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.
The layers are shown, from bottom to top, in the graphic to the right.
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.
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-level networking equipment, such as some hubs, cabling, and repeaters. The physical layer is never concerned with protocols or other such higher-layer nonsense.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.