 So we want to go in and understand how link communications work. What are the signals that we send? That's where we want to start with communications. But before we do that let's Talk a little bit about performance of communication systems Because later when we analyze or compare different systems We will compare them based upon Performance and I think many of these things you will know or you've had exposure to already so we'll give some examples of What do we care about with respect to performance of communication systems? so Data communications is about sharing information getting information from one point to another We want to talk first about what types of information. We've already mentioned Different types of data images videos audio Mention some of them. What types of applications? All right, we may talk about different types of internet applications web browsing email file transfer voice calls And then we'll try and talk about well how much information Do different types of applications typically transfer? So try and measure how much information we need to transfer and then Start to introduce. Well, what's a good communication system? If you get a job to go work for a company and you need to Choose between two different systems to connect your company's office to another office You need to be able to choose the best one So we'll move into how do we measure good ones? What's effective communications? Then how can we measure the performance and we'll finish with some Examples of what applications require information and applications Sorry Where are we? Can I have some answers? How big is a web page? any any ideas a few kb kb meaning what? Kilobytes, okay, everyone understands bytes bits kilo mega giga nano picot Don't worry. There's a Practice lesson that you can do over the weekend that will remind you of all those things Okay, I'll show you about that towards the end But okay a few kilobytes, okay bytes remember one byte eight bits. Let's Eventually will well, how do we measure information? One measure is bits or bytes. Okay, how many bits in that piece of information? So a web page a few kilobytes any other suggestions will test in a moment Let's have a look at some web pages and measure and see how big they are our demo yesterday I we visited this this website. How big is the web page? Let's see. Let's save the page and No, I'll save web page complete. We'll see why that's the case in a moment. Let's save it and then see how big it is save it Where am I? It is 4k a few kilobytes was a good good answer Exactly well 4k that's a bad answer. It's exactly 233 bytes this file Okay, the web page the HTML page that small one welcomed to ITS 3 2 3 was 233 bytes 4k's was an approximation from a different piece of software ignore that bad answer So less than a thousand bytes less than a kilobyte this one, but it was a very simple web page Not all web pages are that simple. Why is it 233 bytes? It's text. It's HTML What does it tell you that if it's 233 bytes? 233 characters most likely Okay, it's just a text file with Some some strings of some HTML tags and welcome to ITS 3 2 3 if we counted how many characters? word count on that file word count is a program that tells us there are 12 lines 25 words and 233 characters inside that file Okay, if you open the file in the text editor and went through you count 233 characters each character is represented by a single byte That's common with most operating systems How do we represent a character as a byte? How do we represent a character as a byte? What's the name of the common system? Let's look at this text file in the binary form and it will come up terrible on the screen here But you'll take some notice Because it's it's a little bit condensed This is the binary form of the text file. So It's zoom out so we can fit it in For example, what it says is this H here in the last HTML tag is represented by these 8 bits Okay, this is just the binary view of that text file. Why do we get these 8 bits for H? Why isn't it? What's the name of the system or the how do we? ASCII okay ASCII encoding you all know that where you it's a mapping of Letters characters To binary or it goes to decimal numbers ASCII is usually 7 bits, but it's usually represented as 8 bit number There are other encodings. That's just the common one so any text Characters that we type for example, we can easily convert to binary to bits zeros and ones Back to our web page Maybe a different website sit website. Let's Let's save as Save page as let's just call it SIT. I think this will do it This is just a short way to give me the size the SIT dot HTML file. The basic web page is about 40k 40 kilobytes But there are files with this web page images Style sheets JavaScript all of them come and are needed to display the web page as is and they are saved in this directory Which makes up another hundred eight hundred and thirty six kilobytes. So it's getting close to one megabyte for that one web page So how big is a web web page? It depends upon the content on that web page, but you're talking the order of Kilobytes to megabytes typically Okay, you go to Google and do the same you'll see it's probably the similar size to this Many different websites many images much much bigger Okay, how big isn't We won't do all of these an email. Well email usually just text although you can add attachments So if it's just text the number of characters Is represented by a single byte and Maybe there's some extra information attached to the email like source address. Who is it from who's it to subject and so on We're not do that. How big is a photo? So emails well depends on how long your email is Again in the order of kilobytes plus attachments. What about a photo? depends upon what the resolution pixels so Specifically the resolution is the what the width and the height Okay, so x by y and what else does it depend on? So yes a higher resolution or higher width and and height the format The type of file a jpeg a bitmap Png a gif or other formats. What else resolution format color How do we represent color? bits so a pitch is really a set of pixels With by height a set of dots each pixel each dot is a particular color So the number of pixels in the image is the the resolution is the the width times the height and each pixel is a color and we represent that color by some binary value So the color depth or the bit depth of that pixel impacts upon the size I've got some examples of the same picture Where here's a picture it doesn't matter what it is I just got it from some NASA website the other day It's hard to see that this it's 4859 pixels by three two three nine Width is four thousand eight hundred and fifty nine height is three two three nine So that's the number of the the resolution And the size is about 47 megabytes Why is it that? let's Have a look I need my calculator We have four eight five nine pixels across and Down we have Three two three nine pixels. So that's the number of pixels or dots in this picture Sorry, that's the number of pixels this 15 million pixels each pixel is a is a color and the color is represented by some binary value and Typically how how many bits do we use to represent the color it varies so Black and white would be how many bits To zero one black or white although we could then have gray scale different scales this one I think is 24 bits Three bytes per pixel. So that's the number of pixels Times by 24 will give us the number of bits in this picture There are 15 million pixels each pixels represented by a 24-bit value. That's the number of Bits in the picture How many bytes? Dried by eight Dried by a million convert to megabytes 47.2 megabytes. Okay, that's the size of the image in fact the size of the file So this file Is simply the width times the height times by the number of bits per pixel the color depth How big are most of your pictures are they 47 megabytes you take a photo? Different compressed This picture was I don't know if it says anywhere this the format of this picture was tiff TIFF It's a format that's used commonly for uncompressed data. There's no compression But with many types of media we can compress it either Without losing information or even possibly losing information to make the file much smaller. I think I have it here Same picture almost the same two megabytes Okay, you check the difference 47 megabytes two megabytes 47 two Almost no noticeable difference The two megabyte picture is a JPEG JPEG Defines the format and it defines how to compress the information It is actually loss lossy compression in that that it throws some of the information away So it's not exactly the same picture Some of the informations that was in the original picture is thrown away just to make the file size smaller So it uses things like compression we will not cover in this course, but things like if there's a repetitive pattern I don't know. There are a hundred blue pixels in a line Instead of storing those a hundred blue pixels store some value that tells you that there are a hundred blue pixels And when you decompress it shows those hundred blue pixels, but it's more advanced than that But compression is used there. So photos are in the order of megabytes Depends upon the format the size and the color depth how big is the song? Let's measure all these in bits So we can compare How big is a single song? about three four five bits No three four or five Megabytes, okay in the order of several megabytes three megabytes four or five megabytes Again the answer is it depends depends on what? song length Okay, say a typical song is three four five minutes The encoding or the format mp3 is Defines a way to encode the audio a song is audio to encode that into a digital form Okay, the audio is an analog is analog information sound is some continuously varying signal but with Music files we encode it as bits mp3 is one way to do that and again it applies compression What are some other formats mp3? flak m m4a Old-style WMA windows many okay many different formats they Even with mp3 there are different options different rates that will give different quality We'll return to some of those things in a later topic on signal encoding and look at how audio is encoded as binary Songs in the order of megabytes audio CD not a CD 700 megabytes Typically around 700 megabytes how much song how much how many minutes of audio on an audio CD on the old style The ones you buy the actual audio CD is not with mp3 on them, but a true audio CD 60 70 minutes if you measure the length of the audio Why? We'll cover that in another topic. We'll do the calculation of why can we fit 60 or 70 minutes on a 700 megabyte file? We'll not go through it now Let's let's do one more a movie how big is a movie Geobytes megabytes kilobytes What's it depend upon? Okay, good. I think people see this the similar patterns. You see the formats Which is the way that the data the information is encoded into binary and Whether it applies compression or not like what are some formats? What are some formats that you've heard of? mkv mp4 Maybe there are many others The size of the picture. What is a video a video is a Picture It's a photo That changes at a regular rate It's just a picture and then a few milliseconds later a new picture is shown. It just flicks through So it's just photos which have X or horizontal and vertical resolution Okay, and have a color depth each pixel But also the video change the picture changes at some frame rate Refresh rates so talk at refresh rates Formats resolution refresh rates impact upon the size Let's have a look at an example Where can I find a movie? movie YouTube correct But hard to measure the the information. I've looked up some data. Okay found a movie and I Didn't download it But it gives us some information about the movie. That's why I've chosen this website It gives us the size of the file About ten ten gigabytes Okay, the size of the fire But it also gives us It also gives us some information about that The video inside this file remember video is normally Video plus audio So there's an audio track at least one plus the video track. So we usually treat them separately in this case They say that the video gives us some information. Let's see if we can calculate The resolution If you cannot see at the back at small, I'll I'll write these numbers again in a moment. The resolution is 1920 pixels across 800 pixels down So each frame each each picture is that size It doesn't tell us the color depth We'll make an assumption in a moment as how many bits per pixel It also tells us the frame rate 23.976 frames per second about 24 frames per second. So the image changes 24 times per second So every second I think there are 24 images The duration of this movie is two hours 15 minutes So from that we can work out how many Pictures for the entire duration and then how many bits for the entire duration It also tells us the bit rate We'll come back to them in a moment, but let's just look at the picture and Calculate how big it is? Close this one Let's just make notes of what we have. This is the the resolution the number of pixels across and down we had a frame rate of 20 about 24 frames per second FPS frames per second where a frame is just one picture How many bits per pixel? Let's make an assumption that it's same as normal that it's a typical one is 24 bits per pixel Yeah, let's let's start with that so I Don't know the answer whether it's 24 or it's a lower value, but 24 is typical that is this is the number of pixels Okay, and then a typical color depth is 24 bits per pixel So what we want to know is how many? Bits in our movie and the other thing we had as our movie was two hours 15 minutes Okay, or 135 minutes. I Want to know how many bits to store this movie and I think Multiply multiply multiply multiply them all together and you'll get the answer that is how many pixels multiply these two together How many bits per frame? So this is one frame How many bits per frame multiply? These two by 24 bits per pixel. So we have the number of bits per frame. How many frames per second? Multiply by 24 again. These two numbers are not connected. It's a coincidence that they're both 24 Then we get bits per second. How many seconds? Well, we know the number of minutes so we can find the number of seconds and therefore get the number of bits Find my calculator again what we have 1920 by 800 24 bits per pixel 24 frames per second Yeah, we have 60 seconds in one minute and We know it was two hours 15 minutes or 135 minutes The answer will be the number of bits Okay, seven by ten to the power of 12 bits in this movie Convert it to a maybe a nicer number Convert it to gigabytes the gigabits first Giga is ten to the power of nine So that's the number of gigabits gigabytes divided by eight 900 gigabytes eight hundred ninety-five gigabytes Approximately so we took the total number of bits converted to Gigabits and then gigabytes Let's record it eight hundred ninety-five gigabits A gigabyte I'm getting confused Let's write it down gigabyte Uppercase B for bikes. That's the size of that image of that of that video But if you remember on the web page the file was much smaller than that Okay This is the fire if you think of the uncompressed video There's no compression the raw video the pixels across down each color and then every frame is displayed But the same with audio and images we can compress video and Actually compress it quite a lot For example if you have if you have a sequence of the movie which things don't change much Then each frame is almost the same as the previous frame so you can store that quite more efficiently So the raw video is 895 gigabytes. Yep. Yes, and this is just for the picture not the audio How about the audio? All right, you'll calculate the audio yourself but Generally, the audio is smaller than the video if we get time will calculate This is just the video part component. Where are we? but The file is 10 gigabytes not 890 gigabytes So in fact it's compressed and the file contains not just the video, but it also contains audio and In fact the information gives us a little bit more Another characteristic given here is the bit rate The video bit rate. There's another one for audio The video bit rate is 8851 kilobits per second This is think of this is after compression The number of kilobits per second of this video on average It's usually average because it goes up and down so after compression after encoding the average rate of this video is 8 0.8 megabits per second and The audio is 1.5 megabits per second Let's use those two numbers just to finish off This is the video only We didn't do it for audio compressed after compression or after encoding the video became 8 According to the website somewhere 8851 kilobits per second and the audio 1509 Kilobits per second That's what the website says anyway. Let's just check so 8851 kilobits per second how long for the entire movie we'll calculate 8851 kilobits per second how many seconds Well times 60 times 135 because that's the 135 minutes of video kilobits That's megabits gigabits divided by 8 8.9 gigabytes the video only compressed, okay 8.9 or about 9 gigabytes for video and while we're here The audio was 1509 kilobits per second Times 60 convert to minutes times 135 to convert to the entire duration kilobits megabits gigabits 1.5 gigabytes okay so audio about 1.5 gigabytes Video about 9 gigabytes total 10 10 and a half gigabytes, and I think if you check the file size It's it's about that Approximately the total and that I think the file size was 10 point something gigabyte So compression worked quite well on video. It goes from 900 gigabytes down to 9 gigabytes It's compressed by a factor of 100 So very efficient there Again, what's the size of a video depends upon the resolution the frame rate the pixels The color depth the bits per pixel and the duration and the format How much compression is applied? Any questions? Some of the calculations I went through quick, but I think you'll work them out with a few examples later and some quizzes Any questions? You can find for other examples. How big that piece of information is So if I want to transfer a movie from one device to another one computer to another I've got much more information to be able to transfer than say a web page or an email or just a photo So when we build communication systems, we need to look at well, what are the requirements? How much information do we need to transfer and? What do we expect in terms of performance? How long are we willing to wait for that transfer? Let's do a couple more slides just to finish for today different types of information sometimes we classify as well first information can be stored as in an analog or a digital form Many of our examples will look at a digital form, but we'll see later some more examples of analog data Audio is a good one video is also represented as analog data, but commonly today we convert it into a digital form When you talk on the phone If you're talking on your home Landline telephone line, you're sending analog data, but if you're using your mobile phone It's actually analog data going into the microphone on your mobile the mobile phone converts that to digital It sends it as bits. So there's a conversion process So analog voice calls radio music audio video And they can be either analog or digital video video conferencing video streaming And sometimes we distinguish a third type as the others We've got audio Video and data where data here means it's not audio or video Messages emails webpages files all the other applications that we exchange data What we'll do next week is look at How can we measure whether a communication system is good at sharing data? And we'll look at delivery accuracy and timeliness and then look at performance metrics like data rate delay Throughput and up a few others, but we'll do them next week. Okay, so we'll stop there