Network
The Network layer is where the frames of the Data Link layer become packets. The best way to think of the Network layer is as the mailroom clerk of the OSI model. The clerk receives mail and directs it to the appropriate couriers, while the Network layer translates the frames it receives from the Data Link layer into more logical packets, which can be routed to other networks (like sending it to a courier). At the Network layer, you can begin to actually communicate across a Network, but the service is called "unreliable" because no connection can be established. Communication over the Network layer is something like throwing a message in a bottle into the sea - you cannot verify that the other person ever reads the message. It should be noted that most of what we call "routing" occurs at the network layer - that is, network traffic is routed from one network to another at this layer, allowing for inter-network (as opposed to intra-network) communication.Transport
At the Transport layer, the Network layer's packets are sorted and organized into "segments." This is different from the idea of packets in the Network layer in one fundamental way: the segments of data over the Transport layer contain information on the connection and the transmission of data. The Transport layer removes the uncertainty of "throwing the message in the bottle" that we experience over the Network layer by attaching to segments (which are basically continued packets) information about the state of a connection. Thus, the Transport layer uses the LLC sub-layer of the Data Link layer to establish connections between hosts. The protocols that are typically associated with the Transport layer are:
TCP: Connection-Oriented, reliable - unlike the "message in the bottle," it can verify that a segment or packet reaches the location, or note failure.
UDP: Connectionless, unreliable - it is like a "smarter" mesage in the bottle service; it makes a best-effort delivery but cannot establish a connection or verify receipt.
Session
The Session layer is the layer that initiates and terminates the transport layer connection-oriented services. While the Session layer is not widely used by protocols, it is important in that it is responsible for managing the connections that we value and that the Transport layer provides.Presentation
The Presentation layer translates the segments of information from the Transport layer into data that can be used at the Application layer. It is something of an intermediary between the network node's processing area and the network node's actual networking area - it can interpret the segments or packets it receives and change them into "data formats" that we all know and that the PC can recognize.Application
This is the top of our cake, utilizes the layers below it, and includes the functions that we are most familiar with - the end-user application protocols such as FTP and HTTP, the vital services like DHCP and DNS, and several obscure applications. The Application layer does indeed cover a wide variety of protocols and services, but don't let this overwhelm you. In general, when trying to decide if a service or protocol is an application level one, ask: "Does this facilitate networking, or does networking facilitate the service?" If the answer is the latter, you know it is an Application layer service.We Shall Meet Again...
Although you have enjoyed learning the OSI model, don't assume this is the last encounter you will have with it - you would be far mistaken. Actually, the OSI model permeates the theoretical foundation of all networking hardware, software, and standards. It is the common denominator of the networking (at least as far as the Network+ exam is concerned), and will reappear in just about any discussion of any networking topic. So, be sure to keep the OSI model in the back of your head and continue to study it!ProProfs Free Online Network+ Certification School provides free study aids for the Network+ exam such as study guides, practice exams and practice questions. It's free online Comptia Network+ learning center is located at: http://www.ProProfs.com/certification/comptia/network-plus/index.shtml
