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Grid: Scalable Ad Hoc Wireless Networking Douglas De Couto http://pdos.lcs.mit.edu/grid
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What is “Ad Hoc”? 802.11 “Ad hoc mode” Single-hop communications Bluetooth: master/slave All communication goes through master device We will mean multihop wireless networks without infrastructure, possibly mobile.
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Talk Outline Motivation Research Results Geographic forwarding Grid location service (GLS) Capacity of ad hoc networks 802.11 performance Testbed Implementations In-building net Rooftop net
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Application: Smart Devices Internet Access Point Print E-Mail Share Remote Control
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Application: Rooftop Nets Game server School/Homework Server Internet Access
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Application: Community Nets Cheap Incremental Automatic
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Application: Disaster Services Disaster may have damaged phone system etc. Want to avoid N 2 plans for N services to communicate
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Goal: Networks out of chaos AFDBECGJIH
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Direct Contact Scales Badly AFDBECGJIH “Hello J!”
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Solution: Multi-hop Forwarding AFDBECGJIH “A to J: Hello!”
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Design Challenges Finding routes Cope with mobile nodes Conserving battery power Coping with malicious/faulty nodes Scaling to large networks
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Completed Research Scalable routing: Geographic forwarding Distributed P2P location database Low-power forwarding Understanding capacity limits Avoiding malicious nodes Current research: 802.11 link selection
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Geographic forwarding (GF) Packets addressed to id, location Next hop is chosen from neighbors to move packet geographically closer to destination location Per-node routing overhead constant as network size (nodes, area) grows Requires location service, which adds overhead N1 N2 N3 N4 N5 N3’s radio range N7 N6
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A E H G B D F C J I K L Each node has a few servers that know its location. 1. Node D sends location updates to its servers (B, H, K). 2. Node J sends a query for D to one of D’s close servers. “D?” Grid Location Service (GLS) overview
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level-0 level-1 level-2 level-3 All nodes agree on the global origin of the grid hierarchy GLS’s Spatial Hierarchy
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3 servers per node per level n s s s ss s s s s Node updates servers with GLS protocol sibling level-0 squares sibling level-1 squares sibling level-2 squares
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Queries search for destination’s servers Queries search with same protocol as updates. Guaranteed to find closest location server. n s s s s s s s s s3 x s2 s1 location query path
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Geographic forwarding is less fragile than source routing. DSR queries use too much b/w with > 300 nodes. Fraction of data packets delivered successfully Number of nodes DSR Grid GF + GLS performs well Biggest network simulated: 600 nodes, 2900x2900m (4-level grid hierarchy)
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GLS properties Spreads load evenly over all nodes Degrades gracefully as nodes fail Queries for nearby nodes stay local Per-node storage and communication costs grow slowly as the network size grows : O(log n), n nodes More details: Li et al., Mobicom 2000
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802.11 Capacity Enlarge network by adding nodes, area Constant density Ideally, there is more “packet-hop” capacity, due to spatial reuse of spectrum But: more nodes producing traffic to be forwarded across network
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802.11 packet-hops can scale
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Per-node capacity depends on traffic patterns “Random” traffic patterns won’t scale Per-node capacity decreases like O(1/sqrt(n)) “Local” traffic patterns scale, capacity remains constant if number of hops follows a power law distribution (e.g. GLS) More details: Li et al., Mobicom 2001
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Implementation and Testbeds Software distributions for Linux, BSD PC, iPAQ Works with unmodified Internet software Two Grid nets deployed
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LCS Grid Net 5 5 5 5 5 5 5 555 5 6 6 6 6 6 6 17 static nodes on 5 th /6 th floors A dozen iPaq hand-helds wired gateway
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Roof-Top Grid Net LCS 5 4 3 1 2 6
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A B C D E F A’s nbrs: B, 1 hop (nh: B) C, 2 hops (nh: B) D, 3 hops (nh: B) … C, 2 hops (nh: B) B’s nbrs: A, 1 hop (nh: A) C, 1 hop (nh: C) D, 2 hops (nh: C) … Distance Vector Protocol C, 1 hop (nh: C) *Nodes periodically broadcast route tables *Nodes choose route with fewest hops
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Implementation Click modular software router (userlevel) Portable: userlevel or kernel Rich APIs, e.g. Vector, HashMap, etc. Any 802.11 card with std. “ad hoc mode” Aironet 340/350 cards on Linux/BSD Lucent-based cards on Linux Best performance with driver support for signal statistics (minor patches)
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Grid Protocol All packets have Grid header Own Ethernet type code (not IP packets) Transmitter information: ID, location Control packets Route advertisements (broadcasts) Location queries and replies Data packets Encapsulated IP Link information is included
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Packet Handling Kernel Userlevel eth0 Grid routing process demux IP Stack Route lookup Route table Control packets (broadcast) Encapsulated data packets Grid packets (via pcap) IP packets (via tun/tap) Add/remove encapsulation Applications
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Packet Handling: Control Kernel Userlevel eth0 Grid routing process demux IP Stack Route lookup Route table Control packets (broadcast) Encapsulated data packets Grid packets (via pcap) IP packets (via tun/tap) Add/remove encapsulation Applications
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Packet Handling: Data Kernel Userlevel eth0 Grid routing process demux IP Stack Route lookup Route table Control packets (broadcast) Encapsulated data packets Grid packets (via pcap) IP packets (via tun/tap) Add/remove encapsulation Applications
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Does Grid Find Useful Paths? AFDBECGJIH
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Mistake: Shortest-Path Routes AFDBECGJIH A’s max range
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Link Quality Isn’t Bi-modal
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Obstacles to Better Routing Use low-loss paths, but… Loss rate masked by 802.11 re-sends Changes quickly with time, motion What’s the best metric to minimize? Expected total packet transmissions Fight strong bias towards shortest paths
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How to choose links? Signal strength?
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Should we use “quality”? Aironet “quality”
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Current Approach: Measure loss rates Receiver measures loss rate of sender Receiver ping-pongs loss rate to sender Meaured with broadcast But: each node broadcasts every ~1.3s What period to measure over? How to smooth? Trying exponentially time-weighted avg.
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Installing Grid ipkg install grid Follow prompts, be sure to set IPADDR Is it working?
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Grid Summary Grid routing protocols are Self-configuring Easy to deploy Scalable Software etc. at: http://www.pdos.lcs.mit.edu/grid
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References GLS: Li et al., “A Scalable Location Service for Geographic Ad Hoc Routing”. Proc. ACM MobiCom, August 2000. pp. 120--130 Capacity: Li et al., “Capacity of Ad Hoc Wireless Networks”. Proc. ACM MobiCom, July 2001. pp. 61--69 Link quality: De Couto et al., “Effects of Loss Rate on Ad Hoc Wireless Routing”. MIT LCS TR #836
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End Of Talk Demo…
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Links Aren’t Symmetric
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Application: Smart Devices Internet Access Point Print E-Mail Share Remote Control
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Application: Rooftop Nets Game server School/Homework Server Internet Access
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Application: Disaster Services Disaster may have damaged phone system &c Want to avoid N 2 plans for N services to communicate
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Direct Contact Scales Badly AFDBECGJIH “Hello J!”
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Design Challenges Cope with mobile nodes Finding routes Conserving battery power Coping with malicious/faulty nodes Scaling to large networks
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Topology Distribution Scales Badly 1. “C can reach A and B.” ABCDF 3. Data from F to B. 2. “D can reach A, B, and C.” G
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Geographic Forwarding Scales Well Longitude Latitude AFDBECG “Send towards lat G / lon G.”
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Location Database Longitude Latitude AFDBECG DB 1. “G is at lat G / lon G” 2. “Where is G?”
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Distributed Location Database Each node is DB for a few other nodes How to find a node’s location server(s)? Every node has an unchanging ID hash(ID) maps ID to position in unit square
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G’s Location Server is a Point G hash(G) = 0.1,0.9 x (0,0) H I
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Spatial Grid Hierarchy All nodes agree on the global origin of the Grid hierarchy
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Multiple Servers per Node G c ba
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Lookups Expand in Scope G c ba A ?
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Grid Protocol Overhead Grows Slowly Protocol packets include: Grid update, Grid query/reply. Number of nodes Protocol Overhead (packets per second)
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