2.2 - Five Layers Model - part 1

you in the left video we realized why we need layers we said that networks are very complicated and have various requirements dividing the communication into layers will allow us to simplify things and make communication more flexible in the upcoming videos we'll get to know the five layers model will first take a high-level look and understand the role of each layer so for the upcoming videos will not talk about the details but rather only discuss the responsibilities of each layer in the following videos we'll dive deeper into each layer and understand how it might be implemented in this video we'll cover the first three layers in the next video we'll cover the remaining two the first layer is the physical layer this layer is responsible for transmitting a single bit 0 or 1 over the network while this task is extremely complicated it will not be a far focus for this course just to have some understanding of what this layer is responsible for let's consider the time of transmission let's assume that we have some kind of cable to transmit our data and we use +5 voltage to transmit 1 and minus 5 voltage to transmit 0 what beats does the following diagram represent well it might indeed be 1 0 0 1 that is the case if it takes this long to transmit a single bit however it might also represent other bit streams for instance if it takes only half the time to transmit a single bit then the bit stream might be 1 1 0 0 0 0 1 1 the difference lies in the time would dedicate for transmitting a single bit this is called the bit rate that is the number of bits that are conveyed per unit of time of course achieving a high bit rate is preferable as it means we can send many bits in a very short timeframe however it is hard to achieve high bit rates without getting many errors this is only one of the things that the first layer needs to take into iteration again will not focus on the first layer throughout this course will will however discuss the other four layers in much depth let us continue to the next layer the second layer is the datalink layer it is responsible for transmitting data between two hosts that are directly linked despite possible errors what do we mean by directly linked for now let us simply say that there is no device in between the two devices so if we have two computers here computer a and computer B and they're connected via computer M then computer a and computer B are not directly linked however computer a and computer M are directly linked and so our computer m and computer be another way to put it in that computer and computer M our one hop away from one another whereas computer a and computer B are two hops away that is in order to get from computer a to computer B we need two hops one hop from A to M and another hop from M to B what do we mean by errors the physical layer might provide air Ernest data for example 1 instead of 0 so a stream of bits such as 0 0 0 1 1 0 might be received as 0 0 1 1 1 0 many reasons might cause this kind of errors for instance we can think of a truck literally running over the wire where the bits are transmitted causing some problem anyway the second layer must handle the communication despite these errors the second layer sends data in datagrams that is chunks telegrams in this layer are called frames wait why do we need frames why should we split the data into frames and not just send a very long stream of data will answer this question when we talk about the second layer in more depth that is in future videos friends will usually contain a MAC address a physical address identifying the sender as well as the receiver why would we need a MAC address first as receivers we'd like to know that the frame is intended for us who wouldn't like to waste precious time reading data intended for someone else if the frame contains a MAC address that doesn't belong to us we can simply drop it second for privacy reasons would like messages to arrive only it intended receivers so only them can read the data third as a sender I would like the receiver to know who sent the frame that way the receiver will be able to send his or her response back to me and not to someone else note that we would like the addresses to be unique that is we want one address to identify a single device that way we know that if we send a message to this address it will be sent to the intended device only let's move on to the next layer the third layer is the network layer this layer is responsible for routing that is determining the password that data will travel we can think of this layer as the successful routing app Google Maps when we get in the car and use Google Maps we tell the app our destination and Google Maps finds out the best route for us to drive in notice that Google Maps is dynamic it won't necessarily pick the same route each time sometimes one path will have a traffic jam so Google Maps will prefer another route we said that the second layer has physical addresses called MAC addresses the third layer is responsible for logical addresses such as IP addresses we learn about them in depth when we learn more about this layer in the third layer datagrams are called packets let's look at the following network diagram we have two computers here one in France and one in the States obviously they are not directly linked they're linked via our third layer devices called routers will talk about routers in a later video which layer is responsible for each connection consider the connection here between computer a and router what the second layer is responsible for this connection and for this one here between router 2 and router 5 right again this is the second layer the same applies for each connection between two directly linked devices the third layer is responsible for defining the route that the message sent from computer to computer we will go through routers 1 2 5 8 and 10 and not in another way note that there may be different implementations for each layer for instance we may have different implementations of layer 2 so while the connection between computer a and router 1 might be over an Ethernet cable the connection between router 1 and 2 might be Wireless and used Wi-Fi the connection between router 2 and router 5 my use a carrier pigeon while the connection between router 5 and 9 will also use Wi-Fi the third layer does not care about these changes but the second layer definitely does if a sniper shoots the carrier pigeon that transmits data from router 2 to router 5 the second layer will have to handle it it will also have to make sure that the data transmitted over the air between routers 1 and 5 is valid and without errors all right so far we have covered three of the five layers we said that the physical layer is responsible for transmitting a single bit 1 or 0 over the network again we shall not focus on this layer throughout the course well then briefly talked about the data link layer responsible for transmitting data between directly linked devices that is devices connected via a single hop when we need to get data to transfer between hosts that are connected via multiple hops the third layer comes into play and determines the route that is the path that the packets will travel in the next video will cover the remaining 2 layers and then gain a full image of the five layers model again these videos are only a high-level overview of the layers model we shall take an in-depth view in later videos if you have any questions feel free to post them in the comments below or on our Facebook page

Channel: Brief

Views: 7,103

Keywords: Computer Networks, Networks, Brief, Brief Networks, Computer Science, Five Layers Model, TCP IP, Layers Model, TCP/IP

Id: Q3qqd6Y2FbQ

Length: 9min 4sec (544 seconds)

Published: Sun Dec 30 2018