 We finished talking about first dial-up internet access, so we've got our telephone network, which is made up of our homes and telephone exchanges. So we connect to a hierarchy of telephone exchanges across the world really. And for the most, the first popular internet access was to use that telephone network where you essentially make a phone call to your internet service provider. So using your computer, which connects to a modem, that modem makes a phone call to an ISP's device at an internet service provider. So it's a normal phone connection here. And because it's a normal phone connection, we were limited to just a 4kHz bandwidth for sending our data, and that limited our data rate. But it turns out that the copper wire from your home to the telephone exchange can transmit signals greater than 4kHz bandwidth. The telephone system, when we make a voice call or using dial-up access, we only use 4kHz of bandwidth all the way from source to destination. But it turns out this first segment, which is from your home to the exchange, the copper cabling can support larger bandwidth signals up to about 1 MHz. And what this diagram shows is here's the spectrum of some signal that we transmit from 0 up to 1,000 kHz. It shows that when we use ADSL, or in general digital subscriber line technologies, we transmit a signal across our copper wire. And that signal occupies a portion of the spectrum, and we have three different blocks or three different components. One portion of the spectrum is used for sending our voice to make a voice call, which is the normal telephone service or the plain old telephone service. The first 10 or 20 kHz are reserved for that. But our copper wire can support a larger bandwidth signal. And so we use other portions of the spectrum, this range of frequencies, to transmit data from our computer up to the internet service provider, upstream or uplink. And also at the same time, the internet service provider is using the same copper wire to send a signal down from the ISP to your computer, the downstream signal. And we do all of them at the same time, so we can make a voice call, be uploading and downloading data at the same time using DSL. ADSL, asymmetric digital subscriber line, we have a symmetry between uplink and downloading. In most, or most past internet applications, that's well suited for what most home users or consumer users do on the internet. We usually download much more than we upload. You think about web browsing at least, a main internet application. What do we upload with web browsing? We send a request, a small request to a web server, that web server sends back a large page, which is then viewed on our screen. The amount that we send up from my computer to the ISP is usually much smaller than what comes back from the ISP to my computer. So this asymmetry between having a small upload speed and a high download speed is well suited to web browsing. And that's one of the reasons why ADSL become popular. Many internet applications, we download more than we upload. There's some other reasons why ADSL, it's easier to have a larger download data rate in terms of implementing at the ISP center. The multiplexer is easier if we have more download bandwidth and ADSL has become one of the main forms for internet access, home internet access and internet access in some case for buildings. The main feature that made it popular is that it uses the existing telephone network. In many countries, many people have telephone lines. So to make use of that existing infrastructure, as opposed to having to install new cables to a home is much better to use the existing infrastructure. And the bandwidth available for the copper wire and as a result the download and upload speeds is suitable for basic internet applications, basic voice and video applications. We'll talk a little bit later and see some alternatives. Once we start to have applications that require very high data rates, there may be streaming of multiple high-definition TV channels. There may be ADSLs not suitable, but for most basic voice and video applications and many web-based applications, it's okay. High enough speeds. ADSL is one instance or one case of a digital subscriber line technology. Generally, there are different types of DSL. ADSL is the most popular one. In fact, ADSL itself has been improved over time. It was the original ADSL, supported some example downlink and uplink data rates of up to about eight megabits per second down. And at the same time, 800 or close to one megabit per second uplink with ADSL. But it's been improved over time and ADSL 2, ADSL 2 plus reaches about, or reaches 24 megabits per second downlink. And around three and a half megabits per second uplink using ADSL 2 plus. So by upgrading the equipment in the exchanges, from ADSL to ADSL 2, ADSL 2 plus, potentially the home users can get faster internet access. Note, in this diagram, here's our home, here's an exchange. This is a local exchange and for ADSL to work, there's some special equipment installed in the local exchange, which terminates our ADSL connection. Going back to dial-up, from our home to the ISP, we use the normal telephone network. The ISP doesn't need to be local. With dial-up internet access, I can call an internet service provider in Australia and use their internet access. It's going to be expensive because I have to make a phone call to there, but it's possible. So with dial-up internet access, we essentially make a telephone call to our ISP, which can be anywhere in the world, the ISP. And that ISP has some special equipment to receive that phone call and then send the data out to the internet. With ADSL, it's different in that the internet service provider needs to install some special equipment, a DSLAM, a multiplexer, inside a local exchange. If your local exchange doesn't have this equipment, you cannot get ADSL internet access. So what ISPs have done is they've gone to the telecom companies and said, we want to install our equipment in some telephone exchanges so that customers can get faster internet access. So this is from our home to a local exchange. And then from the local exchange, the ISP has their own network, maybe using coaxial cable, optical fiber, out to the internet. There are other types of DSL. Some of them are listed here, high data rate, DSL, symmetric. So it doesn't have to be asymmetric. We have the same upload and download speeds. Some examples listed here. HDSL, 1.5 megabits per second, up and down. SHDSL on improvement, 5.6 megabits per second. Not so much used for end or home users, more for businesses that want to connect offices together. And VDSL, 100 megabits per second. But the difference, one of the differences, or the three main trade-offs are, we have different data rates, higher data rate the better, but we also have different distances at which they cover. And of course, there's also a different cost in installing the equipment. Looking at the distances, let's look at HDSL first. You don't have this, there's a link on the website, but it's just taken from some ISP's information. It's a bit old, five years old, but it's still relevant today. It shows on this axis the distance from your home tele-telephone exchange, the local telephone exchange where the HDSL equipment is, and the data rate that we can achieve. Focus, let's say, on the blue one, which is the maximum limit for normal HDSL. What it shows is here, if you're close to the exchange, here's 100 meters, so if your home is close to the telephone exchange, you can reach the maximum data rate for download of eight megabits per second. 1,300 meters about the same, up to about 2,000 meters, two kilometers from the exchange, everyone within two kilometers of the exchange can get the maximum data rate of about eight megabits per second. But as you go further away, the data rate drops off. So if you live five kilometers from the telephone exchange, then you may not be able to get a connection. So the technology works better over shorter distances. And we see with the green one, HDSL 2+, goes up to 24 megabits per second. But only if you're close to the exchange within one kilometer, here 24, but only in the first 1,000 meters do you get that speed. If you're living, say, three or four kilometers away from the exchange, then your speed drops down to the same as HDSL 1. So the speed drops off rapidly in that case. So there's a trade-off in terms of the speed, the data rate, and different technologies have different costs as well. With VDSL, we saw there's a data rate of 100 megabits per second, but the distance at which you get that is within hundreds of meters. Not many people live within 100 meters of a telephone exchange. Most people are within, well, a typical, to be in a city or a town to be within several kilometers of a telephone exchange. So HDSL is okay in towns and in cities. But not many people live within 100 meters of a telephone exchange. So VDSL is not used in the same manner as ADSL. It's mainly used as an extension if you have some coaxial cable or optical fiber nearby. We'll talk about that later. Any questions on the basics of DSL and ADSL? Again, we don't cover how it works, how the protocols work, how the signaling works, just the features. The closer you are to the exchange, the faster the potential data rate. And it's not just about distance in practice, it's more about the quality of the wiring from you to the exchange. So if you've got very old telephone lines from your home to the exchange, then the data rate may drop off as well. So it's about the quality of the signal that can be sent, which normally depends on the distance. So that's one form for internet access, common form. Use the telephone network. Another one, use your cable TV network. Most people, many people have telephone lines coming into their home, but in some places people also have cable TV in that there's coaxial cable coming into the home. So as an alternative, make use of that existing cable coming into your home and use that for internet access. Cable TV uses coaxial cable. Typically they support bandwidths of up to, probably close to one gigahertz, so 600 megahertz, maybe 900 megahertz in some cases. So with cable TV, you can send a lot of data across that cable. They're built for television, where some company offers 10, 100 TV channels to watch. But because there's a lot of bandwidth available, it makes sense to also use it to send data up and down, so for internet access. And again, it avoids having to build new cabling from your home. Cable internet access, so it uses the existing cable TV network. Normally they are designed such that within a neighborhood or within a street, the network is shared amongst the users. With ADSL, everyone has their own individual telephone line to the exchange. So that telephone line is just for your use. But with cable TV and cable internet access, often the cable from, say, one street or one small neighborhood is shared amongst all the people who live in that area. So the more people who are accessing and subscribing at the same time, the more you need to share with and the lower the performance each individual user will get. That's one of the issues with cable TV. It's a point-to-multipoint topology. There are multiple users attached to the link, to the network. The more people using at the same time, the lower the throughput that you get. That's not the case with ADSL. The most common standard used for cable internet access is data over cable service interface specification, DOCSIS. And there have been variations, DOCSIS 1, 2 and 3. DOCSIS 3.0 is available now. And these are some example data rates of down link and up link. Six megabits per second, 30 megabits per second, down one megabit per second, up. And nowadays, in fact, with DOCSIS 3, it's common to get 100 megabits per second down and say 30 megabits per second up. And there are other variations as well. But in some countries, 130 is common as well. Typically faster speeds than ADSL. We saw with ADSL 2, we're looking at 24 megabits per second download, here up to probably 100 megabits per second and it's improving to go even faster. But shared amongst multiple users. So it depends upon how the network's built as to how many users you share that with. It doesn't use the telephone network. So one of the things you can do is use the cable for TV, for internet access and use something like voice over IP or simply Skype to replace your home telephone line. Or you just use your mobile phone. So you may not need the telephone line, which can be cheaper in some cases. You don't have to rent that telephone. Any questions on which one's better? And the trade-offs between the two. Try and understand that it's mainly about the trade-offs. It's not one is better than the other, it's that some have advantages and disadvantages. So so far we've looked at just ADSL and cable access. Co-actual cable potentially higher speeds, although shared, they use two different networks. DSL uses the telephone network, cable internet uses the existing cable TV network, usually provided by different companies, different operators. I wanna try and draw a picture that we'll see how successful we are. We have a new toy and I'm gonna try and draw a picture on here. And as a result, you don't have to copy it down and we have some more space on the board. But my drawing skills are quite limited at the moment. Let's try it. Let's, here's our home, okay? And we have our telephone line to some telephone exchange for ADSL access and I'll press the wrong button. See if I can write. And then we have a voice and potentially for our internet access or IP. The idea is that you don't have to draw that in that I'll upload these pictures to the website but we'll see how beautiful they look. So our ADSL internet uses our telephone network. So it assumes we have a telephone line to the home. But in some cases we may also have cable TV, a separate line coming into the home. So that would go to, the cable for cable TV would go to some other equipment operated by the cable company which then connects out to the internet. So here's our internet here and we've got two different options at the moment. So using a telephone line or using cable TV. Now it depends upon where you are. Not all homes or in all countries is it widespread that there's cable TV. In some cases it's just microwave or satellite. In some cases cable TV is more widely deployed than others. The important thing is that both of these internet access technologies use existing networks. To give someone internet access if they don't have a telephone line, if they don't have cable TV, the only real option in practice is wireless access. To deploy new cables into a home and not just into one home but into many homes is very expensive. So the idea with internet access is to make use of the existing networks and reuse them for internet access. Building new networks is expensive. Especially if you want to cover 10,000, 100,000 homes, deploying cables to all of those homes, digging the holes, putting the cabling in is very expensive. So both of these use the existing technologies. What other choices do we have? If we don't want to use cable internet or ADSL, what else can we use? Some of you may have mentioned yesterday optical fiber. So a third network that deploys optical fiber into the home and into the internet. So another network, another technology. In this case, not many people have optical fiber in the home. It's only in the last five or 10 years that companies have started to deploy optical fiber to the home. It's expensive to build the network. That's why it's taken time. Any others? We'll go through optical fiber briefly in a moment. Any others or internet access to the home? I can think of two or two common ones and probably others. Don't use cabling at all. Use wireless and the main other option. These three are available. If you don't use them, the real options you have are wireless. Either using mobile phone technologies, 3G, GSM, we'll talk about later. Wi-Fi in some cases and satellite is another main one. Especially outside of cities in the country in rural areas where either the telephone exchange is a long way from the home or just not available. In rural areas, there's no cable TV, no optical fiber, it's too expensive to cover everyone with optical fiber. So either mobile phone access, 3G, which is generally slow and expensive, or satellite internet access. So the other options are wireless access. Optical fiber is the third one that we drew there. Optical fiber currently is mainly used inside core networks. We're talking about access networks, but it's in the last five or 10 years that optical fiber has been started to be used in access networks. That is, go directly to the end user, to the home, fiber to the home, that's an example. It's possible if we don't have copper or coaxial cables or as well as using the telephone network, you commonly called fiber to the home, FTTH, or to the premise or to the building, if we're talking about to an apartment building. Also uses a point to multi-point topology. Usually, for example, fiber to the building, there's a fiber coming into the building and then that is shared amongst all the people living in that building. Speeds, they're in the order of 100 megabits per second up. So compared to the other two, faster. Typical speeds, it depends upon the operator, maybe 30 megabits per second, but they usually can easily support 100 megabits per second and can even support one gigabit per second. So the speeds for download are much faster than ADSL and coaxial cable. Much higher bandwidth and therefore much higher data rate. And as a result, you can support a combination of normal internet access, voice, video, and high definition video because of the high data rates. The problem with optical fiber is it's expensive because we usually need to install the network. In most cities, the optical fiber is not deployed to homes and buildings yet. So companies need to dig up the holes, lay the cable, and deploy the network and that's an expensive operation. So it's only just starting to happen in some countries. So three main wide access technologies. The main ways for people to get internet access. DSL, coaxial cable, optical fiber. Any questions about that? Does anyone know how much optical fiber access is? Some places, just certain areas in Thailand provide it. Some coaxial cable is more widespread, probably ADSL was the most widely used in South Thailand. In Japan and Korea, optical fiber to the home has been widely deployed and is cheaply available. After the midterm, we'll come back and look at voice over IP and IPTV. And when we talk about IPTV, we'll return to optical fiber and some of these acronyms like fiber to the home, fiber to the premise, fiber to the building, and there are others, fiber to the, these normally mean the optical fiber is laid direct to the building. But that can be expensive. So there are alternatives like laying the cable to some street and then using, for example, VDSL, copper wiring to go to the individual homes on that street with difficulty. An example or alternative is if we press the wrong button what do we do? We use our copper to some special device and then optical fiber and then the internet. So this is fiber here and this is say VDSL. Here we have a short distance from our home to some special device on the street, for example. So along some street there's some cabinet and there's copper wiring from your home using the telephone network into that cabinet because it's only over 100 meters or so, VDSL works well there, providing a data rate of 100 megabits per second. And then from that device, optical fiber to the network operators network and then out to the internet. As opposed to getting fiber direct into the home, which is more expensive. So there are variations of using optical fiber. And we'll return to some of their names and the differences when we look at IPTV. Over the semester I hope to improve my drawing ability so I don't look like a three year old. But we'll see how we go. So in summary, what have we got? Ethernet is not so much used to the home but used within the home for access. In some rare cases you can get Ethernet to the home. But mainly for internet access to a home or to a building, we have using the telephone network and today using DSL, using a cable TV network, co-actual cable network, cable internet access, or with networks recently built providing optical fiber to the home or near to the home. And there are differences in terms of costs and data rates, speeds or performance. We're still talking about mainly wired technologies. Let's look at some wired core network technologies. That was about to the home. What about in our picture? Wrong way. What about in here? I draw this cloud to represent the internet which connects across the globe. What technologies are used inside here? The core network. And then we'll look at wireless including Wi-Fi and 3G. Different approaches for core networks. In the past and still today, because again telephone networks have been around for many years, say telephones have been around for 100 years. So the networks have been built. So when people wanted to start to use sending data instead of just making voice calls, it made sense to use the same networks for their data, for internet access and to use the same technologies. So inside core networks, there are a number of technologies based upon what's called more telephone, based upon the telephone networks using digital circuits. Circuit switching. Least lines. And we'll look at briefly PDH and SDH, the digital hierarchies. Usually using a point-to-point topology. So from one location to another. An example of where we see this if we want to connect our campuses together. So over a distance of, say, 15 kilometers from this campus to the Rungset campus, we want a connection between them, all right? Wireless connection is possible. Or what we've done in the past is we go to a telecom company and say we want to lease some of your network to get a direct connection, a point-to-point connection between our two campuses. And we pay them per month and usually a telecom company has a network already built across the city. They have the cabling in the ground and they provide us a connection via their network using a technology we mentioned briefly, PDH or SDH, depending upon the speed we want. Using the telephone network. Alternatives. There have been different packet switching networks, wide area network technologies being deployed over time. In the past, maybe 20, 30 years ago, X25 will just mention briefly the characteristics of that frame relay and nowadays ATM, not the money machine, asynchronous transfer mode. And of course, nowadays, we mainly use IP networks, new networks built in the core using IP and optical fiber. There's also wireless networks which are available. So this is the core of the internet, how's it built? They cause telephone networks have been around, then the telecom companies started to use them to carry data traffic, not just voice traffic. And they used similar technologies for what was built for voice traffic. Voice, we know we carried it in a bandwidth of four kilohertz and the basic form for sending data across those, if we convert that analog to digital, we convert it into 64 kilobits per second. We saw yesterday with our dial-up mode and we can get 56 kilobits per second with 128 levels. Well, typically we talk about multiples of 64 kilobits per second in telephone networks. We'll see them come up in the next few slides. So companies that provide a telephone network started to use them to provide data access. One of the popular technologies is called PDH. What is it? It's a hard one to say. PDH, Plesionchronous Digital Hierarchy. We'll see SDH Synchronous Digital Hierarchy. Something about the synchronicity of when we send the signals. Again, we will not cover how it works. I don't remember any of the details of how it works. I've started at once. But just some examples. And you may have heard some of these, the T levels or the E levels. When you go to a telecom company, you say we want to, as a company, SIT, we want a connection between our two campuses. We say, okay, and we need eight megabits per second, then that telecom company deploys a circuit, using circuit switching between the two campuses, using one of these standards. In this case, probably, what's called an E2 line. It supports a data rate of eight megabits, or 8.448 megabits per second. If you want higher data rate, then you've got the option of going up to the next level. It's a hierarchy of levels. There were some differences in the US or North America and Europe. So the different areas developed varying standards. So you see different data rates, the T line, T1, T2, T3. Used mainly to connect between sites inside cities and even in the past between cities. And usually leased or rented from a telecom company. And they are dedicated. That is, that entire bit rate is dedicated for the two end points. You don't have to share it with anyone. These have been around for a long time and were used in the core network, but it turns out to be too slow for large networks. So, and they made use of copper cabling, which was originally in the telephone networks. So people developed an improvement that made use of optical fiber called synchronous digital hierarchy. And from our perspective, it's faster. It can provide much higher speeds. Again, it's a hierarchy in that there's different levels, starting at, say, around 51 megabits per second, but going up to 1.25 gigabits per second, 10 gigabits per second, and there are others, newer ones as well. They use optical fiber. So sometimes you'll see the name of the technology used as an optical carrier, OC1, OC3, and so on. And again, in the US and in Europe and other countries, there are slight variations. So in the US, it was called Sonnet and elsewhere, SDH. These technologies are used today in the core of the internet to connect between cities and between countries. Typically use these technologies and a few others. Let's go back. Here's a network of a diagram of a network in the US. Again, it's a bit old. It's five or six years old, but it's nice in that it shows us some of the technologies used. Some telecom company, Qwest, they think of them as a large telecom company covering, providing access across North America. This map shows their links between different sites, different cities. And the legend of the links shows us that the white ones, OC192, what's the data rate? OC192, about 10 gigabits per second. So the white ones are 10 gigabit per second links. The blue ones, OC48, OC12, OC3, and DS3 is one of the PDH hierarchies. DS3 is equivalent to a T3 link on the previous site. So this is an example of a network across the US, a core network, using a combination of PDH and SDH to connect point-to-point links. So customers of an ISP in one city would connect via the access network into some central location, and then for their data to reach somewhere else would go via one of these links. And that link, this 10 gigabit per second link, would carry, of course, the traffic of many customers in this city, many destinations in the other city. Used, SDH is deployed widely across Thailand as well, but I just cannot find a map of it, but there's a large optical fiber network across the country using SDH for links between different cities and different locations. And used between countries as well. So the main links between countries nowadays would use the optical carriers, SDH or Sonnet. The one network that I could find, the diagram, this is for TRU's international internet gateway. So TRU has links to other countries, and this gives a very simplistic view of some of those links. And we can see the speeds, although it doesn't tell us whether it's SDH, but we can guess that most likely the 10 gigabit per second links, even the two or the one gigabit per second links may be in this case optical carriers, SDH. So we see links to Malaysia, to Singapore, Hong Kong, US, and UK in that case. Other technologies may be used as well. We cannot tell from that diagram. Which one's better, SDH or PDH? SDH. SDH is an improvement, it's a new one. PDH was built and has been around for many years, built for the copper cabling in telephone networks, but we see that some limitations of the technology meant it become increasingly complex to build the equipment to go at higher speeds. So we see it reaches what, 140 megabits per second. But SDH was designed to use optical fiber and it was designed to be simpler and much higher speeds are supported by SDH and Sonnet. But there's still PDH networks available, especially if you need lower speeds. Or you don't need so high. I'm not expecting you to remember all these details, I just want you to, when you hear these acronyms to pick up higher, that's about a core network technology. SDH for optical fiber faster than PDH. Not about the individual speeds, but just to be able to recognize that they're about core network technologies using what transmission media. These three will go through even faster, I think. SDH and PDH mainly point-to-point links, in some case ring networks. To build a larger network, people started to use not circuit switching, but packet switching. And that's what's used in the internet today, datagram packet switching. But some technologies were built using virtual circuit packet switching. There was X25, that was improved to become frame relay, and improvement of that was ATMA, synchronous transfer mode. From the other perspective, there's datagram packet switching, which is what the internet has built around today. The example of these virtual circuit packet switching is, again, a telecoms company wants to build a network across Bangkok. So they deploy the links and some switching devices like, or routers throughout the city, connect them together to form their own network. And then, again, as a customer, I go to that company and say, I want connection from this location to this other location, and they would carry my traffic through their virtual circuit packet switching network. X25, very old now, hardly used. Speeds of only up to two megabits per second. For a core network, that's nowhere near enough nowadays. Mainly only used in where networks have existed for a long time and haven't needed to be updated. Many banks use them to connect their tele-machines together to their branch offices. It was improved, and the technology that improved upon it was called frame relay, provided just more efficiencies, higher speeds, and you can think another improvement of that, this shows a typical frame relay network. It's a packet switched network. So these are packet switches, and we send our data into the packet switched network. It flows through the network through a set of switches to some destination using virtual circuit packet switching. And a further improvement really was ATM, a synchronous transfer mode, and supports higher speeds. We're not gonna go through any details of that. ATM's still used today. Has anyone seen where ATM is used? Where? Someone may have seen it. Anyone, if you've configured, if you've got ADSL at home, or if you've used ADSL, and you look at the details of your ADSL modem, you can configure it usually via webpage, then ATM is used to create a connection from there to the internet service provider's network. You can see some characteristics on your modem or some parameters like the ATM, VCI, and VPI, the virtual circuit identifier and virtual path identifier. It uses virtual circuit packet switching, and normally you don't need to configure it, but on a configuration of an ADSL modem, you can find the details about the ATM connection there. So it's used inside, or it's used as a combination with ADSL. It's also used in mobile phone networks, in the wired part of mobile phone networks. For example, from the base stations, connecting them all together, ATM is used. Just as the perspective of layering, we're talking mainly about the physical and data link layer, but there's some overlap between layers. There are both circuit switching and packet switching technologies used in the core networks today. Circuit switching technologies have the advantage in that they use the existing telecom, the existing telephone network, the network that's been there for a long time. Packet switching is more suited to the type of data that's sent across the internet today. There are many technologies used together. For example, ATM makes use of SDH. So it gets very complex if you look at the details, which we're not going to do. I just want you to be aware and to be able to recognize some of those acronyms if they're mentioned in literature and when people talk about core networks. Questions, before we look at wireless. Which one's better, ATM or frame relay? ATM is better. It's newer, it's an improved technology. And nowadays, no one deploys X25 or even frame relay networks. They're quite old. Most new networks make use of optical fiber and even some alternative technologies. Ethernet over fiber. Let's spend the last 20, 25 minutes starting to look at wireless networks. Previous was on wired networks, first at access networks and then quickly about core networks. What about wireless networks? First, generally, wireless communications. Why do we want it? What are the benefits of wireless communications? Unteavored communications means not connected, no wires. In some cases that has the benefit of quick installation, that is we don't have to deploy wires to build a network and it's useful when deploying cables or wiring is expensive. You have an old building, you cannot easily put cables all through the walls so use wireless in that case. Or you have a city. You graduate, your experts on the internet when you graduate and you want to start your own ISP. Two options you have, build a wired network which involves getting permission from the government and all the landowners to dig holes through the city to build all the cables, very expensive. Or put some wireless transmitters up across the city and create a wireless network across the city. So it's convenient in that case in that it's usually much cheaper than deploying the cables. And of course, the other benefit of wireless is we can be mobile. But there are some problems using wireless communications, some challenges that we have to deal with. The communications channel when we send signals is not as robust as a wired communications channel. There are more errors. When we send a signal wirelessly, generally there's more errors, more data is lost. As a result, we need to retransmit more data. As a result, we get lower throughput. So our throughput is usually less for wireless communication. Higher delays, longer retransmission or waiting for retransmissions, varying conditions. Delays go up and down, number of errors and throughput goes up and down because someone's moving, for example, due to mobility. So performance is not as good. Another problem, the spectrum, this range of frequencies that we have available to send our signal is limited. The radio spectrum is limited. Because there's interference and there's no protection from interference, like we don't have cabling to protect the interference or wires to guide the signal, we cannot just add more cables to get better performance. I want to connect two PCs together and I want to transfer data between them at a data rate of two gigabits per second. How much will it cost me? Just two PCs, two servers, I want to back up some data and I want to send it two gigabits per second. What will you do? Using wired communications first. Wired. Hmm? It'll be cheap? Why, how would you do it? How, okay, more specific. Give me a design. Two PCs, I want to transfer two gigabits per second. Are we gonna have a switch or let's even simpler, connect them direct together. No switch, no router, just back up between two PCs. What do I do? What technology could I use? Ethernet, Ethernet, what do we see yesterday? What are some of the data rates for Ethernet? 100 megabits per second is typical. My laptop has one gigabit per second. Most LAN cards, which are built into motherboards or attached to motherboards support one gigabit per second. Pay another 200, 300 bar and you get a separate LAN card and in fact some motherboards have two LAN interfaces. So we could just have on both computers two LAN cards, two cables connecting together and with software, each cable is effectively providing one gigabit per second with two cables. We've got two gigabits per second, okay? So one cable supports one gigabit per second. Just add another and we've got two. If I want three, just add a third cable. Another 400, 500 bar, not a problem. How fast can I send from my laptop to a wireless router using Wi-Fi? Anyone, 54 megabits per second is a typical data rate. Nowadays you can get higher. I wanna go, let's say we've got 54 megabits per second. I wanna go faster. What can I do? Can I add another wireless LAN card? You can buy wireless LAN cards and a TAT or the USB devices are cheap. I can put, I've got one built into my laptop. I put in a USB adapter and a third and a fourth USB adapter. I've got USB adapters, five of them in there. Five times 54 megabits per second? No, because with wireless, you must share that wireless, that radio spectrum amongst all the users. So the 54 megabits per second, in fact, we don't even get that. When multiple people transmit, they are sharing that 54 amongst them. So I cannot get more speed by just adding another wireless adapter. But with wired communications, we generally can. We can just add more cables and we can get higher speed. So there's a problem with wireless communications to getting better performance. It's much harder. We must somehow share the spectrum amongst the different users. Much of the internet, which was designed 20, 30 years ago, was designed assuming wired links. Sometimes some of the internet protocols like TCP don't work so well in wireless links. You may have sometimes recognized that if you download a large file like a DVD via the wireless LAN, there can be many errors and the performance can really drop down. It could be very low in some cases. With a wired link, because there are fewer errors, TCP generally works much better. Another thing which we won't even touch upon in this course is security. Of course, with wireless communications, there's no form of physical security. Our signals go everywhere, so you don't have to be inside the building to receive that signal. You can be outside and therefore it's less secure. With wired communications, my signal is contained inside the wires and inside the building. It's more secure. So there are some problems with wireless. But some benefits, and as you know, those benefits are quite important because wireless communications is used widely today. A simple view of a wireless communications channel. We have a transmitter, generates some energy, has an antenna that converts that electrical energy into a magnetic wave form. We send a signal from the transmitter. It propagates across some distance. As it propagates, the signal energy gets weaker. So the signal starts off at some strength, some power level. Over distance, the energy gets weaker and it's eventually received at some power level. The antennas introduce some gain, some amplification of the signal. And the receiver receives some signal, decodes it and gets the data. Some issues that we care about here. How much power is lost across some distance? Depends upon how far we are. The longer the distance, the more power we lose. Depends upon the frequency of the signal that we transmit or the range of frequencies. The antennas introduce some gain into the system. So the larger the antenna, the larger the signal strength and the further we can transmit with the same transmit power. And related to the gain is the shape of the antenna. The shape of the antenna and the size, in fact, determines some gain, some amplification. So the shape determines how much the signal energy is concentrated in a particular direction. When I talk, at least without the microphone, my signal energy goes out in this direction a little bit behind me, but it's concentrated in this direction. If you hold your hands over your mouth a bit better, then you concentrate the energy a bit more in this direction. It'll be stronger in this direction, that's the idea at least. So with directional antennas, you can concentrate the energy and transmit further in a particular direction. The other thing which determines how far we can transmit or how much power is lost is obstructions. No one can hear me upstairs because the walls are obstructing the signal. Similar with wireless internet access, I can connect to an access point out in the corridor because the signal can go through the wall, but it gets weaker as it goes through the wall. And to go through a second wall and a third wall and even the floor, it becomes too weak to be received outside. So obstructions impact on how much power is lost. How fast can we send? Some things that impact is how we encode our digital data, our zeros and ones as some analog signal sent across wirelessly. We'll impact upon the data rate how fast we can send. We generally assume that the receiver can only decode the signal if it's received at some power level or above some power level, called the received power threshold, or the received threshold or sometimes related to the receiver sensitivity. That is, this receiver here receives some signal. We assume if the signal is greater than some level, some threshold, then the receiver can understand the data that was sent. If the signal received has a power which is weaker than that threshold, then it cannot understand or it cannot decode that data that was sent. It's not so simple as an absolute cut-off, but for designing systems, often we'll think of some threshold and say, if we can receive greater than some threshold, everything's okay. If it's below, nothing's okay. We do not receive anything. We'll see that when we look at equipment, wireless equipment, that's sometimes specified when you buy the equipment. What is the threshold? A simpler model of wireless transmission. What we care about when we build networks, primarily we care about, okay, we have a transmitter and receiver. We care about the transmit power needed to transmit to the receiver. How far we can send the transmission range, the distance. The data rate, how fast we can send and the frequency we use. The four main things that we care about. The last one may be cost. Choose a technology that meets your requirements and is within cost. Why do we care about those things? Some are obvious. Data rate, normally when we talk about internet applications, one of the primary requirements is sending data at some speed. So when I want to choose a wireless technology, I know how much data I want to send. Therefore, I choose a technology that will give me an appropriate data rate. If I know I want to send 10 kilobits per second from my laptop to some small wireless device, then I could choose Bluetooth, for example, which has a data rate of one megabit per second, or even less, or even infrared in that case. A very low requirement of 10 kilobits per second could be supported by an infrared, but if my requirement was 20 megabits per second to send from laptop to some other destination, I have to choose a technology that supports that data rate. So we care about, for wireless technologies, what data rate does it provide? Usually in bits per second. When we build a network, we care about how far we can separate the transmitter and receiver. The maximum distance we can communicate between them, the transmission range, or transmission distance. You want to build a network you want to build a network that covers an apartment building. One of the third-year students sent me an email across the break saying he wants to build a wireless LAN network for an apartment building. There were, I think, 10 floors, about 20 rooms per floor, and he asked how many access points does he need? Well, the main criteria is about how far what's the transmission range of each access point to provide coverage for the entire building. If I can put one access point in the center of the building and it can provide a transmission range that covers the entire building, I need one access point, but that's unlikely. More likely I may need one access point per floor to provide coverage for each floor of the building. One on the next floor to cover the next floor and so on. Or maybe two per floor or three per floor. The more I need, the higher the cost of my network, the larger the transmission range, the better in this case. The more coverage we get. So we care about how far our wireless technologies can transmit. We care about the frequency of the signal that is sent. Why? Two main reasons, cost and performance. Performance first, when I transmit on one frequency, if someone transmits nearby on the same frequency, we'd say that those signals may interfere with each other and the receiver may not be able to receive and understand what was sent. We'll talk more about this when we look at details at wireless LAN, but when you send signals from two transmitters to a nearby receiver, then those signals will interfere with each other and the receiver will not be able to decode what was sent. To avoid that, use different frequencies. I transmit on one frequency, someone else uses a different frequency so they don't interfere. So we care about the frequency so that we can avoid interference. You've probably done that with Wi-Fi. You may have configured your laptop for your wireless access point to use a particular channel, channel one, channel six, channel 11. That's really a different frequency. I use channel six in my dorm, my room, someone else uses channel 11, means that even though the signals are in the same area, they're using different frequencies and do not interfere with each other, giving better performance. So we'd like to choose frequencies that do not interfere with nearby transmissions. It gives better performance. The set of frequencies we can choose for wireless technologies is limited. Therefore, there's a demand for those frequencies. As a result, there's usually a cost involved of using those frequencies. If many people want to use the same frequency, then how do you give them out? Well, make them pay. The one who pays more gets that frequency. In some cases, there are free frequencies. Some special cases are unlicensed or free to use. Wi-Fi is one example. You don't have to pay a license to use Wi-Fi to transmit at the 2.4 gigahertz. What's an example of a licensed wireless system? Expensive. There was an auction last month. There was a 3G auction where, well, maybe it wasn't so expensive for the companies, but they bid for frequencies. That is, there are three companies. There was an auction. There are a range of frequencies available to transmit for 3G wireless access. And those companies paid what, four and a half billion baht for a frequency with a bandwidth of around five megahertz. So those frequencies cost money to use. So when we choose a wireless technology, which is the best technology for our purpose, we need to consider what frequency it uses. The last thing that is important is the transmit power, especially for mobile devices. With your mobile phone, it transmits a signal at some power level. The more it transmits, to transmit that power, it uses its battery. So the more power it transmits at, the more of the battery it uses, and therefore the less time that battery will last. So again, when we choose a wireless technology, choose one that will consume less battery of our device. So a lower transmit power is better. It won't use up the battery, but a lower transmit power leads to a lower range. And that's a problem. So all these are related and cost. Choose a technology which is cheapest. What have we got to finish today? Two or three slides. And why are we doing that? Pass that around. Still on frequencies. When we send signals, we send them, our signals contain multiple frequency components normally, not just a single frequency, but a range of frequencies. It's usually sent with some, we'll refer to as some center frequency and some bandwidth. The set of all frequencies available is the spectrum of a signal. So we talk about the frequency of a signal and also the bandwidth of the signal. So with the 3G auction last month, I cannot remember the exact frequencies, but the frequencies are in the range of 2.1 gigahertz. But in fact, each license covered a range or a bandwidth of five megahertz. So a company paid for a set of frequencies with a width of five megahertz around about 2.1 gigahertz, plus or minus a little bit. So when you send a signal, we usually talk about a center frequency and in fact, the signal has a bandwidth. It covers a range of frequencies, not just one frequency. Why do we care about the frequency and bandwidth? Some general relationships between them and the data rate. Usually a higher bandwidth and frequency gives us a higher data rate, which is hand faster. That's a good thing. But higher frequency leads to a shorter range. We don't want that sometimes. So we have a trade-off here. Also, different frequencies are obstructed by different materials. Green laser? Does it go through the wall? No, it will not. On the other side, you will not see the laser. It's a signal, a particular frequency. My Wi-Fi signal will go through the wall, at least partially. Different frequencies are obstructed in different ways. Interference. If you choose a frequency that someone else is using, they may interfere with you. My wireless LAN in the laptop uses a frequency of around 2.4 gigahertz. Bluetooth uses a frequency of the same area, 2.4 gigahertz. In theory, if you transmit using Bluetooth and wireless LAN close to each other, and at the same time, they can affect each other's performance. They can interfere. Some microwave ovens cook food using 2.4 gigahertz. A microwave oven can interfere with your wireless LAN, in theory. Because they transmit signals at the same frequency, and can cause interference. So don't put your laptop in the microwave oven. It will not work. And cost. Different frequencies have different costs. The example is the 3G auction, where a range of frequencies with a bandwidth of five megahertz cost around four and a half, what was it, four and a half billion bar, I think, for the license. For a 15-year license to use that frequency. In other countries, maybe 10 years ago in Germany, an expensive one, it was around two trillion bar that they made from that 3G auction. So, quite expensive in some cases.