 So we're going to look at, explain first what is a wireless LAN, a wireless local area network, specifically focusing on IEEE 802.11 wireless LANs, which is a standard for wireless LAN. So we'll explain the difference between the standard, the name, and some of the terminology. And then we'll look at the different protocols used in wireless LANs and some details about the physical layer transmission. And throughout this we'll give some examples of some wireless LAN devices and how they operate. And your assignment will start, your assignment will have multiple parts. So actually some mini or some multiple phases. So three phases most likely where you do one phase, there's a deadline, you submit that, and then you move on to the next phase. So like three small assignments. And they will involve using some wireless LAN access points. So you'll learn more through the assignment. First some quick introduction to what we normally view as a wireless LAN and what we deal with when we use a wireless LAN. The most common topology of a wireless LAN is that we have a client, your laptop, your mobile phone, some mobile device, some cases fixed devices. But we call it a client. And it connects wirelessly with an access point. A device which then connects usually via wires onto a wired network. So what this example shows is this is a wireless link between client and access point. And the access point has a cable plugged into it which then connects onto some other network, some wired network. We're not going to focus on how this works. That's typically Ethernet, a wired LAN. For example here we connect into a switch and maybe some other switches or a router and that goes out to the internet. And on this switch maybe some desktop PCs attached. But that's all the wired network. The wireless LANs we're focusing on how do we communicate between the client and the access point via the wireless link. The connection from the access point to the rest normally is Ethernet but in theory it could be any technology. It could be an ADSL link. It doesn't have to be Ethernet. But many of the devices that are available and available cheaply today take an Ethernet cable as the wired link. So the terminology we use to refer to the end user device, the client and the device that connects the wireless portion of the network to the wired portion is the access point. So an example of our access point. And you see them around because you connect them to use the SIT wireless LAN. You see them attached to the walls and ceilings. And as you may see they have ports on the back to plug the wired LAN into. Some terminology, a wireless LAN is a general type of network. So a local area network using wireless technology. There have been multiple different standards that have been created to implement wireless LANs. The most common one is the one that we use today, the standard. So let's say the general name is wireless LANs or a wireless LAN. One of the standard that we use today, a set of standards is called IEEE 802.11. In fact there are many, many individual standards within that family. So there is some 802.11, ABG, some extensions which we'll talk about later. But that's the standard or a standard for wireless LANs. In the past there were other standards available. You didn't have to use IEEE 802.11. There was another one which came out of Europe for a while called Hyper LAN. But it just didn't take off, it wasn't popular. So the main technology used for wireless LANs today uses the standard IEEE 802.11. The idea is that if your client implements that standard and it should be able to communicate with any access point that also implements the standard. If they use a different standard they may not be able to communicate. Another name or another term that you often hear is really a marketing term, Wi-Fi. So sometimes you'll hear them effectively meaning the same thing. A wireless LAN, IEEE 802.11 and Wi-Fi. But wireless LAN is the general name. IEEE 802.11 is a specific standard. Wi-Fi is a marketing term. So there's an organisation that certifies products to be Wi-Fi certified. I don't know if this has any label on it but many products may have a Wi-Fi sticker on them saying that they've been certified by some organisation to be called Wi-Fi. But it's more of a marketing term. I will use all three of them, whichever one's easier. Sometimes Wi-Fi's easier to save than 802.11. So just some terminology about wireless LANs. Or WLANs is the other one, W-L-A-N-S. So here we have a WLAN, Ethernet. Here's the wireless part. Really 802.11 is concerned of how to get data across this one link, across this wireless link. So it focuses on in our layered stack two layers, a physical layer which is how to transmit bits. We've got digital data. We've got transmit bits as some signal, some radio signal across the air. So our medium is the air. We need to send some radio signal. That's the role of the physical layer. And then the data link layer, layer two is how to make sure that use of the medium is efficient. And especially one thing that comes into play and we'll spend a lot of time on is how do we share that medium when we have multiple people wanting to send data at the same time? When we have multiple laptops wanting to communicate via one access point, who gets to send first? And that's one of the roles of the data link layer. We will see some different specs, some specifications for equipment and standards as we go through this topic, but just some summary. And like some of my slides, this is getting a bit old, at least the data rates. This is when 802.11 was not so popular, but the most common standards, how fast do they support what data rate? Well, in the past it was 11 megabits per second was the maximum data rate. Most products today like this one support 54 megabits per second and probably over the last two or three years, most devices you bought even support faster. 802.11N supports 300 megabits per second and even up to 600 megabits per second in some cases. So we're talking about tens of and nowadays hundreds of megabits per second as a data rate. That's the data rate. What you get when you send your data across the wireless link is because of overheads is usually much less. So the throughput that we achieve is usually around half of that. We'll do some calculations later, not today, but we'll see the relationship between the throughput and the data rate, but usually approximately around half of the data rate and shared, which means the more people using that wireless or communicating at the same time, the less an individual user will get. We need to divide that throughput amongst multiple users. So if we have say five users and we had a throughput for a total throughput of 25 megabits per second, approximately each user gets five megabits per second. We divide the total divided by each user and we'll explain why that's the case. So we're really talking about end user throughput in order of tens of megabits per second. How far can we send? Indoor meters, tens of meters, outdoor hundreds of meters. The distance, the transmission range depends upon the, as with all wireless communications, it depends upon the transmitter and the antenna on the transmitter, the transmit power used, the antenna on the transmitter, similar on the receiver, the antenna is used, and the receive sensitivity, the capability of the receiver device, and especially on the obstructions. Your signal needs to pass through walls, it will be attenuated as it goes through walls, it will get much weaker, and therefore you will not be able to transmit as far if we have different obstructions. So that's why indoors it's around tens of meters, outdoors when there are few obstructions, say in an open field, you can get hundreds of meters in transmission range. And with special antennas, you can get kilometers of transmission range. But typical cases in buildings, tens of meters. How much power do we transmit with? Usually around from one milliwatt up to around, up to 100 milliwatts. So when my laptop transmits, it depends upon the laptop, the device, it's in the order of 30 or 50 milliwatts for my laptop. I cannot remember the value, but we'll see later. Typical devices in the order of milliwatts, tens of milliwatts, maybe up to 100 milliwatts. However now you can buy special devices or some devices which will transmit at a higher power if you want to transmit over a larger distance. Why do we care about the power? We care about the data rate because we know we'd like to send our data as fast as possible, download or whatever. Care about the range because I'll put an access point in my home, I'd like to get coverage of the entire home. So I care about how far it transmits. Why do I care about the power? Cost? Health? Cost? Cost. Okay, my device that transmitted at a higher power may have a higher cost. Small cost. Health? Why health? Okay, if you transmit a high power signal close to the human body, it can have an effect on the body. But in terms of hundreds of milliwatts, even at 1 milliwatts, it's not going to, in most cases, have an effect. Unless you have it very close to you, 100 milliwatts shouldn't be a problem. What else? Why do we care about the transmit power? The battery of the device. The battery of the device. If it's your laptop, especially your mobile phone, it runs on a battery. When you transmit the higher the power you transmit at, the more of the battery you consume. The higher the transmit power, when you're sending data with Wi-Fi on, on your mobile phone, your battery will not last as long as if you're not transmitting data. So the higher the transmit power, the less time our battery will last. So that's important. The higher the transmit power, the further we can send as well. One thing that we don't often think about, but also the higher the transmit power, the further we send, and a negative of that is that the more people we may interfere with, and that can cause problem with performance. What else? Frequency. The frequency range that's the most commonly used with wireless LANs is 2.4 gigahertz, plus or minus a few tens of megahertz. So the center frequency of our signals that we send are around 2.4 gigahertz. Another frequency band is around the 5 gigahertz, but that's not so common. But many products support it nowadays. This is in what's called the... It's an unlicensed band. It's the ISM band, which means industrial, scientific, medical. The ISM band is an unlicensed band. A range of frequencies that you're allowed to use and you don't need a license to use them. So originally developed for... So for industrial purposes, you build your own wireless network and for testing. But generally used nowadays for many consumer wireless technologies. Bluetooth, wireless LANs, for example. Unlicensed, you don't have to pay a license to use that frequency to transmit, meaning cheaper, and anyone can use it. And the fact that anyone can use it is also a disadvantage, because the more people that use it, the lower the performance that you as a user will get. You have to share amongst everyone. So we need certain rules or procedures such that we can fairly share our resources amongst multiple users. So cheap, but we have the problem that there's no control about who uses it. Anyone can use it. Security, are they secure? Link level security, that is sending from here to the access point. There are techniques available that provide satisfactory level of security. That is almost as good as we can hope for across a wireless LAN. There are some... So they use... For security, we provide encryption of the data. We encrypt the data before we send it so there are techniques like WPA, WPA2 that provide satisfactory security for most purposes. However, you need to turn them on. You need to enable such techniques in your network for them to take effect. If you don't use encryption techniques, then your network or your wireless LAN is considered insecure in that whatever you transmit, someone nearby can receive and see what you sent. So unless you explicitly use the security techniques, your network is insecure. So a quick summary of some characteristics of wireless LANs. We'll go through how they work and where some of this data comes from. Who? Most of you have used the wireless LAN here on campus. Who uses one at home or in a dorm? A wireless LAN. What's the basis? Most people? Some people at least. Who has set up their own wireless LAN? A bit higher. So you've configured an access point for example and set it up. So for those that haven't, that's what the start of your assignment will involve. So it's not so hard. Next week I will give you some access points. You'll form some groups and we'll get started on setting up your own wireless LAN. So still on some terminology and some very basic concepts. So we have an access point which connects our wireless link to our wired link and to the rest of our network. And we have clients that communicate wirelessly to an access point. We say that those clients associate with an access point. So there's a procedure to associate an association. And a client associates with just one access point at a time. You cannot be associated or in the normal case you cannot be associated with multiple access points. You're associated with one access point. Of course having a single access point because of the limited transmission range we cannot have a single access point that covers all of this building. In fact a single access point is the floor of the building. So when we want to provide coverage to users larger than the transmission range we use multiple access points. And that's what we have across this campus. Almost two or three per floor in some cases now. We have access points with the intention of providing coverage no matter where you are in the campus. So for a larger area we use multiple access points. And we connect to some wired switch a LAN switch which then may connect onto either other switches, desktop PCs and then out to the internet which is not shown here. The set of clients so for one access point we may have multiple clients associated. The set of clients associated with an access point and that access point are called a basic service set, BSS. So this is one basic service set or a basic service set. Sometimes generally called a cell I don't think we'll refer to as a basic service set. When we have multiple access points all operated by the same organization and all configured to work together then the set of basic service sets that is all clients, all access points is called an extended service set. And it should be easy for a client to move between access points within the same extended service set. And there are mechanisms for doing that. That is you're currently associated with an access point outside because that's nearby but then you move you're on your phone and you move and as you move down the stairs you associate with another nearby access point because you move outside of the range of the one on this floor. That process of changing your association from one access point to another is referred to as a handover. We hand over from one to another from one basic service set to another. As long as they're within the extended service set that's normally quite easy. One thing you may have seen or you actually all have seen is that an extended service set an extended service set ID an ID so we can recognize it. And the one that you use in SIT WSIT that is the name of the extended service set covering this campus. More specifically it's called an extended service set ID WSIT. So that's common across all the access points inside SIT. Each of them are configured and there's a parameter in the access point that says select the ESS ID and all of them have been set to be WSIT because they're all in the same extended service set. What else can we say about this? So we use wireless LAN we use an 802.11 to communicate from here to the access point and then the access point most typically uses Ethernet to send our data via a wired LAN maybe to a router or maybe just to a desktop PC inside the campus for example. So this may be my office PC this is an access point out in the corridor my laptop connects to the access point or associates with the access point sends data to the access point which then takes it and sends it across the wired LAN to some switch eventually into my office in the office. What's the standard used for wired LANs? Ethernet standard is IEEE 802.3 so that's the standard used for wired LANs or generally called Ethernet so same organization and they use the same or similar techniques so there's a very a number of similar protocols used between the two also some other standards and also for example the frame formats when you create create a frame to send there's a frame format used for wired LAN and then it's converted into the frame format used for Ethernet there are some similarities and the addressing scheme used is the same we'll see the address that your PC gets IP address but the hardware address the hardware address of your PC is the same format as the hardware address used for your laptop the wireless LAN interface it's this 48-bit Ethernet address or MAC address about the standards 802.11 so in fact the family of 802 standards covers wired LANs, Ethernet gigabit Ethernet, wireless LAN we're going to focus on and many others, Y-Max Bluetooth fits in there token ring, older wired LAN or wired networks and within this 802 IEEE 802 architecture the standards all follow some same approach that is there's a physical layer standard and in fact there may be many variants of the physical layer standard MAC layer what does MAC stand for say up there MAC is medium access control there's a MAC layer in each of the standards and then there's a common logical link control standard used and that's used this what's called 802.2 is used for all of them it's the same for wired LAN as wireless LAN protocol, MAC standard and there's specific physical layer because sending data across a wireless link across a short distance or across tens of meters requires different techniques to sending data across a copper wire and similar sending across kilometers in Y-Max requires or can use different techniques than in wireless LAN so they have different physical layer standards and in some cases different MAC standards and then all the common logical link control from what we will see in some of our examples we will not see much about the logical link control one thing in common is that they have the same hardware address format the same address format we will focus on these two the physical layer and the MAC layer so the physical layer and generally the data link layer getting data across individual link how do we do that within here both of these are in fact multiple standards multiple enhancements have been made over time there's the original one made back 15 years ago and then there's the over that time there's been enhancements improvements and with respect to the standard a letter is added to the end there's two letters we've got past z z so we have multiple different physical layer standards so I've tried to draw that here there's the one MAC standard which has been fixed although there's been amendments but the main technique has been fixed since the start and there are multiple physical layer standards which really have been improvements over time increasing the speed the data rate so the original one was simply 802.11 and then there were two improvements that came out at the same time 802.11a and b they used different approaches 11a offered a data rate up to 54 megabits per second 11b up to 11 megabits per second so we refer to them as 11a and 11b and then there are other improvements as technologies and the cost of implementing them got cheaper technologies got better 11g came out and now 11n most new devices support 802.11n and the old ones at least maybe b and g sometimes a and there are newer ones AC and AD in addition to the different physical layer standards there are also improvements for other parts of wireless lands improvements on security for example 11i there was an original security standard WEP which had some flaws it didn't work well in some cases it was insecure so they developed enhancements improved security standards for wireless lands providing quality of service which is about for example making sure your voice traffic gets priority over your web browsing so you can give some special service to some applications and lower service to other applications how to do that managing the spectrum, the frequencies and many others so we're up to 20 close to 30 different amendments that are available now and they keep working on new improvements sometimes they are called a standard like the standard 802.11g but more precisely it's called an amendment to the standard it fixes the standard or changes the standard we're not going to try and explain all of the enhancements or amendments we'll talk about the differences between the main physical layers and focus on the common MAC layer before we go into how the physical layer works and some assumptions let's look at my laptop and see what we have for our wireless LAN connection so on my computer I can look at the configuration of my wireless LAN interface using this program I have config and the name of my wireless LAN interface WLAN0 and it tells me some information about my interface, my network interface what can you recognize there what information does it tell you that you know about what about Ethernet why has it got Ethernet there in fact I don't have a wired cable plugged in, I've just got wireless access on the laptop at the moment from the perspective of the operating system the similarities between the 802 802.3, 802.11 my operating system considers this at least from the network layer perspective as just a general Ethernet interface the same as that's my wireless LAN and my wired LAN although I've got nothing plugged in the method of encapsulation is also called Ethernet they consider the same from the perspective at least of this program this is my wired Ethernet interface this is my wireless LAN interface focusing on the wireless LAN one at the top what else may you recognize MAC address the hardware address how many bits how many bits 48 bits this is in hexadecimal each hexadecimal digit is 4 bits so 12 hexadecimal digits it's a 48 bit address the hexadecimal is just to make it shorter to write down it's actually 48 bits in length so this is the hardware address of my wireless LAN interface normally with hardware addresses they are assigned by the manufacturer of the device when I by the laptop there's a wireless LAN chip on the motherboard in fact and that comes with a hardware address assigned by the manufacturer of that chip with the intention that they are globally unique that is another laptop that comes off the same production line the next one would have a different hardware address, a different MAC address that's the idea what else do you recognize here we have an internet address so an IP address so when I connect to the SIT wireless LAN I get an IP address assigned to my laptop 10 10 97 1 5 0 and related to that a subnet mask to say internet address identifies the subnet I'm on and that's identified by this 255 255 248.0 although we're not going this is outside of the scope of what we're covering there's an IPv6 address and some information about my interface that it's up means it's on some capabilities it's running it supports broadcast and multicast and U is the maximum transmission unit the maximum size of data that I can transmit across this interface 1,500 bytes and some statistics about the number of packets I've received and transmitted including bytes okay now it doesn't say much about the wireless LAN characteristics of that interface it's all general it's about an IP address which is not specific to wireless LAN a hardware address which is not specific to wireless LAN because my wired LAN has the same type of hardware address so to look at something about my wireless LAN interface okay two parts so I said that the hardware address is intended to be globally unique will they run out how many in theory do we have or 48 bits gives us 2 to the power how many is that get my calculator with a 48 bit hardware address that's 2 by 10 to the power of 14 possible addresses it's not quite that many because they're structured in some way but that's what they're 10 to the power of 9 or maybe say 10 to the power of 10 people in the world so that's what 1000 addresses per person it shouldn't run out just yet so of course this is not just for laptops but for wired devices because the same address structure is used for wired LAN and other devices, mobile phones access points many devices use them but should be enough for the medium term that's not a problem but a problem maybe is and is possible that it may not be globally unique because nowadays you can change using software the MAC address at least you can set the MAC address of your device to be something else so you could set the MAC address of your laptop to be the same as mine and therefore it's no longer globally unique so it's now possible to change when you send data to set a different MAC address but when they're manufactured the idea is that they will be globally unique the way that that's provided and some of you have seen this before is that the manufacturer is assigned an identifier and that is the first 6 hexadecimal digits and when the manufacturer creates a device it assigns the last 6 digits and the first 6 hexadecimal digits identify the manufacturer of the device let's remember what mine is and bring up a website that tell us that may tell us the manufacturer IEEE the organization that manages these addresses has a website and you can search sorry, what was mine 8C, A9, A2 so those first 6 digits in the MAC address should identify the manufacturer of my device and we search their database Intel mine is an Intel wireless LAN card so Intel is assigned these 6 digits and when they create each wireless LAN chip they select the last 6 digits with the intention of them being unique so you can look up how you manufactured your device from this IEEE website let's look at some more details about the wireless LAN interface so another program I can use is IWCONFIG IFCONFIG to configure my interface my network interface IWCONFIG gives me specifics about the wireless interface and the name WLAN0 now we see some details WLAN0 the standards it supports IEEE A2.11 it supports B, G and N so the different physical layers B there was A2.11 the original one and then 2 extensions A and B this one my card supports B only it doesn't support A it also supports G and also supports N it provides 11 Mbps maximum G 54 N maybe up to 300 Mbps which one does it use it only uses one at a time depending upon what the access point supports so if this access point which I associate with supports 11B and G and my laptop supports B, G and N they'll most likely negotiate to use G they could use B as well but if this doesn't support N then of course we cannot use it both need to support the same standard and they can use either of them B or G and then it's up to you the user to select do you want to use B or G G is the faster of the two my extended server set ID associated with an access point and that access point is using the WSIT ID the mode managed is just to say that my interface is connected to an access point the managed mode means that I'm a client and I'm in a mode that I'll only associate with an access point there are other modes where you can connect direct from laptop to laptop client ad hoc mode is commonly called so there's different modes that you can use wireless LAN the most common one is client to access point managed mode as called by this software the frequency I'm using the precise frequency 2.462 GHz there are multiple different frequencies available to use in this 2.4 GHz range there are different channels available and you select and use one of them to communicate with the access point the access point and the access point is identified normally by its MAC address its hardware address so the access point also has a MAC address and it's the same format this 48-bit MAC address so that's the identifier of the access point sometimes called the BSS ID the basic server set ID the bit rate the data rate, 1 Mbps why 1 Mbps because I haven't send any data if I access my website and then run this command again when I actually send data it will move up to a higher data rate so now it's reporting 54 Mbps so that's the data rate I most likely use to transfer that data we will see that wireless land devices can support a range of data rates, not just one and they can automatically switch between them the transmit power of my wireless land device 14 dBm how many milliwatts calculate the number of milliwatts that I transmit at anyone have a calculator or a mobile phone that can calculate 14 dBm how many milliwatts 25 milliwatts 10 to the power of 1.4 and that dBm, remember the general formula for decibels the is 10 log in base 10 of some power level so some power in the absolute value and the power in dB so in our case with dBm dBm, the M stands for milliwatts more precisely it should be written as or it could be written as dBm it's decibels relative to 1 milliwatts so the way to work that out quickly is that the power level in milliwatts gives us dBm so if dBm, if we have in our case 14 dBm equals 10 log in base 10 of some power level in milliwatts so therefore you just find out what p is which is 14 divided by 10 is 1.4 equals log p and therefore p in milliwatts equals 10 to the power of 1.4 which is about 25 milliwatts when we talk about wireless communications transmit powers, receive powers we often talk in using a decibel scale dBm is common with wireless it's the power level relative because decibels is a ratio between two power levels dBm means the power level relative to 1 milliwatts 25 milliwatts divided by 1 milliwatts is a factor of 25 convert into dB and you get 14 decibels so my laptop transmits at 25 milliwatts which is a typical range of most at least laptop devices 20 30 milliwatts some may be higher powered the retry limit RTS fragmentation we'll talk about we'll see them later as we go through the lecture notes how they come into play in the wireless LAN MAC lab power management is off power management is a feature that you can turn your wireless LAN interface into a mode such that it sleeps in the idea of saving power saving battery power it sleeps in that it turns off or goes into a low power state an idle state or let's say a period of time half a second and then wakes up every interval and checks with the access point whether there's any data to send or receive so in particular to receive so if there's no data to go between the laptop and the access point it turns off the wireless interface saves power on your device but what if there's some data that's come from the wired network to your laptop then every so often your laptop turns on turns on the wireless device and sends a special message to the access point to check if there's any data and the access point informs it and if there is then it can wake up and receive that data so there's a way for the the wireless device to sleep as a way of saving battery power it's off in my case there's some statistics about transmit and receive packets and the last thing we may see is the link quality some measure of the quality of the wireless signal being received by my laptop received from the access point that I'm associated with access point identified by this address 70 out of 70 in this case and the measure of the receive signal so this is some some relative measure and this is the absolute measure of the receive signal minus 40 dBm which is what 10 to the 10 to the power of minus 4 milliwatts which is 0.1 microwatts so that's the signal strength the receive signal strength from my laptop is this one and it gives some quality of level that's considered very good any questions before we come back to our lecture notes so just an example of some details that we can see about a wireless LAN we've got a name of our network extended service at ID different addresses different physical layer supported data rate transmit power let's look at the physical layer we're not going to go into details about how the physical layer standards work we some of us don't have enough background on some of the physical layer or the wireless communications to understand and it's not so important to understand some of the aspects of performance in wireless LAN we'll just talk about the characteristics some of the specifications of the physical layer and some general concepts from our simplistic view we think the physical layer takes bits from the data link layer and converts them into some radio waves that are transmitted from the antenna from the transmitter to the receiver how does that work well there are different things that the 802.11 physical layer defines is the modulation technique the analog signal that is transmitted how to shape that into some efficient form so that we can send data at a high rate and that the receiver can receive it and understand what's transmitted so different modulation techniques are available it defines the physical layer defines what frequencies to use and what bandwidth of the signal we should use we've given an example of my laptop used a frequency of 2.462 gigahertz the normal bandwidth used of wireless LAN signal is 20 megahertz and I've drawn a diagram similar to this before let's try again so this is the spectrum of the signal that I transmit where we'd say the center frequency is 2.462 gigahertz this is the frequency and gigahertz and this axis and this is the power strength when I transmit a signal the signal actually has a range of frequency components and the bandwidth is here is the bandwidth is 20 megahertz in fact here the lower point is about what is it 2.452 here and this would be 2.472 because the difference would be 20 megahertz or 0.02 gigahertz so that's the signal transmitted by my laptop that is in these frequency in this frequency range the signal has high power outside of those frequency range the signal has very low power almost zero or effectively zero so I transmit across a range of frequencies so the physical layer defines the bandwidth and the frequencies used this we call the center frequency of that signal and in wireless LAN there are several different frequencies available to transmit at we'll see a plot of that later the exact set of frequencies available the other part defined by the physical layer is some timing or how to synchronize the sender and receiver that is when we transmit some signal that the receiver can detect that signal represents the first bits or which signal represents the first bits of a frame, the start of a frame so some synchronization issues we're not going to attempt to explain how they work what we care about are some practical characteristics of the different physical layers the speed, the data rate how many bits per second can we send with each physical layer how far can we send and we'll talk about because it depends on different factors the transmit power of the transmitter and the receiver sensitivity transmit power is how much power I transmit with that's easy the receiver sensitivity is how weak a power can the receiver receive and still understand that data transmitted what's the minimum power level it can receive and still decode or demodulate the signal and get the data out so that's usually a characteristic of the receiver electronics and usually a specification of your equipment the frequency and related to the frequency the number of channels available I have a plot let's see if I can find it this illustrates the range of frequencies available in wireless LAN I said 20 MHz or I was wrong 22 MHz that would be more precise that's the bandwidth of the signal how do we read this and we can see that we have different channels available so channel 1 through to channel 13 depends upon the country and the regulations in that country as to what channels you're allowed to use typically channels 1 to 11 are available 1 to 13 in some countries and sometimes it's different but the most common channels we have is 1 to 11 and the channel is defined by the center frequency for example my laptop was using channel 11 I was using frequency 2.462 GHz and this plot shows the bandwidth when we transmit that signal do you have this picture yes you do have this picture in your handouts towards the end of the where is it at the end of your election notes the wireless LAN flip through a few pages so it shows the different channels available for your wireless LAN device and note for example on channel 6 transmitting at a frequency of 2.437 GHz our signal has a bandwidth of 22 MHz shown here so that ranges from so channel 6 the frequencies that we transmit at at least at significant power ranges from 2.437 minus 11 MHz 2.426 up to 2.448 GHz that is a bandwidth of 22 MHz a difference between these two 0.022 that's from here to here and there are other frequencies available you can transmit on channel 5 2.432 is the center frequency what's important here is that some channels overlap that is when you transmit on channel 5 you're transmitting a signal in this range your the frequency components you transmit overlap with some of those most of those in fact with channel 6 what that means is that if my laptop is transmitting on channel 5 and someone else's laptop is transmitting on channel 6 a receiver that was trying to listen to my laptop an access point on channel 6 and 5 would also receive most of the signal from the other laptop they would interfere with each other and in fact the receiver most likely would not be able to understand any of the data sent they would cause some interference and the receiver could not decode the data being sent so that's a problem if there's two devices transmitting on overlapping channels by overlapping I mean that because the bandwidth covers a range of frequencies if there are two stations or devices transmitting on overlapping channels a receiver will hear both of those signals and normally would not be able to understand either of them so the non-overlapping channels is considered important and we see in this set that the main ones are channels 1, 6 and 11 of those 11 or 13 channels available 14 is a special case only available in some countries of up to 13 the maximum number of non-overlapping channels available is 3 even though there are 13 channels available only 3 are non-overlapping using non-overlapping channels can significantly improve performance and we'll explain that shortly any questions on that picture before we move back to the slides it shows the channels available and it shows the bandwidth consumed by each and in particular it shows an example of 3 non-overlapping channels channels 1, 6 and 11 so the frequency you choose to transmit at is important and in particular the set of non-overlapping channels is important let's just see where we can finish today let's look at some of the characteristics there's this slide which goes up 802.11N and then I've got another picture which shows two others two newer standards but this captures most of the information this compares the different physical layer so the amendments to the original standard the original one 802.11 released in 1997 the frequency 2.4GHz or in the range of 2.4GHz modulation technique we're not going to describe it uses a spread spectrum technique for modulation this is the non-overlapping channels available three non-overlapping channels the more the better what it means is with three non-overlapping channels is that we can have three different pairs of communications happening at the same time in the same area without interference I could be transmitting to one on channel 1 another person transmitting to another access point nearby on channel 6 and a third transmitting to another access point on channel 11 and even though they are in the same vicinity they will not interfere with each other because they're using non-overlapping channels so that's an important practical characteristic the number of non-overlapping channels available original maximum data rate supported was 2.4GHz per second and the range was approximately it depends upon the building and so on around 20 metres indoors maybe 300 metres outdoors approximate range that didn't last long because as soon as that was released the people who created the standard realised that it wasn't fast enough that they could do better and within two years 11B and also 11A were released at the same time so they provided two different options 11B was released to be a released to use the same approach as the original 802.11 so that you could use the similar equipment it would be backwards compatible 11B used the same frequency and in fact the same modulation technique same number of channels and it provided an increase of the maximum data rate up to 11Mbps and about the same range as the original one whereas 11A released at the same time higher data rate much higher five times higher 54Mbps is the fastest we could send with 11A more non-overlapping channels a different modulation technique which led to the higher data rate the main difference is that use a different frequency band one characteristic of that higher frequency the transmission range was lower we couldn't endorse not much difference but it could be significant in some practical cases but the main difference and the reason why 802.11B become more popular than A A was faster, B was more popular one of the reasons was the backwards compatibility you could have let's try and draw it you could have a network let's consider a university or an organization, SIT which has an older organization that had their network of access points throughout their building and many clients, many people had laptops and let's say the laptops originally they supported the original 802.11B the access points what was supported by this upgrade to 11B because they used the same frequency the same transmission scheme is that you could quite easily upgrade your access points to 11B and maybe some of your laptops were even upgraded and it was very easy to also support association between the old laptops using the original standard and the newer laptops using 11B with these newer access points because of they using the same frequency the similar radios were used in the new access points so just by upgrading the access points and some of the laptops we can still use the old 802.11 in some of the laptops it means you don't have to upgrade every piece of equipment at the same time and we see that with 802.11G and also parts of 11N we'll see that but it's both frequencies so 11G was the same 11G was another improvement four years later released speed went up to 54 megabits per second the key point it was backwards compatible with 11B and the original 11 which meant that an organization could upgrade some laptops and or access points to support B and G and those access points could support laptops or clients using either 11B or 11G so you don't have to upgrade all the equipment at the same time and that's especially important to support organizations that have already built a network rather than requiring every user to change their laptop they can just upgrade the access points and as people get new laptops they'll upgrade to the higher speed whereas 11A required a different radio a separate radio inside the access point and therefore people didn't buy 11A devices it was faster than B but not so widely supported nowadays you can buy devices that support all of them because the electronics has become cheaper the radios are cheaper you can buy devices that support A, B, G and N all in the one device sometimes they have two different radios so the speed is not always the most important thing that drives the acceptance of technology other factors impact A2211N is the main one available today and this is missing it supports both 5GHz and 2.4GHz it supports both higher data rates in fact now up to 600Mbps but it requires some special characteristics the range all around the same just to finish one last picture this is similar from the original A2211 B, A, G, N and the newer ones which are expected in the next year or two 11 AD and 11 AC but we see with B, A, G and N bandwidth around 20MHz N supports both 2.4 and 5GHz and N goes up to 600Mbps why? I had 300Mbps in my table N supports the case of you can use twice the bandwidth normally 20MHz is used but N supports you effectively double that bandwidth you transmit a much wider bandwidth signal gives us double the data rate so they're the most common ones today the next two almost for very special special purposes use completely different frequencies 2.1GHz sorry 60GHz and 5GHz the 60GHz is for very very short range communications centimeters maybe a meter say video transfer of a very short range we talk about gigabits per second there but they're expected in the future and AC still using 5GHz but again much higher bandwidth used what we'll do next week is we'll talk about why for example using multiple non-overlapping channels can improve performance interference effects performance and then we'll focus in detail about the MAC layer medium access control layer enough for today