D: OK .
C: OK .
C: We can start ?
D: OK .
C: OK .
C: So , welcome to our next meeting .
C: Uh , today we will have a talk from Miguel Sanchez concerning his uh work related to ad - hoc networks .
C: We have here a guest , a former ICSI member of the NSA group , Jordi Domingo Pasqual .
C: He is , uh , assistant professor at UPC .
C: And maybe Jordi , you have a few words concerning your work C: one or two sentence
C: and because we will have tomorrow a talk from him in more detail concerning what 's going on .
E: Eh , you you want me to say C: Yeah , only a short introduction ,
E: Oh !
C: ve very short introduction , please .
E: Of course .
E: Eh , well , I 'm working in the em Broadband Commu
C: Uh , sorry , one interruption .
C: This is not related to hearing ,
C: it 's only for fixing it .
C: It 's not a earphone .
E: Ah , it 's not a hear C: No .
C: So .
E: It works like like this .
C: Yeah , yeah .
C: Like this one .
C: Yeah .
E: Ah ,
C: It 's only a microphone .
E: that 's E: Ah ,
C: Yeah ?
E: that 's uh much more comfortable .
C: OK .
C: Yeah .
C: OK .
E: Thank - thank you .
E: I 'm uh working in the Broadband Communications Group
E: and eh ,
E: eh
E: well , the research topics at this moment is uh , I P - eighty traffic monitoring and characterization and em eh tool developing tools uh for quality of service , eh performance analysis .
E: That 's eh  And we have several projects eh both Spanish and European projects , ongoing
E: eh in this area .
C: OK .
C: Thank you .
C: And after the talk of uh , Miguel , I will give you all a few slides concerning the project proposal .
C: I will um say something about in which direction it will go
C: and
C: uh
C: Maybe it was my fault that I assumed that the pointer of Miguel concerning UMTS services was deployed to everybody ,
C: but it was only sent to me ,
C: so in the meantime everybody got , I believe , this pointer from me ,
C: and
C: maybe we It 's a homework , in principle , for the next meeting , to get  a little bit familiar with these things .
A: Worse than no work .
C: And um
C: so I believe we will not have today a very hard technical discussion ,
C: maybe more a few topic and items we will focus in the next days , until the next meeting .
C: And when we are ready with our meeting .
C: Everything is done .
C: Then , uh , we have to read these number
C: and we go around here , every member .
B: OK .
C: So .
D: OK .
C: Miguel , please .
D: Thank you .
D: Well , eh , I I have prepared , eh , a really short talk about some of the topics
D: I I have been doing some w some research work uh before coming here .
D: And eh , well , mainly I will present
D: uh  Here you have more or less the sketch of the presentation .
D: I will present some ideas and some concepts about what ad - hoc networks are ,
D: about the issues in media access control in wireless networks ,
D: also about routing in such kind of networks
D: and , eh , I will make some comments also about uh the current standards of wireless local area networks
D: and finally , I will present the blueprint of of my research project or my research uh proposal here .
D: So the first thing is eh what an ad - hoc network is .
D: You know , uh , an ad - hoc network is made of wireless nodes .
D: These nodes of course can be mobile nodes ,
D: and , eh in this network there is no network infrastructure .
D: So eh the only active thing we have is the mobile nodes .
D: And eh
D: end - to - end communication , is sometimes , uh  done by direct communication from one node to the other ,
D: because they can talk to each other .
D: But sometimes it is not possible , this direct communication , because nodes , end uh source or and destination nodes are um are too far away .
D: And in this case , it can be done , communication can be done , by uh using multi - hub routing .
D: So
D: an intermediate , or a set of intermediate nodes will route or will forward the packet from the sender to the destination .
D: So
D: uh in a sense , what we have in this kind of networks , is that , uh nodes are as both arced as both uh endpoints and also routers of other neighboring nodes .
D: And this makes a uh difference from other kind of network .
D: And well , regarding the purposes of what can this kind of network be used for , eh we can say that , well , cellular networks are infrastructure - based networks ,
D: in this kind of ad - hoc networks , we don't have anything uh , with the exception of the network nodes .
D: So , this uh network can be useful for eh uh occasions where no available infrastructure is present ,
D: like for example , in in eh after some kind of of eh um catastrophe catastrophic event ,
D: or , also when even when there is a network available , we don't want , for different reasons , to use the available network .
D: This latt later case can be , for example , the use in military systems ,
D: where maybe we don't want to use the other the enemy network infrastructure , for evident reasons .
D: Yeah .
C: Hmm ?
C: But you can also , I believe , uh to have an ad - hoc network , where one node is then connecting to the outer world .
C: For instance , what I have in mind , you have a car , where you plug in different devices and they talk to each other , for instance ,
C: but one device decided to go to the G S M network or GPS network , or whatever kind of things .
D: That 's OK .
C: So there 's a certain kind of task uh , for one node within the ad - hoc network to provide connectivity to the outer world ,
C: right ?
D: That 's O K .
C: OK .
D: That 's perfect , you know .
C: Yeah .
D: This in this case , we will say that this node is acting as a as a gat external gateway , from this network to connect to other networks ,
C: Right .
D: and of course the underlying uh protocols can can be , of course , uh IP - based .
D: Uh , OK .
D: Here you have several several uh , proposed , uh , scenarios for for used this networks .
D: When no network is available ,
D: OK ?
D: And another another application , for example , NASA is is uh studying , is uh the use of these networks , these ad - hoc networks , for sending multiple probes , uh , um uh missions .
D: So , for example , all of you are aware of this uh the last Atlas , the last successful uh Mars mission ,
D: the cellular vehicle was only one , and was uh walking around , uh roaming around the base station , which in this case was the gateway uh node of this network ,
D: but it was only one mobile node , of course .
D: Nothing prevents ,
D: except well maybe the cost , to send not only one mobile rover , but ten tons of them , or  or at least uh several of them .
D: So , uh
D: higher area could be covered , could be explored
D: and uh of course , all the different nodes could also use some kind of ad - hoc networking , , ad - hoc routing , to extend the distance , the maximum distance , than uh a single radio transmission will would allow .
D: OK ?
D: And in the other case , or the other area uh of of of application , is the so - called network of sensors ,
D: where we can deploy a set of sensors in a maybe uninhabited area ,
D: let 's say , for example , the forest ,
D: and we can uh take some information
D: uh , for example for for fire - fighting , we can just launch from a plane , several small sensors ,
D: and uh
D: these sensors can be for example , uh checking that there is no fire ,
D: they can have some kind of uh let 's say infrared technology sensor ,
D: so we can cover a huge area with only a small set of of sensors , and without any other infrastructure .
D: Yeah .
B: Um , is that really an ad - hoc network ?
B: Because the sensors are not moving .
B: And I think it 's makes no sense with moving sensors because then you don't know where the sensors are .
D: Yeah .
B: So .
D: Well ,
D: You know , it 's an interesting question .
D: Uh , the the main thing about an ad - hoc network is not that nodes move .
D: It is the way of topology changes can happen .
D: The idea that mmm , any change is possible at any given moment .
D: But you can of course ask , " but if the nodes are not moving , what kind of topology changes can we expect ? "
B: Yeah .
B: That 's it .
D: Well , 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 .
C: Yeah .
D: maybe the batteries are uh  exhausted in some nodes because maybe uh they are not being able to take uh , let 's say , any sun to recharge its cells ,
B: OK .
B: Mm - hmm .
D: uh maybe some of them can be uh just uh out of order for other eh environmenta environmental conditions .
D: So uh the idea is that even when these nodes are not moving , they can start and stop working at uh , let 's say , higher pace .
D: 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 on - off uh rate , high rate .
C: Mm - hmm .
C: Let me go back to your uh uh example .
B: Hmm .
D: Yeah .
C: Have in mind some routers are burnt here , at ICSI or whatever kind of thing ,
C: what will happen then even in a fixed network ?
B: Mm - hmm .
C: So , if some sensors are getting out of uh service and whatever kind of thing , so the topology has changed , even if no node is moving .
B: OK .
B: Mm - hmm ,
C: Yeah .
B: OK .
C: And how to provide the connectivity and the intercommunication things wi within this network .
D: Thank you , Joe .
B: Yeah ,
B: I see .
B: Mm - hmm .
D: Well ,
D: uh , you know , and finally , uh , another non - military applications , where uh maybe a network is available , like for example , a room like this ,
D: where we have , in fa in fact , a wired infrastructure ,
D: but this uh possible application of ad - hoc networks could be uh , any place , any meeting room , where we arrive with our computers
D: and eh we just can communicate seamlessly to each other without any kind of access point , any kind of uh mmm external network ,
D: because the kind of work we want to do is just only local based ,
D: it 's only to chat to each other or to exchange files with the person , which is in front of us .
D: OK ,
D: but we n don't want to product pro provide , eh , access to the internet or something .
D: OK ?
D: And , of course , the same eh the same scenario , could also happen in a place where there is no uh , infrastructure available .
D: Let 's say a meeting room uh , in in the middle of the desert ,
D: OK ? ,
D: without any other network .
D: So
D: the point or or the question could be for some of us , the idea of saying " OK ,
D: That sounds OK ,
D: but why not to use a longer transmission range , so we can reach any destination in just one hop ? "
D: This seems to make sense because , well , uh we are this way avoiding all the routing problem
D: that , well , we will talk a little bit uh , about routing later ,
D: but uh
D: it it seems that i probably that the problem is is not an easy one ,
D: because uh a node 's moving , the route is changing ,
D: so w one can think that routing in this uh scenario is is a complex thing .
D: But the problem is if w we just uh try to put more power in each tr transmitter ,
D: so this uh transmitter can reach any destination in only one hop ,
D: what we are doing is uh to rising the power needs of each node ,
D: and also we are reducing the network uh throughput ,
D: because we are only allowing one transmissio one transmission at a time .
D: Because uh , well , we cannot allow several transmissions , several high - powered transmissions at the same time
D: because one is interfering with the other .
D: OK ?
D: We are D: uh I think I 'm not telling it yet ,
D: but we are assuming that we are using a single common channel for all the nodes .
D: OK ?
D: So that 's why we cannot allow several transmitters in the same channel at the same time .
D: OK ?
D: So , the idea of hav having a longer transmission range is not a good one .
D: or at least it has the drawbacks um , um , I was telling you .
D: Eh , let 's speak a little bit about the media access issues in in wireless networks ,
D: 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 ,
D: what what we have is a set of nodes , each one at a different location .
D: At at at a at an at a given moment this eh node , this dash node is is transmitting
D: and uh , the arrows are representing this transmission
D: and the receiving nodes , these three nodes , are receiving the transmission ,
D: and I 'm trying to uh show the area , the covered area , for of this transmission by means of this uh dashed eh line circle .
D: OK ?
D: So , uh , any transmission has a maximum transmission range ,
D: and uh in the simpler way , we can see this like a circle with the center in the transmitter and a certain radius .
D: OK ?
D: The value of this radius is uh  in principle , is a fixed value
D: and it depends on the uh power of the transmitter .
D: Of course , uh this is only our uh simplife simplified model
D: and of course this can be uh improved uh a lot ,
D: OK ?
D: And in fact the real behavior is not exactly this one ,
D: but uh for for what I want to say it 's it 's OK ,
D: it 's enough .
D: OK ?
D: So
D: what are the differences uh , in this in this eh wireless transmission , regarding uh to the wireless to the wired ones .
D: What what are the d the main differences .
D: 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 .
D: And the point I 'm 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 ,
D: it won't be able to hear anything
D: so , he will conclude that no other transmission is going on ,
D: and he will start transmitting .
D: And if he does , what will happen is that this node and this other node will also receive this transmission ,
D: and this will produce , at least at these two nodes , a collision or a m a m a a re a bad reception .
D: OK ?
D: So none of these two simultaneous transmissions will be properly received at this node nor at this other node .
D: OK ,
D: eh , this problem is what is called D: Oh , sorry .
D: uh , the D: I think this is the previous one ?
D: This is the German C: Uh .
A: No .
A: One
C: Yes ,
D: OK ,
C: this one .
D: thank you .
D: Eh
D: this is what is called a " hidden terminal " problem .
D: OK ?
D: because well uh this node is not able to hear this other node because of the distance uh , between them .
D: But Uh , this problem , as you can see in the in the picture , is uh really producing at least some reception problem at these two nodes .
D: OK ,
D: so it is it is a problem that we have to address
D: and it is a new problem compared to wired uh media access control .
D: And , uh ,
D: well , some devel some protocols have been developed to avoid this problem and to try to address this particular behavior of wireless transmissions
D: and the main idea is to use some kind of a reservation mechanism .
D: So
D: we are um reserving a certain , and a specific uh , area of space so we can guarantee that nobody is trying to use the same space at the same time .
D: OK ?
D: This is the basic idea .
C: Mm - hmm .
C: Um
D: Yeah ?
C: Miguel , can you give some examples for such kind of protocols ?
D: Yeah .
D: Yeah , in fact C: One is really really D: This one .
D: This one , n next slide is C: OK .
C: Yeah , yeah .
C: OK .
D: OK ,
D: in eh the the the main idea and and the base of most of this protocols , is what this the so - called RTS CTS uh dialogue .
D: The idea is that ,
D: uh ,
D: of course , this is only intended for unicast transmissions ,
D: I mean , only for when a node wants to transmit something to a neighbor node .
D: OK ?
D: But not to broadcast . Uh , to all the nodes .
D: OK .
D: For this for this eh , work , the transmitter has to send a a a reservation packet called RTS , " Request To Send " , before eh being able to send data .
D: And the receiver is expected to return back another packet called CTS , " Clear To Send " , to allow the transmitter to send the data .
D: If the transmitter receives successfully the CTS packet from the destination , then it will be able to send eh the packet , the data packet .
D: If not , it will h uh , back off , and it will retry uh sometime later .
D: OK ?
D: So the main idea C: And the media is is is still occupied during that period ,
C: or ?
D: Uh .
D: Yes .
D: Yes .
C: OK .
D: The idea is uh , if I want to send you something , I first of all , I need you to answer me to be sure that you are my neighbor now
D: because maybe you are off or maybe you are uh too far away ,
D: and also because that way Miguel , for example , it may be is not being able to hearing me will hear your CTS and will know that you are about to receive uh data from me .
D: OK ?
D: So
D: in fact , forcing the receiver to transmit for a short period of time is also uh , used to signal all the uh receiver neighbors that a recept a reception is taking place .
D: OK ?
C: Yeah .
C: I didn't get it .
C: Um , what happened when I sent an RTS packet ?
C: Is then the media still occupied for me ?
C: Or , is anybody allowed also to send RTS packets ?
D: Or everybody C: Because if I go to an intermediate node , maybe you are communicating to me
C: and I am the intermediate node to the final destination Michael .
C: And Michael send also some RTS packet in the time and and uh uh uh Jordi too and whatever .
D: Yeah .
C: Then you will never come to a really C: then you 're always exchanging RTS and non - acknowledgement packets for the RTS .
D: OK .
D: Two two things .
C: So that 's uh that 's uh , my my problem .
D: First of all , eh , this this mechanism is only a media access controlled mechanism ,
D: so it is not connected with routing eh , anyhow .
C: Then , fine .
D: And , second point ,
C: OK .
D: eh
C: So it 's only related to the direct neighbor within the scope of the transmissions ?
D: RTS D: Yeah ,
D: RTS packets can collide to other RTS packets .
C: OK .
D: So , this mechanism is not uh preventing collisions ,
D: it is preventing only data collisions .
D: OK ?
C: OK .
D: So the D: And and you can say " OK ,
D: but what is the what the benefit is ? "
D: The benefit is that , eh RTS packets and CTS packets are short packets ,
D: really short packets .
D: Let 's say eh eh 30 bytes or less .
C: I assume so .
C: Yeah .
D: while data packets can be much mor much bigger .
D: Let 's say uh one - K , five - K - s .
D: OK ?
C: Yeah .
D: So
C: Fine .
D: a collision in this short packet is only wasting a small amount of time .
D: And , uh
D: if that way we can warranty that there is there are there is no collision uh uh , between data packet , well , this this is this means a a huge eh increase in network throughput ,
D: because we well , we we only waste really small time colliding .
C: OK ,
D: OK ?
C: but that means , in principle , that on the MAC layer you have always a RTS CTS packet between neighboring nodes ,
D: Yeah .
C: and you send afterwards the data .
C: But the data will be maybe delayed in the intermediate node because you have always be assured to be uh s uh uh RTS and CTS packets are changed before  before you can forewarning the data ,
C: right ?
D: Yeah .
D: It is a reservation mechanism ,
C: Yeah .
C: OK .
D: so
D: if reservation doesn't work OK , if uh one reservation don't succeed then , uh , this transmission should have to wait .
C: Now the p
D: until the reservation D: OK ,
D: yeah ,
D: let me C: Maybe it 's better to to uh , draw it on the C: Oops .
D: l let me C: So , maybe my idea first .
C: So this is what , node one ?
C: This is node two
C: and this is the source ,
D: Yeah .
C: and this is node three that is the final destination .
D: Yeah .
D: Fo - fo for forget forget about routing .
C: Yeah ?
C: Yeah , yeah .
C: OK .
C: But - but anyway , I would like to do that in the ad - hoc network .