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Wireless Mesh Networks

Wireless Mesh Networks. Paul A.S. Ward Shoshin Distributed Systems Group University of Waterloo pasward@ccng.uwaterloo.ca. Outline. Introduction Modeling and Capacity Multi-radio and Multi-channel Systems Congestion Control, Fairness, Load Balancing and Quality of Service

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Wireless Mesh Networks

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  1. Wireless Mesh Networks Paul A.S. Ward Shoshin Distributed Systems Group University of Waterloo pasward@ccng.uwaterloo.ca

  2. Outline • Introduction • Modeling and Capacity • Multi-radio and Multi-channel Systems • Congestion Control, Fairness, Load Balancing and Quality of Service • Deployment Issues Wireless Mesh Networks: CNSR 2006 Tutorial

  3. What? A multi-hop wireless network, typically with • limited/zero mobility • powered • acting as an access network Other names include: • infrastructured ad hoc network • community area network Wireless Mesh Networks: CNSR 2006 Tutorial

  4. Variations: 802.11 WLAN DS • The distribution system of a WLAN using wireless forwarding AP Distribution System AP Gateway Internet AP Wireless Mesh Networks: CNSR 2006 Tutorial

  5. Internet Community-Area Networks J. Jun, M. L. Sichitiu, Department of Electrical and Computer Engineering, North Carolina State University Wireless Mesh Networks: CNSR 2006 Tutorial

  6. Residential Broadband Source: Broadband & Dial-Up Access Source: Leitchman Research Group Source: Victor Bahl, MRFS 2005 Wireless Mesh Networks: CNSR 2006 Tutorial

  7. Processing One PC-day of computation Storage 1 GB disk storage(2 DVD quality movies) Interconnection 100 MB broadband data (3.5 hours of music) 1 MB voice telephony(15 minutes talk time) 1.6 KB SMS (10 messages) Bits ≠ Value Broadband: 1¢ per MB GPRS: $1 per MB SMS: $600 per MB What can you get for a $1? It’s the Bandwidth (and Spectrum) that’s expensive Wireless Mesh Networks: CNSR 2006 Tutorial Source: Victor Bahl, MRFS 2005

  8. What not wire the last mile? The Last Mile: Connection between a home and local hub Scale & legacy make last mile expensive • ~ 135 million housing units in the US (U.S. Census Bureau 2001) • POTS (legacy) network designed for voice & built over 60 years • Cable TV networks built over last 25 years The Truck Roll Problem: Touching each home incurs cost: customer equipment; installation & servicing; and central office equipment improvements • In our estimate building an alternate, physical last mile replacement to hit 80% of US homes will take 19 years and cost ~ US $60-150 billion Source: Victor Bahl, MRFS 2005 Wireless Mesh Networks: CNSR 2006 Tutorial

  9. Relevant Deployments • Packet Radio – 1970s • Ricochet – mid- to late 90s • Roofnet – post 2000 • 802.11 cards w. Soekris single-board PCs • Nortel: Taipei: 10,000 nodes • WLAN replacement • With multi-channel, single radio 802.11a backhaul • Max length: two hops Wireless Mesh Networks: CNSR 2006 Tutorial

  10. Key Properties • Wireless • Multi-hop • Contrast with WLAN • Stationary • Powered • Gateway • Single Authority Domain (?) • Contrast 3-6 with ad hoc networks Wireless Mesh Networks: CNSR 2006 Tutorial

  11. Modeling and Capacity • Modeling Wireless Communication • Modeling Multi-hop Networks • Capacity Calculations Wireless Mesh Networks: CNSR 2006 Tutorial

  12. CSMA/CA MAC ~ 802.11 if (carrier(busy)) BEB; else { wait DIFS ; if (carrier(busy)) BEB; else { transmit(msg); BEB; } } Wireless Mesh Networks: CNSR 2006 Tutorial

  13. CSMA/CA MAC ~ 802.11 BEB() { WT = random(0 to CW) wait(WT); } Try again: double CW on each attempt, up to 1023 • Freeze countdown when medium busy • RTS/CTS may be used to avoid collisions Wireless Mesh Networks: CNSR 2006 Tutorial

  14. Single Node Communication (1) 2 • Locality of transmission • can communicate only within transmission range • Broadcast medium • Transmission range is “Tx” • Reality Check: • Reception range is not a uniform circle • Packet loss occurs due to signal degradation, especially caused by multi-path fading 4 1 3 Wireless Mesh Networks: CNSR 2006 Tutorial

  15. Single Node Communication (2) • Interference: multiple transmissions within neighbourhood of the receiver will cause a collision at the receiver • Collision is not observed at the sender • Interference range is “If” • Reality Check: • Interference is a function of SNR • Carrier sensing is often assumed to be equivalent to interference range 4 2 3 1 Wireless Mesh Networks: CNSR 2006 Tutorial

  16. Multi-hop Model • Model as graph: • Vertex is computer node with wireless card • Edge between two vertices if the nodes can communicate • Reality check: what does “communicate” mean? • Is this undirected? Wireless Mesh Networks: CNSR 2006 Tutorial

  17. Capacity • Constraint satisfaction problem on network graph • Depends on traffic • which depends on routing • activity • link-resource constraints • Link contention • which depends on node location • traffic direction • medium-resource constraints Wireless Mesh Networks: CNSR 2006 Tutorial

  18. G1 G2 G3 G4 1 2 3 4 GW Network Feasibility: Streams • Streams – link-resource constraints G1 <= L12 G1+G2 <= L23 G1+G2+G3 <= L34 G1+G2+G3+G4 <= L4G Gi >= 0 • Depend on routing and activity Wireless Mesh Networks: CNSR 2006 Tutorial

  19. G1 G2 G3 G4 1 2 3 4 GW Network Feasibility: Links contend? • Links – medium resource constrains • Collision domains • Over-estimates contention • Cliques 23 CA CB 12 4G contends 34 L12+L23+L34+L4G <= C23 L12+L23+L34 <= CA L23+L34+L4G <= CB • Depends on link-contention: • bi-directional link assumption is common Wireless Mesh Networks: CNSR 2006 Tutorial

  20. Link Contention • Link-contention model options • Measure it • Endpoint of one link within interference range of endpoint of other link • bi-directional assumption • 2-hop contention: Endpoint of one link is within transmission range of the endpoint of the other • Under-estimates contention Wireless Mesh Networks: CNSR 2006 Tutorial

  21. Link Contention • Reality Check: • Tx < If • interference is not all or nothing • a node cannot identify its entire contention neighbourhood • realistic vs. omniscient neighbourhood information Wireless Mesh Networks: CNSR 2006 Tutorial

  22. The Computational Problem • AG <= C • A – usage matrix • G – stream throughput vector • C – medium capacity vector (bandwidth) • Use the actual single-hop medium bandwidth • The problem (for absolute fairness): find Gmax = max G : For all active i, Gi = G Wireless Mesh Networks: CNSR 2006 Tutorial

  23. 2G G Example: Clique Model B G G G G G G 3G 4G 5G 6G Gmax = B/18 Wireless Mesh Networks: CNSR 2006 Tutorial

  24. B 2G G 21G Example: CD Model G G G G G G 3G 4G 5G 6G 6G 10G 15G 18G 15G Gmax = B/21 Wireless Mesh Networks: CNSR 2006 Tutorial

  25. Accuracy of Computation Wireless Mesh Networks: CNSR 2006 Tutorial

  26. Accuracy of Computation Wireless Mesh Networks: CNSR 2006 Tutorial

  27. Accuracy of Computation Wireless Mesh Networks: CNSR 2006 Tutorial

  28. Statistical Data • Compute fair-share point on hydra graph • Compare to calculated capacity • Clique model under-estimates by 0.07%, on average • Collision domain model under-estimates by 2.3% on average • Deviation in ~10% in both cases • Provided RTS/CTS is not used Wireless Mesh Networks: CNSR 2006 Tutorial

  29. Why RTS/CTS is Bad in WMN Wireless Mesh Networks: CNSR 2006 Tutorial

  30. Summary • Two capacity models: • Clique • Collision domain • Clique is more-accurate (single channel) • But requires information that may not generally be available • Collision domain approach is very close to accurate • Within the deviation of the two models • Computed with readily available information • Neither model is accurate in the presence of RTS/CTS Wireless Mesh Networks: CNSR 2006 Tutorial

  31. Multi-channel and Multi-radio Wireless Mesh Networks: CNSR 2006 Tutorial

  32. Design Issues • Location-dependent contentionA transmission interferes with both sender’s and receiver’s neighbours • Spatial channel reuseNon-contending flows can proceed simultaneously • Distributed coordination • Hidden terminal problem • Relayed trafficInter- and intra-stream contention over links Wireless Mesh Networks: CNSR 2006 Tutorial

  33. Capacity Problems • Efficient use suggests multi-channel routing Wireless Mesh Networks: CNSR 2006 Tutorial

  34. Approaches • Multi-channel, one radio • Cheaper • Switching delay • Same (or worse) delay as single-channel in a given hop • In, then out separately • But better over multiple hops Wireless Mesh Networks: CNSR 2006 Tutorial

  35. Approaches • Multi-channel, multi-radio • More radios: more expensive • But still relatively cheap • But they interfere with each other • Use one in 2.4 GHz and one in 5 GHz band • Access vs. backhaul separation • e.g. Nortel approach • Multi-radio backhaul Wireless Mesh Networks: CNSR 2006 Tutorial

  36. Multi-radio capacity • Generalize capacity models by creating N sub-graphs, one per channel, and then using the same basic approach on each sub-graph • Clique performs poorly in three-channel, two interface case • Collision Domain is largely accurate Wireless Mesh Networks: CNSR 2006 Tutorial

  37. Analysis: multi-radio backhaul • How much gain do I need? • Factor of “C” or just use one channel that spreads over the range of the “C” channels • Any gain, since the channels are already there • Performance gains of more than a factor of three can be realized • two-interface, three-channel model • But only with careful channel allocation • Delay is substantially reduced because of parallel operation Wireless Mesh Networks: CNSR 2006 Tutorial

  38. Congestion Control, Fairness, QoS • Congestion Problem • Causes • Tradeoff on fairness and capacity Wireless Mesh Networks: CNSR 2006 Tutorial

  39. G G GW 1 2 S1 S2 S1 S1, S2 S1, S2 S1 S1, S2 S2 G G The Problem Wireless Mesh Networks: CNSR 2006 Tutorial

  40. Solutions • Separate traffic into different queues • Some benefit • But MAC solutions do not scale to network fairness • AQM solutions • But the bottleneck is not at the gateway, and shifts • Rate limit at source • Only known method that works • How do we tell the sources what to limit to? Wireless Mesh Networks: CNSR 2006 Tutorial

  41. Solutions: Distributed • Gather the information • Required for the computation of the fair-share • Compute the fair-share rate • According to the network model • Enforce the computed rate at stream origins • Leaky-bucket style Wireless Mesh Networks: CNSR 2006 Tutorial

  42. Stream Activity • Don’t want to waste bandwidth for inactive (temporary non-existent) streams • Additional constraints: For all inactive k, Gk = 0 • For now: two-valued activity information • It is dynamic=> a method to gather the information is necessary Wireless Mesh Networks: CNSR 2006 Tutorial

  43. Fair-share Computation • Feasibility assumption: • Feasible requests => WYWIWYG • We can compute the fair-share rates from: • a network feasibility model + • a fairness criterion Wireless Mesh Networks: CNSR 2006 Tutorial

  44. Stream Activity Processing • Who computes what, and when? • Our choice: • All nodes compute Gmax on their own • The stream activity information is distributed as soon as it changes Wireless Mesh Networks: CNSR 2006 Tutorial

  45. Stream Activity Distribution • Keep it simple • There are up to 2*N streams in a one-gateway WMN • Activity status of all (possible) streams can be piggybacked with DATA packets • Requires no change of IEEE 802.11 MAC Wireless Mesh Networks: CNSR 2006 Tutorial

  46. Piggybacking Issues (1) • Version control rule is necessary • Our rule: • A node hop-wise closer to the origin is more up-to-date on the activity of its streams • Requires symmetric spanning tree routing Wireless Mesh Networks: CNSR 2006 Tutorial

  47. 1 2 3 4 GW Piggybacking Issues (2) • Carrier packets are required • The “one-way” problem • Alleviate it with promiscuous mode • The stream deactivation problem • Monitor the upstream links from children • The silent subtree problem • When information is unavailable, assume thestreamsare active Wireless Mesh Networks: CNSR 2006 Tutorial

  48. Experiments • CBR over UDP / FTP over TCP • Fixed shortest-path routing • No RTS/CTS • Two link-contention models: • o+cl = • omniscient neighbour info (interference range) • cliques • r+cd = • realistic neighbour info (transmission range) • collision domains Wireless Mesh Networks: CNSR 2006 Tutorial

  49. Chain Experiment (1) Simulation result with Plain TCP Wireless Mesh Networks: CNSR 2006 Tutorial

  50. Chain Experiment (2) Simulation result (TCP) with our fairness rate-control algorithm Wireless Mesh Networks: CNSR 2006 Tutorial

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