 A broad coverage, M.T. students want to join as well, that's okay. A broad coverage of network technologies and very broad, just mentioning some of the terminology of how we classify different networks and then mentioning some example network technologies but not going into detail of how they work. So mentioning the basic things like what are their names, what are their advantages and disadvantages against other technologies, what are the data rates that they offer in terms of wireless, what is the distance at which they transmit and so on. So that's what we'll go through in this topic. We aim to list and compare popular, future, in some case, technologies for different types of networks. Local area networks, LANs, and wide area networks, both wide and wireless access. So you're familiar with some of the technologies in use today. Not all of them and not how they work but just the basic concepts. First, what we're going to do is categorize networks by different criteria and then we'll go through some examples. So we have a computer network made up of devices or stations, whether it's end user devices like PCs, laptops, mobile phones, other devices like switches, routers, access points, and of course connected via either cables or via wireless links using different media and we connect them together to form a network. Well, there are many different types of networks and technologies to build a network. What are they? Well first, how can we categorize different types of networks? One way is based upon the geographical coverage. How large they are. That's one way to classify networks, categorized networks. And you've seen these or you've heard this categorization before because you've heard there's a LAN, a local area network, and a wide area network. The names tell us that one is larger than another. A wide area network is a network that covers a large geographical area, local area network, a local area. They don't tell us the exact size, but we can categorize different technologies based upon the coverage. There are some other names that you may have heard of or you may come across. Starting from the smallest size, body area networks. Networks that cover centimeters or meters. Body area networks referring to devices you may carry on you and they connect together. You can think most people today carry many different technical devices like mobile phones, laptops, tablets, but not just them. Headpieces, so a Bluetooth headset, maybe a watch on your hand. In the future you may be wearing different types of sensors in different situations, maybe not in the normal life, but if you're working in a particular job you may have different sensors. And all these devices can be useful if we can connect them together, form a network amongst them. So there are different technologies used for body area networking. What's one that you may know of? One technology that you know the name at least that may be useful for connecting across this very small area. Devices you carry with you. What's a technology that you may have used, some of you probably have used, that connects devices that you carry with you together? Bluetooth, where have you used Bluetooth? Or Bluetooth, often or sometimes used for a headset, connect to your mobile phone. So there's an example of a technology that's used for body area networking. So Bluetooth is commonly used for connections across several meters in a very small coverage area. If we go larger then we get local area networks. Networks for connecting devices inside a home, inside an office and even a little bit larger across a campus or inside a building between multiple buildings. So here on this campus we have two buildings. We can say that there's a local area network connecting all the computers on this campus. So that's the typical coverage area. Buildings, homes, offices, campuses for local area networks. Yes, we've changed rooms. So LANs, two technologies used for LANs nowadays. What are they? The cable and what's the name of the cable? The easier one that most people know, Wi-Fi or wireless LAN. That's the wireless technology. And what about the wired technology? What do we normally call it? Ethernet. Ethernet is a common name. And there are names. So for our wired LAN, we'll see this in later slides, but a common name is Ethernet. So when you plug the LAN cable into your laptop, into your PC, that's using the technology called Ethernet. It's a wired LAN technology. The specific standard, so for most of the network technologies we deal with, there are standards that define how they work. For Ethernet, a wired LAN, the standard is developed by the organization called IEEE. And the standard number is 802.3. So the common name is Ethernet. The general name is wired LAN, or simply LAN. The standard name is IEEE 802.3. And even more within this standard, there are many extensions, variations of it. We'll see some of them mentioned in later slides. And you also mentioned the wireless form. Wireless LAN is the general name, a LAN where we use wireless instead of wired. A common name or more of a marketing name is Wi-Fi. Generally refers to the same thing. There are other types of wireless LANs, not just Wi-Fi, but today most wireless LANs use the technology called Wi-Fi. And there's a standard developed by the same organization and 802.11 for wireless LAN. So there are the two common LAN technologies we have available. There are others, which are just the most common ones we'll see. Moving up to a larger area, we have different technologies for covering, let's say, a metropolitan area, a man. Not so common, but it's in use. That is, connecting office buildings across a city or an internet service provider providing a network connecting different points of presence in a city. So all across Bangkok, for example, that may be called a metropolitan area network and then a wide area network. And there's some... It's hard to distinguish sometimes between them. Some people would call a technology which is covering a city also a wide area network. But wide area network covering cities, connecting cities, connecting countries together, covering the globe. What's an example of a wide area network technology? Anyone think of them? Not so common, because we don't, as end users, we don't usually directly access them. We usually access via a wired LAN or a wireless LAN. YMAX is an example. You may have heard of that. It's a wireless technology for long-distance links. There's a link from this building to a building at Rungside for our other campus, providing a wireless link between our two campuses. It's considered a wide area network technology. And there are others. We'll mention several as we go through this topic. Depending upon the area that we want to cover, there are different requirements from the user's perspective. For a body area network, then what are we doing? We're transferring... Usually we need wireless technology to make it convenient. We're usually transferring, what, multimedia from a headset to a mobile phone. We're sending voice. Sometimes we're sending some amounts of data if we're sensing things. Very specific applications in body area networks. You don't use your body area network for downloading large files. But inside your LAN, you have different types of applications which are more common. And different requirements on the technologies. And in a wide area network, for connecting this building to the other building, or connecting Bangkok to Singapore, there's a different amount of data that needs to transfer across that network. Therefore, different technologies are needed. So there are trade-offs between the data rate needed, how fast we can send the data, the distance, and the cost. So let's go through some other ways to categorize networks, and then we'll see some examples of these technologies. That's based upon geographical coverage. So we can talk about networks being different, depending upon how large an area they cover. Another thing maybe based upon the users. A common classification is either an access network or a core network. An access network is one where the end users, you and I, use that network directly. Whereas a core network is a network that carries the data or traffic from a set of users between two points. Maybe best described by this diagram of some internet here, where we have the hosts as these computers, the things that you and I access to use this network. And in this simple case, we have clouds to represent some network technology, whether it's Wi-Fi, Ethernet, or some other technology, we don't care at this point. And those clouds are connected together by routers. And we have different networks connected together to form one large internet in this case. So this may be a wireless LAN network. And it's built for end users to use. And all of the end users on this access network send the data to this router. And then the data is sent, if they're destined to some people over here, the data is sent across this core network to the next router, across another core network, and eventually down to the destination access network. The difference between the access and core network in this case is the end users have direct connections to the access network. The core network carry the traffic from different access networks. The end users, the computers are not attached to the core networks in this diagram. That's the main difference in this case. And the technologies used for both access and core networks also may differ because there are different requirements for those technologies. This is a large internet. Then this network here, this core network in the middle, needs to carry traffic potentially from these users, from the users on this access network, and from all of these users may pass through this core network. So this must be designed to carry the traffic, not just of the users on one access network, but the users on multiple access networks. So when we talk about a large network, we often distinguish between access and the non-access or the core network. Other terminology used include a backbone network, the backbone of the internet, or a transport network, which is a bit confusing because we know about transport protocols, so try and avoid that one. But a core network or a backbone network carries the traffic of other networks. Think of your internet service provider. You connect, you as the customer of the ISP, you connect to their network via some access network, and then your traffic that goes out to the YouTube, Facebook, wherever on the internet, goes via the internet service provider's core network and via some other internet service provider's core network. And eventually to the US, eventually to Google's or Facebook's servers. So often the core networks correspond to networks owned and operated by telecom companies or internet service providers. Have we said some of this? I think we may see it if we haven't said it already. We'll see it when we give some example technologies. Another way to categorize networks based upon the transmission medium, wired or wireless. Some use wired, some use wireless. What's the difference? With wired transmission, it's easy to control where that signal carrying the data goes, where it's transmitted. And as a result, we can normally get better performance in terms of better higher data rate for the transmitting of the data. With wires, we can normally much easily protect our signals from interference from other people transmitting. When I transmit, I know it's not data communications or common one, when I'm transmitting video across this cable and transmitting via a USB cable, other data, they don't interfere with each other because using the wires, the signals are contained basically in the cabling. There may be a very, very small amount of interference. But if I transmitted via Wi-Fi from my laptop and via Bluetooth with my laptop at the same time, those signals may interfere with each other, leading to lower performance. So with wireless, interference is a problem. Generally with wired technologies, with the same cost as wireless, we get higher data rates, less errors, and a more predictable transmission medium. That is, more predictable may be that the delay is about the same all the time. The number of errors is about the same. Whereas with wireless, we have lower data rates, more chance of errors, we send something and it's received incorrectly or it's not received at all. And the chance that things vary over time. The data rate varies over time. The number of errors, the delay varies over time with wireless. It's less predictable. So why do we want wireless? Well, it allows mobility and it allows convenience. We can move and communicate at the same time, which we cannot do with wires attached, and even if we want to be fixed. So my laptop's fixed, it's not moving anywhere. Rather than having to plug in the LAN cable, just use the Wi-Fi, wireless LAN. It's convenient. What else? We can categorize networks based upon how the links are configured. Often we'll distinguish between a point-to-point link and a point-to-multipoint link. Point-to-point is simply we have one link, two devices attached to it, and they communicate direct with each other. Point-to-multipoint, we have multiple devices attached to some link. One can send, the others can receive that. That becomes important, or that the performance is impacted upon what configuration we use. We'll see when we look at Wi-Fi, this issue of sharing this medium in this point-to-multipoint communications, we have a medium, a link, shared between multiple users, more than two. Performance becomes an issue, and complexity increases. Another way we can characterize networks is whether they support users being mobile or not. Some are fixed networks, some are mobile networks. I think you understand the difference between the advantages and disadvantages. With a fixed network, the users or the devices attached to that network do not move. As a result, it's easier to design the network because you can predict how many people are going to be in the network. You can predict how much traffic is going to travel across that network. Therefore, you can design the network to meet those requirements. An example of the wired LAN in this campus. We know approximately how many PCs are in this campus. One in each lecture room, 40 in the labs, one in each office, plus some printers and so on. When we build the network, connect them all together, we know how many devices we can predict how much data they're going to send. Therefore, we can choose a wired technology that will carry all the traffic needed for all those devices. We can design it and meet the requirements. Now consider a mobile network situation. And the example is the Wi-Fi access in this campus. How many students are going to be using Wi-Fi in this room tomorrow? Anyone? I can predict how many PCs will be connected to the wired LAN in this room. One. Two at the maximum if I connect my laptop. How many devices will be using wireless LAN in this room tomorrow? Devices include phones, tablets, laptops. Cannot. It's going to vary. When there's no one in here, zero, when there's a lecture in here, again it's going to vary depending upon who's using the device at some point in time. When the lecture's over, back to no one using it. So as the users move around, the requirements on the network technology change. So for one hour, we may need a high-speed network to provide coverage for the 30 users in this room. And then at another time, we need it somewhere else because the users move. So the problem with mobile networks is difficult to predict the number of users and therefore the amount of data that's going to be sent through the network and what capacity is needed to carry that data. So it's harder to design mobile networks than fixed networks. Of course, the advantage of mobile networks is that we can move. A very quick coverage talking about how we can... Common ways to classify networks based upon the area covered, geographical area, based upon... What have we got? If I can remember, based upon wired or wireless, fixed or mobile, the point-to-point or point-to-moldy point, and there's one more that I forgot. Anyone remember? What's the other classification? Based on users, core or access network. So geographical coverage, users, that is, core or access, the link, point-to-point, point-to-moldy point, mobility, and fixed or mobile, and what was the other one? I still can't. The medium. Wide or wireless. So we're going to go through... We're going to go through some example technologies which fit within these categories. First, we're going to talk about wired network technologies looking at access network technologies. So the classification we're starting with is... Let's focus on wired first. And let's focus specifically on how you and I accessed the internet. And before we go through the technologies, we're going to go through the survey. Who has internet access on campus? When you're here at SIT, who can access the internet? Everyone. How do you access the internet? That's the security aspect of it. What technology, what network technology do you normally use to access the internet? Wi-Fi. Most of you would use Wi-Fi. Some, if you go to the lab, you're using the wired LAN, tablet, phone, you probably use Wi-Fi. So it's not a wired access, but that's one access technology that we use. What about at home or in your dorm? Anyone have internet access? Or when you're not at SIT, you don't get free internet. At home in dorm, Wi-Fi. And with Wi-Fi, I think you'd probably know there's your laptop and some access point or your laptop or phone. So maybe in your dorm, the building owner provides some access points throughout the building. It gives you internet access. So that's a one common approach. Any other ways for you to access internet if your building owner is not so nice? Okay, share the internet service. Is it wired or wireless? Wireless. Which... Okay. So you do similar, but... there's your laptop, and you have a shared, maybe in your room, a shared wireless router. And then that connects to some device in the wall probably via your telephone network. Maybe. Or via some cable network. So in some cases you'll use technologies like ADSL for internet access. So from your... house, then here's some modem and has a cable going out plugged into the wall which connects to the telephone network. Or maybe it's got cable internet access which is coming a little bit more common than Thailand. So we also would consider ADSL, cable internet access as an access network, in particular, wired access network technology. So even in the case where your building owner provides you wireless internet, what about from that building to the rest of the internet? What is the connection? Again, most likely the building has one cable going to the telephone network or to the... via cable network to an internet service provider. So we're going to look at these... some of these wired technologies here of how we can access via this point to the internet. Wi-Fi, ADSL, cable in some cases. Any other way you access the internet? Fiber to the home, okay? So similar here. This cable, usually we've referred to co-actual cable and nowadays also available optical fiber. So we have... coming into the home some cable. Telephone cable, a co-actual cable or maybe optical fiber. Coming into the home or coming into the building, okay? Anything else? Must be some other common way. When you're not in home and you're not in SIT getting free internet access, how else? Cellular, mobile phone. You... 3G, true. AIS and so on. This is your mobile phone for internet access, okay? So that's another common access technology. We're going to look just at the wired ones first and then later we'll look at the... we'll look in some detail about Wi-Fi and 3G, a mobile phone access. First quick mention, wired LAN, ethernet. And then we'll look at telephone access and then co-actual cable and optical. All right, we're talking about wired. We'll talk about wireless. We'll mention wireless as well, one of the common ways for the initial form of access from your PC to some modem or router is via ethernet, via wired LAN, okay? 802.3 is the name of the standard. But it's... it's varied over time. It was originally... so it's a very popular LAN technology. Originally it was point-to-multipoint but mainly point-to-point, a switched network. And the data rates... original ethernet was... original popular one was 10 megabits per second. Now we're up to a common... is 1 gigabit per second. And there are standards for... and practical implementations of 10, 40 and even 100 gigabit per second ethernet. For end-user devices, the main advantage or a main advantage of ethernet and why it's become popular is because the devices and the network to configure it has been easy and cheap, okay? So the end... the network interface cards, in the past you had to buy them separately for your PC and plug them in. You even had to buy separate cards for your laptop at some stage. But now most of the... the LAN interfaces are built on motherboards, small chips, very cheap, very easy to install. And the cabling is generally easy to install. The cabling... do we have a cable? The cables, these LAN cables here, because they bend very easy, it's... as opposed to some rigid, strong cable, it's very easy to lay them wherever you like. It's very easy to put them through the wall cavities, up through the ceiling and so on. Whereas if you have some... cabling which is much more rigid, so shielded cabling, it's harder to bend them and to install them, it's much more difficult. Similar with optical fibre. It's much harder to deal with optical fibre. For example, cutting the cable compared to using these LAN cables. So that's an advantage and a reason why it's become so popular today. And because everyone uses it, people want to keep using similar technologies, they keep improving upon it. And that's why we get these higher data rate technologies. And also because of its popularity, mainly in end-user devices, everyone's laptop has a LAN port, everyone's computer. Most offices have a switch and so on. It's been adopted to non-LAN applications. That is wide area network applications. Now internet service providers, large companies use Ethernet to transfer traffic across cities and between cities. So a wide area network. Especially the 10, 40 and 100 gigabit per second technologies are designed mainly for wide area networks, not so much for LANs anymore. We're not going to talk how it works, just a reminder of what it is. I've got a picture somewhere. What's this? This is a picture of our five layer stack. That's not what I want to show you, but for those who can't remember from previous courses, you don't have this, don't worry. When I talk about data communications, we normally talk about a, from perspective of a protocol architecture or a layered stack. And there are different views of the layered stack. When I talk about layers, I use this stack, these five layers, starting from the bottom being the physical layer, dealing with the data, the transmission of signals, data link layer dealing with getting bits and frames across a link, the network layer dealing with getting data across a network, across multiple links, transport for getting data efficiently between applications, and the application layer protocols to support specific applications. For example, HTTP supports web browsing, SSH remote login, SMTP email. That's not the point about Ethernet. That's just, if I talk about layers, we use this five layer stack. Have a different picture. Where, okay. Again, you don't have these pictures, but if you need them, I'll provide links to, they're on the website already. The standards for wired and even wireless lands, the most common ones in use today are created by the same organization, IEEE. It's an organization that creates standards for many things, for many electronics and electrical devices. And within the data communications and specifically for local area networks and some wide area networks, the standards are named 802, 802. And then for the different variants, 802.3 for wired LAN, Ethernet, 802.11, and there are many others, up to 20-something. 802.3 for Ethernet, 802.11, 802.16 for YMAX, Bluetooth is in there somewhere, 802.15, and there are others. So that's just about this organization, IEEE, that creates standards. Again, there's a... This is the one I was looking for. Again, no need to copy this down. There's a link on the website to a page that includes this diagram that explains the later Ethernet technologies, 40 and 100 gigabit per second. The main point here is that Ethernet, wired LANs, has improved over time. First standard, which is 30 years ago, was called Ethernet, provided a data rate of 10 megabit per second. That was the original 802.3 standard. Ignore this column. It was improved, and the most common one and still in use today is fast Ethernet, 100 megabit per second. Most likely when you plug a cable into your laptop or PC, you're using 100 megabit per second fast Ethernet. Ignore this one, not so common. Then 1 gigabit per second Ethernet. What does my laptop support? Most new devices today support up to 1 gigabit per second. In fact, they support multiple speeds. They support the latest 1 gigabit per second, as well as they can drop back to the slower speeds. Where can I see it? In my laptop, I can look at the characteristics of my Ethernet interface, using this program called ETH Tool. It provides us many details about the LAN card in my laptop, the wired LAN card. The things that I just want you to recognize is that the supported link modes, especially 10 base T, 100 base T, 1000 base T, 10 megabit per second, 100 megabits per second, 1000 megabits per second. The base T is just a specific standard. My laptop supports all three data rates. When I plug a cable in, the data rate used depends upon or is the maximum that the two end devices support. If I plug a cable into my laptop and connects to a switch, if that switch supports 1,000 megabits per second, they'll most likely use that speed. But if that switch only supports 100 megabits per second, my laptop will drop down to 100 megabits per second. Most switches in offices and in SIT support 100 megabits per second. Some will support 1 gigabit per second. What else do we get to? Where's our pictures gone? And there's 10 gigabit per second Ethernet, not so common in end user devices, but connecting network devices together. One switch to another switch, one router to another router. 10 gigabit per second Ethernet. And more recently, they've been improved to provide 40 gigabit per second and 100 gigabit per second Ethernet. These two are mainly used for wide area networking or very special cases of LANs. They're not so much designed for end users like you and I. They're designed for connecting data centers together, for example, when there's a large amount of traffic to send at 100 gigabit per second, only when there's a lot of data to transmit. It's not so useful for end users, but important when there's data centers, internet service providers connecting different customers' networks together. They're the newest technologies. What does your tablet support in terms of speed? I know it's not wired, but what would your tablet support in terms of your iPad? What data rate? How does it compare with these? You don't know. You bought this tablet and you don't know how fast it is. You bought it for the color, didn't you? In the order of? With wireless LAN, we're talking about the order of tens of megabits per second. Still around here. 54 megabit per second is one common one, but 802.11n supports more than 100 megabits per second, up to 300, maybe even 600 in very special cases. With wireless LAN, the most common data rates are around here. We'll also see later, when we talk about wireless LANs, there are newer technologies that provide up to gigabits per second. Generally, the same cost, wireless, lower data rates than wired data rates. How does Ethernet work? Some of you may be studying what is it, ITS 327, this semester with Dr. Conwood, and you may discover the details of how it works. We will not. How else do we access the Internet? A very common form for accessing networks, accessing the Internet, is via a telephone network. It's common because telephone networks have been around for around 100 years. So 30 years ago, 20 years ago when the Internet was not so popular, but telephone networks existed, and then as users wanted to access the Internet, get on the Internet, how do they connect, will make use of the existing telephone network to access the Internet. So the telephone network is a common way for Internet access. And it's still used. The telephone network, which connects across countries, is referred to as the public switch telephone network. So talking just about the telephone network for phone calls, not for Internet access, but for fixed landline telephones, PSTN, the public switch telephone network. Sometimes you'll hear of, or you'll come across the acronym POTS, Plain Old Telephone Service. That is the service of giving someone the ability to pick up the phone and make a phone call. So this service provided to the users of making a phone call is called POTS, or the Plain Old Telephone Service. Of course it's been extended to provide other services like Internet access. From your home, when I say home it includes dorms and other buildings as well, but from your home, the main form for connecting to a telephone network is via a twisted pair copper cable. So you have a cable and you have copper wires. And those copper wires come in pairs and they're twisted around each other so to shield them from interference from other pairs and other transmitting devices. So an example of twisted pair, not just for the telephone network, LAN cables also use twisted pair copper wiring. So if you cut this open and look inside you'll see the copper wires in there. It's the same cabling or the same wiring as used in the telephone network. So from your home to the public switched telephone network copper wiring is used. And most homes in many countries, not in all countries, but many homes before the internet came along already had telephone access. Fixed telephones. So with the wide availability of telephones everyone has a telephone line to their home that it made sense to make use of those telephone lines for Internet access. And the first main use we referred to as dial-up Internet access. Anyone still use it? Anyone remember using it? Dial-up Internet access, the noise, the modem creates some connection, makes some noise and you could send data how fast anyone remember? 56 kilobits per second was the limit speed limited by Nyquists basically. 56 kilobits per second. That was the data rate supported by dial-up Internet access. It was improved over time. One improvement was because as people wanted to send more and more data via the Internet they needed higher speeds for voice for video over the Internet. One was called ISDN and it had services digital network and it provided 64 kilobits per second, 128 kilobits per second but it didn't become very popular. In some countries in Germany it was become popular for some time but something else took over DSL technologies, digital subscriber line technologies. They still use your telephone line but now with ADSL we can get data rates not 56 kilobits per second but up to 24 megabits per second. 24,000 kilobits per second. Let's have a look at some of those technologies. Some very basics of the network. What's the telephone network look like? The public switch telephone network we have so this is just the telephone network not Internet access. The old telephone network we have your home from your home you connect to a local exchange a telephone exchange and the cable there is your home the twisted pair copper wiring going into the exchange and everyone in your neighbourhood connects to the same exchange so there are many wires going into this telephone exchange and of course there are exchanges all across the country providing service for different areas and when you want to call and talk to someone in another city then you make a connection via your line to your local exchange and there may be a hierarchy of exchanges so there may be a central exchange for Bangkok and the local exchange connects to there and then up to some Chiang Mai exchange and then to a local one and then for the person who you're trying to call there's a link to their telephone so there's cabling between the telephones and the exchanges and between exchanges the exchanges use switching to connect these cables together when you make a phone call there's some signaling that goes on such that when you call this person the network knows that you need to connect via this local exchange to the Bangkok central exchange to the Chiang Mai and then to the local exchange here and it connects those conceptually it connects those lines together when you make another phone call and call someone else in another city then the connection would go via the appropriate exchanges so the switching is used circuit switching in particular is used in the telephone network these are still copper wires copper cabling between exchanges originally it was copper wiring this usually coaxial optical fibre satellite links between exchanges now how do we get internet access this is for making a phone call dial up internet access we use the same telephone network but there's some internet service provider they have their own network and they connect via some router to the larger internet so there are two different networks and there's the internet and your internet service provider connects them together for you so what you do from home is not with your telephone but now you have your computer connected to a modem computer connected to the modem and the modem makes the telephone call across the network it connects to your local exchange it doesn't call some other user it calls your ISP your internet service provider has a special device to receive the connection from your modem so essentially you make a telephone call from your home modem to the ISP these students enjoyed this class so much last year that they were coming again but we use the telephone network to connect to the ISP and then that ISP the internet service provider connects out to the internet so your data when you're accessing some website goes via the telephone network to your ISP and then that sends it out via a packet switch network to the destination on the internet this is dial up internet access what the modem does is converts the digital data from your computer into an analog signal to be sent across the telephone network your telephone network carries an analog signal the telephone system limited the bandwidth to 4 kilohertz 4000 hertz and you can calculate using Nyquist theorem that with we... you remember twice 2 times the bandwidth 2B log M the number of levels that is if we have a bandwidth of 4 kilohertz the maximum data rate we can achieve is 2 times that multiplied by log of the number of levels in a simpler case if we have a bandwidth of 4 kilohertz we can get 8 kilobits per second the point isn't to remember Nyquist the data rate is 2 times the bandwidth log of the number of levels in our signal M the telephone line has a bandwidth of 4 kilohertz where you told me that your data rate supported 6 kilobits per second and therefore you can calculate how many levels was used by that modem to send the signal if we have 4 kilohertz bandwidth what is it? M equals 128 128 log of 128 in base 2 is 7 7 times 4 times 2 is 56 28 times 2 the 4 kilohertz was a limitation of your telephone line the telephone line carries signals with a bandwidth of 4 kilohertz and what your modem does is converts some digital data into an analog signal with a bandwidth of 4 kilohertz and that signaling of converting the data to analog if it uses 128 levels then we can get a data rate in theory of 56 kilobits per second 128 levels at that point in time was about the best that they could do without getting extra noise or errors on the system if we went higher we get a higher data rate but the performance because of errors was much lower so that's why our limit of our data rate was around 56 kilobits per second with our dial-up modem the telephone system limited our bandwidth and therefore limited our data rate not so useful nowadays but for a second so the thing that took over dial-up access for home internet access was digital subscriber line DSL your telephone system used 4 kilohertz bandwidth for your voice call and dial-up internet access only had that 4 kilohertz available and that's why there was limited in the data rate but it turns out the copper wires support a signal with a bandwidth of larger than 4 kilohertz much larger 1 megahertz is the typical bandwidth supported by copper wires so the copper cabling actually can transmit a signal with a bandwidth of 1 megahertz 1000 kilohertz the telephone normally uses just 4 kilohertz because when you talk you only need about 4 kilohertz dial-up only made use of that 4 kilohertz what DSL does is makes use of the rest of that bandwidth the remaining up to 1000 kilohertz so this is some representation of the spectrum or the bandwidth of our signal that can be sent across the copper wires we can send signals from frequencies from 0 up to 1000 kilohertz with DSL the first portion of that spectrum is reserved for the telephone the voice calls we only need 4 kilohertz but we include some spacing in there so up to 20 25 kilohertz that's the signal for sending the voice over the telephone network then the rest is used for sending our data our internet data and with ADSL ADSL we divide the spectrum into frequencies used for uplink and downlink upload and download from your home and with ADSL it's asymmetric they're not the same we see here the upstream or uplink we have what about 175 kilohertz bandwidth whereas for downstream we have 750 kilohertz in this example so with dial up we have 4 kilohertz bandwidth available for our voice or and remember with dial up internet access you could either talk on the phone or use the internet you couldn't do both at the same time when you're using the internet with dial up someone could not call you so you could either use voice or use that 4 kilohertz and convert to digital say 56 kilo bits per second for your data your internet access with ADSL and more generally we talk about DSL digital subscriber line technologies we have still our 4 kilohertz for voice and because we don't just use the 4 kilohertz we use the full 1 megahertz available for the copper line we use some portion of the spectrum for uplink in this example it's around 175 kilohertz it differs in different technologies uplink or upstream and at the same time here we've got 750 for downlink is from your device to the internet service provider downlink is from the ISP to your modem note here we have the voice or data via your phone with DSL we can make a voice call and we can be uploading data and downloading data at the same time because we divide the spectrum into those 3 portions we use frequency division multiplexing and with ADSL the A means asymmetric it's asymmetric in the uplink and downlink they're different and as a result we get different data rates with uplink and downlink this is just an example of ADSL the split here is not the same in all technologies in all cases just to show an example so you can make a voice call and transmit data at the same time and again there are different signaling schemes used for the data transmission we still have an analog signal here the data we want to send is digital so we convert at your modem your ADSL modem converts the digital data into an analog signal using some signaling coding scheme and that's sent across the telephone network we have a link picture here's our example network topology here's your home your computer you have an ADSL modem and your telephone attaches and we have this special device a filter we'll talk about in a moment and from your home we have this is the telephone line coming out of your home and it goes to some telephone exchange a local exchange inside that telephone exchange is a special device to receive the ADSL signal an ADSL multiplexer or a DSL access multiplexer a DSLAM so this is operated by the internet service provider inside the telephone exchange if we go back to here that's at this point so we connect from your home via the telephone network to the ISPs device which is normally inside one of the exchanges so we shift that to inside an exchange and the ISP also has the connect you can connect to the other telephone exchange via the public switch telephone network but the ISP importantly has its own network and connects out to the internet so when you make a phone call here your signal just goes across the telephone network and out to someone else's phone when you are downloading and uploading data over the internet the modem converts that digital data from your computer into an analog signal and sends that across the telephone network it goes to the DSLAM here which converts it back to packets to digital data to be sent across the internet via the ISPs network so the voice goes via the public switch telephone network the data, the internet data goes via the ISPs network what's the filter for? the filter is just to separate out any interference from the the signals from the computer on the telephone the voice signal to make sure that the signals from the different portions of the spectrum do not cause any interference on each other you can get problems if you don't use a filter in that case you can get data and voice access at the same time using the same cable, the access cable from your home anyone have ADSL internet access in dorm or home no one yes do you know how much it costs per month 600 baht a month and you know the speeds that you get per second down downlink and uplink maybe about 1 megabit per second it depends so there's a typical example 600 baht a month for ADSL 13 megabit per second downlink and 1 megabit per second uplink asymmetric because the downlink has a much larger bandwidth available than the uplink so it's very common about home internet applications or at least it did in the past so very common today not just in Thailand but in many countries across the world for home internet access there are other DSL technologies not just asymmetric there are others we will talk about them tomorrow any questions on very network technologies how we can classify them mainly useful for the later topics when we talk about the individual technologies and we're going to go through briefly how do we get internet access and we've mentioned dial up ethernet and now briefly ADSL