 Everyone's happy with the quizzes most people passed fine. So I must make them harder the quizzes Maybe shorter time five minutes instead of ten minutes on Monday. We we Talked about several things about the TCP IP protocol architecture including addresses We saw an example of web browsing Where in web browsing using HTTP the sim simple description of what happens is that? Your browser sends a request for a web page to the web server The web server sends back that web page in a response as well as some other information saying everything was okay And then your browser displays that HTML on your screen and we What do we do we saw? So I accessed a real website and I use some special software on my computer to record all the packets or all the messages sent and Received by my computer this software called wire shark allows us to then look at those messages sent and received We'll see that I'll use that as demo software through the course and maybe in the assignment. You'll get to use it yourself There's a lot of information here that you will not understand yet, but over time learn that what we want to move to is look at some of the performance aspects of computer networks When we send a message We use mold from my computer's perspective it uses multiple protocols and we saw with web browsing and we can see it in here is that we have some data that we want to send in This selected packet. It's actually the response from the web server this orange one The web server is sending my computer the laptop the web page from server to laptop So the data that's coming from the web server to the laptop is the web page That's what we want to get from one computer to another remember data communications getting data from one location to another The data is the web page How big is the web page? Well, in fact, we can see that here. We could expand You could count the characters and count how many bytes that represents in fact This software gives us a summary down here this HTML web page Contains 105 bytes may be hard to see but there's 105 bytes in the web page So the amount of data that we want to get from server to laptop is 105 bytes but sent Across the network and into my laptop We'll see that there's much more than that was actually sent Well, what was actually sent into my laptop? one protocol use was HTTP and We'll see with many protocols What they do is they take the data that we want to deliver and they attach some extra information to that data and We call that extra information commonly a header Some extra information to support the operation of the protocol in the case of HTTP That extra information if I expand is listed here and many of it many of the things may not make sense, but The server sends me the data the web page as well as the date and time at the server Some identifier of the server software and Many other pieces of information that may be useful for the protocol to operate correctly So not just is it not just data is sent from server to laptop, but other information as well that supports the protocol For example in HTTP it sends the type of content in this case It's HTML if it was a JPEG this value would be different saying that the content in this message is a particular type all this information is Defined as part of the protocol as part of HTTP We do not send that HTTP message directly across the link and across the network. In fact, we use another protocol to deliver it TCP So TCP takes the information from HTTP and then puts it inside its own message and also adds extra information And we will not go through all of them But we'll see again if we expand we see TCP attaches more information Num sequence numbers port numbers check sums TCP sends all of that to the internet protocol IP which then attaches more information Again many different things including and common in protocols the source address The IP address of the computer that sent it and the destination address who we're sending to This information is included inside the message that's sent across the internet It's included as part of the IP header and Ethernet does the same so most protocols attach extra information so that they will operate correctly a way to visualize that is We go back to our slides on this one. This is this example of web browsing This is the example of the request It could also be the response The data that we want to get from one point to another let's say the data here Or the user data the data that the user is interested in Is in fact what we say encapsulated inside another message The application layer protocol HTTP attaches extra information. We call it the HTTP header So we have the original data plus a HTTP header So this all happens inside the sending computer that HTTP message is Then delivered to TCP which attaches some extra information for TCP to work. We'll call it the TCP header We're not too concerned at this stage of what's that this extra information There's many different things that can be attached the point is that we Include this header information and we'll see that if impacts on performance IP attaches a header So in fact we have the original data the HTTP header TCP header IP header and Ethernet the data link layer protocol attaches ahead and The physical layer although it's not shown very well here the physical layer you think just takes a sequence of bits All this information is just binary values the data the headers I Skip one The header means we attach it at the front of the message We sometimes can attach something at the end and we call it a trailer So even it may attach a trailer some information again Treat that as a sequence of bits The role of the physical layer down the bottom is to convert those bits into some signal to transmit across the link So this is just my picture of some signal some electrical signal for example and That's sent across the link the receiver Processes that and maybe sends it across the next link and so on until it reaches the destination So in fact to get this user data from one point to another We don't just send that 105 bytes web page We send that user data plus all these header information and trailers in some cases the one Consequence of the way that protocols work like this is that all this header information we can think of as overhead It's information or it's Bits that are sent across our network or link that are not the real data but some extra information for the protocols to work and They reduce the performance of our network or lower the efficiency of our network as we'll see in some calculations Let's put some numbers to that in our example how big are these headers Let's go through a simple example the one we captured last week Before we go the numbers So one way to to visualize the information sent across the network is these types of diagrams that show them the entire message And that message contains some headers from different protocols some trailers sometimes and the original data the user data but we'll try and draw a picture like that and They have particular sizes and that may impact on performance and I'll record on the board these numbers In our case when we're sending the web page from server to the laptop The user data Just write the data in our case is 105 bytes so the amount of See the user data that is the information that we want to get from one computer to another is 105 bytes Okay speed Our goal is to get that 105 bytes to the laptop But what do we actually send across the network? Well, there's that data There's a HTTP header attached to that how big is the HTTP header if we select here It shows us down the bottom here. It's 436 bytes. You cannot see this 436 bytes means all the information inside here which includes This 200 ok message the date the server all these other fields of information Total is 200 and a 436 bytes So I'll just record the numbers here. I won't write B for bytes here everything all the units of bytes But in fact, we also attached the TCP header and it's 32 bytes. This is not to scale And if we count the IP header It's another 20 bytes of information and the ethernet header the last one we have captured is 14 bytes so what I've just drawn on the board is At least from my computer's perspective the entire message received it contains the original data the web page But it also contains other information the ethernet 14 bytes And in fact, why shark shows us the total size 599 bytes about 600 bytes Does it add up? Check anyone have a calculator Mobile phone a laptop says the user data is 105 bytes The web page that we're transferring. That's the real thing that we want to transfer is 105 bytes Long doesn't doesn't quite add up does it? There's some special thing that happened here The web page is 105 bytes, but in fact before the web server centered It actually compressed the the the page It took that 105 byte web page and compressed it down to 97 bytes Okay, that doesn't always happen. It did happen in this case. That is this data is 97 bytes if you add up these numbers You should get 599. I hope so maybe there's something missing. It's there 1315 19 you ends up in a 9 So the message That is sent and received by my computer my laptop contains 599 bytes It contains all this header information and some data, but in fact the original data was 105 bytes Some compression was applied to reduce that 105 byte html file down to 97 bytes It's a special case that happened in this web page transfer How efficient is this web page transfer transfer from the web from the user's perspective? If we look at just this one message, how efficient are we in transferring the user data? Let's put a number to it Can we measure the efficiency of this? The delivering of the user data To my laptop the goal here is to get the web page to my laptop and then my browser can display it That's from the user's perspective. That's what I want to get that web page But the protocols in operation includes an extra information for them to work correctly in Total there was 599 bytes Delivered across the network or to my laptop But from the user's perspective only 105 bytes were real data useful data The rest was what we may call overhead That gives us a measure of efficiency In at least this simple this single packet 105 bytes a real data divided by 599 bytes of Total data is about 17 percent Or 0.17 if you do this calculation or as a percentage about 17 percent that is of the total amount of information sent Only 17 percent represents the web page So we'd say that for this single packet We're about 17 percent efficient in using the network to deliver that user data So we can start to analyze the performance of our network by considering in just a simple case Considering the amount of user data delivered Relative to the amount of total data sent through the network We want the efficiency to be as high as possible. What if my User data was not 105 bytes, but the web page was bigger What if the web page was increased to say 1000 bytes or about 1000 bytes Well, we increase the size by about 900 bytes. I'll say about let's do it quickly the mathematics. So if the web page was 900 bytes larger Then the total message size Would be nine nine hundred bytes larger Because the headers would be still the same size or very close to They do not change So if this is increased by 900 bytes, then this is increased by about 900 bytes We've got about 600 plus 900 is 1500 bytes So if the web page was 1000 bytes Then the total message size would be about 1500 bytes 1000 divided by 1500 is what two-thirds 66 percent efficient There's some approximations there, but it's close which would be better better use of our network and of our link because We're spending more percentage of the time sending real data as opposed to sending headers increasing the size of the user data if The headers are fixed increasing the size of the user data Increases the efficiency of the use of the network, which is important If you have a small amount of user data You still need to add the headers and it becomes less efficient than having a large amount of user data What else can we say? Any questions on on this? Okay Are the size of the headers fixed? I'll say roughly yes. Okay, the size of the header Actually depends upon the protocol So TCP if you look at the standard defines the structure of the header and what information can be included It may vary a little bit in some cases But this is a typical size in the order of 10s 20 30 bytes for TCP and Usually if one packet is 32 bytes, the next one will probably be the same may vary a little bit IP is usually by default 20 bytes HTTP can go up and down it may be hundreds of bytes So yes, it may vary a little bit, but normally over a short period of time they're fixed The data may vary a lot you download a small web page you download a large web page so the data can vary from Tens of bytes hundred bytes up to 1,000 bytes and larger So the data varies much more than the header size Usually ethernet is also fixed there It's different protocols have different header sizes as we see here all different and The exact values. We're not too concerned about here. It's just the point that For every protocol we normally add some extra information and that incurs some overhead and That reduces the efficiency of our data transfer Because to deliver the real data. We actually need to transmit more any other questions We'll continue on some different performance calculations Let's see if we can get this point clear Okay, easy Not not not easy Why is it not easy? the calculation We've done two things so far we look at a message that sent and I've captured the message I've recorded the message the exact piece of information received by my computer that one message and I can see that the amount of the size of the web page was 105 bytes that was the size of the web page contained in that message But there was other information in that message as well What we call the headers for each protocol and I can check the size of them Total message size if I add them up was and it shows in the software about 600 bytes web page size About 100 bytes or 105 bytes So what percentage of the total message is the web page or 17%? 105 by 599 expressed as a percentage That's in other words the efficiency of that web page delivery in that one message If we increase the user data, but keep the other things fixed For example increase this up to 1,000 The headers and extra 500 then it's 1,000 divided by 1,500 which is about 66.6% so Real data or user data divided by the total size We'll see some more calculations over this topic and some future ones There's one slide I think we Which is related that we skipped over one or two lectures ago this just described some of the Notation or the terminology that we've started to introduce It's common for many protocols not all but most protocols We have some data to carry But we have some extra information and That's carried usually in a header sometimes It's called a trailer because it's added to the end the headers added to the start of the data trailer to the end and The this process of adding the header or including the data in a larger message is called encapsulation What's inside the header many different things may be source and destination addresses It may be sequence numbers some parity check many different things Ignore the second point about a protocol data unit. We won't use that terminology Segmentation we will cover later where necessary again The main point is that we have our message that contains the header and the data I've said a message sometimes I also say a packet again some terminology Generally the messages we send across a network or a link we can refer to as a message or maybe more commonly a packet a Packet of information, so I think a packet contains header plus data But we'll see other terminology as well sometimes they're called messages segments data grams frames It's just in different technologies and different fields. We use different names the general name or the more common one is a packet One packet, but we'll see some others come up along along the way in this course This leads us into this issue of Well, how do we measure the performance of networks and of links? How do we measure the performance of communication systems? Well, there was one example look at the efficiency of the data transfer We'll look at some very basic performance measures for different internet applications and give a few more examples first applications What's an internet application or a networked application? Well, let's distinguish between say a standalone application and a network or a distributed application a standalone application like you install Microsoft Office on your computer To to use Microsoft Office you don't need a network connection You can create documents on your own computer. You don't need to talk to some server or some other computers to use that application in most cases We'd call that a standalone application. It just runs on a single computer to perform its purpose It has some user interface Some GUI that you can click on and do things on and some application logic That is the the code behind it that does all the processing So we have standalone applications. They just run on a single computer But some applications would call them network or generally distributed applications to work They must communicate with other instances of that application on other computers web browsing For web browsing to work if I just install Firefox on my computer and have no internet connection It's not of much use to me So your web browser would say is a network application in that for web browsing to work We need to communicate from my Firefox application to some web server so Firefox has some user interface of course some application logic that is to do the Processing and to save files and so on but it also has some communication mechanisms Some features that allow it to send a message to the server and receive a response back So we distinguish between standalone applications and network applications or internet applications in general We're gonna focus on the communication mechanisms Well some examples and I think you know a lot of these you use a lot Some internet-based applications file transfer email web browsing remote login you Connect into another computer and perform some operations database and many other Applications instant messaging. Well, in fact that may come later other internet applications that we can categorize differently are Multi-media or real-time applications still we use across the internet But they have different requirements in terms of performance things like audio and video streaming you're watching YouTube or maybe more Precisely your Or a more accurate representation. You're streaming some radio to your computer Your inner voice or video chat with someone using Skype talking to someone across the internet video or voice call Gaming applications where you're playing a game and it's sending data to a server And that's going to many other users at the same time. So you interact with other users Collaborating sharing your desktop with other users We'll distinguish between these multi-meal multi-media or real-time applications and the ordinary or traditional internet applications Like just web browsing and email. What's the difference? Well, the main difference is about performance and what they require for performance These traditional applications are about transferring data from one location to another the data as an email a file a web page It's important that that data that we receive is accurate the file at the server and The file I receive should be exactly the same. So accuracy is the most important feature then with Real-time and multi-media applications sometimes accuracy is not so important, but more important is timeliness anyone play online gaming What's a measure of online gaming that you may know? You check whether the server is ping the ping time that is ping in online gaming you connect to a server and You want to know the delay between sending a message from your computer to the server Sometimes it's called the ping a ping or the ping time. It's in the order of milliseconds Ping is we'll see is an application for Doing exactly that checking the delay between one computer and another the round-trip time I don't think it stands for anything Maybe it's a background and They've made it stand for something you ping something you tech check whether it exists What's a good ping time anyone play games for a game Yeah, anyone else anyone know a Ping time a good ping time if you're playing a game if the delay between you and the server is 10 seconds. Is that good? No in the order of milliseconds Milliseconds tens of milliseconds So for that application to work well Timeliness is important. That is getting message to the other Computer in a short amount of time Similar when you're on a voice call with Skype or a similar application If there's a large delay between you and the other person, it's very hard to have a conversation In the order of hundreds of milliseconds so We have different requirements in terms of performance That it's more complex than that but roughly some applications Accuracy is the most important with multimedia and real-time timeliness is more important last thing for this topic We want to look at three or four different ways to measure performance common ways There are many ways to measure performance, but we'll look at three or four common ways to measure the performance We say the performance metrics the things we use to measure performance just Well, there's five listed here. We'll go through I think three in detail bandwidth data rate throughput Delay and packet delay variation We're going to spend some time on data rate throughput and delay The others will either not touch or cover later first bandwidth in many communication systems We at the physical layer we send signals Okay, so we take our bits and transmit it as some signal some electrical signal along a cable some radio signal through the air and those signals can Have some frequency or range of frequencies so the frequency of a sine wave for example and The range of frequencies that we use will see is a very important measure of the performance of a communication system The range or the set of frequencies that we can send in a particular communication system We call the bandwidth And we'll see that the measure is in Hertz, so the bandwidth used in AM radio When you listen to an AM radio channel on your car in your car Then the bandwidth for that signal for sending the audio from the radio channel to your car is About 10 kilohertz 10,000 Hertz Different systems use different bandwidth signals And we'll see that the larger the bandwidth the better the performance in these other metrics, but we'll see the larger the cost We're not going to cover that in this topic because in the next topic when we go into water What are these signals will go in and define this in much more detail? But we'll come back to bandwidth in the next topic The next two or three will spend a bit of time now on because they're important and you know about them already We often with computers we care about getting bits from one location to another a file Some audio but represented as bits the speed at which we can get those bits from one side to another is called the data rate the number of bits a communications channel or a network can transmit in some period of time Units being bits per second What's a channel think of that as the transmission system or transmission line or part of so I connect my Blue laptop to my gray laptop via a cable I'm interested one thing. I'm interested in in terms of the performance is how many bits I can get per second from one laptop to another and That's the data rate the rate at which the data could be sent across that transmission system or channel Let's look at that with a few examples I'm going to plug my cable in here Set up my second laptop and we'll see if we can communicate between the two in a moment Turn off my wireless. I don't need that So I've connected my two laptops directly via a cable. I'm not going to use Wi-Fi or anything I've just connected by a single LAN cable. So I'm using the wired LAN the blue LAN cable How fast can I send data from one to the other any guesses have a guess One's two years old one's five years old or four years old 100 megabits per second. Let's have a look. So we care about how fast we can deliver the data between two computers The data rate. Well, I can actually check on here. I plugged the cables in we're using What's called generally called ethernet a wired LAN technology and the speed at which I can send and Received depends upon my LAN cards Depends upon the capabilities of my laptops In fact, this blue one is a bit older than this one and much cheaper I've got a program that will tell me the speed or the data rate of my ethernet interface actually first From yesterday I have config tells me the IP address. I Don't have one at the moment It's disconnected. It's connected now. Let's try again. Nothing works Try now. We're right Sorry, okay. My ethernet wired LAN interface has an IP address 192 1683.2 And I've set this blue one to be 192 1683.1 Okay, so they were connected. They both have IP addresses, so they should be able to communicate How fast can they communicate? I've got another program ETH tool ETH tool tells me some information about my ETH my ethernet interface Need a password to check this There's a lot of information here. Well, I'll just focus on the one thing we care about speed 100 megabits per second that is My LAN cards on both computers have negotiated when I plugged in the cable they did some negotiation and They check which speed both support and they come up with 100 megabits per second My gray laptop actually supports 1,000 megabits per second, but my blue one does not it's a bit older So they use the lowest common or the highest common speed that they both support So that's the data rate from now my link 100 megabits per second 100 million bits per second to be sent Let's send some data Across that link and measure How fast the data is delivered? So the data rate let's make note is 100 megabits per second Let's do a speed test and and check the speed My blue laptop actually has a has a web server and I have some files on it that I can download from the web server So I'm going to use my gray laptop to download a file and also measure how the speed of that download Instead of using a web browser because I don't care what the file is. I just want to measure and time How how long it takes to measure download a particular file? I'll use some command line program to do it Or I type in the URL and I recall from the web server the file name I've set this up before let's explain on the blue laptop. I have a 50 megabyte file I've just called it Meg 50 dot pin. It's just a binary file 50 megabytes in length on the blue laptop I'm going to use a program W get to download that file using HTTP So this is the IP address of the blue laptop. This is the directory the file name So when I press enter my gray laptop is going to download this file And it's also going to report how long it takes She remember things set up downloading 100% done average speed 11.2 megabytes per second That was the speed measured in the file transfer in this case Data 800 megabits per second here Note the uppercase B bytes megabytes per second Let's convert drawing the dots there megabits per second Multiply by 8 to get megabits per second my link supports 100 megabits per second But when I downloaded that file in that specific case the speed I delivered the data was 89.6 megabits per second Why why is it less than 100 because of these overheads? a because of the packet The file of 50 megabytes when we send the packets across the network. There's some extra headers added In fact, it's split into multiple packets and each packet has extra headers In fact, if we captured with wire shark, we would see that But this just shows us well for that 50 megabytes to be transferred If you include all the headers and there are even some other overheads the average speed you get is 89.6 megabits per second How efficient is my data transfer? What's the efficiency in this case? The maximum speed my link supports is 100 million bits per second But when I perform that data transfer, I got 89.6 megabits per second. How efficient was that data transfer? It's 89.6 percent because The data rate was the maximum was 100, but I only got 89.6 89.6 percent So we can say that that data transfer was 89.6 percent efficient in the way that it used the link So here's my data rate think of this as the maximum speed at which we can send bits Across the link or across the network What is this number? Well, we call that the throughput The data rate is the map or the number of bits we can send across a link The throughput is a measure of the rate at which the real data for example the user data is Successfully delivered to the destination Because if we have overheads like packet headers That doesn't count towards the throughput So we'd say this 89.6 is Is throughput 11.2 megabytes per second or 89.6 megabits per second both are performance metrics and both Considered when we're designing and building and selecting technologies for computer networks So coming back data rate is the maximum speed at which we can send bits across a link across a network But actually when we send data across that link there may be some overheads like packet headers So throughput is a measure of the speed at which we can do it deliver the real data across that link So ignore if we don't count all the overhead then we get the throughput and The throughput will be less than the data rate less than or equal in most cases less from the user's perspective I downloaded the file 89.6 megabits per second is the most accurate performance metric. It's the speed at which I receive the data So that's important the throughput Download another file. Let's see if we get the same we may Yeah, we got the same now. Let's create a smaller file. I Just downloaded a 10 megabyte file and We got 11.2 megabytes per second the same as before Let's download a smaller file and see if we can get a different value. I need to create one That one's not a very good example. It's too small. Okay. Let's ignore that. Sorry My example is not good for that one. My point I was trying to make is that sometimes the throughput will vary that is depending upon the protocols we're using Sometimes depending upon the the size of the file the data that we're downloading This number can go up and down the throughput even when the data rate is fixed So our data rates our maximum our throughput is what we actually achieve And it may vary in different conditions. I can't create conditions to change it at the moment both Both performance metrics are used to measure Different technologies data rate the maximum speed throughput the actual speed The problem with throughput even though it's more accurate is that it's hard to measure in some cases. It's hard to predict It may vary So we don't know what it will be in advance data rate. Normally. We know based upon the technology my wired LAN 100 megabits per second What about Wi-Fi? Let's switch back to Wi-Fi on my laptop here Enable the Wi-Fi Now my laptop is connected to the SIT wireless LAN through that access point. What do you think the data rate is? Okay, let's see. I can actually see The data rate IW config shows me some details about my wireless LAN and my wireless LAN Bit rate. It's called here data rate 24 megabits per second that's Try again 48 megabits per second It's now 48 megabits per second So think of the data rate from my laptop to the wireless LAN access point on the wall is now 48 megabits per second before it was 24 In wireless LAN in fact it changes Depends upon what I'm doing on it and it also depends upon the the signal quality how strong the signal is It will go up to 54 megabits per second in this case and in this stage it's reached the maximum So the maximum data rate for my laptop to this access point is 54 megabits per second, but in fact there are lower ones let's download a file long URL I've got some files Have to find a file to download Just bear with me One of the lecture notes from one of our future lectures Get that not found two things one downloading a file So I provided a URL for a file on the ICT server This long one is just a PDF file on the ICT server and this program downloaded it the file was about three megabytes in size throughput two megabits per second so What do we have we had a data rate of 54 megabits per second my wireless LAN When I downloaded that file I received a throughput two megabytes per second Which is 16 megabits per second and efficiency Maximum speed or the data rate 54 megabits per second I achieve 16 megabits per second So efficiency is this 16 divided by 54 Which is what around 30 percent a bit less Compared to my wired LAN Data rate was 100 megabits per second throughput about 90 megabits per second and same with the efficiency 90 percent or 89 point six percent wireless LAN lower data rate, but more importantly lower efficiency Because wireless LAN has more overheads incurred than my wired LAN So both the data rate is lower, but also the efficiency is much lower about a third of what we achieve so The data rate is normally part of the technology It's specified as part of the standard The throughput may vary depending upon the situation and Will be less than the data rate and the efficiency from our perspective is what percentage of the data rate do we use? So simply in this case throughput divided by data rate expressed as a percentage If I connected my two laptops using Bluetooth and tried to transfer a file I'll have a different data rate maybe one megabit per second and get a different throughput and a different efficiency most likely So depending upon the technology we have different rates and different throughputs and the Efficiency and throughput why is it less than data rate? One of the main reasons is because of the overheads of the packet headers But there are other reasons as well when we look at individual protocol some questions We're starting to look at some performance aspects of network So everyone's taking notes on how to do the calculations when the quiz comes up. I'll be able to calculate these for example If I give you this picture Or some information like this you'll be able to calculate the efficiency if you know the user data and the total packet size Well, if I give you in a question the data rate is 100 megabits per second We measured the efficient efficiency to be 89.6 megabit 89.6 percent What is the throughput? 89.6 megabits per second. So some simple calculations Okay, any problem How about bandwidth is a question? What's what's the impact of bandwidth was a question? We'll not try and explain it here. We impact need an entire topic to explain the relationship between bandwidth and these other factors, but I'll give the hint that The effectively the larger the bandwidth the larger the data rate we can achieve And we'll see the relationship more precisely, but the bandwidth that we have available Impacts upon the data rate other things do as well. So Why is my data rate 100 megabits per second? Why not 500 megabits per second? Why not 10 gigabits per second? Well, it depends on different factors including the bandwidth but other things as well so bandwidth will impact upon data rate and throughput Depends upon the overheads caused by the different protocols used In all of my examples I used HTTP If I used a different protocol, there may be different overheads and we'll get a different throughput and efficiency Generally, we want the highest possible choose a technology that gives a high data rate and a high throughput at least high enough for the needs of your users That's why we're interested in these metrics one more calculation recall When we looked at our web page that single packet we saw that we had 600 bytes of The total size of the packet and the user data was 105 bytes And we did a calculation and found that the efficiency in that case was about 17 percent that is 17 percent of the size is the real data So in that case we had an efficiency of about 17 percent So if I delivered that web page There was a web page of 105 bytes. If I delivered it across my wired LAN What throughput would I get? I downloaded that web page across my wired LAN my LAN cable What throughput? 17 percent of the data rate If my data rate was 100 megabits per second And we say the efficiency and we calculated this 17 percent from the packet size 105 bytes of user data is about 17 percent of 599 bytes 105 divided by 599 Gives us if we're 17 percent efficient about 17 megabits per second throughput Much lower than we saw with our real data transfer There are two more metrics that are commonly used We'll spend some time on one of them delay the other ones packet delay variation So data rate the the maximum speed at which we can send bits Throughput the actual speed at which our data is delivered considering all the overheads All right, so those both are about speed Delay is about time how long something takes In your game the pin time is a measure of delay The time it takes to get one message or a message from one point to another So that's another important metric the delay Another one which we will not cover At least at this stage is the variation or the the variation of delay between packets But I've listed it here, but we'll not cover this one. We'll cover delay But we'll cover delay next next week. Okay. We need some time to go through some examples So to finish for today what we've done We'll skip back some slides Again, we're almost at the end of this lecture on protocol architectures Protocols often introduce extra headers when we send our data the protocol adds some extra information to support its operation Depends upon the actual protocol how big it is So in my computer I take some data One protocol attaches a header sends it to the next protocol to go to work It may attach some header Sometimes that they attach a trailer and then out of my computer what is sent some signal That represents all of that information That entire message the entire packet Those headers have some impact upon performance We can draw the entire message like this We label the headers depending upon the protocol being used We can look at the size of them and calculate some efficiency in the data transfer And now we're starting to go through well, how do we measure performance different ways bandwidth is one We haven't touched upon but two we've introduced data rate and throughput data rate the maximum speed We can send bits through a communications link or a network Throughput the actual speed at which we deliver data successfully across that link or network And next week we'll continue with delay And that finishes this topic and we'll move on to how to transmit signals through a link I'm going to stop there because we need more time for delay and if you have questions come and see me Otherwise, I'll see you next next Monday