Lecture 14: Managing Mobility. Rik Sarkar

Transcription

1 Lecture 14: Managing Mobility Rik Sarkar

2 Communica9on with mobile nodes How do cell phones work? How can we talk to mobile nodes without the cell infrastructure?

3 How do mobile phones work? There are large wireless antennas (Base sta9on) The base sta9ons are connected to each other by high speed wired network A mobile phone connects to the nearest base sta9on, and uses that to communicate with rest of network Each base sta9on maintains a list of cell phones in its region, and their ongoing calls and communica9ons

4 What happens cell phone moves Suppose it moves away from base sta9on A and close to base sta9on B Then A sends all informa9on relevant to the mobile phone to B, so that B can con9nue all the data/voice communica9on without interrup9on

5 Loca9on maintenance Suppose we wish to make a call to cell phone c. We need to know the base sta9on B that is currently servicing c s communica9ons How do cell phones find this out? Each cell phone has a home region determined by its phone number For example, a phone with a Berlin number has home in Berlin. Whenever it moves to a new loca9on, it informs the home sta9on about its current loca9on

6 Star9ng a call to a moving phone Suppose phone c is currently in Frankfurt A call to c first goes to Berlin Berlin redirects the call to Frankfurt Suppose c moves to a neighborhood of Frankfurt: It s9ll has to update Berlin : long communica9on A call may come in before the update has completed Solu9on: Phone c also leaves a pointer at Frankfurt A call coming to Frankfurt gets forwarded Small moves do not need to be updated to Berlin

7 Can we work without home bases? Home bases may fail Home can be a bozleneck May cost a lot of communica9on on updates Can we do coordinate based greedy rou9ng for mobile devices?

8 Methods we have already seen Use Geographic Hash Tables (GHT) Essen9ally the Home base scheme Use double rulings Each node is a producer, stores data on a producer curve Too expensive each update costs sqrt(n)

9 Papers Baruch Awerbuch, David Peleg. Concurrent online tracking of mobile users, SIGCOMM'91 Jinyang Li, John Jannoa, Douglas S. J. De Couto, David R. Karger and Robert Morris, A scalable loca5on service for geographic ad hoc rou5ng, MobiCom'00

10 Communica9ng with mobile users Move opera9on Update the system when a node moves Find opera9on Find a user loca9on to ini9ate communica9on Must be distance sensi9ve Update should have small cost on moving to nearby loca9on Find should have a small cost when searching for nearby users Local moves and queries are common

11 Model Graph G (like the wired network of base sta9ons) Weight w(e) for each edge e dist(u,v): length of shortest path Diameter D(G) max distance between any pair of nodes addr(x): Current loca9on of node x Assume we have a rou9ng method on G

12 Distributed loca9on informa9on Store pointers to loca9ons of each user in various nodes Update these as users move Allow some pointers to be inaccurate Pointers at nearby loca9ons should be accurate: updated frequently Pointers at far away loca9ons can be updated less oden

13 Hierarchichal pointers There are directory servers at different distances that store these pointers A level i directory server stores loca9ons of nodes within distance 2 i Each node stores a pointer at a directory server at each level i

14 Forwarding When a user x moves, update the directory servers at all levels: farthest is at about D If node has moved a distance d, update upto log d lowest levels, leave pointers Addresses are up to date at lower levels : for local communica9ons Low communica9on costs

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16 Forwarding pointers Server at level 1 always has the right address The server at level i stores a pointer to a server at i- 1 Therefore, node is always reachable Problem: may be a long chain of pointers

17 Upda9ng addresses Keep a counter c i (x) : Distance moved since last update to level I When c i (x) exceeds 2 i- 1, update directories at all levels upto i : Remove all old entries Create new entries upto level i

18 Finding a mobile user Check servers at increasing levels 1,2,3.. Will find within distance 2 i when checking server at level i Next: What if there are no sta9c nodes to serve as servers? What if all nodes are mobile?

19 GLS: Grid Loca9on Service Each node is assigned a random id from a circular set Store loca9on at more nearby servers, fewer far away servers

20 Recursive par99oning: Each node belongs to a unique square at each level. A level i square contains exactly 4 squares of level i- 1 Quadtree

21 Loca9on Servers for node B Inside each sibling square on each level, choose B s closest node: Node with least id ader B

22 Loca9on query A wants to find B A only knows B s id Neither A or B knows B s loca9on servers

23 Loca9on query A has the loca9on of some other nodes Send request to one closest to B That node repeats Guaranteed to reach B!

24 Loca9on query A has the loca9on of some other nodes Send request to one closest to B That node repeats Guaranteed to reach B!

25 Loca9on query Suppose 21 is B s closest node in A s level 2 square: no node in in this square Suppose X in range in A s level 2 sibling square 21 has informa9on of all nodes in in level 2 sibling square including the one closest to 17 (B) X

26 Loca9on query Query always goes to B s closest node at the next level Finally finds a node that has B s loca9on X

27 Update loca9on servers A can update loca9on server without knowing it A routes to a square S with geographic rou9ng First node in performs a loca9on query for A Query will go to node closest to A This is A s loca9on server! Update it! X

28 Ini9aliza9on Level 1 square exchange en9re informa9on Update loca9on servers Then loca9on servers can be built with the update protocol X

29 Summary Solves loca9on problem using geographic rou9ng Distance sensi9ve Completely distributed: everyone is a server Works with complete mobility, since server ids are not needed Worst case query cost not bounded. Nodes at nearby level 1 squares may be far in the quadtree

30 Student presenta9ons 31/1 Andreas Krause, Carlos Guestrin, Anupam Gupta, Jon Kleinberg, Near- op5mal Sensor Placements: Maximizing Informa5on while Minimizing Communica5on Cost, IPSN 06 How to place sensors to get good results Sorabh Gandhi, Subhash Suri, Emo Welzl, Catching Elephants with Mice: Sparse Sampling for Monitoring Sensor Networks, Sensys 2007 Ac9vate a few sensors such that any event large enough is detected Nisheeth Shrivastava, Subhash Suri, Csaba, Toth, Detec5ng Cuts in Sensor Networks, IPSN'05. Use a small number of sensors to detect if the network is par99oned Luiz F. M. Vieira, Uichin Lee, Mario Gerla, Phero- Trail: a Bio- inspired Loca5on Service for Mobile Underwater Sensors, WUWNet'08 Loca9on for mobile underwater 3D networks

31 Jaspreet Singh, Rajesh Kumar, Upamanyu Madhow, Subhash Suri, Richard Cagley, Tracking Mul5ple Targets Using Binary Proximity Sensors, IPSN'07