important because, like the lion’s share of browser functions, computer applications sometimes need more than desktop resources. It’s more typical than you would think for the communicating components of several network applications to come together to carry out a requested function. Here are a few good examples of these kinds of events:
■ File transfers
■ Enabling remote access
■ Network management activities
■ Client/server processes
■ Information location
Many network applications provide services for communication over enterprise networks, but for present and future internetworking, the need is fast developing to reach beyond the limits of current physical networking.
The Presentation Layer
The Presentation layer gets its name from its purpose: It presents data to the Application layer and is responsible for data translation and code formatting. Think of it as the OSI model’s translator, providing coding and conversion services. One very effective way of ensuring a successful data transfer is to convert the data into a standard format before transmission. Computers are configured to receive this generically formatted data and then reformat it back into its native state to read it. An example of this type of translation service occurs when translating old Extended Binary Coded Decimal Interchange Code (EBCDIC) data to ASCII, the American Standard Code for Information Interchange (often pronounced “askee”). So just remember that by providing translation services, the Presentation layer ensures that data transferred from the Application layer of one system can be read by the Application layer of another one.
With this in mind, it follows that the OSI would include protocols that define how standard data should be formatted, so key functions like data compression, decompression, encryption, and decryption are also associated with this layer. Some Presentation layer standards are involved in multimedia operations as well.
The Session Layer
The Session layer is responsible for setting up, managing, and dismantling sessions between Presentation layer entities and keeping user data separate. Dialog control between devices also occurs at this layer.
Communication between hosts’ various applications at the Session layer, as from a client to a server, is coordinated and organized via three different modes: simplex, half-duplex, and full-duplex. Simplex is simple one-way communication, kind of like saying something and not getting a reply. Half-duplex is actual two-way communication, but it can take place in only one direction at a time, preventing the interruption of the transmitting device. It’s like when pilots and ship captains communicate over their radios, or even a walkie-talkie. But full-duplex is exactly like a real conversation where devices can transmit and receive at the same time, much like two people arguing or interrupting each other during a telephone conversation.
The Transport Layer
The Transport layer segments and reassembles data into a single data stream. Services located at this layer take all the various data received from upper-layer applications, then combine it into the same, concise data stream. These protocols provide end-to-end data transport services and can establish a logical connection between the sending host and destination host on an internetwork.
A pair of well-known protocols called TCP and UDP are integral to this layer, but no worries if you’re not already familiar with them because I’ll bring you up
