 We're focusing on how to get our digital data across a link still and we introduced the concept of framing Framing in the previous lecture where we said that we divide our Data that we want to send into chunks and we call those chunks frames. There are some other names We'll see we'll come across where the frame is Contains some actual data, which we refer to as payload but it also contains some other information that helps the communications work correctly and Some of those things that it may the other information it may contain include the address of Who's sending who's supposed to receive? Maybe a sequence number to count the frames so that if we send a thousand frames the receiver can check that it's received the correct a thousand frames in order and some other information and that extra information we contain inside Either the header or the trailer or both Where the header is broken into a set of fields and each field has some values The exact structure of a particular frame depends upon the the standard Wi-Fi For wireless LAN communications say from your your phone to an access point uses one particular standard and defines one type of frame wired LAN uses something different Your ADSL home connection uses something different again, so there are different frames for different technologies But they all follow this general structure of having header payload and trailer although not all three are necessary We'll see different examples and then we started to look at Different ways that we measure the performance of our communications our link communications We know data rate is one measurement the data rate is depending upon the The signaling technique that we use and how we transmit our data as as signals So we usually have some signal element or a signaling rate and each signal element contains One or more bits and from that we can determine the data rate in practice You can think data rate is part of a specification for equipment or a technology you buy your laptop in built is a LAN card and The standard defines that the data rate for that LAN card is either 10 100 or 1000 megabits per second You as the end user cannot change that that's built into the technology you buy your new mobile phone and the data rate for for Wi-Fi says that You the standard for Wi-Fi says the data rate goes up to say 300 megabits per second or with LTE as a mobile phone Data communication system goes up to say 40 megabits per second That is usually defined based upon the technology and it may vary We'll spend some time talking about delay the time it takes to get a message from one point to another But we also mentioned error rate That is what percentage of things Arrive in error or don't arrive things could be bits frames packets So there may be different names, but some measurement of how many errors are there? often that's a characteristic of the link or Sometimes even a specification of the link saying the maximum error rate for this link is say 1% Sometimes we cannot control that From the end users perspective if we are aware of the error rate We can do something about it, but we may not be able to reduce it We said in our frames we have header and trailer So we can say that they're not the actual data. So we'll count them as overhead stuff that we must send But don't carry our real data that we want to communicate to someone so we count them as an overhead and throughput will Instead of just using data rate to talk about speed we'll use throughput to indicate the speed at which our real data Gets delivered to the destination So we need to take into account now overhead and We may need to take into account errors If we send data it gets delivered, but there's an error Then it's not really the effective data delivery. So we wouldn't count that towards throughput and some Technologies have rules such that they cannot be transmitting all the time They'll spend some time not transmitting and that leads to a calculation for throughput Efficiency is how well we use something some utilization so if I buy a link and for my home ADSL and I pay 500 bar per month for a 10 megabit per second downloads data rate Then I want to use that 10 megabits per second as much as possible So efficiency is a measure of how much I use or utilize that that link So we talk about efficiency usually as some ratio between throughput and data rate If I have a data rate of 54 megabits per second, but I achieve a throughput of just 20 megabits per second The maximum I can get is 54, but I'm only getting 20 We can say we're 37% efficient in using that data rate There's all some other calculations of efficiency Some of the main performance metrics we care about We've seen a couple of examples Let's look at delay in in more detail and we'll see some more examples as we go through delay How long does it take to get a message from A to B? So let's look at the different components or the different things that may cause delay First we call our knowledge of signals and a quick drawing of a signal we We can draw our digital signal one of the the characteristics of our signal. We said was a signal element and another way we can look at how many signal elements per second and we can talk about a signaling rate and the bits per signal element Combined with a signaling rate gives us our Data rate so that are some of the concepts we've seen before just a reminder that is With a signaling scheme we will define the duration of signal elements, let's say one millisecond So we'd sent one signal element every one millisecond that would give us a signaling rate of 1,000 signal elements per second so the rates the inverse of the signal element duration and The signaling scheme would define how many bits per element the basic case. We looked at was one bit per element But we may have more we could have two bits per element or four bits per three bits per element and so on we saw different schemes If we know that if we know we're sending signal elements at some rate and how many bits per element then we can determine the data rate The question with regards to delay is how long does it take to send us a message of so many bits out of my computer? So let's put some numbers to those and let's say just for simple calculations. The signal element duration is one Millisecond One millisecond per signal element therefore a rate of one thousand per second Let's say we transmit two bits per element We have a scheme such that every signal element can represent two bits So we'd get a data rate of two thousand bits per second 1,000 elements per second two bits per element two thousand bits per second So if we have a data rate of two thousand bits per second we can think that When we transmit a signal from my computer to the destination computer Every one millisecond. We're sending two bits out Every one millisecond. We send a signal element and Then we send the next signal element and so on and it takes one millisecond per signal element So we can use that to determine how much time it takes to send a certain number of bits so if we have a frame frame size of say 10,000 bits We'll keep it simple and in bits at this stage Then we want to send this frame of 10,000 bits. How long does it take to send? Well, we're sending two thousand bits every second. We have to send 10,000 bits therefore the The time it takes is five seconds and we'll call this the transmission delay We'll see the names on the slide So quite simply one component of delay is how long it takes to transmit that data that message or frame Using our particular data rate for our technology 10,000 bits at 2,000 bits per second takes five seconds easy or you can look at the detail Okay, one millisecond per one signal element So how many signal elements are there? 5,000 signal elements One millisecond each takes five seconds Transmission delay is one aspect of how long it takes to get a message from one point to another But there's more This is the time to transmit the signal Containing the data out of my computer But then that signal propagates It propagates across the link to the destination is received We transmit a signal it propagates if we think of a waveform it Attenuates as it goes across some distance and then it's received the other component of delay is how long does it take? each signal element or Each bit to get to the other side To propagate from transmitter to receiver How do we determine that? How long does it take a signal to get from one point to another? distance It's going to depend upon the distance. How long does it take a signal to get from? My laptop up to the access point on the wall. It depends upon the distance and the speed Speed of what not the data rate the data rate is how many bits per second my laptop can transmit out But then if you look at every bit or every signal element representing each bit How long does that signal element take to propagate the wave propagate up to the access point? So it depends upon the distance The speed of what? Well the speed of the signal in propagation and There are different speeds. So we'll talk about so a second component of delay will say is Propagation what are we? Let's see if I've missed something. I think we've got it. We're going through these delays in this Slide propagation delay This depends upon how far our signal needs to propagate and how long it takes to propagate and That depends upon the medium. Remember we have different types of media. We have wired and wireless With wired we talked about optical fiber co-actual cable twisted pair. So electrical wires a Glass and plastic fibers where light Propagates through and with wireless we have radio waves that propagate through the air How fast are these signals travel? What's the fastest speed that they can travel? Light speed okay, so if we think of optical fiber in the best case Light comes in at one end point and travels at the speed of light and through to the other end point How fast does electricity travel through copper conductor? It's not light But it the speed is close to the speed of light Okay, it's maybe it depends upon the materials It may be 2 by 10 to the power of 8 or 2.5 by 10 to the power of 8 meters per second but most signals propagate at a speed approaching the speed of light and If I don't tell you otherwise you can assume that our signals always propagate at the speed of light and You remember the speed of light for the exam, which is what? 3 times 10 to the power of 8 meters per second remember that that one so the propagation delay depends upon the distance and The speed at which we send so if we have a distance of our link of Between the transmitter and receiver and let's give it some numbers 30 kilometers we have a long link a long cable between our two points and Let's consider the speed of propagation the speed of the signal What propagation delay do we get to send? 30,000 meters 30 kilometers At 300 million Meters per second It takes point zero zero zero one seconds We'll keep the unit in seconds So there's two parts for getting our message from A to B It's how fast can we transmit the bits out of our computer out of the transmitter and then if you think every bit Propagates through the medium and that takes some time as well And we call that proper delay which depends upon the distance the speed of the signal not the speed of our transmitter This is not data rate This is the speed of our signal the physics of it The total delay we can think is the summation of those two one way to Realize that it's the summation of those two is to Visualize them. I'll show you what I'm trying to draw in a moment or explain Let's say we want to understand why the transmission delay and the propagation delay are additive in this case if I see any homework from other courses or any phones or games for the rest of this semester, then You lose attempts on the quiz Okay, you want attempt instead to or zero attempts instead of two If you want to do your homework for your other courses then do it in another room not in this one So why does the why can we add the propagation and transmission delay? Let's let's think of it in a simple perspective the transmitter. What we'll try and draw is the bits being transmitted Here's our transmitter Think if we send a signal for each bit and that takes some time So we transmit bit one out of our computer It takes some time to transmit then we transmit the next bit We move on to the next bit immediately after the first bit Then we transmit bit three bit four bit five in our case. We had 10,000 bits. We keep drawing How long does it take to transmit the 10,000 bits? We'll not be to scale We had 10,000 bits in our frame Well, we calculate that we know that every bit at 2,000 bits per second every bit takes half a millisecond the number of milliseconds This takes a half a millisecond to transmit another half and We go if we do that 10,000 times the total time is five seconds You see a few people leaving they're going to do their homework. Good. That's fine. That means The exam will be easy for them in this course Each bit in our example takes half a millisecond. Why because we said 2,000 bits per second Calculates if we did the inverse of 2,000 bits per second we get half a millisecond so Transmit the first bit takes half a millisecond then another half for the second and we keep going after 10,000 bits It would take us five seconds to transmit This is the transmitter computer. Let's see what happens at the receiver at the receiver So this is the transmitter then we propagate across some link and eventually receive those the signal representing those bits Focus on the first bit. It's transmitted at let's say at time zero. We start transmitting We finish transmitting the first bit at time 0.5 milliseconds When does the first bit arrive at the receiver? When is the first bit fully arrived at the receiver? If we if we start our time we start transmitting at time zero Then at 0.5 We've send the first bit that's coming out of the computer, but we said the signal representing that bit must propagate through the link How long does it take to propagate? Well, we calculated this point zero zero zero one seconds point one milli seconds So if we transmit at time zero if we count a milliseconds now and finish transmitting at time point five at what time has it received 0.6 Why? We said the propagation delay is zero point zero zero one seconds or Point one milli seconds So here just to keep the numbers. We're dealing with milliseconds It takes zero point five to transmit one bit another point one to get to the other side At what time does the second bit get received? 1.1 if we think of the time that we start at zero plus point five Plus another one a point five gives us to one millisecond plus the propagation delay 1.1 Milliseconds it's received So what's happening here is that while we're transmitting? We're transmitting the next bit the previous bit is propagating across the link they're happening in parallel So the second bit is being transmitted While the first bit is traveling across the link similar the third bit is being transmitted The second bit is traveling across the link They happen in parallel and if you follow that through at what time is the entire 10,000 bits received There's some delay to get there the fifth bit as you'll see it's half a millisecond difference so 1.6 Because the transmission time is half a millisecond for each plus the 0.1 for propagation five seconds or 5,000 Maybe easier 5,000 milliseconds. What time does the last bit arrive? Well the 5,000 plus that zero point one Propagation delay and if you don't believe me go through and draw 10,000 bits and calculate those times You'll see that Well the end result. What's the total time to get that message from transmitter all the way through to receiver if we started at time? Zero we've received it in its entirety at time 5,000 point one milliseconds Which is just the summation of the transmission and propagation delay 5,000 milliseconds plus the zero point one milliseconds So we can calculate them separately and add them together to get the total delay delay is additive Any questions on this example of transmission and propagation delay? Just note that the reason we can add them together is because While we're transmitting bits. I think they're coming out of the computer The previous bits are flowing across the link They're propagating so the total delay in this case is our 5,000 milliseconds plus zero point one millisecond Which is what we arrived at so let's return to the slides and we'll see the general concepts So delay is the time it takes to get from one point to another There's actually different things that contribute to the total delay. We've just seen two of them. We've seen trans the time to transmit data onto the link and that Depends upon the number of bits to send and the data rate and Propagation delay the time for the bit or the signal element representing that bit to propagate through the link and That depends upon the distance of the link the number of meters and the speed at which that signal propagates in meters per second But there may be other things as well Our transmitter and receiver are computing devices They don't do things instantaneously. They take some time to process So sometimes we'll try to count the processing delay queuing delay will mention later but we'll We can visualize that in a different way Here's our transmitter and destined and would source and destination Let's say the the person at the source computer presses send We want to know how long does it take to get that message to the destination computer Well, it depends on three main factors with a single link Inside the computer When they press send your computer goes to work and does some processing The application reads the message maybe puts it into the right format sends it to your operating system Sends it to your network interface card your LAN card or Wi-Fi chip and that takes time and Those the delay incurred there we call processing delay So the time spent inside the source computer before we actually send anything will classify as processing delay Then we know that it takes time to get that data out of the computer Depends upon how many bits we have to send and what data rate we can send out and that's Gives given as transmission delay So if if we want to visualize transmission delay think of it How long does it take to get the bits out of your computer out onto the link? But once the bits are onto the link they must flow across the link or propagate and that takes time depending upon the distance and the speed then they arrive and at the receiving computer as Each bit arrives. It's processed again The computer takes time to process those bits and passes them up to the end user the total delay from when I press send and to the the person at the destination computer sees the message and The summation of the four components Processing at the transmitter plus transmission plus propagation plus the processing at the receiver so really three Three different types of delay Transmission propagation and processing delay We would like to be able to predict those things in different communication networks predict What will the delay be in this particular network? Or understand if we get a particular delay Why is it that value when we ping another computer and it says the delay is 10 milliseconds? Why is it 10 milliseconds? Why not one millisecond one second? These are the three three components so far. There's a fourth component That arises primarily when we have multiple links here. We're focusing on just transmitter one link receiver a Fourth component would be if we have two links We transmit across one link to an intermediate device and then that transmit across the second link to the destination The processing or some storage inside that intermediate device will classify sometimes as queuing delay It must wait in the queue for its turn to be sent That's another component, but I think we'll return to that Through some different examples later focus on these three How do we calculate them? Transmission delay we've done here's the equation Number of bits divided by the data rate So we've seen an example of that Propagation delay the distance of the link in meters divided by the signal propagation speed and If I don't tell you then you can assume the speed is the speed of light in Some questions or an exam. I may say assume the speed is 2 by 10 to the power of 8 meters per second But if I don't say use 3 by 10 to the power of 8 meters per second Different materials different media have different speeds in fact So we can calculate those two if we know the the link characteristics I know my data rate. I know how many bits I want to send If I know the distance of the link and I have the speed of light I can calculate transmission and propagation delay How do I calculate processing delay? How fast is your computer? Well, it depends. There's no one answer. It's not easy to determine how fast it will take Someone's phone to process a message and send it versus my laptop versus a super computer Okay, they'll have different processing delays So it's hard to calculate processing delay. We don't have a way to do it in this in this course But the good thing is that usually it's quite small compared to the others Nowadays computers are quite fast at such that the time spent processing a message in in RAM by the CPU Is very very small compared to these two components? Transmission delay and or propagation delay will be the major contributors to the total delay So in in data communications often will assume the processing delay is so small we can set it to zero compared to the other two Will often assume the processing delay is zero. We have no way to calculate it easily for this course It depends upon the actual computers being used and what are they doing? What software? They're running whether they're doing something else at the same time. It's very Hard to predict that But the good thing is it's usually very small Again, unless I tell you assume it's zero, but sometimes I'll say let's assume the processing delay is one millisecond in which case you can Use that in finding an answer queuing delay is The time spent waiting in intermediate devices usually for now. We're going to assume that small or zero Okay, we will return to that in later topics after the midterm and talk about why do we have queuing delay? How is it different from processing delay? It'll come up when we talk about the internet queuing delay Let's focus on the first three. So let's consider some examples first simple example just not extend from the Well same approach as the previous one. We have a link from a to b It's 10 kilometers long The data rate is one megabit per second and let's assume the speed Just to make it a little bit different that the speed of The link I've looked up the characteristics of optical fiber and other media and some common values are 2.8 By 10 to the power of 8 meters per second slightly less than the speed of light I have a frame of 100 bytes. How long does it take to get from a to b? Do a quick calculation of the the total delay in that case You can use these equations to calculate the transmission and propagation delay link from a to b The total delay is the sum of the processing plus transmission plus propagation plus processing In our case, let's assume the processing is zero. I haven't said anything about it So we only need to find the transmission and propagation delay So let's do that First say the transmission delay Transmission delay depends upon How many bits we want to send and how fast we can send them? 800 someone says that sounds correct, but let's calculate it for everyone. It depends upon the data size divided by the data rate What's the data size? Well our frame we have a hundred bytes and the data rate one megabit per second Here is when it's useful to make sure that you use good prefixes and or be careful with the Bits and bytes here we have measured in bytes, but here we have megabits per second So we may multiply by 8 to convert the top one to bits 800 bits divided by 10 to the power of 6 or 1 by 10 to the power of 6 bits per second 800 by divided by 1 we get 800 divided by 10 to the power of 6 Becomes 10 to the power of minus 6 which you remember is micro Okay, so Dividing by 10 to the power of 6 or even simply a one to remember if you divide by mega you get micro So we get 800 micro something. Well, what is it seconds because? Bits divided by bits per second the bees cancel out and we get seconds s comes to the top We get 800 by 10 to the power of minus 6 5 by 10 to the power of 6 seconds or 800 micro seconds So just the full approach for calculating the transmission delay Do the same but for the propagation delay distance divided by the signal speed Just remember propagation. It's about the physics the distance in meters the speed in meters per second It's not about the the bits. We're sending We have 10 kilometers divided by our 2.8 by 10 to the power of 8 Meters per second That's a zero there convert to meters 10,000 meters divided by 2.8 by 10 to the power of 8 and You'll use your calculator. And what do you get? 35.7 something something Micro seconds Okay, your calculator there will solve that so the total delay is just the summation of the components 835 What point seven micro seconds? So very easy to find the delay of a link Calculate the transmission and propagation if also in the question. I said the processing delay was for Micro seconds, then you'd add that on as well The processing delay is just additive and I would not never ask you to calculate Processing delay in this course. I would just give it to you say the processing delay of each computer is two micro seconds So the total you'd add on two for the transmitter plus two for the receiver and other four micro seconds If that was the question any questions on this simple example One more to to make sure it's a hundred percent clear for everyone another Another example we're in Bangkok We want to send to the US LA in the US and we're going to use a satellite So we have a satellite up in space and We have a ground station on Bangkok and in LA that we're going to utilize And they have antennas pointing up to the satellite So here's our scenario or our our network in this case because we're going to actually have two links We want to consider we have our office in Bangkok. We have a satellite dish on the roof and it will transmit up to the satellite in space and Then the satellite is quite simple and all it does is whatever it receives it then transmits down to the receiving ground station Let's try and we'll put some numbers to that and then calculate the delay the total delay from Bangkok to LA Let's say we have a frame we want to First calculate for a single frame to send one message of say a thousand bytes One message to LA. How long does it take? It's going to depend upon the distance of Each link in fact we have two links here. We have an intermediate system We're going to send the frame up to the satellite once that's received the satellite will send that entire frame down to the receiving station So I'll give you the characteristics. Let's say the links are the same The distance up to the satellite from earth and then down from the satellite to earth is the same All right approximately the same depending independent of the location so a Common location for satellites an orbit is the geostationary orbit, which is 36,000 kilometers above earth So the satellite is that and an orbit of 36,000 kilometers such that it appears to be all always over our head As a satellite rotates and as the earth rotates it appears fixed above our heads That's the idea of this orbit and the link to transmit up is One megabit per second and Down is the same okay to make your life easier and calculating we transmit up and down and Let's say there's a processing delay the satellites are quite slow That's four milliseconds just to add a number in there so we can see it used in the total delay you run out of We ran out of space Processing delay of at the satellite only of four milliseconds let's find the Response time So to make it simple assume the link and the uplink to the satellite and the downlink down to earth again is the same characteristics same distance same Data rate find the response time Response time is the time to get to LA and get a message back And let's assume that the message that comes back. It's the same size as the one we send them We'll add one more No, let's say the the gateways at Bangkok and LA is so fast that the processing is zero Effectively zero there only processing at the satellite. It takes some time there because it's a old satellite Find the response time in this case Remember the components of delay we can think every device has Potentially some processing delay or some processing delay it may be zero in some cases, but every device has some processing delay and Think from the link's perspective the link between two devices There's some delay to transmit onto the link and Some delay to propagate across that link So the simplest way to remember this is that Devices have processing delay links have transmission and propagation delay Everything's additive. So if you have multiple links and multiple devices you find the processing delays of all devices You find the propagation and transmission of all links add them all together So we'll do that to get the answer for this question and just to be to be clear in my example We transmit one frame up 36,000 kilometers to the satellite once that frame is fully received The satellite spends four milliseconds processing that frame say looking at the frame checking the header and so on Then it sends that one frame down to the gateway in LA and the distance down is approximately the same as a distance up 36,000 kilometers and For this case, we'll say the same data rate as well the processing delay the two end points is zero So we can add that in that's easy and then for response time when LA receives the frame is sends back a frame and For simplicity, let's assume it's the same size a thousand bytes So it will transmit a frame up to the satellite satellite will process for four milliseconds and then transmit down so For the response time. We can think that there are actually four links up Bangkok to satellite Satellite down to LA and then LA up to the satellite the up link and the down link to Bangkok For response time in this case four links turns out they're the same characteristics So let's calculate the propagation and transmission delay for the link Transmission delay the data size We transmit a frame with a house and bytes divided by the data rate one megabit per second 8,000 bits divided by one gives us 8,000 micro seconds propagation delay the distance 36,000 kilometers divided by the speed and here will assume the speed of light 36,000 divided by three is 12,000 divided by a hundred is 20 micro seconds times by a thousand is a hundred and twenty thousand 120 milliseconds Be careful. It's kilometers Okay, 36 million meters 36 million divided by 300 million is 0.12 or 120 milliseconds so Propagation layers in milliseconds Transmission delay is in micro seconds. Let's convert this one to milliseconds to make our life easier So that when we add them up, we're using the same prefix So let's go back to our picture and Make note of those times The propagation delay to go up Is a hundred and twenty We calculated the propagation delay to go up the transmission delay on the up link. We calculated to be eight So it takes eight to transmit up a hundred and twenty to get there It's such a large distance that the propagation delay is very large compared to prop transmission There's also for processing in the satellite So once the frame gets there we spend four milliseconds processing and Then the satellite transmits down and we should calculate the transmission and propagation delay But the numbers are the same same frame size same data rate same distance So it takes another eight to transmit down Plus a hundred and twenty to propagate there's no processing at these nodes so from Bangkok to LA in one direction zero processing plus eight to transmit plus one hundred and twenty to propagate Plus four to process in the satellite Eight to transmit down another hundred and twenty to propagate down and zero processing in LA gives us a total of two hundred and forty two hundred and Sixty two hundred and sixty milliseconds these numbers in milliseconds How long to get back? It's the same time to get back. Okay, everything's the same in the return path So the response time it's two hundred and sixty milliseconds to get there Two hundred and sixty milliseconds to get back response time is five hundred and twenty milliseconds so for delay Think about the processing delay in devices and Think about for links the propagation and transmission delay and then separate the network This is a network in this case where we have two links up or two links to the destination So we have two links. I treat them separately calculate the delays and then add them all up it would be slightly different if we have different data rates on the downlink for example and Response time is simply to go there and also to Sam back about half a second and this is The the distance especially is a realistic number Geostation is satellites are 36,000 kilometers away from earth What's the major contributor to the day in this case? Well, it's the propagation delay If I have a higher data rate Let's say I had 10 megabits per second If the data rate is higher Transmission delay goes down So the major contributor here is the propagation delay processing is small usually Propagation is a big problem with satellite communications. So whenever you use a geo stationary satellite If you use it say for internet applications To send a message to the web server and then get a response back Irrespective of where the web server and how fast it's going to be it's going to take about half a million a half a second To get their response back, which is quite noticeable when you're adding other delays of web servers in the internet. So Internet access Typical applications via satellite links the delay is usually a little bit too large to make it usable It makes it quite inconvenient because the satellite is so far away the propagation delay We can is the main contributor How do we reduce the delay a new satellite? Closer to earth we can't change the speed of light The satellites at that distance the only thing we can do to reduce the propagation delays to bring the satellite closer to the earth What's the problem? The earth rotates and and if you're close to the earth they the satellites rotate much faster than the earth rotates So if you look up you see the satellite moving and it's no longer there after 10 or 20 minutes So you no longer have coverage from the satellite With a geo stationary satellite you look up and it's rotating and you're rotating so when you look up It's always above you. You always have coverage if you have a lower orbit You need if you just have one satellite you may only have coverage for one fifth or one tenth of the day The satellite only comes above every so often So if you want to have lower orbits you need multiple satellites Network of satellites one follows the other and that becomes costly and complex Let's return to our slides and see what we've missed In a later lecture. I'll show you the What in a later set of slides will got the picture of multiple links, but I also have it here to show you You don't have this in the slides that maybe come in a later lecture, but I think from the satellite example You can almost see this already that is We've looked so far at a link, but in a network which has multiple links. It's the same concept We have a source that wants to go to a destination. It doesn't have a direct link There may be multiple links. So we send to an intermediate device in our example The satellite was an intermediate device which then sends on to the destination and we can extend this to have Many links many intermediate systems. You don't have this in your lecture notes this picture. It's okay You don't need to draw it because you've already drawn it with a satellite system but what we saw with a satellite case is that When we have multiple links When we have multiple links, it just becomes the processing delay in the first device Transmission propagation of the first link Processing delay in the second device transmission and propagation of the second link and processing delay in the last device That's what we had in our satellite example Except we had zero processing delay here for here and Zero here and we calculated the propagation and transmission for each link The last thing to mention is that and will not have any examples of it is that in these intermediate devices Sometimes they need to send data from other people as well. Not just your data The satellite receives a frame from you But it also has a frame some from someone else to send It can only send one at a time Therefore the second one may have to wait It may wait in a queue and be later That's why in intermediate devices will often see especially in the internet and large networks There's an extra delay called the queuing delay which is the time your frame spends waiting in that device Waiting for others to finish being completely sent So that's where queuing delay will come in when we look at networks in detail Four components of delay transmission and propagation we can calculate processing Must be given and queuing may come up In later topics The last part here is how do we deal with errors? But instead of starting that will I think finish with one one more example with just the five minutes left one last example a Question about the mid-term exam. It will be closed book mid-term exam Last example just to illustrate and We did this We did this in the previous lecture we can ping use an application to test the delay between different devices When we ping it's an application that sends from my computer a message To some destination computer Which we denoted here ICT web server and when that server receives the message It sends back a short response and it reports the time. I'll run it again with a smaller window and The thing of importance is the time here. This is the response time or round-trip time and it keeps repeating this Every one second send a message Get a response Why is the delay in the order of 20 or 30 milliseconds the average across those 14 messages received was 30 milliseconds The average delay it varies is about 30 milliseconds from my computer To a computer which is on the third floor of this building Okay, so the question is why is it 30 milliseconds? Well, we now know it's a combination of those four different components From my computer to the computer downstairs. There are multiple links. I'm using Wi-Fi at the moment So there's a link from my computer to the access point on the wall That's the first intermediate device. There may be some processing delay there Then from the access point. There's a cable that goes down. I think it goes directly down to the third floor That's the second link Then it goes into another intermediate device and eventually into the server just a PC on the third floor So we have at least three links. There may be another one. I'm missing so there are multiple links each have their own propagation and transmission delay In theory, we will calculate them. I know the message size It's actually 84 bytes. I Know the data rate for Wi-Fi I'm using 54 megabits per second so I can calculate transmission delay. I could estimate the distance Two and a half meters divided by the speed of light and estimate the link distances and find the propagation delay But also the intermediate devices would have some processing and most likely queuing delay Especially the intermediate devices in SIT. Everyone else is doing the same thing when I'm sending Everyone else is in here sending things from their phone and other places via the same network So some of those intermediate devices would send receive my frame But they also receive hundreds of other frames at the same time So mine have to wait in a queue before it's sent So that contributes to the queuing delay and that's hard to measure. We will just observe it in some cases So now you know why? The ping delay the ping time or response time Well, what contributes to that delay? processing propagation transmission and queuing delay and they may vary over time as we see here what we'll do on Thursday is finish these slides on just talking about how we deal with errors and I think that will finish for the this topic will finish for Leading up to the midterm and we'll say a little bit about the midterm on Thursday but in short it will be a closed book exam bring your calculator and questions similar to past years Exams and similar to the quizzes, so we'll see you Thursday