 We've spent a lot of time this semester so far looking at how to get data as signals across a link and more recently how to make sure that that data is delivered correctly in the presence of errors. We have for example error detection, error correction, retransmissions and to make sure that we deliver the data efficiently we have flow control mechanisms. So we've been focusing on links. We're going to move on today but the last thing we want to talk about links is multiplexing and the concept is very simple. Even though there may be 10 or 15 slides it only takes us 10 or 15 minutes to cover this topic. We'll talk about the main concept. What do we mean by multiplexing? The concept is if we have a link from A to B we know the techniques for getting data from A to B. We know that we transmit a signal that signal is at a range of frequencies and usually we denote some centre frequencies so if we've seen plots like this in the frequency domain we transmit a signal that signal has some bandwidth. In this picture the width of the green shape is our bandwidth and we have a centre frequency and we encode that data, the zeros and ones using one of many different techniques we've studied. We've looked at shift keying techniques, amplitude shift keying, frequency shift keying, we've looked at non-return to zero, Manchester encoding and so on, other encoding techniques. The media that we have available we've looked at examples of wire to wireless media, it could be a cable or we could be sending a radio wave wirelessly. So we send signals to carry our data from A to B. The question arises is if we have more than one piece of data we want to send across the same link how do we do it? And the scenario you can think of is instead of A being a single computer think of we have A as one location like our campus here, B is at Runxit campus. Multiple users, multiple users here on our campus and we want to communicate with multiple users at the Runxit campus. How do we do that? How can we send the data of multiple users from one location to another? Well the first approach, how can we support multiple users? Give every pair of users their own link. For example, if as a simple case there are four people or four users or four computing devices here at Bunker D campus and they want to send to a corresponding device at Runxit campus then one approach we could do to support multiple users is each pair has their own link, maybe their own cable from computer source A to destination A at Runxit and then from another source at Bunker D to another destination at Runxit and so on. For every user that wants to communicate have your own cable. It would be like you have a cable to someone you want to send to at Runxit and everyone else in the class has their own cable. I think you can see that that's not very efficient use of resources. We're not scale very well as we add more people we can't have a separate link for every pair of users. So the concept, every pair of users has a link between them and they transmit their signal. So in this picture I show the signal for example source A transmits a signal carrying its data to destination A where that signal has some bandwidth B and is centred about some frequency F1 and we get the signal and we've received the data and at the same time the good thing about this approach everyone having their own cable is that we can all transmit signals at the same time in parallel getting our separate data to the destinations and if the cables are designed correctly they will not interfere with each other. So the good thing here is that we can transmit a signal along the cable from the two blue computers and at the same time another signal between the two green computers they can be using the same range of frequencies they're both centred on F1 the same bandwidth but they were not interfere because the way we designed the media is that they were not interfere with each other especially with wired media and similar with others. Of course it's very wasteful of resources we have a cable for every pair of users wanting to communicate and it's hard to expand deploying cables is expensive. I want to have a link from here to rungsit well let's say it's a wired link if I want to put a new link in I need to dig a hole under the ground or at least pay someone who has some conduits under the ground to lay my cable through and that's actually quite expensive if you want to cover across an entire country then digging the hole under roads under cities is a very expensive task and we don't want to have to do that very often. So having a cable per user just most cases doesn't make sense especially when we cover long distances. So what multiplexing is about is taking the data from multiple users and sending it on one cable or one link in general and there are two main approaches. So the approach in general of multiplexing from our two locations they bunk it in rungsit we have one link between the two locations and we introduce a special device at each end. So we have our sources and our destinations each source has data to send they send it to this special device a multiplexer and the multiplexer combine the signal from those four sources and creates one output signal that goes across the link. So the signals from the four sources are going to be combined in some manner and then send the multiplexer will send a single signal to the demultiplexer across the link and the role of the demultiplexer is to take the received signal and then extract the data and send it to the correct destination the data from source age or the signal from source a needs to be sent to destination a and so on. That's our concept of multiplexing we use a multiplexer and demultiplexer or MUX and demux. Somehow we need the multiplexer to combine the four produce one output signal. So there are two basic approaches one is that we combine the signal using a different set of frequencies and that's called frequency division multiplexing and the second approach is we combine the signals by transmitting at different times for each user time division multiplexing. Let's look at them recall think each user has data to send but focus on the the data from the perspective of the signal each user has a signal to send a data it has some bandwidth B and let's say it's centered about some frequency F1 maybe F1 is 10 kilohertz and the bandwidth maybe five kilohertz so that the bandwidth says it ranges from one frequency up to another centered on F1 they all have the same bandwidth for this example. What we do with frequency division multiplexing is that those four sources send their signal to the multiplexer bandwidth be all the same center frequency F1 and the multiplexer creates one signal that comes out which combines the four signals from the sources in such a way that they are shifted to different center frequencies if you think this is the signal being transmitted by the multiplexer at the first range of frequencies contains the original signal from source A centered in this case at frequency F1 the signal from source B which previously was centered at F1 is being shifted and is centered at F2 but it has the same bandwidth and for source C it's at F3 with the same bandwidth and source D F4 the same bandwidth the idea is that we can transmit one signal but that signal actually contains the signals of those four different users we must transmit them at different frequencies because if we transmitted everyone's signal at the same time at the same frequency that all overlap with each other and effectively interfere with each other it's the same as is everyone talks at the same time in this room we're all transmitting at the same frequency so you're all interfere with each other and I will not be able to hear you or you will not be able to hear me so what we could do transmit signals at the same time but at different frequencies and the role of the demultiplexer is to extract the individual signals and send them to the corresponding destinations so what it receives around the F1 frequency it sends to destination A what it receives around the F3 frequencies sends to destination C so this is one way to combine the signals from the multiple users and send across one link the signal sent across that link from multiplexer to demultiplexer has a bandwidth at least four times B in this example the bandwidth of each user their signal is B B Hertz if we're going to combine them and make sure they don't overlap then we need a bandwidth of at least four times B and in practice we usually have some spacing between those signals so that they are not exactly on top of each other but there's some spacing so it's greater than four times B that is the signal sent between the multiplexer and demultiplexer across our link has to be four times the bandwidth of the signals from the original users larger bandwidth we know has a higher cost we generally want to minimize the bandwidth this scheme requires a higher bandwidth here if we have a thousand users and it will be a thousand times B a thousand times the bandwidth of each individual user that's one of the issues with using this approach we need to if we have a bandwidth a fixed bandwidth for this link it determines how many users we can support at a particular bandwidth for that user this concept is commonly used and you see the similar concept used in wireless systems or cable TV for example cable TV the TV there's one cable a coaxial cable that comes into your house but the signals from multiple TV stations are sent along that one cable and they're sent at different frequencies so they don't interfere with each other and all your TV does is tunes into the right frequencies it filters out so cable TV is similar except there's a single destination but multiple sources but the way that it combines the signals from the multiple TV stations is using frequency division multiplexing you give a different frequency to each TV station the frequencies used in this so refer to as we assign channels so we may say this is channel one channel two channel three channel four and you know that in TV and radio stations they're assigned channels or channel numbers in many cases and that's meaning it's at a particular center frequency and using frequency division multiplexing another view of that the concept of frequency division multiplexing is shown here where we transmit the data from all users at the same time so on the same time scale but we transmit using different frequencies so the channels correspond to frequencies in this case this is a case where say we have six users we have six channels we transmit their data at the same time but using a different frequency for each if for example we have our link and has the link has a bandwidth of the total bandwidth equals 100 kilohertz and the bandwidth per user is 15 kilohertz that is each user generates a signal which occupies 15 kilohertz but our link from A to B has a bandwidth available of 100 kilohertz how many users can we support using FDM how many how many users can we support how many channels we can transmit a signal with a bandwidth of 100 kilohertz each user wants 15 kilohertz if we have six channels each user has 15 kilohertz with six channels that occupies what 90 kilohertz so if we have some spacing between those channels then that will probably bring us out to our 100 kilohertz if we try to fit in seven users seven times is 105 kilohertz we'd need a bandwidth of 105 kilohertz to support seven users but we only have 100 so here we'd have a upper limit of supporting just six users in this link a similar example is with Wi-Fi our Wi-Fi access point on the wall here with Wi-Fi the bandwidth allocated for the range of Wi-Fi signals what is it wireless LAN the total bandwidth is 80 megahertz and again we have different channels when we use Wi-Fi we can when an individual computer transmits a Wi-Fi signal they don't use the full 80 megahertz they use a particular center frequency one channel and the channel bandwidth one channel one computer transmits at 22 megahertz how many channels could we transmit on at the same time using Wi-Fi without interfering with each other three if we use four it would go up to 88 megahertz we only have 80 megahertz available each channel occupies 22 megahertz each user when they transmit a signal their signal has a bandwidth of 22 megahertz so what we could have is one user transmitting at the lowest frequency range or the one low channel and another user transmitting at another channel such that their signals do not overlap and a third user transmitting at the same time and the higher channel gains that none of those three signals overlap so there's no interference between the users three individual parallel transmissions happening at the same time has anyone know the number of channels supported in Wi-Fi anyone set up their Wi-Fi access point or router at home you can sometimes select the channel you've seen or maybe you see the channel number when you connect or you often see channel numbers like channel one channel six channel eleven it depends on the country there's between eleven and thirteen channels available with Wi-Fi but we said we can only have three channels that don't overlap it turns out that you can have channel one channel six and channel eleven and they will not overlap when we transmit but if you use channel one and channel two there's an overlap and if I transmit on channel one and your device transmits on channel two they will interfere with each other so that's not very effective so in fact with Wi-Fi in the 2.4 gigahertz band there are really three non-overlapping channels that we can use so you're often people often use channel numbers one six and eleven they're the three non-overlapping channels it's frequency division multiplexing give each user a separate frequency such that they were not interfere with the others the other approach give each user a different time slot to transmit so that they were not interfere with the others time division multiplexing here we'll transmit a signal between multiplexer and demultiplexer with the same bandwidth as the original source users so we will not go up the 4B but we'll transmit the same bandwidth but to transmit the data from user A we'll transmit for one short period of time so plotted versus time the signal here we'll transmit the data for user A for one period of time then we'll stop transmitting his data and move on to the next user's data and transmit for a short period of time user B's data and then after her data sent we send user C and then user D and then we come back to user A's data is transmitted for a short period of time and we keep going like that we assign short usually it's usually short but we assign time periods for each user time slots they are called you take in turns talking essentially so if multiple people want to communicate then one speaks for a little bit then the next one then the next one then the next one then we come back to the first and we take in turns all transmitting at a bandwidth of B we divide by time we divide the space in the signal by time the channel the link only requires a bandwidth of B in this case so coming back to our first example with time division multiplexing if our users required a bandwidth of 15 kilohertz so this was FDM if we use TDM time division multiplexing if the users had a bandwidth of 15 kilohertz what's the total bandwidth needed for our link with time division multiplexing we just need the same as for each user 15 kilohertz that is we don't need separate frequencies for each user we all use the same frequency but at different times so here's a good thing about TDM the users require 15 kilohertz for example our link only needs to support 15 kilohertz whereas with FDM if our users are the same 15 kilohertz our link would need to support 100 kilohertz in our example if we I think we had six users in our example so FDM requires a larger bandwidth larger bandwidth larger cost that's the problem what's the problem with TDM time division multiplexing slow in what way is it slow you're on the right track it may be slow or performance worse under the same conditions what why the user only wants to use 15 kilohertz say okay so they still get their 15 kilohertz the problem with TDM is I transmit for some period of time and then I stop and wait for the others to transmit effectively you don't get to send your data all the time you only get to send a portion of the time so back to our slide user A is not that or the data from user A is not being sent across this link all the time it's only being sent one quarter of the time with FDM the data from user A is being sent across the link 100% of the time at a different frequency from the other users here it's four times worse performance because we only get to send one quarter of the time the other three quarters of the time the link is carrying other people's data so it turns out that in terms of performance FDM requires larger bandwidth four times as much bandwidth which is a bad thing but the bad thing for TDM you get four times as less performance that is your data is only being sent one quarter of the time through the link so effectively they become the same in terms of performance so it's a trade off some differences in how to implement them as to which one's better so they actually arrive at the same performance in theory because we either use four times the bandwidth or we get four times the less time to transmit and we also know of course bandwidth relates to data rate if we use four times the bandwidth we can get a higher data rate so in other words if we use TDM and 100 kilohertz we could get a higher data rate per user but they only get to be sent one sixth of the time so the data rate per user becomes the same with FDM and TDM under the same conditions for the link between A and B the users will get about the same performance TDM is used in a number of other we'll see some example other digital transmission systems it's common that nowadays when it's easier to generate the signal and split by time and often more efficient it also has the advantage with TDM is that in some special cases in the normal case the assigning of time slots to you to user is fixed it's always A, B, C, D, A, B, C, D if there's four users they get one fourth of the time to transmit a more dynamic scheme would be to assign time slots based upon demand if user A has a lot of data to send users B, C and D only have a little bit of data to send maybe assign two time slots to user A and only one to B, C and D so more complex schemes could be dynamically allocate time slots so that the users get better performance if they need it and then later when they don't need it you can give it to someone else and that's outside of the scope of our topic today that's it multiplexing user link to support the data of multiple users by either transmitting the signals of those users at different frequencies FDM or transmit the signals of those users at different times time division multiplexing allowing the data from multiple users to be transported across one link this approach give the users a separate link is often sometimes referred to as space division multiplexing give them different space physical space a different link which is not common because in this case it's very expensive to finish where they used FDM is used in broadcast cable TV radio transmissions some of the older long distance communications that a telecom company would set up links to cover Thailand so to have links to go from Bangkok to Chiang Mai in the older cases they may have used FDM for some wireless transmissions optical fiber used today uses the concept of frequency division multiplexing you have a single fiber you transmit the data from multiple users by sending a light at different frequencies what's the inverse of a free of frequency or invert right what's the the the not direct inverse is proportionally in inverse proportional to the frequency is the wavelength okay remember wavelength equals the speed of light divided by frequency speed of light is fixed so wavelength division multiplexing is just another name for frequency division multiplexing frequency division multiplexing give the users different frequencies or give them different wavelengths if you have a different frequency you get a different wavelength just turns out when we talk about optical and light sources we refer to the wavelength as opposed to the frequency but it's the same concept your home ADSL internet if you have internet using ADSL at home that the basic concept is to use frequency division multiplexing you have a range of frequencies that you can send across your copper line the copper telephone line and this is one example where the frequencies are split into three channels or three ranges the lower frequencies are used for transmitting your voice when you make a normal voice call using your telephone line a plain old telephone service you transmit with the lower frequencies on that one copper telephone line a range of frequencies are used for upstream traffic sending from your computer to the ISP upload and a range of frequencies used for downstream or download traffic and those the signal sent across the telephone line are split based upon frequency for voice upload and download some you probably haven't heard of for TDM especially links across cities between cities and between countries that are run by large telecom companies they use nowadays time division multiplexing some of the names of the technologies which we will not talk about Plesioncranus digital hierarchy pdh synchronous digital hierarchy and sonnet some of the technologies used for wide area networks we may mention them in a later topic when we come to lands and wands we will not talk about multiple access right now