Suppose I have an electric dipole with me, which is basically two equal and opposite charges separated by some distance. And imagine I go far away from this dipole. So I zoom out, zoom out, zoom out. In fact, I zoom out so much that the dipole looks like a single dot to me. Then the goal of this video is to derive an expression for the electric field somewhere far away on the axis of the dipole. An axis basically means a line passing through both these charges. Before we begin, a quick question. Why are we doing this? Simple answer, it's in your syllabus. But a better answer would be, we've seen in a previous video that if you have any other group of charges, then the electric field far away is simply radial. Very similar to the field that you get due to a single point charge. But the field due to dipoles are special. Now these field lines do help us in visualizing what the field looks like everywhere. But if you want to figure out, for example, how does one dipole interact with another dipole somewhere far away, let's say? Something that could be super important chemistry because a lot of chemical molecules are dipoles, then we need to look at this mathematically. And that's why in this video, we're focusing on what would the mathematical expression of the field look like on the axis. So let's zoom back in and we've got to figure out what the electric field is somewhere on the axis. So let's choose a point somewhere over here. But we're going very far away, okay? And that's why I put the dotted lines over here. And that distance from here to the dipole, we're calling that as R. Now one immediate question is, where should I consider R from? Should I consider R from this charge to this charge? Is this my R? Or should I consider it from this charge to this point? Or should I consider it from the center? Most textbooks and derivations do that. It's a very safe way of doing things. But I like adventures. So I'm not going to consider R from the center. I'm not going to follow the textbook. I'm going to consider R from here. Why? My argument is, look. When we're going far away, it really doesn't matter whether you're considering R from this charge, whether you're considering from here, or whether you're considering the R from the center. Because from far away, this looks like a dot to us. And so the final answer should not depend upon where you're considering R from, from here or here or from the center. You should get the same answer. And so I see an opportunity to be a little adventurous and do things a little differently from the textbook and see if we get the same answer. Sometimes you might make mistakes and that's okay because you'll learn something new. So let's begin. How do I, how do I start? Well, I want to know the electric field over here. And there are two charges. So I can use the superposition principle, which says you find the electric field here due to this charge alone. Find the electric field here due to this charge alone. And then add them vectorially. So first let's draw the vectors. What would be the direction of the electric field at this point due to this charge? Well, remember, electric field is basically force acting on one a coulomb of charge. So if I keep one coulomb charge over here, it gets repelled by this positive charge. It would go to the right. And so the electric field due to the positive charge would be to the right. So let's call that electric field as E plus. And the electric field due to the negative charge where it pulls the coulomb. So the electric field due to the negative charge would be this way. So that would be E minus. And so the total electric field would now be a vector sum. But since they're in the opposite direction, we basically have to subtract them. And so the immediate next question for you is, what do you think will happen when we subtract them? Do you think the two fields will cancel out or not? Can you pause and think about it? All right, my first instinct would be to say that, hey, point P is so far away from the dipole that it's pretty much the same distance from minus Q and plus Q. And if the distance is the same, then the electric field values are the same, which means they should cancel out. But that's wrong because if that was the case, then the field everywhere would be zero. And we know the field everywhere is not zero. There is an electric field somewhere, which means this tiny distance matters. And we need to consider that. So if I consider that, we can see that point P is slightly closer to the negative charge compared to the positive charge. And therefore the electric field due to the negative charge is slightly bigger than the electric field due to the positive charge. And as a result, we will find that the net electric field would be in this direction. Does that make sense? This should be the total or the net electric field. All right, so now let's go ahead and figure out what that net electric field is. So from superposition principle, that net electric field will be, because this is bigger, I'll say this minus this. So it's going to be the electric field by the negative charge minus the electric field due to the positive charge. And because we're dealing with one dimension, I don't have to worry about vectors. I can just simply subtract them and that'll give me the answer. All right, what is the electric field due to the negative charge? We've seen the electric field due to a point charge expression. It's going to be KQ by R square. So it'll be again, good idea to pause and see if you can substitute yourself and see what you end up with. Okay, let's do this. So if I consider the field due to only the negative charge, that would be KQ divided by R. What is the R over here? Well, the distance is R. From here to here is R. We consider that as R, so it's going to be R squared minus what is the electric field magnitude due to the positive charge? Well, that's going to be KQ divided by this distance squared. The distance from here to here is R plus D. So it's going to be R plus D, the whole squared. And what's interesting is, remember, D is much, much smaller than R, right? Because you've gone very far away. And so can I neglect D over here? Can I say R plus D is pretty much the same thing as R? Well, you can't because if we did that, then you'll get the same problem. These two will cancel out, you'll get zero. So you have to be careful when you're approximating. That's what makes this interesting. So this means we need to simplify it a little bit further before we start approximating. That's what it means. So let's simplify a little bit further. What I can do next is KQ is common, so I can take K times Q out. And what I get inside is one over R squared minus one over R plus D, the whole squared. If I simplify even further, I will now get, let's see. If I take common denominator, in the denominator, I get R squared times R plus D, the whole squared.  Okay, what do I do next? Well, let's see. I can expand this.  So R squared cancels out. And now let's see if we can approximate. We can use this. If I do that in the denominator, I could say, hey, let's ignore D because R plus D is pretty much the same as R.  And I can do that because that's not giving me zero. As long as you're not getting zero when you approximate, that is valid. So when I approximate in the denominator, I get one over R to the power four. What do I get in the numerator? Well, I have a D squared and I have a two RD. Can you see that D squared is way smaller than this term, R times D? Here, let me give you an example. Let's say R was, I don't know, maybe a hundred and D was, I don't know, maybe 0.1. Then D squared would be 0.01. But R times D would be 10. 100 times 0.1 is 10. And so if you compare, 10 is much larger compared to 0.01. And so you can see, I can neglect this. And therefore, I can now say that the net electric field is going to be, okay, let me write that over here. I can write that over here. So I just give this as two RD, two RD. And let's see what I get. One R cancels out and I give this to be three here. And so if I write it down, I end up with, let's put two K out. So I get two times K times Q times D. D divided by R cubed. And so now can you tell why if we had directly negated D over here, we would have gotten zero because we would have neglected this term and this term. And so the whole numerator would have been zero. Because we simplified, we got a choice of just negating this term and we considered this term. This is how we approximate. Beautiful, right? And here is our mathematical expression for the field on the axis of the dipole. But let's see if we can make sense of this. That's the final thing we'll do. The first thing I want you to focus on is see how it's dependent on the R. It's one over R cubed, which means that the dipole field is falling much faster than the radial field, which is one over R squared. Does that make sense? It kind of does. If we go back and look at our dipole visualization, you can, if we compare this with the radial one over R squared field, then you can see from the field lines itself that the dipole field, as you go further away, look at that, it goes farther away from each other much quicker, much quicker than the radial field. And so it makes sense that it should die out much faster than one over R squared. And so one over R cubed kind of makes sense. The second major difference we are seeing between the radial and the dipole field is that the radial field only depends upon the total charge, whatever is the total charge. If this was not a dipole, then it will only depend on the charge. But look at what the dipole field depends on. The dipole field depends on the product of the charge and the distance between them. If I had another dipole, or if I took the same dipole, and let's say I doubled the distance, but I made the charge value half, the product would stay the same. And so when I go far away, it feels like it's the same dipole. So the identity or the strength of the dipole is not in their charge or in the distance, but it's the product. The product decides the strength of the dipole the product decides how strong the field is going to be. And that's the reason this product for a dipole is given a name, it's called the dipole moment, which is represented by P. And so finally, finally, we can now write the expression for the electric field on the axis is two times K, which is the Coulomb's constant times P divided by R cubed. And what is the direction of this electric field? You can see over here everywhere to the right, the electric field will be towards the left. What if I go towards the left of this dipole? The electric field would still be towards the left because positive charge will be dominating and so it'll be pushing. And so again, if we go back to that, you can see everywhere on the axis, the electric field is to the left. Let's come back over here. And it's for that reason we said, let's say the dipole moment is also to the left. The direction of the dipole moment is also taken to be towards the left. So the dipole moment is from negative charge to the positive charge. And so we can say everywhere on the axis the electric field is in the same direction as the dipole moment and so vectorially, I could say they have the same direction. Of course, one small detail is if you were to look at the field inside the dipole, then it would be from positive charge to the negative charge. It would be in the opposite direction of the dipole moment. But remember, this is the field far away from the dipole. So when I'm looking at that field and not looking at the field inside the dipole, I'm looking at the field far away from the dipole. So I'm always looking at the field outside the dipole. So remember, this is the expression for far away outside the dipole.


Hi everyone, Joe for jasphyscasebrakes.com yep, we did it well look we had a little bit of help We we got so close we ended up doing these brakes and these brakes right here, and you know what? We were feeling nice. We just said screw it. We won't include the wild card box in there But we will do everything in fact This was the only one kind of kind of lingering and we said listen We'll cancel this if we do this We'll do the promo and everybody rallied around it and made it happen. So thank you very much We're giving away $800 of brake credit so that to the people involved in all of these brakes right here or randomizers aside from that wild card We're gonna take all of your names. We have a big master list which I'll show you in a second We'll randomize all those names top eight get a hundred dollars of brake credit each So there's that brake credit promo about to do right now. So let's go to the 622 promo tab and there it is So if I highlight this right here, you'll see six right there in the bottom right-hand corner So that's the the Palakins only number block the optoprism mixer the hit parade hockey helmets NT 17 NT football 17 Not to be confused with with NC 17 now We're a very family friendly show plus the filler pack number 63 and prism basketball first offline box number five So you can still see six right there on the bottom of your screen because that counts all the all the Information in those cells. So there's the look at a little bit closer right here. So there's the mosaic 63 filler Then from there on down is the actual NT football break itself. So yes, if you won mosaic spots You did get double spots And that mosaic 63 not 62 From Paul on down that's the prism basketball first off the line break number five There's the autograph mini helmets hockey mini helmets There's the optoprism break And there's the ten spots For that random number block the Palakins only for that encased. So that's a total of 167 names. That's actually pretty decent odds for for a promo of this caliber top eight after 167 is is pretty good Usually this balloons to like two three four hundred names But this time pretty small so odds are a little bit better Let's grab all those names. So again, there should be 167 names And once again, thank you everybody for getting in. It's a nice little promo Which generally it's we were probably gonna do these breaks anyway We might as well do these breaks Especially when there's some sort of promo happening here. So thank you very much I really appreciate your support of just be case breaks calm. Let's roll it. Let's randomize that whole list Wow a full 12 times six and a six So after six the top eight hundred dollars of break credit each yes, you can win multiple break credits Good luck everybody Cross your fingers one two three four five six seven eight nine ten eleven and 12th and final time after 12 times Right after 12 times Good luck So yeah, a lot of sad people will hopefully not that sad you were involved in a break Maybe you got some nice hits out of there, which I I appreciate you getting in But if you didn't I appreciate you trying at least you had a chance at trying to get a little bit of money back from us And hopefully the people in the top eight whether you hit if you hit hey There's a nice cherry on top to end the evening if you didn't hit tonight with those top eight then Listen, you got a little bit of break credit back and you can join in some more breaks on jaspy's case breaks common fire again I will email this out to everybody after this video and Please check your if you have multiple emails that you that you use to shop with us Please check those check your spam folders So and check it out. All right So now the worst spot is spot nine AJ my apologies clothes unless you're in the top eight my apologies close Mino cigar Apologies to everybody, but I appreciate you trying sad times But very happy times for the top eight Congrats to Paul Jared Greg Sean Sean Matthew Shearer miles Evans and Joshua Edelitz with a spot that he won in that mosaic break Congrats to you. There's the top eight $100 of some some jaspy's case breaks calm break credit Going your way. Love that stuff. Thanks again. Thanks everybody really appreciate it. I'm Joe for jaspy's case breaks calm I'll see you next time for the next one. Bye. Bye. Good night


Hey there friends, how's it going? My name is Kevin and today we're playing roller coaster tycoon. It's taken me a while to play these games It's actually amazing that I haven't I've played park attack and I've played planet coaster and now here we are and we're gonna make a Fantastic park where no one dies. I would assume But let's get started. All right a blank slate. How is this gonna start? What are you doing? Why are you here? I gave you a friend. Oh God other people are showing up as well. All right He'll be our security while we set up the park. Then I'll get rid of them. Okay. We've got our first ride It's not tested, but I trust it. Go ahead. Also. I couldn't get the slide working. That probably doesn't inspire confidence I can't figure out a slide, but I figured this out. So use this thing No, that's the customers enjoying their time here.  And yes, I just called myself God Sorry about that folks. Sorry They was complaining. There was no path to get through from the exit So I had to remove some of the track for the go-karts. They're gonna start thinking that ship literally crashed here Oh, oh there goes another one. She looks like she's done the time for a life. Their life won't be for long though Oh those poor people. I know how it feels trust me. I've been there. We need more cars That's the problem.  Yeah go-karts go All right, who's first for the call me Kevin simulator 2020 we got some volunteers here. Oh, yeah That is exactly how it happened I got hit by car by the way if you don't have the context Okay, it didn't explode isn't exactly how it happened, but it's like the movies You know they dramatize stuff to make it more exciting. Look at this. There's just a line of people to be just like me. Oh My god, you've received a fine most dangerous most criminally negligent even the word tidiest parking Incoming sorry folks All right, I'm gonna make the go-karts free in a Let's see national newspaper campaign or parked value has gone up to 15,000 somehow even though I'm sure a lawsuits are like 15 million at this point What is that noise? Oh, it's the go-karts coming in. It's like it sounds like angry bees. I'm putting all like the facilities right here So they all have to make a queue in this area where the go-karts come in first aid is here There's a winning combination. All you got to do is charge people for first aid and we'll make a fortune There we go. Now that is a busy street This is the high street now all of a sudden all people are just getting off the ride This is perfect time for the call me Kevin simulator 2020 to strike. There you go. That's a multi-kill That person with the afro just didn't care I'm gonna make an enclosure here and we're gonna have a little viewing gallery to what animal can we have in here? I think a grizzly bear that should hold a grizzly bear, right? And the thing is I don't want this to be like animal centric So I might just put a few animals in the same enclosure. Why are they circling like that menacingly? I don't like that.  Oh god I don't think they need it. Okay. One of them is dead. Anyway, I don't know who died I think it was the poor camel. That's a shared meal. At least they're willing to share their food I think the problem is I put in too many predators and not enough prey and the last thing you want Someone saying about your park is there are too many predators Maybe they're getting riled up because the rides there, but no one's using it Okay, that didn't sound good. I don't almost think that sounded like a wooden fence and caging some animals Breaking, but no that can't be And again that that sounded kind of the same, but let's not fear the bull ride thing is open We definitely need more prey just put in a load of horses here. There we go. A lot of wild horses in here now I think this is just agitating them actually that's not even me adding in sound effects like I usually do That's just them screaming Henry. The horse has been killed by a predator.  I Can't keep giving you prey like this it takes so long to even place them because you guys are all over the place I need a better system now. We got some sort of like uber eats delivery here offer fix sake They somehow avoid it. How can I solve this issue? I think the animals just exploded there There we go, that's what I'm talking about straight into the pen It's like the uber eats of the animal kingdom nature's so cool stop giving me alerts I got more food coming in for you folks. Don't worry. Look here. They come we Have Look they got away, okay We got like a 50% success rate here people coming in three have made it in three have Somehow saved but they're locked in that cage now willy D is trapped in an enclosure. Sorry willy D Oh, God the fence is falling apart. Wait. Is he dead? Are these animals dead or sleeping? Okay, they're sleeping good. We've enough human deaths on our hands None of mine animal deaths. No God here comes more food.  Sorry. I thought she was gonna clear it over the enclosure there That would have been a sick jump to be fair That's a lot of alerts and can I get some staff or something to take care of this fence cuz it's kind of falling apart I like how the fake bull riding thing has barbed wire, but the actual grizzly bears only get a wooden fence Okay, I put an animal keeper in here.  They're just gonna starve. Okay. The mechanic is fixing the enclosure Fantastic the humans are like no that was our only way out. What is going on with this right? They don't seem to be going anymore.  I like to call them Oh god, they almost didn't hit anyone there, but those people conveniently walked in the way. That's insurance fraud. Oh Okay, that wasn't that was people were just gravely injured or possibly killed. I know things are going wrong Okay, the animal protection society are threatening to confiscate Gabriel the gorilla because his health is very low Everyone cares about Gabriel the gorilla and no one cares about Who are you hold on Karen Kay who's fleeing from Tyler the tiger?  I can't hear the ambient bird song over you guys screaming. Oh There they are. Oh, that's kind of nice I'm concerned because this road is getting a bit busier than this road now And I feel like the high street should have some risk to it Ignore the crumbling. That's a different type of risk entirely. Oh sweet Jesus This is absolutely falling apart the mechanic needs to get his act together and build more wooden barriers. Oh Jesus Christ Okay, that was too fast. Could we see that again, please? Oh God we're getting feckin emergency helicopters in and everything what is going on other taking away my animals Look at all these alerts. Okay, this is quite the intense ride because it goes all the way up then all the way down and just Everyone Jesus Christ. That's a weapon of mass destruction So it is this is fecking chaos like even just stopping and watching this for like five seconds I'm like, oh my god. There's so much to digest here. They're taking away my animals you to extreme neglect Why does no one do this for the people? They're all being neglected to Jesus Christ. That is brutal There we go.  It goes so fast and up so high and then suddenly goes right on down This way don't like roller coasters. You can't see what's coming. I miss my animals You can take my animals, but you can't take my right to buy more of them Well, you probably can but you didn't and you really really should to be honest I just bought a ton of chimps and I will buy one panther and we'll just see well He is similar with the tribe. He's immediately starting to murder them. This experiment didn't work at all He's not even eating them. Come on. That's not what the circle of life is all about You're just killing them for the sake of it quick distract him deliver some people on quad bikes here come the people there You go. There's a distraction for you. Oh Christ I'd say computers back in the day wouldn't even be able to process what's going on there You need a beefy computer to handle that much death.  That's kind of nice Those birds flew by and I thought they were bodies because I'm just so used to that. There we go It's done. Oh, there's nothing fecky about it.  Although. It's not that crazy. It's just tame It's tame golf actually. There's one single hole and then that that's the end of it You can move on to the quad bikes or something. I just like golf, okay?  All right, this guy kind of deserves to die though He's doing pretty bad and this person's not letting him finish his turn.  This is chaos. There's another one coming. What is going on? I put my own custom song on for the golf There we go. Select a genre. Hmm. What I describe this as trance How I'm gonna go with alternative and enjoy your Have a good time blank. I've won another award tidiest park. That's nice. I'd ignore the other two. It's like, oh my god What's that screaming? Oh, that's just a crazy golf Yeah, it's pretty intense. Oh hole in one by the little girl there great job Kelly T You're kind of making everyone look bad though. You're going in with the panther Kelly T All right, you're next on the chopping block. Oh, she's getting away. She's getting away. Pick her up Pick her up. All right. I have her at the ready. Here comes the ride. Drop her. Drop her. Yes. Take that Kelly T Oh my god. She survived. She's the only person. She's overpowered who got sick. Come on That little oh no, right drop her. All right. Good. Good. Come on in come on in. Okay. Yes, Kelly T is gone Poor Kelly. All she did is get a hole in one a tame golf. Oh for fuck's sake. We got another hole in one All right, come with me. I actually like that screaming. It really sums up the park Can we make every ride scream like that? There we go. Yeah, it's just it's just coming from everywhere and they're oddly in sync as well God that is awful. It's just a trademark of the parks. I make that there's got to be like a lot of screaming on the rides All right, that might be a good time to end it. They're all deltries God this was a horrible horrible back If you want to see more of this game, let me know there's so much more room to expand like we just did a tiny chunk Look at all the land available. Oh, it looks like Ireland reminds me a home, which I hate I'm here right now and all it does is rain. No, but I think we're gonna leave it there I hope you enjoyed the video. I had a lot of fun with this game. This was a nice quirky thing to play I hope you're staying safe in the strange times we find ourselves in and I hope to see you next time I appreciate watching as all those folks and that's about it. Bye for now


So we look at, we said yesterday there are tasks involved with getting data across a network as opposed to just individual links and one of those tasks will call switching. So we'll talk about that in this topic and the other one which is a little bit easier to understand is routing, finding the path, finding which set of links to go by. But before we talk about switching let's talk about what do we mean by a network, a communications network, we'll call it a switch communications network but I'll explain some of the terminology we'll use via this example, a simple network to illustrate the components. We have our end user devices around the outside, PCs, any computer that the human may use, servers, any device that we want to allow to communicate with other devices on the network. We may call these stations generally so whether it's a server or a mobile phone we can refer to it as a station. Other times we may call it a host. So we use the name station or host to mean the same thing that the devices that we want to allow to communicate but because we don't have links between every pair of stations for example station A and F there's no cable directly linking them then to communicate between A and F we need to send via other devices and these other devices are shown as the green boxes and we can call them nodes or specifically switching nodes and we'll see some other names in other contexts for these devices, these intermediate devices. Now at this stage we'll refer to them as just nodes, switching nodes or just switches. We'll see why switching comes into it in a moment. So the idea is if A wants to send data to F because it doesn't have a link directly to F it sends to some other node inside the network. So this cloud represents the the communications network made up of these nodes. The lines are links whether wired or wireless but some form of link and we can think that the stations attached to the network and these internal nodes deliver the data from one station to another. One of the problems we said was finding the path so now from sending data from A to F we need to send it via these internal switching nodes. Which ones to send via is one question. Do we send four seven six or four five six or four one two three five six to get from A to F? That is what we'll call routing, choosing the best path. And that's the next topic. To discuss this topic we'll assume that we can choose a path. We will not say how but let's assume that when we send data the path is chosen. It's already known. So we have stations that want to be able to communicate. We have a network of switching nodes which provide connections hopefully such that any station can communicate with any other station. And you see in this case that that's true. If any station wants to be able to communicate then they just send it to the network and there's always a path to each other station. Some aspects of this network so we've got stations, nodes. These nodes their goal is to just send data for other people. The stations may create data. They are the originators of the data. They may be the consumers of the data. They receive it and process it. Whereas the nodes their goal is just to forward data for other people. So they have different purposes in here. Sometimes they may overlap but often the nodes their only job is to receive from someone else and send to someone else. Typically the stations will connect into a node via a point-to-point link which is usually dedicated for that station. So A has a link into node 4 and that link is only for communications between A and node 4. No one else uses that link. The data from no other device travels across that link. Only data coming from A traverses this link or data destined to A traverses this link. That's why we say it's dedicated just for A this link. Whereas the internal links between nodes sometimes they will be using multiplexing. That is they'll carry data from multiple users across the single link. For example if A was sending data to F across the path 4, 5 and 6 and at the same time B wanted to send to E across the path 1, 4, 5, 6 then you're thinking at this link between 4 and 5 it would need to carry the data from two different users. It would need to carry the data from user A and also from the user B. What would commonly do is use multiplexing on this link such that when A and B send at the same time the data arrives at node 4 and that would combine it using FDM or TDM and send it to node 5 across the single link. So often multiplexing is used internal between the nodes. Usually dedicated point-to-point links are used from stations to nodes. An example maybe if you were using TDM let's say that the data rate of the link from A to 4 was 1 megabit per second and that was the case for all the station-to-node links 1 megabit per second whereas the internal links say from 4 to 5 maybe the data rate is 5 megabits per second so that it can send data from A to F at 1 megabit per second at the same time be sending data from B to E at 1 megabit per second and to do that this link needs to have a data rate larger than 1 megabit per second enough to support the data from both sources both users. So multiplexing would usually result in this being a much higher capacity link than the station-to-node links. So that's the network that we'll deal with or an example of the network we have node stations links what else can we say this network often is not fully connected so there's not necessarily links between every pair of nodes node 2 doesn't connect to node 7 node 6 does not connect direct to node 4 so that's a cost-saving measure and we don't necessarily need to have links from each node to every other node if you can imagine these nodes are spread across the country across Thailand are in different cities different locations then you connect usually nearby nodes in in a manner such that there are usually several paths between any pair of stations so I think in this case from A to F there are several different paths we can take and from B to E there are some different paths that can be taken and similar from other pairs of stations so that's desirable to have as few links as possible then it's cheaper but have enough links such that there are usually multiple paths between pairs of stations having multiple paths helps for redundancy something fails node 7 fails a can still send data to F by 4 5 and 6 so having multiple paths is useful there things like load balancing a is sending by the or someone is sending data 4 7 6 and the the capacity of the links and the nodes is starting to be reached that is the amount of data going through it is using all of the capacity so I may want to send their data via a different path which is less congested to balance the data not to have all the data going across one path but to balance it across different paths so that you get better performance so that's load balancing so we'd like to have multiple paths but few links to make it cheap so it's usually not fully connected that the technologies we're about to talk about switching and and the the issues we're dealing with often relevant for large networks wide area networks networks that cover cities between cities across a country and between countries it's not necessary something that we use at least we don't have a network this complex say inside our home we would only need one such switching node inside your home it's much simpler but in larger networks we may have a much more complex net situation so this just summarizes those concepts data is transmitted from source to destination through a network of switching nodes those green boxes those switching nodes don't care about the data they don't care that you're sending a voice communications a web page they just treat it as any type of data and their goal is to send it on to get to the destination we refer to the collection of those switching nodes as a communications network and the devices the end user devices are attached to that network and we can call them stations another thing we may call them is hosts or just end user devices usually the station to node links are dedicated they're just used by that station sending data from and to that station whereas node to node links may be multiplexed they carry the data from multiple stations at the same time and we don't usually have fully connected networks so the two main issues with how to communicate when we have such a network which path to take which is routing and another is well how do the nodes forward the data on different modes for how how to get the data through those nodes which will refer to a switching the idea is that we'll send data to one node if it knows the path you can see node 4 has three different output links for the data from a to f if it's going to go on the path 476 then you can think the data comes into 4 and it switches it out to node 7 yes if it was data going across a different path maybe a to e through the path 456 then that data will be switched to the link to node 5 so these devices we call switching nodes because they can think they have a switch in inside which tells it where to send the data on the output as it comes in and the way to do that there are really three different approaches for for dealing with the data delivery we call it switching and we'll go through those three approaches today the first approach is circuit switching who has a telephone at home not a mobile phone but a landline telephone anyone few people still have them okay or you I'm sure you've used them in the past so telephone networks have been around for a long time maybe a hundred years more than a hundred years so the telephone was invented I guess in the early 1900s late 1800s so telephone networks have been the main communication network in use compared to the internet which has only been around for about 40 years and we'll see that circuit switching was designed for and used in telephone networks when I say telephone networks I mean not necessarily mobile phone but or not using voice over IP but the home landline telephone so the concepts were of circuit switching as suited to telephone networks and in telephone networks what data do we send voice okay we're not sending web pages if we if you imagine a time before the internet and we just had telephones there were no web pages the data being sent was just voice people talking so we think the data or the application is to make a voice call so the networks were designed for transferring data which is voice they were extended to then support non-voice communications but the the main design was around transmitting voice so let's look at how circuit switching works the nodes in circuit switching connect the original source eventually through the final destination and this is what those green boxes look like in the old telephone networks so the green boxes in our network remember they data comes in they switch out to one of the output links in an old telephone network this is what a switch looked like the way that works say in a local neighborhood you have a telephone at home and there's a cable going from your home into a local telephone exchange so there's your cable you're the station connecting to the telephone exchange which is the node and everyone's home had cables going into that telephone exchange and what happens is if you want to call someone you pick up the phone and you tell the operator I want to call Steve and the operator connects so there's a port here that connects from your to your telephone line and a port to Steve's telephone line and what the operator does is basically connects them together with a cable connects them together so that now when you talk the signal carrying your voice travels across your telephone line from your home it goes into the telephone exchange and then it's hard to see but it goes into one of these cables and then out the other end and then it goes into the telephone line of the people the person you're talking to so that's the role of the switch it connects those two telephone lines yours and Steve's together and once that connected the data the voice just travels from the original source through the first link effectively through one of these cables and then on to the destination and with telephone systems that was an analogue signal and still is so when you talk an analogue signal was sent from your telephone across the cable to an exchange and then it would go across this cable and then out to the destination link and the result was once you'd made the call and you were connected as you talked it's as if you now have a link from your home into the destination destinations home the call call ease home the person you're calling and as you talk the signal that's generated just travels across your link through the switch and then direct through to the destination it's as if you have one long link in this case and that's what circuit switching does the first step is to set up that link to set up what we call a circuit from source all the way to the destination and once it's set up you can communicate today it's just done in a digital manner okay so that these are quite old but some of the switches in telephone exchanges are just computing devices that do the same thing but automated a circuit switch someone you what do you do you pick up the phone and then what do you do to call someone press the number what happens then there's some waiting for someone to respond and so what's happening in the meantime is when you press the number that triggers your phone to send a special signal to the telephone exchange effectively saying you want to call this person the number identifies the destination sends a special signal to the exchange and the exchange receives that and works out that okay I need to connect your telephone line to Steve's telephone line if you're trying to call me and it does it of course inside a computer in this case and connects or it starts to set up the connection and as it creates that it sends that special signal to Steve's telephone and what happens then it rings okay the the call ease telephone rings when it receives that signal and when they pick up the telephone then that sends a special signal back to the switch the switch now knows that the the person being called accepts that call sets up a connection or sets up that circuit so finalizes the connection between the two links and then sends the special signal back to the the callers phone and then they hear the the the sound going from ringing to now it's connected and then they can talk so that's how circuit switching works there's let's go to the three phases at the start before we send data we establish a circuit we call it a circuit because once those links are connected it's like we've connected a electrical circuit and we can send a signal all the way along that circuit a circuit sometimes we call it a connection we establish a circuit by sending some special messages from source through the nodes to the destination if the destination accepts the connection that is they pick up the the phone then a special message is sent back through the same set of nodes telling them that they've accepted the connection and telling them that they must finalize the establishment of this circuit and it gets back to you and that's when you can start communicating so we establish a circuit first which is really allocating some resources inside the nodes and for the links and then we can transfer data so in a telephone network that data you can talk so it can send a voice signal analog or digital so the same concepts can be applied for non telephone networks and at the end when you're finished talking you may hang up your phone which sends a special disconnect message to the nodes saying those nodes can remove the circuit they no longer need to connect from this link to this link and that disconnect will terminates the circuit terminates the phone call for example so three phases establish a circuit transfer data and then disconnect we're not going to cover the details of how you do that in a telephone network or in other networks just the concepts now an important point with circuit switching is that it's you can think it's all done at the signal level that is especially telephone networks and analog signal is sent across the cables to the nodes which tell them to set up that circuit and once the circuit set up that analog signal is transmitted across each link to the destination yes a motor remember a modem it was about converting your computer digital data into an analog to transmit so you'll say your dial-up modem or even your ADSL modem connects via the telephone network with a dial-up modem it's like making a phone call you dial in you make a phone call from your computer to an ISPs really receiving computer so yes that was using circuit switching with ADSL it uses circuit switching well there's only with ADSL it's different ADSL it's just from your home to the the local exchange and then it no longer uses the telephone network then your computer data goes out to the internet so with a dial-up then it was using circuit switching with ADSL it's it's changed a bit but you don't need a modem for circuit switching just your own home telephone does that for you once you establish a circuit there's a link from source all the way to destination even though it goes via multiple links the way that the circuit is set up when you transmit a signal at the source the circuit is set up so that signal will flow through all links and get to the destination as it was transmitted let's let's have a look at some examples and see some details some more examples about a telephone system don't worry too much about the names of these blue boxes so the example here is that we have some people their home telephones and this first blue box this end office is a telephone exchange so there are telephone exchanges spread across the city usually to cover a particular neighborhood in particular area so all the people living in that area have a line going into their local telephone exchange here it's called an end office and in other neighborhoods there are other telephone exchanges and of course alright first example a wants to call be a has a link into the local exchange so does be so when a makes the call it sends a special message to the local exchange which recognizes the person that calling be is also on this local exchange so the this is the switching node okay the local exchange is the device which does the switching and what it does is it effectively creates a connection between a's link and B's link or it sets up the connection tells be so these phone rings when and be accepts sends back a special message and that's when the connection is a finally established once the connections established when the person that can telephone a talks the telephone takes your audio and sends a analog signal that analog signal goes across this cable into the exchange think it goes across this internal cable and then comes out across this cable to be so it's as if we now have a link all the way from a through to be and we just send a signal and it goes right through that link if C wants to call D then it's a little bit more complicated because D is not on the same exchange they're in a different neighborhood so there's a hierarchy of telephone exchanges in this case this intermediate exchange connects the two neighborhood local exchanges for the same concept C sends a special message to the local exchange saying I want to call D the local exchange recognizes from the phone number that it will need to go to the the higher level intermediate exchange which covers say the city and that goes via a link here it's called the trunk line or a trunk link and the intermediate exchange recognizes D is that this local exchange so starts to establish a connection sends it through to D when D answers a special message comes back and as that message comes back it tells this local exchange connect D to this trunk and here connect these two trunks together and as the message comes back connect this truck to C's line C knows that D is answered and now when C talks the analog signal goes across here through this exchange through the trunk line and flows all the way through to D again as if we have one long cable going from C all the way through to D that's the idea there in in real telephone networks there's a much larger hierarchy so you have local exchanges exchanges in cities between cities and so on so there's a global hierarchy of telephone exchanges a little bit about the trunk line again the terminology is not important for this course we're not studying telephone networks but just to explain the concept this trunk line would be shared by all the users at this local exchange okay here's an exchange maybe there are 100 people connected into the first exchange so when they make calls to people on other exchanges their data must travel across this one trunk line so we would use multiplexing here this line would carry the data from multiple users at the same time as opposed to having a single line for every possible user so the idea is that when we establish a circuit we must have enough resources to support that circuit to support the connection let's say there are 100 people on this telephone exchange and that this trunk line has a capacity to support say 50 telephone calls at a time okay so each telephone call consumes some of the resource of the nodes and of the line so there's enough capacity here enough bandwidth to support let's say 50 telephone calls at the same time but there are 100 people here so what happens when you establish a circuit when C calls D that exchanges or more more generally the switching nodes allocate resources for that circuit saying okay if this connection is accepted then I'll guarantee that we'll have enough capacity to support this this circuit this phone call especially on the trunk lines so this device would do a check when the connection is tried to be established it would check is there enough capacity on this trunk line to support one more voice call if yes then it proceeds and each node would check that this node would check is there enough capacity across this trunk to support our new voice call from C to D if so everything proceeds and the connection is established if not the connection is not established so let's say currently there are 50 people already making a call via this trunk line to somewhere else then C tries to call D what happens C there's a special message sent to the local exchange at this local exchange that recognizes C wants to call D it needs to use this trunk line currently this trunk line has 50 calls in progress the capacity is 50 voice calls so the connection requests from C must be rejected and this exchange or switching nodes sends back a special response saying no you cannot connect maybe you get some network busy signal on your phone not necessarily the destination being busy okay so sometimes you try and call someone you get a special sound that indicates that they on your mobile you get a message but you on a landline you get a special sound indicating that you cannot call that that person because they are busy similar there's the concept that the network may be busy C tries to call D but they get some indicator saying that the network somewhere along the line there's not enough capacity to support your phone call the calls rejected you may may get that maybe in busy periods maybe around New Year if you try and call people it's possible that the network doesn't have enough capacity to to support one more call and that's a problem using this approach it's both an advantage and disadvantage the advantage is that we establish a circuit once it's established the resources in that path are guaranteed they're allocated for a seated cork to D but if if the capacity is currently full it means we cannot start a new connection so it's good for what the people who have started the call are guaranteed to be able to transfer their voice but it may be a problem if we reach full capacity we cannot have any new connections and that becomes a problem if someone makes a call but doesn't transfer any data it can be inefficient let's look at another aspect let's try and illustrate one other thing of circuit switching here's a simpler network a wants to station a wants to communicate with station B and there are two switching nodes in between so the path is from a to 1 to 2 to be once the circuit is established I will not draw that but once the circuit is established a transmits a signal and with circuit switching we deal with analog signals being transmitted I'll try and draw what happens once the circuit is established we can think internal for node 1 there's a link between the link from a to 1 and from 1 to 2 and similar internal to node 2 it connects those two links together the result is that when the data is transmitted and if we look at analog data then a transmits an analog signal if I try and draw that some sine wave okay that transmits along this line what happens when it gets to node 1 well node 1 knows that anything that comes in on this line as a signal comes in it knows because the circuit was established to send that same signal essentially through the node and out onto this line and it keeps going and it arrives at node 2 the signal the analog signal because the circuit's been established it knows that anything that comes in on this line from 1 which originated from a needs to go through here and out on this line and that's my sine wave and eventually received by b the establishment of the circuit allows a signal to traverse all the way from a to b through the nodes and it's as if there's now a link all the way from a to b that's the result of circuit switching from amb's perspective once the circuit is established from the data perspective it's as if we have one link all the way through okay of course there are two nodes in there but their role are just to receive the signal and transmit the signal across the next link so from the signal perspective a transmits and b receives so in terms of timing well transmission delay depends upon how much data we have to send and how fast we send and importantly propagation delay it's as if we just have one cable from a to b so circuit switching creates a link from a through to b and now the performance just depends upon the characteristics of that that link created from the circuit so in telephone networks this data would be your voice but it has been used for other non-voice applications as well and the way that the resources are allocated is that this link or the circuit once it's established is dedicated for the user's amb if a doesn't transfer transfer anything if it's if a makes a phone call but stops talking so no one's talking then the link is still established no one else can use it and that's one of the disadvantages of circuit switching the advantage is that once the circuits established it's guaranteed that amb can use that no one else can use it the disadvantage is if if it's established but amb are not sending any data then it's inefficient because we have this link or this circuit here but no one's using it and the way with circuit switching works is no one is allowed to use it once it's reserved for amb so circuit switching I think they are the main concepts that we need to be aware of a path is established before the data transfer begins and in doing that in establishing the circuit that the nodes check do we have enough capacity to support this new circuit depends upon the application as to how much is needed so they if there is then they reserve that and a reservation is that this capacity across this link is reserved for these two users no one else can use it it's what a reservation means so we check if there's enough capacity if there is then we accept the circuit if not we we reject the circuit this works well with voice traffic so it was designed for telephone networks where voice was transmitted the reason it works well is because with voice the amount of data that A sends to B and similar B sends back to A is quite predictable when someone's talking on the phone one user talks for about 30% of the time 30 or 40% of the time the other person is talking about 30 or 40% of the time and the other times are no one's talking maybe you're waiting for the other person to talk so from many years of using telephone networks the people the telecom companies measure those statistics and they know quite accurately how much data needs to be sent they know that voice occupies a bandwidth of about four kilohertz they know how much time spent is spent talking so they know and can predict how much capacity is needed especially across trunk lines the result is that once you establish your voice call you can talk clearly to the other person so designed for voice traffic sending voice data it doesn't work so well for non-voice applications like internet applications with voice the amount of data being sent is predictable or quite quite stable what about when your web browsing for example think of your computer when your web browsing over a period of say 10 minutes how much data does your computer send or receive or maybe easier when does your computer send a message when your web browsing again when you click on a link so you click on a link that triggers your computer to send a request some web pages of course have some automatic refreshing and updating so that triggers data to be sent but with web browsing what happens is usually maybe you click on a link and then you spend tens of seconds maybe a few minutes reading the page looking at it so during that time nothing's being sent so with web browsing and many other internet based applications there's usually a lot of traffic needed to be sent a lot of data and then nothing for a large period of time and then another burst of data and then nothing because you click on another link and so on and it's quite unpredictable okay a different person visiting a different website will generate a different amount of data so circuit switching is not so good for that because with circuit switching before we send data we establish a connection and we reserve resources no one else can use them but then if we start to send our web browsing data across that connection then a lot of the time we're not using it only for a small portion of time do we send actual data other time we're reading the web page and not sending and that's wasting that connection wasting the resources so it's not well suited to our typical internet data applications where is it used public telephone networks so the telephone network across the world private telephone network so say inside our campus we have telephones linked together and a private exchange a private switching node and some private is a spelling mistake data networks especially or a large example is banks so think of a bank which has branches across the country it has some central office for keeping all the records and it has ATMs thousands of ATMs a private data network many banks in the past build private data networks that they connect the branches and ATMs together such that instead of sending voice across that network they send transaction logs events from those different locations to keep track of what's happening so they'd have a network which would connect from ATMs and branch offers back to a central office but nowadays circuit switching is only really used in telephone networks no new networks really take advantage of it for data communications because it's not well suited for applications where the amount of data sent varies over time this is just another example with a call to through different switching nodes so with circuit switching and we'll compare it to the other approach in a moment but with circuit switching we'll see we reserve resources for a duration of a connection that's good in that the resources are guaranteed for that pair of users so we get guaranteed quality think of if we use circuit switching today in the internet if we did then think that from your computer you create a circuit through to the YouTube web server okay you're guaranteed to be able to transfer a 10 megabits per second for the duration of that circuit which means when you're streaming the video it's always going to be at 10 megabits per second no one else can use that capacity it's guaranteed and allocated for you so it'd be reserved through the network that would give you high quality the delay would be quite small the delay depends really of now upon how long the link is and how fast we transmit it doesn't depend so much about the intermediate nodes with circuit switching quality is good but very inefficient if I establish a circuit I reserve 10 megabits 10 megabits per second but then I don't use it for five minutes from the network perspective there's 10 megabits per second of capacity unused for five minutes for example I stream a video then I go on and do something else that doesn't require that capacity so very inefficient another small thing with circuit switching is that the devices use the same technology of the same speed think of when you call someone on the telephone then inside that circuit switching network both telephones transmit and receive the same form of signal a four kilohertz analog signal circuit switching is used in telephone networks but it does not good for internet applications so in I guess the 60s and 70s as people realize that they want to allow computer applications to send data across a network they develop new approaches and the new approach is called packet switching and that's what's used today in the internet in packet switching now think of your internet application has some data to send from one station to another so here's our two stations we have a network they'll be switching nodes inside but from the application and for the stations perspective they have some data that we want to send we break it into smaller chunks called packets and the same way that when we looked at flow control and error control we took our data and split it into frames a packet's just a different name and we send one packet at a time into the network the switching nodes deliver those packets to the destination and there are two different variants of how to deliver them the packets usually contain a header to indicate who does this packet belong to who's it going to and also in what order does this packet come in in the application data a sequence number for example so we'll we will look at the two variants for how to send the the packets through the network and then talk about well why using packets is it better than using circuit switching what advantage does it have there are two types of packet switching datagram packet switching and virtual circuit packet switching let's look at datagram packet switching first and then we'll through an example and the example is we have a source station a destination station and it has some data to send and the green circles are the switching nodes and the links between them and what the source station does is it breaks the data the original data into multiple packets another name for those packets is datagrams so they have many different names frames in some cases generally packets sometimes we refer to them as datagrams and we break it in this example into three different packets packet number one two and three and the source station sends to the first node in order first sends packet one packet two packet three and then that node will send those packets to the next node and it will follow some path through the network the nodes will choose the path we haven't touched upon how yet so sometime later let's say this first node sent the three packets to this second node and packets one and two are then being sent down to this node in the middle packet three is still being sent to the second node at some time later one important issue with datagram packet switching is the packets may take different paths and that's here packets one and two which belong to the same original set of data took this path through the middle node and in this example packet three took a different path so that's possible in datagram packet switching because in this case the nodes treat the packets independently what that means is if I'm a node I receive a packet I look at the destination alright it needs to go to this computer and I make a decision to send it to the some next node thinking it will get to the destination the next packet I get I do the same thing but I don't care about the previous decision so I receive packet number two okay who's the destination let's send to the next node and then I receive another packet and I make a decision and it may turn out over time that this node chooses to send some packets in this direction and some in another direction just because those three packets belong to the same piece of original data the internal nodes don't care about that they treat the packets independently the advantage being that that makes it very simple the nodes receive a packet send they don't need to keep track of past packets the result being is that if packets can take different paths some packets may arrive before the others in the sequence so packet three by chance arrives before one and two because it took a different path maybe this path the links are much faster and the delay is much smaller such that the packet gets there first so the result is that this last station a last node in this case puts them back into order before it sends on to the destination so it must reorder so that's a disadvantage of datagram packet switching in that we must deal with the fact that some packets may arrive out of order and put them back back into order and that's what's used in the internet today so the internet uses a protocol called IP the internet protocol and it uses this concept the source sends packets to the nodes and each node as it receives an IP packet it looks at the destination who does this need to go to decides to send it on to a next node and they don't care about the the previous packets and it makes it very simple and one of the reasons why the internet became so popular because of the simplicity of that protocol for sending data why why do packets help compared to circuits what was the problem with circuit switching now compact itself what was the problem with circuit switching look any election notes it's written there why what's the problem and then we want to know well why use packets what's the first what's the problem with circuit switching or a problem right but forget about packets for a moment what's the problem with circuit switching what is a problem we highlighted a disadvantage again it can be inefficient in using the resources okay with circuit switching a good thing is that we establish or we reserve resources so think of every link when we establish a circuit some resources are reserved across that that's good because I'm guaranteed to get some level of performance but it's bad in the cases that if if for a short period of time I'm not sending anything I've got nothing to send then in circuit switching those resources cannot be used by anyone else and that becomes inefficient you can think there's a link as a capacity of 10 megabits per second I reserve 1 megabit per second no one else is allowed to use that one of that 10 megabits per second it's reserved for me or for my connection if I've got no day to descend still no one else can use it that's the problem with circuit switching or a problem how does packet switching help in that case so that's a problem with circuit switching why can packet switching help alleviate that problem fix that problem well the idea is that if we break our data into packets and we think over a particular link if there are many people trying to send data with circuit switching each pair of users would have to reserve a part of the capacity but if at some point in time no one is using that no one is sending data it's inefficient with packet switching the idea is that as multiple users are sending packets if one user has nothing to send then it's likely that the packets of the other user can be sent and utilize that link resource so it can be more efficient let's see if we can illustrate that I won't be very good but we'll try some picture to try and illustrate that or a simple example let's say we have we'll focus on a particular link we do here's one node and we'll have another node I'll explain this in a moment this is node one this is node two and we have many users wanting to send so a b and many others wanting to send the others over here let's say c d and so on and we have a link so the scenario will focus on a particular link between two nodes inside a network this link needs to carry the data from many users from a say through to see from be through to D and others at the same time with circuit switching let's put some numbers to this to make it maybe easier to understand let's say the capacity of this link is 10 megabits per second with circuit switching we often talk about say voice calls but it's for us I think it's easier to think in terms of data rate megabits per second as a capacity that's the link capacity capacity as an eye and let's say for simplicity for the numbers that every we're going to use circuit switching first and then when someone wants to create a circuit they need to reserve one megabit per second so they need to reserve one megabit per second so with circuit switching first think of every application that creates a circuit asks for one megabit per second how many circuits can we support on this link 10 okay what would happen let's say a creates a circuit to see b to D so they've reserved two megabits per second once we have 10 there we've reserved 10 in total 10 different circuits when another person wants to create a circuit they send a special message it gets here and this node one would reject and say no my link has capacity for 10 if I've reserved 10 at this point in time a request for more resources will be rejected so with circuit switching that's how we allocate resources now the problem is if we have 10 10 circuits reserved let's say currently we have 10 circuits open so we've reserved the 10 full 10 megabits per second the problem is that with circuit switching is even though this is reserved if a is sending no data to see they've reserved us a connection but they're sending no data and similar maybe be has reserved a connection but he's sending no data to D then really the link has a capacity of 10 megabits per second but at some point in time maybe we're only using 8 megabits per second because even though some of reserved capacity they're not using it and that's a waste that's the disadvantage of circuit switching because what could happen if the 11th user came along if the current use was 10 sorry if the current use was 8 out of the 10 if the 11th user come up came along then it would make sense to allow them to use that link but it's not possible with circuit switching because it's reserved and allocated so how can packet switching help us in this case well I don't know how to draw up but these users same case the applications want to send one megabit per second but they don't reserve it say we still have a capacity of 10 megabits per second but with packet switching each user sends packets from their link into node one as my packets drawn there and similar be is sending packets at different times into node one and so other users and what node one does is takes those packets and sends them out to node two packets from different users and sends them to node two the idea is that currently maybe there are 10 10 users sending at 1 megabit per second so a is sending at 1 megabit per second b is and so on there are 10 different users sending packets at 1 megabit per second then everything would be okay because our link has a capacity of 10 megabits per second 10 users sending in to node one at 1 megabit per second node one can send out at 10 megabits per second and that would be okay if if now there were 10 users sending at say slightly less 800 kilobits per second 10 users sending at 800 kilobits per second what comes out across the link if we're 10 sending in at 800 kilobits per second what comes out is 8 megabits per second so during the stages when the users are not sending so much even though they may want to send most the time 1 megabit per second if they send less then they become spare capacity so if there are 10 users sending at 800 kilobits per second the amount we use is 8 megabits per second and we have 2 megabits per second of spare capacity so an 11th user could then send some packets in and the packets for the 11th user could use up that spare capacity and maybe the 12th user so if we don't reserve resources with packet switching what it allows us to do is especially in cases when the users are not sending at the maximum in this case at 1 megabit per second that the spare capacity can be allocated to other users it's hard to illustrate but you can think okay if if B is not sending then it means that we've got some spare capacity on this link and we can use the packets from some other user the 11th user can be sent when B is not sending the idea is that with many internet applications the rate at which packets are sent varies over time sometimes A is sending a lot sometimes are sending nothing and sometimes B is sending a lot sometimes are sending nothing so over time if we're lucky what we get is that for most of the time maybe is a period when A is sending nothing so we can send B's packets and when B is sending nothing we can send A's packets so we can more efficiently share that capacity amongst the users now there's a problem that arises what if there are 11 users sending 1 megabit per second what happens there's no reservation but 11 users start their application to transfer data and they're all sending 1 megabit per second into node 1 what happens well 11 megabits coming in capacity of 10 megabit per second going out not all packets can come out well for a short period of time at least they can be queued here put them in a queue and make them wait until we have some capacity to send them out so that's works well in the case when there's a maybe a small burst of sending for most of the time there's only 10 users sending at a low rate but then there's a small time when say 11 users sending at the maximum rate 1 megabit per second so we can't of course send it all out so some of the packets would be queued here and wait a little bit and then maybe A slows down and then those queued packets can be sent in the spare capacity so with packets we can adapt in a much more dynamic way so that spare capacity can be used when when it's available and in the case that we reach capacity then the packets will be delayed a little bit they'll be queued and that's where we get queuing delay with circuit switching we just reserve no one else can use it very good for those users but very inefficient when our data sending rates vary that maybe it's not the best illustration of the difference but hopefully you pick up with packet switching we can send packets from different users and use up the capacity in a more efficient way than if we allocate in circuit switching and it works very well on the internet because in the internet many of the applications send at varying rates sometimes I send a lot then I send nothing when I'm sending nothing you can send your packets and over for a large number of users that averages out to be quite efficient so datagram packet switching split our data into packets send them one at a time there's an alternative called virtual circuit packet switching use packets and have the same advantages in that we can be more efficient in using the link but try to pretend to be like circuit switching so we create virtual circuits remember with circuit switching with real circuit switching before they send data they establish the circuit there's a special message to establish the circuit amongst that set of nodes in datagram packet switching there's no such process we just send the packets with virtual circuit packet switching we establish a virtual circuit first say from one source to a particular destination and then send the packets and one difference that we see with this example let's say our source station has already established this circuit or this virtual circuit along this path then in virtual circuit packet switching the packets that the source sends follow that same path always they will not take a different path which is different from datagram packet switching and that the packets may go in different directions this has the advantage in that a the packets take the same path so we're we're likely to get reliable or predictable performance as opposed to sending in many different paths and also has the advantage is that we inform these nodes in advance that we're going to send packets through them so they can allocate resources in datagram packet switching when a node receives a packet it didn't know it was coming so it's very simple with datagram packet switching but there's no advanced notice that you're going to receive you know one megabit per second for the next five minutes with virtual circuit packet switching we get some advanced notice at the nodes that there will be packets coming from this source for some duration we can plan better what's the problem with virtual circuit packet switching compared to datagram packet switching what's the disadvantage the other links are not used that is we always use this one path that can be a problem in that if this path becomes busy okay it's because there's also packets so be careful this path may still have packets from other users it's just that the packets from this pair of users take this path but if becomes busy then we cannot redirect quickly the packets to another path yes correct other problems there's a delay upon startup you want to transfer data with virtual circuit packet switching and also with circuit switching we first send this special message to the destination and get a response back then we send our data with datagram packet switching I just immediately send data so the delay upon startup can be an issue next week though we'll just summarize some of those advantages for today next week we'll look at some issues of packet size and this will look at this example in a bit more depth but this is a some timing and you see on the left circuit switching in the middle virtual circuit packet switching on on the right datagram packet switching there's a delay to set up the connection take some time to get our special message from source to destination and the response back then we send data same with virtual circuit packet switching take some time before we send the data with datagram packet switching just send the data immediately that's good because there's no delay upon startup we'll do a little bit more analysis of that next week and do some calculations there and I think we'll stop there so next week we'll return to those two slides about the timing do some example calculations and then summarize some of the trade-offs between these three switching techniques


We have a time for a recap again remember we have been going through some of the materials we have already covered in this course in various modules by way of checking if our learning and understanding of the concept is correct. In this module like other recap we shall be trying to assess the learning process its efficacy and hopefully its efficiency as well. This is something for you to consider through these quizzes I would like to assess or I would like to give you an opportunity to assess yourself if you have been learning properly throughout this course in different modules. These quizzes they are in the form of questions, a question or statement which would have four options. Only one option is correct and you will have to choose it by yourself by writing on a piece of paper your correct answer and then you should be checking against the correct answer which I would be sharing. You must also remember that in one of the recap I said there are three types of students type A they would like to read the question very carefully and write down their answer type B would be googling very quickly to find the right answer and type C would be those who would not like to do anything. By this time I hope that there is no one amongst you who falls in the category of type C. Let us go to our question. Question says Ahmed asks his friend Abdullah to find him a used iPhone 12 Pro Max. If he i.e. Abdullah succeeds in finding one and buys it from someone for 90,000 rupees and then sells it to Ahmed for 100,000 rupees by disclosing the purchase price. This will be an example of A, Juwala, B, a sale involving Ghurr, C, Murabaha, D, Salam. This is a tricky question in the sense that A is Juwala. I don't think I have explained Juwala in the previous modules but I wanted to throw a googly. So this is a googly question and you should choose your right answer. Is it A? Is it B? Is it C? Is it D? I would like to, previously I have chosen A, I have chosen B, I have chosen D. I don't remember if I ever chose C. So let's see if Murabaha is the right answer. It is correct because in this case the seller is disclosing the profit. When seller discloses the purchase price which in this case is 90,000 rupees and the seller is selling to a purchaser Ahmed this camera for 100,000 so there is a disclosed profit of 10,000 rupees which is a basic characteristic of a Murabaha contract.  B is wrong or A is wrong, B is also wrong and D is wrong as well. On previous occasions I have been giving you explanations why the wrong answers were wrong. But this time because we are well advanced into this course I would not like to give these explanations you should find the explanations by themselves. So this is your job to find why the wrong answers are wrong. However I would like to explain the term Jualla because we have not covered Jualla so far in this course. Jualla is a kind of agency contract. Remember we have been using Wakala as an agency contract in case of Wakala the fee Wakala fee or agency fee must be disclosed and agreed between the two parties i.e. the agent and the principal. This is a basic requirement of Wakala. In certain situations the principal party may not want to give any amount in the form of a fee if the agent has not performed. So in case of Jualla this facility is there. This used to be used by the people in the Arabian Peninsula at the time of the Prophet peace be upon him. For example if someone's camel has gone into the desert and the party is actually searching for it. So that party can make an announcement saying that if anyone finds me my camel I would be giving certain amount to that person as a reward. Now that certain amount can be fixed as well. So the person can say that I would give 10 or 15 dirhams to someone who would find my camel or it is just I would give some reward. Now if someone finds the camel from the desert and brings that camel back to the owner then the owner would be giving a reward. So in this kind of agency there is a lot of flexibility on the agency fee. Many contemporary practitioners suggest that Jualla should be used in the investment slash fund management industry as well to ensure that the fund manager performs. So according to this view if investors say investment manager if we if you find me a rate of return of 15% or more then I would pay you your management fee of 2%, 1.5% and so on. If you do not find me a rate of return of 15% i.e. if the rate of return happens to be 10% on the portfolio I would not be paying you the management fee. So this is an implication of Jualla contract. In another contract especially in Bahrain the pearl divers used to have a Jualla arrangement with some pearl divers. So if a pearl diver is going into the sea before going into the sea the person could have an arrangement with a Jualla. The Jualla would say let's enter into this Jualla agreement which would allow me to have the first right of purchase. In the evening when you come back with your findings you would show me your pearls and I would then suggest a price if you like it I would buy it even before you go to the market to sell it. So that is called first right of purchase which has been used in quite a number of similar situations.


Is it ever okay for a Christian to marry an unbeliever? Well, in short, no. Many people are looking for love and sometimes we're looking for love in all the wrong places and sometimes you meet them in the right places but they might not be the right person. Meaning that person might have all the qualities that you see outwardly that appeal to you, maybe their looks, maybe their mind, maybe their personalities that work ethic so forth. But there's one thing that needs to be there, needs to be present and that is that person needs to be a Christian. There's one passage that's brought up often that really doesn't involve marriage but it can involve marriage. Though marriage was not the point that Paul was bringing up in 2 Corinthians, it does still apply. That is in 2 Corinthians 6 verse 14. He says, do not be bound together with unbelievers for what partnership has righteousness and lawlessness or what fellowship has light with darkness or what harmony has Christ with blout or what has a believer in common with an unbeliever or what agreement has a temple of God with idols. So the point is this, you cannot be as a believer married to an unbeliever even though this is speaking about just in relationships and so forth, but how can two walk together unless they agree and the most important thing that you need to agree upon is your salvation, is your faith, is your walk with Christ. The problem that's gonna be is that this person is gonna have an allegiance to the world while you're gonna have an allegiance to God. There's gonna be a problem because when kids enter into the picture, how are they gonna be raised?  What is going to happen when there's gonna be chaos? And then you think about how the family dynamic is supposed to operate. Well, you're gonna have a bit of a problem. Remember, we're told in Ephesians 5, wives be subject to your own husbands. How likely is that gonna happen? In many cases, when the person is not a believer and they're gonna reject the biblical style of doing things, instead they're going to embrace what we call social norms where the woman has equal footing with the man and she can even be the head of the household. Well, that does not comport with what the Bible teaches. The Bible says for the husband is the head of the wife as Christ also is the head of the church. Well, Christ being the head of the church is not gonna matter to the person who's an unbeliever because Christ is not the head of that person's life. And so you've already started things off in a bad position. If the situation is reversed and it's a woman who is the believer and the husband is an unbeliever, well, well then the husband doesn't have to apply this particular passage to this life where it says husband in verse 25, love your wives just as Christ loved the church. Well, he's not going to have to respond to that because he is not in Christ. And so you already have something bad to look forward to.  Now the next question that might come up is what if I'm dating an unbeliever? If I'm dating an unbeliever and everything about this unbeliever seems right except for that, well, should I keep dating this unbeliever? Or what if the unbeliever seems to be receptive of Christ and they are kind of gravitating towards it but they haven't professed Christ? What should I do? Should I stop dating that person? Well, the answer is still the same. Yes, here's the reason why. Because it might be that the person that you're dating is just really just putting on a show at performance. Maybe not necessarily intentionally, but unintentionally because they may also like you as well. Still, you are still fellowshiping with someone not for the purpose of bringing them to Christ but for the purpose of some maybe potential matrimony. That's not why we do so. Now, if you're sharing the gospel with this person and this person becomes a believer, well then amen, that's fine. You need to be upfront and tell that person that I cannot enter into a relationship, a marriage with you because you are not a believer. And so unless things change, well then fine. Now, don't do so to pressure the person into becoming a convert which might mean they become a false convert. No, we want a genuine believer and so I would say take a step back and then focus on being a light to this person without any sort of physical interest whatsoever. Nothing that is pointing towards marriage but just your intent for that person to come to know Christ. For that person to be married to the actual bridegroom. Another question that comes up is then what if I'm married to an unbeliever's show? I'd no longer be married to this person. Shall we get a divorce? Well, the answer to that is different than the other two. That is no, you do not get a divorce. That person is your husband. That person is your wife. When you say I do, then you do. How do you know that person is the right one for you? When you said I do. Now forever that person is to be your wife, your husband. God honors his word. He expects you to honor your word. Is there going to be some difficulty? Sure, but it's difficulty that is the making of the two people that decided to do so. The Bible has also told us that this believing spouse can cause or help the unbelieving spouse to become a believer, to end up becoming sanctified as this person walks and there's plenty of testimonies where there's been somebody who was saved who had to go through the time and maybe it's not a short time, maybe it's a long time, maybe it's tumultuous but there's plenty of testimonies where the unbeliever watches the spouse and then sees how the Lord has worked with that person's life and then decides to place their faith in them because they see, as Jesus says, that person at home has let their light shine and then the spouse saw it, glorified God and became a believer. But in the grand scheme of things, there is no fellowship, except the believer is trying to evangelize and to help lead that person to a right relationship with Christ. Otherwise, there should not be any real fellowship with that person, especially if we're looking towards marriage. So I hope this helps, even though that passage is not, even though that passage is not specifically for marriage, it does involve marriage, just like it would involve even business relationships, just like it would involve other relationships. You are not to be unequally yoked, paired together with someone who has a spiritual interest that is the antithesis of what yours are. Amen.


What's up everybody? Jason here for jaspyscasebrakes.com. We are doing 2017 Panini Dauners football five-box blaster break number four random teams on our website This is also an early bird break and chase Mahomes rookies Remember guys if you are an early bird you bought one of the first 18 spots on this break You were getting entered a winner free spot and again 32 total spots everybody gets a random team then if I just remember No veteran common ship at everything else will and again Mahomes PSA 10 Dauners rookies are going between 16 to 2,000 currently guys And remember this is right now. Who knows after Sunday if he wins another Super Bowl Super Bowl VP has a legendary game It's only gonna go up guys potentially go up to 3,000 who knows Remember in every little blaster box you get 11 rookies or raider rookies one memorabilia card Which is a blaster exclusive one parallel on three inserts So good luck guys here's the dice roller We're gonna use that for the early bird customers that won a spot are gonna potentially win a spot And then we'll run on my customer names and the teams there as well So let's roll it and it's a two and a five for seven times number customer number one gets a free spot One Two three four five six Seven seven times we got two and a five Taylor there you go man. Congratulations Just want a free spot in this break Now let's go seven times here on the list of customer names and teams one two three four five six and seven So seven times Travis down a guy All right seven times seven times seven times one two three four five six Seven Jaguars down to the Washington football team So Travis D. You have the Jaguars Taylor with the Bears Gary Myers with the Cowboys Tyler Brenner. You have Falcons Mike with the Fort enters Sean with the Broncos Gary with the Seahawks Matt Al with the Bills Dylan with the Rams Tyler you have the Chargers Gary with the Eagles and Panthers all over you have the Colts Taylor your free spot was a Texans guy with the Bucks and Steelers a Matt with the Dolphins Dylan with the Bangles Matt with the Browns Chiefs Tyler you have the Patriots Matt with the Packers Steve with the Ravens Taylor with the Giants and Matt with the Jets and Saints Sean with the Titans Scott Scott with the Coronos as well as the Raiders Tyler with the Lions Bradley with the Vikings and guy with the football team and Matt out. Good luck, man.  I'll give you guys a quick minute. I'm doing much better, man Feel like my cough is pretty much still have it here in there, but ever since I got back It's it's been much much much less.  It's almost almost fully gone Plus, I can't believe it's only been like a month All right, Tyler. I don't think no one's gonna trade the four teams for your chiefs for the chiefs man Honestly, I haven't seen any my home. So I don't expect Matt to do that. It's called the trade window Well, we've got a try though, right? Guess you never know All right, last five boxes guys. Let's rip it all and we'll go through them Remember guys if you're part of the hit crate number one Mystery crate which I'll announce the names really quick. So Matt Gale David J. Steven After this, I want to do a quick roll call so that we have time to contact you via email or Instagram So let you guys know that we're gonna start it The way you guys can draft your hits if you have the opportunity to Like I said, but if not, we choose for you based on value and we did post up a second one. So if you guys love The hit crates and want to get back into it or maybe you were hesitant because you wanted to see what's gonna pop out first We do have a second one up What's the weather like in Louisville today? Holiday last few packs here. All right, you guys. Good luck Jake, but Joshua our dogs or our Joshua dob. They should say DG Westbrook. You got our first relic and it's a to say Jones This last name is but Dalvin cook They're on Conley and an Alvin Kamara for the Saints It's gonna tailor nice. We're getting some good names so far in this box. Oh Looking at the wrong red Chris Godwin back-to-back the rookies a raider rookie I Zay Jones Carlos Anderson our Joshua dogs Juju Smith Schuster Relic DD Westbrook Dalvin cook again Minnesota Vikings Brad Cameron Wake press-proof O.J. Howard elite series rookie Alvin Kamara Zay Jones Our Joshua dogs man too many our Joshua dogs more padding the homes DD what man? These are like the same blaster boxes While we're getting a lot of delvin cooks our Joshua dogs and DD Westbrook with Alvin Kamara's Jamal Williams Game Bay Packers Matt with that one another Alvin come on. These are like such identical boxes Robert Woods and Chris Godwin I know somewhere there has to be like exactly the same because if we could do the same thing for all these guys It has to be some of my homes Remember he was a tenth overall pick. He wasn't like drafted top five Ratchel O.J. Howard Davis Webb damn Davis. What? the Sean Kaiser Zay Jones and a Darius Stewart another our Joshua dogs March on Lattermore and DD Westbrook All right guys final box legendary series Jerry Rice Brat Kaya Derek Barnett Well, there's a Mitchell, but we need a Mahomes There's a little Taylor with that one. It's like Panini already knew Peter man, so we're getting the number two overall pick right here but Not the number tenth overall pick Mitchell onto foreman. I don't know how much is gonna be down Panini to be honest, man It's got to wait. I guess like everywhere else like everybody else. I guess until they post it up Keenan Allen All right guys Unfortunately, no Mahomes East All right, you guys are there you go all the other rookies here Lot of DD's a lot of dogs a lot of a lot of the same Kamara's down and cooks Godwin's Good names though. There's that got a couple minutes at your biscuit though But there you go guys. That was Don wrist five box blaster break number four Hopefully we can borrow these two redeem ourselves guys get some my homesies. Appreciate you guys