 So we we know some basics of getting data across one link now We want to look at what if I want to get data to some location that I don't have a direct link to and The common approach is I have a link to one device and that device has a link to another device and maybe so on and We send my data across a set of links or across a network So we want to look at the general concepts of how to get data across a network how to find Which links to send your data via to get to the particular desired destination routing and Some of the protocols to make that data delivery efficient. So we talk generally about networking but We'll come back to Some aspects of protocols and talk about how people who design networks Split the functionality into different components We already know two components or two One way that we split the functionality. We've mentioned the physical layer Think of the physical layer is that all of those things that deal with getting a Signal or getting data across a link using a signal What is the bandwidth and frequency of that signal? What is the? Signal encoding technique to be used. Do we use NRZ? Do we use Manchester encoding? frequency shift king What media to use? So different technologies will have different choices there and and they would implement Different protocols and we say all of those are part of what we call the physical layer Getting physical signals from one point to another Then we looked at error detection and correction and flow control Even if we can get data across a link. We need some other features Make sure we don't overflow the receiver make sure that if with data goes wrong that we can retransmit Those are classified as the data link layer Again about a link, but focusing on the frames getting frames across the link But there are other aspects that we need to get networking to work correctly Let's look at them. This is one of the pictures from the first lecture. I think Communications across the link. How do we convert the data into signals to be sent across that link? We've looked at that How do we make sure that that link can efficiently and reliably transmit the data flow control error control are two parts of that There are other parts as well. Maybe addressing Giving each device on the link and address is another part. So we split those tasks into two different Groups and we refer to them as layers. So we'll see why we call them layers in a moment the physical layer Getting signals across the link the data link layer making it reliable and efficient We've covered that already But what if we have a network? What if we want to get data from our source to destination and we need to go via a set of links and a set of intermediate systems? then We can think the physical layer and the data link layer the techniques which are used a specific to each link in this network or path This is try to capture a general system, but let's say there were four different links from source to destination Then we can think link one There's a particular physical and data link layer protocol used Maybe it's using Wi-Fi here where the frequency is defined to be The 2.4 gigahertz a bandwidth of 22 megahertz and the encoding is defined The second link maybe it's a LAN cable different range of frequencies different signaling coding technique different Transmitting and receiving devices those Choices are defined by the physical and data link layers and same for the other links and they may be different Even if we can get data across links There are some other challenges to deal with to get my data from the source to the destination So we don't have everything to to get communications across the network Some of those challenges are listed here. I Want to send from my source to destination? When I send to this intermediate device and it gets the data How does this intermediate device know to send it to the next intermediate device to get to the final destination? That's one challenge. It will deal with how does it know where to send to next? Such that the data actually goes to the destination that I want it to go to How to select which intermediate system to go via? Think we have a set of links My computer has a link to one that one intermediate system and a second link to another intermediate system I want to reach a destination in the US Which link do I use to send my data and? Every intermediate system may have that challenge and that's called routing find a route or a path from source to destination Those issues for getting data from source to destination via the intermediate systems are really key parts of building a network and We'll group them together and refer to them as the network layer And we'll spend some time on that over the next few topics Some other challenges Let's say I I know the path to send the data via I send the intermediate system A will send to be and then on to see and and so on I Send my data to a a gets the data Then a which is a computer fails for one second. There's an error in the computer The data that it just received is Lost so the link communications work correctly The the error control across the link was successful the data got to a and Now a was about to send it on to be but there was an error in the computer such that it could not send that data It was lost deleted from memory What happens now? Well somehow we need to Fix that error. We've essentially sent data from the source to the destination. It's not going to get there It's going to be lost It's lost at system a Not due to the links, but due to a failure of one of the intermediate systems one approach is a When it sends data to the destination it waits for an acknowledgement to come back If it doesn't receive an act within time it retransmits So error control can be implemented from the source to destination So we will need error control both across the links and Across the entire path Because it may not just be the links that fail it may be the intermediate systems that fail So what happens if something goes wrong if there's a failure we will need ways to deal with that another thing we need to deal with is that as You go out and get a job you may want to create an application that runs on this source computer and destination computer a networked application Hey for a business for a game so that a client and server will be able to communicate with each other You would like to be able to write the application such that you as the programmer don't have to deal with Retransmissions you don't have to write the code that checks have I sent the data? Am I waiting for the act have the timeout expired yet? You don't want to have to write the code that does the the parody check and the error detection That should be hidden from the from the application It should be easy to write an application that just says send to destination Magically the data is to the destination and it sends data back from the applications perspective That magic will be dealt with what we say the lower layers The physical and data link layer to get across the link the network layer to get it the data By the intermediate systems and other layers for the transmissions for the retransmissions so there are some features that will implement a Final layer that will support applications and doing common things so that the programmer doesn't have to program Them themselves and we'll see that Almost at the end of the course So there are a number of other challenges for For communications across a network What we do because all of this to get network communications working correctly is very hard The way what we solve hard problems is we can apply the principle of divide and conquer If we have a very complex problem Split that problem into smaller simpler problems Solve those smaller problems individually and hopefully the resulting solution gives us our overall answer The concept of divide and conquer is applied here and The way that it's named or talked about in data communications is we divide all these problems into layers That's why we've said a physical and data link layer that the name of the the grouping is called a layer and We layer one on top of the other such that at the end We build up a system that allows us to communicate from source to destination effectively This concepts are used a lot in data communications in telecommunications talk about layers We design our solutions from a layered perspective. We implement The hardware and software from a layered perspective and we'll go through some of those examples In this topic the advantages of divide and conquer is that we make things simple Very complex problem is hard to design a solution for hard to implement without bugs or errors So by breaking into smaller components Getting a correct solution is much easier. It also helps when we want to upgrade or change things Our idea will be that I Design a solution for getting data from source to destination My solution will be independent of how the links work That is my solution will work if the link from a to b is a Wi-Fi link But if I change that link from a Wi-Fi link to a wired LAN The same solution will work the same techniques will work for the end-to-end communication So we can upgrade or change components without affecting the other parts and that's very useful for Upgrading networks upgrading devices and so on We don't have to Upgrade the whole device. We just do a software upgrade or upgrade one part of it. It also helps in that we allow Allow us to select different implementations from different companies the idea again is that The link here the Wi-Fi link for example the transmitter receiver So long as we define How the link should communicate with a higher layer Then we can choose an implementation from one company. Maybe the transmitter is manufactured by Broadcom Broadcom one company and the receiver a different company and As long as they all follow the same standard Doesn't matter which company we use to implement that standard. We can still communicate from end to end So that's some of the advantages of splitting things up into individual components Some problems are that often will lead to designs which are not optimal with respect to performance We'll have overheads which which can be significant in some cases We don't want to spend too much time on on explaining why we use layers We want to look at what are some of those layers so we can get into them in the next topics That's this is just a bridging topic to get us into the the different actual technologies A general view now of our network We have links the purple ones So we'll have We'll have the physical layer and the data link layer are going to deal with how to get data across each link In addition We need ways to get data from the source via the intermediate systems to the destination For example, which intermediate systems to send via and how do they send data? We'll classify that or call that a challenge regarding to the network support How do we build the network? Finally to get data from an application on the source to the destination We need some other features that will support that application Maybe some retransmissions maybe some encoding of data into the correct format We'll group that into what I'll say is application support, which is needed on the source and destination so three groupings of the types of solutions we need to give and people have come up with different names and different subgroupings of those and They're all referred to as layers So we'll look at two and focus just on one of those two in the 1960s and 1970s ISO and ITU come up with a Protocol architecture, which defines across those three general groupings application support network and link seven specific layers Seven specific groupings of the solutions that are necessary for network communications We'll briefly list them and then we'll look at an alternative that we'll use in the rest of the course At the bottom go from the bottom up ISO is an organization that creates standards. You may have heard of ISO in other contexts They created or developed this standard for Open systems interconnection. This is back to front. Here's a mistake ISO created OSI not ISO here, even I made the most confused the two and They define seven different layers seven different groupings And the names are listed in this picture physical data link network transport layer session presentation and application layer Approximately the groupings The bottom two deal with getting data across a link The network layer is about how do we communicate across a set of links across a network and The top four are about how do we support applications? So that the programmer doesn't have to deal with all the the complexities of writing applications We've seen the bottom two in play the physical layer. How do we transmit data as signals? We know some techniques Data link layer. How do we transmit in an efficient way? flow control error control or examples there The network layer we're going to spend the next two topics looking at How do we get data across intermediate systems to the destination? How do we choose the route to take the path routing? Then the top four and This is where it gets a bit complex and it may be no longer used anymore We'll see that they define the transport layer is how to reliably get data from Source to destination we transmit data If it's lost at an intermediate system will eventually retransmit That was defined to be part of the transport layer How do we transport data from source all the way to the other endpoint the destination? Many applications we Establish a session you access a website You visit that website for a period of time and you access Multiple pages on that same website You visit the the front page then you click on a link to go to another page and so on so we can think all of those Accesses to the same website but to different pages think of that as a session Maybe you're using that website for ten minutes. Maybe you log in do some things and log out. That's a session Well OSI seven layer model to find some ways to to keep track of that session To to manage that session to set up the session to say our session starts now Data is communicated and then to stop the session so they define what's called a session layer Presentation layer is mainly about Presenting data in the right format one computer may have the data Encoded with some encoding scheme ASCII and The destination computer needs a different encoding scheme UTFs eight a different Or UTF 16 in a different language So the presentation layer was defining ways to convert the encodings so that two computers that support different encodings can still communicate encryption comes in there or compression One computer has the data compressed he wants to send it to another computer Which supports a different compression scheme or doesn't support compression? Convert between them is one role of the presentation layer and at the top Some techniques which are specific to applications For example Web browsing is one type of application and There are some protocols or the main protocol for web browsing HTTP defines if you want a web browser You send a request to the web server and it sends back a response The formats of those requests and responses and how to deal with them fit within the application layer. They are Techniques to support one type of application web browsing in this case There are other techniques for other types like email database access games and so on Seven layers It's good to remember the names of those layers because when you go and read material They'll come up the seven layer model the OSI seven layer model will always be mentioned in in data communications Turns out it was developed in the 70s and different companies made use of it But it nowadays it's no longer popular There was another model or protocol architecture beach which became more popular and more used and that's what we're going to look at So even though this is no longer used in practice the concepts are still talked about You'll see if you take it at a test in another course I'll often ask what are the seven layers in the OSI protocol architecture. I'll ask you about them in This course is an introduction We're not going to use all seven of them. We're going to have a simplified view Turned out around the same time in the 70s another group of people were working on different approaches for networking and One of the solutions became Eventually what we have today the internet So look at a different model that is used in the internet today and in fact, there are others IBM had their own solution Apple had another solution Novel and others had different systems. Some are no longer widely used others are used in very specific domains like in telecommunications But let's look at one which is widely used today and What we'll use in the rest of this course I? Will call it the TCP TCP slash IP protocol architecture or simply the internet protocol architecture Let's explain the history and and and then define what we mean by it Where did the internet come from? When did the internet start? When did the internet start what year? Maybe what decade? 80s 70s. Well, it's hard to define what is the internet is that may be the first question Well, people think the origins of the techniques that we use today in the internet Started really the design started in the 1960s with some of the the first Computers connecting together using those technologies in the late 60s early 70s 1970s. So the internet started In the 60s and 70s. Where did it start? the most major work was done in the US where a Research agency called ARPA ARPA wanted to create in a network Mainly to connect Universities some universities in the in the US and some government organizations like research labs Defense organizations they have many old mainframe computer or expensive mainframe computers in these universities and government organizations But no one can access them Okay, no one can remotely access them So they wanted to create a network so it was easy for people it won university To use the computer at another university so that's some of the motivations for a Network created by this organization called ARPA so ARPA net and It was very simple at the start using just the first test just two computers and grew to five and so on and slowly grew and Eventually took off The two key protocols used in that which are still used today Are called TCP and IP IP will see as a networking layer protocol IP deals with How do we get data from this source via these intermediate systems to the destination the internet protocol? TCP the transmission control protocol is for application support. It is about How do we make it easier for the applications to send data without worrying about all the details of the intermediate systems? So TCP and IP were the the main two protocols But many others were developed around that but often people refer to the set of protocols as TCP slash IP so we'll talk about the TCP IP protocol architecture and the protocols developed in the 60s and 70s are still used today But they have been extended and improved over time So we will study IP and TCP shortly and Different organizations have played a role in extending improving those protocols over time So the internet architecture board the internet engineering task force have the the the goal of Updating those protocols so the internet keeps expanding Whereas in the OSI model Organization got to that and define there are seven specific layers and gave them names in the internet They just implemented it. They didn't define the names or define exactly how it should work They just went and did it and that was one of the reasons why it was successful so there's no official set of names for the five or the layers that will be used and Sometimes so we referred by different names TCP IP protocol architecture protocol suite internet stack and many other names You'll see there's no standard But for the the rest of this course I'll define What I'll call the TCP IP protocol architecture with five layers And you'll see similarities to the seven layer for OSI model So let's go through them bottom up The physical and data link layer the same as we've mentioned before how do we get signals across links? How do we do that efficiently and reliably flow control error control? That's the same as what we've seen The network layer How do we get data via those intermediate systems? How do you find a path? How do you forward that data through that path? And we will see That the network layer is built around IP the internet protocol So we'll spend some time studying the internet protocol Then the top two layers if you remember OSI there were four at the top Turns out that we can simplify that and just group it into two different layers The transport layer How do we get data reliably? For applications from the source to the destination and we'll see TCP is a main technique there, but there are others and the application layer How do we support different types of applications web browsing email voice calls video conferencing and so on? That's the the names of the the five layers that we'll use in this course remember them and Over the next few topics we'll see examples of the top three. We've covered the bottom two already We've covered how to send signals We've covered some techniques for the data link layer that is ARQ flow control they part of the part of the data link layer There are other techniques So what we're going to do is cover How do we communicate across a network and then at the end of the course will get to the transport and application layer be aware that Different textbooks different people will talk about different models. Maybe even different names Okay, so there's no fixed standard for these names You may see with four layers or six and with different names, but that's what we will use in this course So let's let's finish this topic with some maybe more concrete examples and some things that you may have heard of To set set us up for the next topic First coming back to our picture I'm flicking through here So now think of the layers each layer will contain one or more protocols that implement a solution for some problem So the physical layer we may choose the the encoding technique Do we use Manchester encoding or NRZ that would be chosen for a particular physical layer In terms of the devices Those devices implement those technologies in those layers Looking at this case the source and destination computers Cover all of the layers in our case all five layers The source needs the layers for application support Application and transport it needs to deal with the networking and it needs to deal with a link So the source and destination computers have to implement all five layers But the intermediate systems may not have to implement all five layers For example, this intermediate system may have to implement the physical and data link layer for this link A separate physical and data link layer for this other link it has two links So I think it may have two physical layers and two late data link layers For example, this may be implementing Wi-Fi and this link using a wide LAN or ADSL Different technology have a different implementation And then a common network layer to deal with or if I receive data on this link How do I send it out onto this next link? That's what the network layer will do with and which next link do I send it out to But the intermediate system doesn't need to support applications in this case The intermediate system only has the goal of forwarding data on behalf of others It doesn't create the original data and it doesn't consume the data at the end point So in terms of implementing this intermediate system, we don't necessarily need the top layers to be implemented And that is the same in different intermediate devices So where'd we get to? In any device, one or more protocols may be implemented in each layer The end devices, the source and destination Which we'll refer to as hosts Will normally implement all layers in the stack or five layers The intermediate devices usually do not implement all layers. They may but often they don't need to and Sometimes will refer to a device based upon the highest layer it implements We may say the Wi-Fi access point implements The physical layer I forgot we number layers by the bottom up one two three four five Go bottom up If our Wi-Fi access point only supports the bottom two layers physical and data link layer Sometimes we'll talk we'll talk about that device as a layer two device It supports layer one and two. It doesn't go above layer two Or a data link layer device Just some terminology We will not talk about much, but as as you go out into practice you'll hear people use So Maybe a modem Which just deals with the physical signals maybe a layer one device A Ethernet switch or a Wi-Fi access point which may just deal with it that the link technologies is a layer two device A router which will explain later connects different networks together. Maybe a layer three device You don't have to remember them at this stage But again, we may see the terminology come up when you read websites and so on Let's finish with some maybe some implement some examples of where these layers are implemented and one final Example of seeing some packets So with respect to the end user Devices that is consider your your computer Sorry, we're flicking consider your computers the source or the destination. He wants to send data across a network If it's using the TCP IP five layer protocol architecture, it would implement all five layers Where does it implement them? He is a rough split of where the layers would be implemented the physical and data link layer Related to how to send signals and We often associate that with the network interface card in the older computers It was a separate card that you could insert into your computer PCI slot and In laptops for and computers today Usually the card is built on or as a chip on the motherboard With just an adapter or a Wi-Fi network interface card same with other technologies with ADSL with Different technologies think there's a network interface card where you physically plug the cable into or which has an antenna that transmits and receive signals That's usually dealing with the physical and data link layer Part of the data link layer may be also in your operating system There may be drivers your operating system uses some drivers some software to control this hardware Those drivers may implement part of the data link layer But roughly we think the the bottom two layers deal with in with the hardware implemented on hardware How to get data across a network and how to support applications? Well, that the the general data delivery is the transport layer in the network layer When you install your operating system most operating systems come with the protocols that fit within these two layers The most common protocol for the network layer is IP And for the transport layers TCP So when you install your operating system, there's some code that implements both of those protocols plus some others So it comes with your OS if you have an open source operating system You can go and find the source code for IP and TCP across a set of files the application layer deals with different types of applications and As often implemented in those specific applications an example The web browsing application uses HTTP To support web browsing HTTP is a protocol which we say is at the application layer Where is the source code for HTTP? Where is it implemented? Usually in the browser or in the web server if you install a web server at the other end point So when you install Firefox Then as part of that install comes the code for HTTP The application layer protocol and same with other applications out Implement themselves those application layer technologies There's some consequence here of the split Normally the end user you and I we can only control the applications We cannot modify how the operating system works much in some cases where we are limited in what we can do with the operating system We can't change the code in some cases So to make changes to how things operate usually we only can deal with the application layer you want to Create your new game online game And you need a server and you need your game client talks to the server and send some commands about where people are in The environment and so on You may create your new own application layer protocol Your own protocol that defines the messages that are sent between your game client and your game server When to send them what they contain So you would be dealing with writing an application layer protocol Let's finish with an example of Seeing seeing some simple messages when we're using TCP IP Where do we get to? This just lists the names of some protocols in those layers It's it's hard to split sometimes between the bottom two data link and physical. So often Ethernet wireless LAN or Wi-Fi cover both So I just grouped these are the names of some technologies within the bottom two layers We've mentioned some others are new here in the network layer everything sent it around IP But there are other supporting protocols, which we'll mention over time At the transport layer the two main protocols are called TCP and UDP. There are some other Less popular ones less used ones At the application layer, there are many hundreds of protocols for different types of applications for web browsing HTTP file transfer FTP email SMTP connect remotely to another computer SSH tell net To connect to databases. There are different protocols. MySQL has a different protocol to connect to a database and Many different applications different games will have their own protocols here. So there are hundreds here Let's consider just one an example to finish today using HTTP web browsing. I'll access a website and Then we'll try and discover what protocols were used in each of those layers to access that website and While I access that website. I'm going to Let me just check that I have internet access. I Currently have Wi-Fi enabled my configuration of my wireless LAN network interface Please remember this my IP address So this is the address of my computer on the on the Internet 10 10 1 0 5 56 What it actually means we'll discover when we look at the internet protocol in a later topic I'm going to access a website from my computer. I'm going to access the ICT server and While I do that. I'm going to record all the messages that my computer sends and receives How do I do that doesn't matter? I'm going to use software called TCP dump to record everything that my computer sends and receives on my wireless LAN interface and write it to a file Some capture file not so important here What's my password anyone? So I'm what I'm doing just recording everything like a screen capture But now this is a packet capture record all the packets all the messages sent and received Now I'll access the website Let's hope it works as With all my demos on the network something goes wrong. Why is it not connecting? Is someone else downloading on Wi-Fi at the moment and interfering with my connection? We're waiting for ICT Okay, we got there all right, so I access the website and This is the web page display okay, so The protocol being used at the application layer is HTTP My browser sends a request to the web server ICT saying please send me the file Cpe.html And when the web server receives that request it sends back the file Let's see how that work And what does it send back if we look at the page source? That's the actual file. Okay, just some HTML Now let's look we'll stop this capture stop the record I actually recorded 7,000 packets My computer during that time sent and received 7,000 different messages called packets here let's look at them and We will not look at all of them will select some to look at and I'm going to use other software called Wireshark to look at them And I'm just bringing up so it's easy for you to see You will see Wireshark in in some maybe other demos, especially in the lab next semester Basically, it shows me those 7,000 packets one by one the packets are the messages sent and received and It shows me We'll zoom in on one of them in a moment, but it shows me the time from when the first one was sent Who sent it? Who do they send it to and something about the protocols the length and some other information? So the 7,000 packets or so on I hear I want to find my Message that Where I receive the web page with HTTP I send a request to the server and I get a response back Let's try and find the one. I know we use HTTP. So I'll do a filter and I know it was my computer What was my address everyone remember? 10 dot 10 dot So I'm just going to filter out and just display the packets Which involve my IP and use HTTP? We've cut it down to five. I know from past experiences the first two okay, so Let's look at them as an overview first You can zoom in a bit The summary information says at this time 49.39 relative to the start time My computer that's my source my address sent to this computer and I know that's the IP address of the ICT server Again from past experience. That's the IP address of the ICT server It sent a message from my computer to the ICT web server and the The top most protocol being used the top most layer the protocol was HTTP the length of the message was 787 bytes and in summary The information in the message not all of it, but it said get CPE dot HTML Remember the URL I typed in was the address of ICT dot SITTUACTH CPE dot HTML so the message really in HTTP the web browser creates a request saying I want to get this web page sends it to the server and This is the response see the addresses are swapped It took a six six and a half seconds. It was very slow. We saw This is the server sending back to my computer and we'll look in the details It says it's okay, and it actually will see that the web page is included in this response message What I'm going to do is look at the details of this one selected packet and it's shown shown in this This space on the window and I'll try and draw the packet while we go first the data The original payload sent from the web server To my browser What did my browser want? What was the data that the user or the browser was interested in? The the CPE dot HTML that the web page Okay, that's the content that the user wants. Hello CPE was the the content But it's actually encoded or it's formatted according to HTML. That's the payload here. It's listed as the line-based text data HTML if we expand that you see Here's so this is one packet one message inside that one message includes this data You can see it's the HTML of the web page The total size of that it's hard for you to see down the bottom you can't in fact Is 288 bytes? Sorry if we Trust me. It says 288 bytes down there. It's very small to see anyway 288 bytes of payload Let's draw that We just draw the packet and it just has one last example and it won't be to scale but think This is the the payload contained 288 bytes But that packet also had a header attached we know we generally have payload and some extra information to support the protocol to support which protocol HTTP in this case So HTTP in that one packet There was also a HTTP header What was in the HTTP header all of these things the date of the response date and time the server that sent the response Something about when the fire was last modified the content length and many other things the encoding That some definition of the content type Think of this is the header for the protocol HTTP in this case How big is the header? It's 412 bytes Let's draw that not to scale this is the HTTP header and it was in our case 412 bytes That's our HTTP message containing header plus payload and That was created at the application layer the topmost layer But then the application layer used another protocol to send that it used the transport layer protocol underneath it to send that HTTP message What was the transport layer protocol? TCP the transmission control protocol So another protocol is used to send the HTTP message in this case is TCP the header is 32 bytes It contains things like port numbers sequence numbers Acknowledgement numbers from our concept of sliding window and some other fields So we draw that as an additional header This is the TCP header in this case. It's 32 bytes We're almost done. This is the TCP payload The TCP packet contains a header of 32 bytes The payload of that is the HTTP message This is the process of encapsulation. We put one inside another and Then we're going to put this TCP message inside one more Let's see it so we can finish our drawing The TCP message is in fact inside is carried using the internet protocol So there's an IP header of 20 bytes containing source address Destination address And some other fields in the header It was 20 bytes That's the IP header and The IP payload is all of that inside We think of the IP message contains a header plus the TCP packet inside last one the IP Message sent Using the Ethernet protocol, which is used for the wired LAN or wireless LAN. It's the same for both in fact I use Wi-Fi Contains a destination and source address and the content type. It's 14 bytes Eth for Ethernet and that's the Ethernet payload This is the entire Ethernet frame Ethernets at the data link layer Total size 766 bytes This was sent from actually this was received by my computer from the server and and What we're seeing here is that the protocols at each of the layers are in play or have been used in this case and You can almost go backwards that at the top most layer is the most is the innermost message HTTP at the application layer TCP at the transport layer IP at the network layer the data link layer Ethernet Which is in this software is similar It's the same as what Wi-Fi at the physical layer this software doesn't show the signals being transmitted It's it only sees down to the data link layer. I can't see any more the actual signals That's to do with the hardware So I think my frame of 766 bytes Actually contains an IP message a TCP message and inside that HTTP The next topics will look at the general approach for networking How to find a path through the network and how to send a path through a path and then IP IP addresses and so on