 We've looked at a number of performance metrics ways to measure communication systems. We've mentioned delay and the last thing We got to in the previous lecture. We we did some calculations of okay, if I have so many bytes to transmit Transmitting at some rate it takes some time to transmit those bytes What we will call the transmission delay or transmission time, but also The signal that represents those bits think of a sine wave some electrical signal or radio signal needs to Propagate Across some medium through the air across some cable that takes time to the propagation time so let's look at the the different components that contribute to delay and Then Give some examples or can we go straight to an example? Let's go straight to an example. Let's say we need to change lecture room We can need to move from this room. It's too hot in here. So we're going to move down to a lecture room downstairs How long will it take us to move? Well, I want to be faster than one hour. We need to continue our lecture How long is it? Do you think it would take or how do you calculate how long it would take this class of students to move? 10 minutes, let's calculate how long it would take and we need to make a few estimates What I want to show is that okay the delay the delay of many systems We use the same concepts whether it's a real system or a communication system the delay to do something Depends upon a different different factors Let's make some simple assumptions. You don't need to copy this down. This is just a Light example to get us started. Let's say how many people here our class Should be 45, but maybe not everyone comes Maybe we've got 40 students and my aim is to move This class of 40 students down to a lecture room downstairs Are we on the sixth floor to under the fifth floor? Okay? So let's say I want to do it in an ordered manner I don't want everyone to let's say we need to do it quickly I don't want everyone just to run to the door and go to the other lecture room I want to do one by one one person at a time and Walk in a nice ordered line down to the room downstairs There are different factors in play the first thing I'm going to do is I Want to make sure everyone leaves and I want to record your name as you leave so I can keep track of you So we'll line you up in a line Maybe at the door line on a single file line And let's say that takes some time to put you in a line before you leave this room and let's say to line up takes some time 30 seconds say I know you fast and want to leave Let's I'm just make up some numbers here. You'll see the significance later and Then what I'm going to do is I'm going to stand at the door and let you out one by one So you've got 40 people to get out of this room and I'm going to control the speed at which you go out of the out of that door. How many people can go out of the door At what speed do you think? If you just walk slowly out of this door How fast can we get people out? Not if you all jam through but just one by one through the door one For one person how long? One second. Okay, let's say we can do one student per second out the door one student per second So imagine we're in a you're in a big long line We've lined up it took 30 seconds to line up and then I say go and the first person goes out One second later the second person goes out and the third person and so Now that when the first person walks out of the door They need to walk down the corridor down the stairs into the other room Okay, and how far away is one of the lecture rooms downstairs? Let's say I don't know some distance between this room and the next room of Anyone guess? Ah 40 meters Okay, let's say it takes it's 40 meters from the door here down the corridor downstairs to the next room How long does it take before if you're the first person in the line? How long does it take you to get there? How long will it take you to walk 40 meters? What does it depend upon 40 seconds? Why? Okay, well one step one second good one For example, I don't know how fast people walk about one meter per second One step one second depends how fast you're going doesn't it? But we want to do it in an orderly manner. So let's say you're going at one meter per second your walking speed Speed of walking One meter per second. So the question is how long does it take to get this class into the other room? What's the answer try and calculate from this information So the first step is it takes 30 seconds to line everyone up at the door Once we're in a line then one person at a time steps out of the door and I control the speed at one Student every second goes out the door Once you step out the door you walk at one meter per second Down to the other room How long does it take you to get to the other room once you step out the door you step out the door it takes you 40 seconds okay to To what to propagate down the corridor down the stairs and into the other room so let's call that The different parts here the time for one person one student to walk there 40 meters at One meter per second equals 40 seconds. We'll call this the propagation time propagation the time to travel But the time for one person to travel How long does it take me to get the class out of this door? How long does it take me to get? Once you're in a line It takes me 30 seconds to get you in this line and then once you're in the line How long does it take me to get you out the door anyone? How many students do we have here? Maybe not quite 40 how long to get out the door not hard 40 students one student per second 40 seconds okay Plus the first 30 to line up those students So let's say you're all seated now it takes 30 seconds to form a line Then we start going out the door One at a time so another 40 seconds to get everyone out so the question is If I start lining you up now, how long until you're all in the other room? 30 seconds to line up Then you start going out the door As the first person goes out the door they start walking and one second later the next person goes out the door How long until everyone's in the second room? again 110 How did you get that? Yeah, you're right So that the answer was 110 I think and I think it's the correct answer Let's see how that we arrive at that First the first step line everyone up takes 30 seconds Okay, once we're all in a line then we start going out The time it takes to get everyone out 40 students at one student per second is another 40 seconds So after 30 seconds we're lined up and then after another 40 seconds everyone is out the door So 70 seconds so far The question is when does the last person get to the next room? Well if the last person leaves after 70 seconds It takes that last person 40 seconds to walk down to the other room So the total will be 110 seconds The time to line up 30 seconds the time to get everyone out the door 40 students at one student per second 40 seconds Plus we want to know when is everyone get there well the first person will arrive When when does the first person arrive if the first person leaves after 30 seconds It takes 40 seconds to get there the first person arrives at time 70 But we care about when does the last person get there 30 seconds to line up 40 seconds to get everyone out And another 40 seconds to get that last person down the corridor down the stairs into the other room 30 plus 40 plus 40 110 seconds in total We will use these same concepts when we look at delay in a network Let's just note that the time for one person to walk to propagate down the corridor is 40 seconds The time to transmit let's say I'm transmitting you Out the door Strange word We had 40 students. It was a bad example same numbers 40 students at One student per second should have used a different number another 40 seconds That's what we'll call the transmission time when we come to computer networks And then the first thing we'll come back to that the time to line up Let's we'll call that the processing time the time to get things ready We've got our students here. We need 30 seconds to line them up to process them get them in the right order then then it takes 40 seconds to transmit all of them out the door and It takes 40 seconds for that last person To propagate from this door to the other door the destination Same I will apply in computer networks instead of dealing with students. We deal with bits and We talk about the rate at which we transmit bits out of a computer a transmission rate or data rate We talk about how many bits we have to transmit instead of how many students how many bits in our data and Instead of talking about the speed at which we walk we talk about the speed of which our signal propagates and We can calculate propagation delay the time for the signal to propagate along the link transmission delay the time to get our bits out of the computer instead of out of a room out of computing device and Processing delay this time to line up is the time it takes your computer to get the data ready So that Example here the data was the students of course in our computer the data is a sequence of bits Sometimes that may help people understand what we cover now delay, but if this makes no sense to you Well, maybe it's not the best of examples, but I hope it helps some people understanding Delay any questions about that Make some assumptions of course about the speed the distance and so on but if we know that data we could calculate So let's try and formalize that let's try to Cover that concept with respect to computer communications Delays the time it takes to get from one point to another Delays additive The total delay is this summation of the components the delay of each step We saw that in our example. It was the time to process the line up plus the time to transmit plus the time to propagate and We talk about four components of delay transmission the time to transmit the data onto a link Propagation the time for a signal or a part of the signal called their signal element formally to Propagate across the link Processing delay the time it takes for the devices to process data Maybe the sending device the receiving device and other devices And one we didn't see in that example, but we'll see shortly is queuing delay Sometimes especially in intermediate devices our data arrives It doesn't get to be sent Onwards immediately may have to wait for other data Before it gets its turn to to be sent so it may have to queue up and wait before it can be sent that happens in some cases So fork parts of delay, let's look at them Let's say we have a link between two computers source and destination and we have users of those computers So what we're showing here is the user uses some application some piece of software on their computer Which talks to the operating system inside that computer? Which then so the user generates data the application processes it sends it through the operating system The OS sends it to the network interface card that the LAN card for example the piece of hardware that then Transmits that onto the link as some signal the signal propagates across the link is Received by the corresponding network interface card Processed by the receiving or destination operating system Processed by the destination application and then the destination user gets the data That's the flow of data here with respect to delay We can show three of those four components like this Think of from when the user creates the data and let's say presses send Until when it gets to the network interface card ready to be sent is The processing delay So that's the time it takes the application to do some operations on the data The operating system to pass that data through some memory structures For the protocols to operate So it depends upon the speed of your computer the application The protocols being used the operating system And what else is running on that computer? So the processing delay is the delay inside the device Once it's got ready to be sent the network interface card the LAN card for example transmits the bits onto the link So with respect to where that happens think of it at this point going from the computer onto the link So we have a sequence of bits maybe a thousand bits to send the network interface card. I didn't bring one today Converts those bits into a signal and transmits a signal out onto that link So there's the transmission delay here. How long does it take? What's it depend upon transmission delay? How do we calculate it calculate transmission delay? What's the general approach? Data size and the answer somewhere in your lecture knows divided by the data rate if I've got a thousand bits and My device can send at a hundred bits per second Data size a thousand divided by data rate 100 will give us 10 seconds So simply the data rate of the device usually a characteristic of the device and how much data we have to send that tells us the transmission delay We send the first bit out it starts propagating as a signal across the link and subsequent bits follow it and Then the last bit comes out the propagation delay of think of each bit propagating across that link Depends upon the length of the link in meters and the speed of the signal Like if it's a light signal the speed of light if it's an electrical signal, it's maybe around two two points something By ten to the eight meters per second. It depends upon the conducting material The signal propagates very fast across the link It's received by the computer and processed similar to at the source computer the Nick the network interface card operating system and application must do some processing that takes time So the delay from human to human One way is the summation of those components the processing in the source device Plus the transmission on to the link plus the propagation across the link plus the processing So if we can find the components we can find the total delay Let's extend that and then we'll go through some examples If we just have a link we've had that case. This is the case where we have two links Or a general network where we send from one source device to some intermediate device Which then sends it on to the destination and you could generalize or extend this to have multiple links more than two Source to intermediate to intermediate to intermediate to destination similar concepts would apply The difference So the source the user creates the data Application processes sends through it's going to send across the first link this intermediate device will receive Do some processing and I've tried to draw a queue here Where the data has to maybe wait in the queue? Why would it do that in some cases this intermediate device may receive the data from the source? Although it's not shown here. It also may receive data from other devices Think of our wireless access point up here. It receives data from my laptop Plus from other people's mobile phones and laptops. So it's receiving data from multiple sources It cannot send them all at once So it sends the data from one of them first before the others Therefore the others must wait in a queue before they are sent That's the concept here the intermediate system. Sometimes the data must be queued up Store in a queue before it's sent out again in the in the best case It doesn't have to wait in the queue it goes straight through but sometimes if this device is busy the data will have to wait Then it's sent on to the destination So we have similar concepts Processing at the source computer Then transmit the data onto the link signal propagates across that link Arrives at the intermediate system. There's some processing at it as it arrives And then the data is put inside a queue Waiting to be sent across the next link So there may be some queuing delay there what we call queuing delay the time the data spends in the queue waiting to be sent Same as if you go down for lunchtime you got to line up for the queue for to order food What's that depend upon depends on how many people are arriving and how fast the shop can serve you So the more people arriving and the slower they are at serving the longer the queue is This is the same concept as more data comes in and the slower that we can send out our queue Length will increase and as a result the time our data spends waiting in the queue the time you spend waiting to get food Will increase the queue delay Once it's your turn to be sent The data is turned to be sent. It's processed briefly Transmitted across the second link Propagates processed at the receiver so when we have intermediate systems we May also have queuing delay With just a link usually there's no queuing delay Queuing delay usually occurs just in intermediate systems in theory it could but it's it's very unlikely We will not consider cases that it does so a simple way to remember these four components is That processing and queuing delay happens inside the devices Processing is inside these computing devices Most cases queuing delay is only inside intermediate devices In this course that's all we're a will assume Whereas transmission and propagation delay depend upon the link characteristics link distance Data rate of the link or at least the device connected to the link So a simpler way to view those components is We have processing delay inside devices We may have queuing delay inside Intermediate devices once we send via to get to the destination and if we look at each link We can calculate or determine some transmission delay and propagation delay So processing and queuing for devices transmission and propagation for links If there's a large network then you just break it into components for each link Think about the transmission and propagation delay and for each device think about the processing and queuing delay and Add them up To get the total delay That's where we say delay is additive the time to get from the first user so a lot to the second user It's a summation of all those component delays We'll get to an example in a moment of those four components transmission propagation processing queuing At least in this course will only attempt to calculate transmission and propagation delay Transmission delay is the number of bits to be sent be in this equation divided by the data rate How much data we need to send divided by the speed at which we can send the data The link data rate is usually a characteristic of the device attached to that link My Wi-Fi device has a link data rate of 54 megabits per second My LAN card has a link data rate of 100 megabits per second My ADSL link at home maybe 24 megabits per second depends upon the device attached to the link the number of bits depends upon What information we're transferring So we can calculate that if we know those two values Propagation delay depends upon the physical characteristics of that link distance and The speed at which the signal propagates in meters per second Distance divided by speed and unless we say otherwise Let's assume that the speed is the speed of light C means the speed of light Approximately and for our course 3 by 10 to the power of 8 meters per second 300 million meters per second is what we will use for the speed of light I will not ask you to remember the speeds of different materials Okay, that's not what we'll cover But sometimes I may give you in an exam or quiz. Let's assume the speed of this copper conductor is some value Then you would not use a speed of light you would begin use the given value So those two we can calculate if we know that information The other two we will not attempt to calculate Processing delay depends upon many factors The amount of data to process how many bits the software That's that's running and how that's actually implemented The computer hardware and what other things are happening on the computer if I'm doing nothing on my computer the processing delay may be smaller than when I'm running some background or applications in the background It depends upon what my computer is doing We don't have a way to calculate that with any accuracy in this course Often not always but often it will be quite small compared to transmission and propagation delay Computers are fast they can process it a Mount of data in a smaller amount of time nanoseconds microseconds and Some of the examples will can consider most times Transmission and propagation delay is much smaller So sometimes we'll assume the processing delay is zero Even though there is a processing delay, it's quite small. That's for let's make it zero just to keep things simple So again unless I tell you In a question the processing delay of this computer is five milliseconds if I don't tell you assume it zero queuing delay Intermediate device data arrives data is arriving from many people many different links possibly It can only be sent one at a time So The first one let's say three pieces of data arrive at the same time only one can be sent across the next link So the other two have to wait in the queue So they have some queuing delay incurred How long is the queuing delay? depends upon how much data is arriving and How much needs this be sent out leave the device and It also depends upon the queuing scheme Maybe we give priority to Steve's data give low priority high priority to my data low priority to your students data in which case calculating the delay would be more complex There's whole courses on queuing and queuing systems. We will not cover how to calculate queuing delay It can be large in the internet When we send data, I think last week I sent data between my computer and the web server in Japan Travelling via other intermediate systems the delay for queuing can be quite large Larger than the other components But again unless I say otherwise, that's a shumit zero So I may say in a question queuing delay is one second if I don't say assume it zero so given that Calculate some let's go through some examples But before we do any questions on the concepts Remember the four components of delay and Remember how to calculate two of them transmission and propagation. Let's try some simple examples Where can we start? let's say we have Computer a connective via a link to computer b So we have a just a simple now a simple communication system with one link and the characteristics The link ten kilometers between two towns or two locations across the city and Let's make it a little bit different the speed of transmission In practice is slower than the speed of light So it depends upon the conducting material Let's say it's this The speed of signal transmission don't confuse this with the speed of data transmission It's not the data rate. It's a speed at which a signal can propagate It's 2.8 by 10 to the 8 meters per second and Let's say our link data rate Simple one megabit per second. I have 100 bytes of data to send 100 bytes of data Calculate the time it takes to get from a to b Spend a few minutes calculating How long does it take to get my 100 byte message from a to b? spend five or so minutes calculating How to do that our components? remember delay four components transmission bits data size divided by data rate in our question data size a hundred bytes Data rate is a hundred meg one megabit per second So we know that we can calculate transmission delay Propagation delay the time for the signal to propagate depends upon the distance of the link 10 kilometers and The speed of signal transmission 2.8 by 10 to the minor by 10 to the power of 8 meters per second So we can calculate that those two components Processing delay in computer a and processing delay in computer b. I didn't tell you what they were so let's assume they are zero and Queuing delay well, there are no intermediate devices. Let's assume that's zero as well. So those two are easy. We just calculate the transmission and propagation delay so Transmission was it data size divided by data rate we've got 100 bytes of data and be careful with units, so we've got 100 bytes of data Let's do it in the full manner divided by a data rate of 1 megabit per second you can't divide bytes by bits So let's put them in the same unit that is it'll be 800 bits divided by 1 by 10 to the power of 6 mega Bits per second If we simplify that convert bytes to bits Times by 8 get 800 at the top 1 mega remember mega is 10 to the power of 6 Bits we convert bytes to bits divided by bits per second. What is the unit's going to be? Seconds do it here. You can check B divided by B over s this inverts the bees cancel out and you get s left over seconds and Then it's 800 divided by 1 800 divided by 10 to the power of 6 is 10 to the minus 6 So 800 times 10 to the minus 6 seconds 10 to the minus 6 is micro so you start to get practice and you can do those and Recognize that's well 800 micro seconds Because divide by 10 to the power of 6 divide by mega and you'll get micro If you divide by micro you'll get mega if you think about the quick calculations on the prefixes or it could have been 0.8 milliseconds That's the transmission delay propagation delay distance divided by the speed of transmission of Our signal and our distance of our link was 10 kilometers and Our speed we said was I don't know why I said it that but 2 by 2.8 by 10 to the 8 meters per second I Think I looked up some material some actual material or not I can't remember optical fiber or something and that was the the rated speed All right, you need your calculator for that one I'll let you do it. What do you get? 35.7 something watt Microseconds good Okay Use your calculator fine, so 800 microseconds to propagate out of computer a sorry to transmit out of computer a and 35.7 microseconds for the signal for each bit to propagate across that 10 kilometer link Distance divided by speed This is Mu or you micro So the total time you can think it takes 800 microseconds for the bits to get out and that last bit Takes 35.7 microseconds to get to the other end of the link So we say the time from when the first bit was transmitted until when the last bit arrives is the summation of these two 835.7 so the total About 836 microseconds Any questions on those two calculations? Transmission and propagation to let it I Skip that step. Why don't we change the units here? Sorry. I skipped that so yes 10 kilometers 10,000 meters kilo times by 10 to the power of 3 Devote 10,000 meters divided by 10 to the point 8 2.8 by 10 to the power of 8 meters per second note that K is not a unit K is what we call a prefix the unit is meters K is the prefix saying multiplied by 1,000 This is meters the unit here is meters per second meters divided by meters per second leaves us seconds So yes, I skipped the step here of expanding the prefix it helps if You start to remember some of those prefixes and you'll with practice You'll start to recognize and recall divide by mega we get micro and it makes your life a little bit easier I see some people calculating on the calculator, and they're writing down 0.0000357 Dealing with such numbers is a little bit harder than dealing with whole numbers 800 plus 35.7 is much easier than 0.00008 plus 0.0000357, okay That's why I try and convert into nice Use nice prefixes at the start any other questions on this one Everyone got this answer Good most people on track if they didn't Another one different scenario We have We have a satellite system We want to communicate from Bangkok to LA and Instead of sending via cables under the ocean We're going to transmit from a a ground station here in Bangkok Transmit up to a satellite in space and Then that satellite operates in a mode that it receives the signal Receives the data and then transmits it again down to some receiving ground station in LA We want to once I give you some information about this we want to calculate how long To communicate between our two locations This is what we'll call a Geo Geo satellite geo a satellite in geosynchronous orbit Sometimes a geostationary satellite What that means is it it's above earth such that as the earth rotates The satellites in an orbit Rotating Such that if you look up and see the satellite as the earth rotates you always look up and see that same satellite It appears above you all the time So the earth rotates and the satellites in the orbit such that as the earth rotates the satellite is orbiting So it's all about always above you That's an orbit which is commonly used for communication satellites for TV for some internet services The orbit is about 36,000 kilometers above earth. So the distance from Bangkok to our satellite We're going to send a wireless signal is 36,000 kilometers and the distance down is about the same. I will not write it same distance All right, because we're in different locations. It must be plus or minus a bit, but 36,000 kilometers 36 million meters Up and 36 million meters down Let's say we have and we'll keep the the rate simple a data rate of both links Let's keep it signal simple one megabit per second That's both links. We in fact have two links in this communication system The up link to the satellite and the down link from the satellite to the receiving ground station So we send our data up to the satellite It may process the data And then sends it down to the ground station in LA Let's say that the satellite takes some processing Delay, there's a processing delay on board the satellite of four milliseconds It receives the data Processes for four milliseconds and then sends down to LA. Let's use the speed of light It's a nicer number to to use in this case not 2.8, but the speed of light for speed of transmission Calculate the delay For 1,000 bytes of data from Bangkok to LA Okay, so there's the scenario. We want to send 1,000 bytes We'll send it up to the satellite the satellite will process and Then send it down To LA how long does that take to get there? quickly calculate the delay and the links are the same Same distance up as down same data rate up as down. I'll just written it once Anyone have the answer? Remember your four components of delay Transmission propagation processing and queuing We we treat this system as a network with two links So in general we have our ground station that's going to transmit to the satellite The satellite will process and then transmit to the receiving ground station So just break it into two links and you can calculate separately In this question. It's even nicer because the links are the same so you calculate once then you've got the answer for both links So let's do that. What have we got? Easy queuing delay zero no queuing delay processing at the ground station. I didn't say anything so zero at the ground station When we receive a message at the satellite Once we've received that entire message then we spend four milliseconds to process it Whatever the CPU has to do with that message Maybe detect if there are any errors in it it takes four milliseconds and then it sends down and Then there's no processing at the receiving one So we're going to at the end will add on four milliseconds of processing therefore we need to know the transmission and Propagation delays of each link Let's do the uplink from Bangkok up to the satellite transmission data size 1000 bytes data rate 1 megabit per second if you remember back our previous example we had a hundred bytes correct in The first example we had a hundred bytes at 1 megabit per second and it took eight microseconds So a thousand bytes is ten times larger. It will take ten times as much. It'll take as 80 micro. Have I done that wrong? All right, let's calculate. I think my memory is not so good from the previous one Let's calculate 8,000 divided by 1 by 10 to the power of 6 8,000 bits divided by 1 by 10 to the power of 6 bits per second 8,000 micro seconds or 8 milliseconds 8,000 micro seconds is 8 milliseconds propagation delay Be careful here 36,000 kilometers 36 million meters divided by the speed of light which is well 300 million meters per second 36 divided by 300 0.12 Or 120 milliseconds again use your calculator for these it's fine To get from Bangkok up to the satellite it takes us 8 milliseconds to transmit But the signal representing each bit takes 120 milliseconds to propagate through space to the satellite It's such a large distance that the propagation layer is quite significant Compared to the other components So it gets up there That's record What do we have on our picture? Processing it here is zero Transmission we have and I'll just do it eight to transmit 120 to propagate for the process So that's eight to transmit up 100 another 120 for the propagation So it arrives at time 128 and then four milliseconds to process So we add that on and then we need to transmit it down and As you realize the transmission delay down is the same as up same data size same data rate So the transmission delay on the downlink is also eight eight milliseconds and The propagation delay it's the same distance at the speed of light is another hundred and twenty Just add them up eight plus a hundred and twenty plus four Plus eight plus a hundred and twenty 260 Don't forget the processing delay in there any questions on that one This was an example of a network with two links It was a simple case where the both links had the same characteristics So we only calculated for once for one link and then it applied for the second But in general the links may be different different data rates different distances So you'd need to calculate separately, but this one's a little bit easier Any questions on delay? What about satellite communications? What's the problem with? Internet access via satellite you subscribe one of the popular ones in Asia. It's called IP star It's it's the tycom satellite the Thai satellite provides a service called IP star for satellite internet through Asia and the Pacific What's the problem with using satellite for say web browsing and general internet access? That we see here these are some realistic numbers or some some of them are at least What's the problem? The problem is the delay Especially the propagation delay Imagine your web browsing Your computer Maybe at your home sends Here's at your home in Bangkok. You've got a satellite dish You send up to the satellite the satellite sends down to some gateway and then on to the rest of the internet There's always this propagation delay of 120 milliseconds up 120 milliseconds down So when your web browsing Compared to your normal access. There's always this additional 240 milliseconds To send your data from your computer to the server Plus when the server sends back the web page an additional 240 milliseconds Which is what 240 plus 240 is about half a second So when your web browsing via satellite, there's at least a half a second delay between You and the server plus all the other delays of the internet transmission and so on which means genuinely with web browsing via satellite internet it can be quite Annoying in terms of responsiveness because there's always this large delay compared to what we're used to So the delay is quite a Significant factor here. How do we reduce the propagation delay? How do we reduce the propagation delay? It's 120 here. How can we make it smaller? Change the speed of light No, you see propagation delay Distance divided by the speed of light we have we can't change the speed of light and The distance for the satellite is in such It's it's such that the orbit is so that the satellite rotates at the same speed as the earth So that we always have coverage of that satellite That's why we have such a distance if we had it lower Maybe a thousand kilometers above earth The satellite would be spinning much faster than the earth does and it'll be there for 10 15 minutes And then we wouldn't have satellite coverage So that's the problem with lower orbits So we cannot reduce that propagation delay and practice We can transmit faster, but still it makes little impact compared to the propagation delay Let's return to our slides We've almost got to the end of this topic to summarize on delay For components remember them remember how to calculate transmission and propagation and also how to apply them in links and in networks We've had a couple of examples two minutes We'll cover this in two minutes What do we need for good performance? When your web browsing how much delay can we tolerate? That's what this is about The response time. What's a good response time? Have a look at these we will not go through much We may see examples as we go through web browsing Some people say that anything over a hundred million milliseconds users start to notice a Response time of a second you press the button takes a second to get back. You'll notice you have to wait a little bit So we want that to be small as possible There was estimates the Amazon which of course sells a lot of things via their website extra response time Means people don't buy as much So it costs money Voice calls we may return to when we look at audio last one if you've ever strip tried to stream video Depending upon the resolution of the video What throughput do we need to get good quality? Well, here's some approximate numbers Maybe low quality YouTube about one megabit per second You want to stream normal TV? standard definition digital TV About five megabits per second High definition about 11 megabits per second It differs in different cases So these are some requirements of performance of applications so what we need to do is Look at how to design communication networks to meet these requirements So we're not going to say much else about these have a look think about the applications you use and think about What gives good performance? When your web browsing what is a measure of good performance and what's a measure of bad performance the different applications in use? Tomorrow will move on to signals and start to look at signals