huge area with only a small set of of sensors and without any other infrastructure Yeah Um is that really an network Because the sensors are not moving And I think makes no sense with moving sensors because then you don't know where the sensors are Yeah So You know an interesting question Uh the the main thing about an network is not that nodes move It is the way of topology changes can happen The idea that mmm any change is possible at any given moment But you can of course ask but if the nodes are not moving what kind of topology changes can we expect it Well Yeah eh the thing is that usually in in a n in a network of sensors what we have or the idea is to have um an important number of sensors and maybe with a high filer rate maybe the batteries are uh exhausted in some nodes because maybe OK are not being able to take uh say any sun to recharge its cells uh maybe some of them can be uh just uh out of order for other eh environmenta environmental conditions So uh the idea is that even when these nodes are not moving they can start and stop working at uh say higher pace Then uh what will happen in a in a regular network uh in a network where you don't have or either mobility or uh this uh uh rate high rate Let me go Hmm your uh uh example Have in mind some routers are burnt here at ICSI or whatever kind of thing what will happen then even in a fixed network So if some sensors are getting out of uh service and whatever kind of thing so the topology has changed even OK no node is moving Yeah OK how to provide the connectivity and the intercommunication things wi within this network Thank Yeah you I Joe see Well uh you know and finally uh another applications where uh maybe a network is available like for example a room like this where we have in fa in fact a wired infrastructure but this uh possible application of networks could be uh any place any meeting room where we arrive with our computers and eh we just can communicate seamlessly to each other without any kind of access point any kind of uh external network because the kind of work we want to do is just only local based only to chat to each other or to exchange files with the person which is in front of us OK but we n don't want to product pro provide eh access to the internet or something OK And of course the same eh the same scenario could also happen in a place where there is no uh infrastructure available say a meeting room uh in in the middle of the desert OK without any other network So the point or or the question could be for some of us the idea of saying OK That sounds OK but why not to use a longer transmission range so we can reach any destination in just one hop This seems to make sense because well uh we are this way avoiding all the routing problem that well we will talk a little bit uh about routing later but uh it it seems that i probably that the problem is is not an easy one because uh a moving the route is changing so w one can think that routing in this uh scenario is is a complex thing But the problem is if w we just uh try to put more power in each tr transmitter so this uh transmitter can reach any destination in only one hop what we are doing is uh to rising the power needs of each node and also we are reducing the network uh throughput because we are only allowing one transmissio one transmission at a time Because uh well we cannot allow several transmissions several transmissions at the same time because one is interfering with the other OK We are uh I think not telling it yet but we are assuming that we are using a single common channel for all the nodes OK So why we cannot allow several transmitters in the same channel at the same time OK So the idea of hav having a longer transmission range is not a good one or at least it has the drawbacks um um I was telling you Eh speak a little bit about the media access issues in in wireless networks Uh you know the easiest eh way to to show how this works is or at least one of I think one of the easiest is that this drawing or this picture what what we have is a set of nodes each one at a different location At at at a at an at a given moment this eh node this dash node is is transmitting and uh the arrows are representing this transmission and the receiving nodes these three nodes are receiving the transmission and trying to uh show the area the covered area for of this transmission by means of this uh dashed eh line circle OK So uh any transmission has a maximum transmission range and uh in the simpler way we can see this like a circle with the center in the transmitter and a certain radius OK The value of this radius is uh in principle is a fixed value and it depends on the uh power of the transmitter Of course uh this is only our uh simplife simplified model and of course this can be uh improved uh a lot OK And in fact the real behavior is not exactly this one but uh for for what I want to say OK enough OK So what are the differences uh in this in this eh wireless transmission regarding uh to the wireless to the wired ones What what are the d the main differences Well the main differences come from the fact that uh in uh in uh wireless transmission we have a special domain we don't have when we are using a wire for transmitting a signal And the point trying to show in this picture is that if for example this other node wants also to transmit while this one was transmitting if we are using some kind of carry detect scheme like for example CSMA or something uh uh derivated from it the problem is that this node will hear the g the m the transmission media it won't be able to hear anything so he will conclude that no other transmission is going on and he will start transmitting And if he does what will happen is that this node and this other node will also receive this transmission and this will produce at least at these two nodes a collision or a m a m a a re a bad reception OK So none of these two simultaneous transmissions will be properly received at this node nor at this other node