Subtitles of the Movie

To understand the physical layer, we need to understand that all of the communication that takes place between the computers is a result of fluctuations of electrical current on a wire. That's all it is. Either the current changes its state during a timing signal, which produces a one, or the current does not change its state during a timing signal and that produces a zero. It is like really, really fast Morse code going through the wires the whole time. So the physical layer defines the electrical, the procedural and the functional specifications of the physical link. In particular voltage levels, if it all happens because of voltage, how much voltage, and how do we control all this timing and voltage changes. In other words, how we know when we produced a zero or when we produced a one? How fast can we do this? The physical layer determines how fast a component can send this information out, and how fast another component can receive it. And how far, how far can we transmit this information and know that it's reliable, know that it's received? And then the connectors themselves: what do we use, so that we know that we can connect all of these pieces together? Because the bits, the zeros and the one's are nothing but electricity fluctuating up and down, and we are keeping track, or the system is keeping track of the timing and the voltage changes. That information is then being interpreted and sent up to the next layer that has even more intelligence; but at this layer, all we are doing is keeping track, and we need to make sure that we are interpreting at this layer in the right way. So this layer determines - for example a category five cable has to have a certain number of twists in it to prevent cross talk, to prevent pairs of wires that are in the same cable from talking to each other, that's just an example. Or the network interface card has to have a certain number of pins in it, and it has to run off of a certain voltage. That Ethernet can only go so far, and then be considered a reliable communication method. All these are defined at the physical layer, but all of the layers go together to produce the communication. The flow goes between all of the layers, out one computer through all the layers, down through the layers, from seven to one, through the wire and then up into the receiving computer, up through the layers one to seven, and this is how the communication works. So now that we have discussed general communication, in our next module, we'll discuss the collision domains versus broadcast domains, that we can create using different devices in a network. In other words, if we are going to put the data out on to the wire, how we are going to control to make sure that those devices that need to get the message can get the message, but at the same time, make sure that devices that had no need to get the message don't even get bothered by it, don't even have to consider that message and determine whether it is for them. So in our next chapter, we'll discuss collision domains versus broadcast domains. That's next. :04:43.0] So in our next chapter, we'll discuss collision domains versus broadcast domains. That's next.