The OSI Model
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[edit section] The Open Systems Interconnection (OSI) Model
At the core of the Network+ exam is the OSI "Seven" Model. The model describes the ways and means that networks use to operate and for communication . Though you will seldom (if ever) encounter or use it in practice, and although you will find it to be a mundane, ambiguous, or even arbitrary model, it is heavily tested on the Network+ exam and therefore for our purposes, it is immediately relevant.
The OSI Model is like a seven-layered cake. Just as the cake starts from the bottom and becomes more ornate as it reaches the top, the OSI Model begins with the most basic layer, the Physical layer, and ends at the layer that we as users encounter, the Application layer. Like a cake, each of the layers depends on the layers below it to operate - for example, Outlook Express cannot receive email when the network cable is disconnected - without the cable, the "cake" crumbles. (OK, this analogy is a bit of a stretch, but just go with it!) The order of the layers, therefore, is quite important and you should have it intimately memorized.
Here are the layers, from bottom to top:
| Name | Mnemonic |
| Application | Always |
| Presentation | People |
| Session | Sales |
| Transport | Trust |
| Network | Not |
| Data Link | Do |
| Physical | Please |
Of course, you are free to come up with your own mnemonic device, but we do recommend you find some way of remembering the layers because it will be a subject of the test.
More important than the order of the layers, however, is the function of the layers. Most exam questions on the OSI model ask you, "Which layer does so-and-so operate in?" or something to that nature. Another type of question that occurs frequently is, "A problem has occurred (Problem description). Which layer is to blame ?" These questions can feel ambiguous or difficult, but most of the time, there is a single, clear answer that makes itself known provided that you are aware of the model and the place each layer takes in the model. We will now cover each layer in detail.
[edit section] Physical
At the base of the OSI model is the physical layer. This one is the easiest to understand - it encompasses most of the physical aspects of the network; for example, a repeater (a piece of equipment that amplifies signals) operates at the physical level because it is only concerned with transmitting the electric signal on the wire - it does not try to interfere with, encode/decode, or otherwise logically manipulate the signal. Think of the physical layer as the "electrical" layer of the model - the physical layer is the layer of low-levelnetworking equipment, such as some hubs, cabling, and repeaters . The physical layer is never concerned with protocols or other such higher-layer items.
Examples of hardware in this layer:
Network adapter
Repeater
Network hub
Modem
Fiber Media Converter
[edit section] Data Link
Directly above the physical layer is the Data Link layer, which can be seen as the layer that links the electric signals of the physical layer to the logical organization of the higher layers. Unlike hubs, switches operate at the Data Link sublayer, because switches can direct intra-network traffic to a specific MAC address. It is the layer that organizes data bits (electric/physical) into frames (more logical). The Data Link layer is traditionally broken into two sub-layers:
MAC Sub-layer: (Media Access Control) The layer that organizes networked devices by a unique MAC address . Every networked device has a unique MAC address assigned by the manufacturer.
LLC: (Logical Link Control) The layer that provides error and flow control to other layers, and that is used to provide both connection and connectionless - oriented services . When you think LLC, think transport layer - LLC is intimately connected to the transport layer as the two work together to provide these two types of services
[edit section] 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. In similar fashion, 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.
[edit section] 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" message in the bottle service; it makes a best-effort delivery but cannot establish a connection or verify receipt
[edit section] 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.
[edit section] 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.
[edit section] 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.
[edit section] 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 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!
Also See: Interactive OSI Model Tutorial
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