Welcome back to digital forensic concepts. We're talking about network basics, in this course and in this module, we're going to talk about the OSI model. OSI stands for Open System Interconnection.And in this slide, we can see the layers, the protocols and the unit exchange. The layers from top to bottom, so from layer 7 down the layer 1. Layer 7, we have the application layer, layer 6, we have a presentation layer, layer 5 would be the session layer, layer 4 would be the transport layer, layer 3 would be the network layer, layer 2 would be the data link layer, and layer 1 the physical layer. And this diagram shows each layer, each protocol, and then it shows the unit exchange. You look at the physical layer, you can see the user unit exchange is bit. Whereas if you look at the network layer, you can see the unit exchange is packet. So different units of exchange happened at different layers. We're going to start and talk about layer 1 down at the bottom. The physical layer is the lowest layer of the OSI model. And we are talking about actual physical components of the network. So we're talking about things including cables, the frequencies that we operate are whether they are wireless or hard wired cables. This is what activates and maintains and ends. The actual physical connection is responsible for transmission and reception of the unstructured raw data, as we saw the units were bits. So it is unstructured broad data that this slayer is dealing with, it also handles our voltages and data rates. How much voltage we're using? What are our data rates for transmission? This is all defined in this layer. It's going to convert the digital analog bits into an electrical or an optical signal to be transported over the network. Data encoding also happens at this layer, if the data is encoded. Our data link layer, which is layer 2, the main function of this layer is to ensure data transfer is error free from one node to another, because when our information is transmitted across our network, it will have hops from one note to another. This layer is also transmitting and receiving data frames, and this is manages the sequence of the data frames. This layer also sends and accepts acknowledgement for the frames received and sent. The frame was you sent, was the frame received? If the frame was not received, it also was responsible for re sending it. This is handled at that later, the data link layer.And it does manage the frame traffic control over the network. So the unit is frames, and this is the layer that is responsible for ensuring that the data is transferred from one node to another without errors, and if there are errors, this layer is responsible to re send those frames of data. Now, the network layer is responsible for packets, because its unit is packets, so it will handle the packet forwarding, it will handle the routing through different channels from one node to another. It acts as kind of the network controller of the traffic cup, and also manages our subnet traffic. It will decide which route to take, and it usually does that by which route would be the fastest.And it is what divides the outgoing messages into packets, and then reassembles the messages, the incoming packets to be sent up to the higher levels of the. The transport layer, layer 4 handles the coordination of data between the system and the host. It decides how much data is going to send, how fast it's going to send it and where it's sending it to. A good example of a protocol on the transport layer would be the TCP, which is built on top of the IP or internet protocol, and it is commonly referred to as TCP IP. We did discuss that earlier. Now the transport layer receives messages from the session layer above. It converts the message into smaller units, what it does that it's going to pass it on to the network layer. The session layer which is layer 5, is what creates the session because when two devices need to communicate, a session has to be created, and it's done at the session layer.And this is involved setting up the communication. An example would be how long should the system wait for response? When is it going to time out? The session layer handles the coordination of communication, it handles transmission between the applications at each end of the session. So two computers are talking to each other, each one is involved in the session, and layer 5, the session layer is what handles how communication is conducted. It makes sure it happens properly so that the session does not end prematurely and cause data loss, and all this is handled at the session layer, layer 5. Layer 6 is called the presentation layer. This layer kind of prepares the data, just like it kind of says it presents it, ensures that it's set in such a way that the receiver can understand it.The receiving device can interpret the data, and this could involve languages, syntax, it sort of plays the role of a translator. It presents the data for the application of the network. One of the important things that happens here at layer 6, is encryption and decryption, happens at later 6, the presentation layer. The layer above layer six is going to be the application layer, and this is the top most layer of the OSI model, and this is the layer that most users are going to interact with directly. This receives information directly from the user, and it displays an incoming information to the user. And good examples of things we're going to see at the application layer are going to be our Web browsers, Telnet and FTP. And these are communications that rely on layer 7, the application layer.
