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CS 598: Advanced Topics in Wireless Networks

CS 598: Advanced Topics in Wireless Networks. Jennifer C. Hou, Professor Department of Computer Science University of Illinois at Urbana Champaign jhou@cs.uiuc.edu http://lion.cs.uiuc.edu. Wireless Mesh Networks.  Is targeted to solve the well-known last-mile problem for broadband

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CS 598: Advanced Topics in Wireless Networks

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  1. CS 598: Advanced Topics in Wireless Networks Jennifer C. Hou, Professor Department of Computer Science University of Illinois at Urbana Champaign jhou@cs.uiuc.edu http://lion.cs.uiuc.edu

  2. Wireless Mesh Networks  Is targeted to solve the well-known last-mile problem for broadband access.  Most of the nodes are stationary.  Only a fraction of nodes have direct access, and serve as gateway to the Internet.  Nodes serve as relays forwarding traffic and maintain network-wide Internet connectivity. Access point DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  3. Why Less is More (Wireless vs Wireline)  Everyone requires residential broadband access  Reality: Only 12% of U.S. households have it (as compared to 50% in Korea; 25% in Hong Kong; 6% in Japan; and 15% in Sweden)  Access is still expensive, and “remote areas” are NOT covered  Wireless mesh networks are preferred to existing cable/DSL based networks and WLANs  $20--$50K per square mile to establish access (approximately 1/4 of high speed cables).  Quick and easy installation (approximately one square mile in 1/2 day).  Low maintenance/operation costs (coverage can be readily extended by installing additional ad-hoc hops).  Inherent robustness (each station has redundant paths to reach the Internet).  More attainable throughput through multi-radio, math-path routing of bandwidth-abundant paths.  Less is indeed more! DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  4. Current State of Art  Wi-Fi clouds have been planned for large cities  Bay Area Wireless User Group (BAWUG) http://www.bawug.org  Champaign-Urbana Community Wireless Nework http://cuwireless.net  MIT Roofnet http://pdos..csail.mit.edu/roofnet/doku.php  Seattle Wireless http://www.seattlewireless.net  Wireless Leiden in Sweden http://www.wirelessleiden.nl  Chaska, Minnesota  One of 29 Wi-Fi clouds in U.S.  18K people, 16 square miles  Built in a few weeks at the cost of $600K  Service charge is $15.99 / month DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  5. Current State of Art  Initial success has been reported in MIT Roofnet, which achieves user throughput rates comparable to DSL subscribers.  However, several performance-related and deployment problems have been identified.  Unpredictable channel behaviors  Inability to locate stable and high-throughput paths due to the shortest path algorithm  Throughput degradation because of intra-flow and inter-flow interference  Lack of incentives (and a pricing mechanism) to forward transit packets  Security holes and lack of privacy protection mechanisms DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  6. Problem 1: Unpredictable Channel Behavior Cause and (Perhaps) Cure

  7. Unpredictable Channel Behavior SNR versus distance Delivery ratio versus distance DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  8. Unpredictable Channel Behavior •Most links have intermittent loss rates •The SNR and the distance are not good predictors of delivery probability DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  9. The Notion of a Link No Longer Exists  A link is usually characterized by its bandwidth, latency, packet loss ratio and patterns.  In wireless environments, most links have intermediate loss rates, and the delivery ratio has little correlation with the SNR or the distance.  This is rooted in the fact that the wireless medium is a shared medium, and the sharing range is determined by  Each station’s transmit power and carrier sense threshold  Intra-flow and inter-flow interference (determined in turn by the node distribution and the traffic distribution)  Multi-path fading, small-scale fading, shadowing  Temperature and humidity variation  Existence of objects or obstacles DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  10. The Notion of a Link No Longer Exists  All the definitive metrics that characterize a link are no longer well defined for a wireless link.  How does this affect higher-layer protocols?  A shortest-hop route consisting of long links give poor throughput.  A legacy routing protocol running on top of such wireless links may never converge, as re-route may constantly be triggered by false link breakage. DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  11. Channel Behavior Modeling  Need a measurement study in order to identify  (i) the factors that will affect the characteristics of the wireless physical layer (e.g., path loss, multi-path fading, interference, shadowing), and  (ii) to what extent and under what circumstances they affect the wireless characteristics.  Need to develop, for each specific environment, statistical models that take into account of multiple factors.  The environment will be specified by, for example, the distance between the sender/receiver pairs, the relative heights of the sender/receiver pairs, whether or not there are obstacles between them (and how tall they are), and the type of potential concurrent on-going transmissions and their effects. DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  12. Transparent Device Driver  A virtual device driver should be designed that  Exports several physical layer characteristics to higher layer modules.  Includes several functional modules supporting  Channel behavior understanding and modeling  Interference detection and mitigation  Cross-layer design and optimization DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  13. Protocol Stack for Wireless Networks Group membership management Application Layer Transport Layer TCP UDP Multicast Routing Routing Network Layer Mobility Management Topology Control Power Mgmt Medium Access Scheduling MAC Layer Physical Layer Power Adjustment Directional Beam-Forming Channels Selection (frequency/code) GPS Positioning & Synchronizing DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  14. Wireless Research in the Stack Group membership management Application Layer Both WLANs and ad hoc networks How to deliver messages to a group so as to ensure causal/total ordering in the presence of out of order messages due to mobility Transport Layer TCP UDP Multicast Routing Routing Network Layer Mobility Management Topology Control Power Mgmt Medium Access Scheduling MAC Layer Physical Layer Power Adjustment Directional Beam-Forming Channels Selection (frequency/code) GPS Positioning & Synchronizing DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  15. Wireless Research in the Stack Group membership management over wireless Application Layer Both WLANs and ad hoc networks: How to instrument TCP so that it does not always take packet losses as indication of congestion. Transport Layer TCP UDP SNOOP, M-TCP, I-TCP, Freeze-TCP, Syndrome Multicast Routing Routing Network Layer Mobility Management Topology Control Power Mgmt Medium Access Scheduling MAC Layer Physical Layer Power Adjustment Directional Beam-Forming Channels Selection (frequency/code) GPS Positioning & Synchronizing DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  16. Wireless Research in the Stack • Ad hoc routing: how to deal with - frequent changing topology - outdated cache DSDV, STAR, DSR, AODV, TORA ZRP • Power aware routing: instead of using hop counts and/or delays as metrics, uses remaining power (or variation) as a metric Group membership management over wireless Application Layer Transport Layer TCP UDP Multicast Routing Routing Network Layer Mobility Management Topology Control Power Mgmt Medium Access Scheduling MAC Layer Physical Layer Power Adjustment Directional Beam-Forming Channels Selection (frequency/code) GPS Positioning & Synchronizing DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  17. Wireless Research in the Stack Group membership management over wireless Application Layer • Wireless multicast routing - Shortest path tree best tree - Frequent changing topology breaks the tree easily  Transport Layer TCP UDP Amroute, core-assisted routing, MAODV, ODMRP Multicast Routing Routing Network Layer Mobility Management Topology Control Power Mgmt Medium Access Scheduling MAC Layer Physical Layer Power Adjustment Directional Beam-Forming Channels Selection (frequency/code) GPS Positioning & Synchronizing DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  18. Wireless Research in the Stack Group membership management over wireless Application Layer Transport Layer TCP UDP Multicast Routing Routing Network Layer Mobility Management Topology Control • Mobile IP and variation • Handoff • Packet rerouting Power Mgmt Medium Access Scheduling MAC Layer Physical Layer Power Adjustment Directional Beam-Forming Channels Selection (frequency/code) GPS Positioning & Synchronizing DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  19. Wireless Research in the Stack How each node adjusts its transmission power so as to • maintain network connectivity • increase spatial reuse • increase network capacity • mitigate MAC interference Group membership management over wireless Application Layer Transport Layer TCP UDP CBTC, COMPOW, CONNECT, Relative Neighborhood graph, LMST Multicast Routing Routing Network Layer Mobility Management Topology Control Power Mgmt Medium Access Scheduling MAC Layer Physical Layer Power Adjustment Directional Beam-Forming Channels Selection (frequency/code) GPS Positioning & Synchronizing DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  20. Topology Control DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  21. Wireless Research in the Stack Wireless devices may go to sleep in the absence of activities •When a node switches to the low- power state •When a node in the low-power state switches to the active state -- On-demand -- Scheduled rendezvous (802.11) -- Asynchronous wakeup Group membership management over wireless Application Layer Transport Layer TCP UDP Multicast Routing Routing Network Layer Mobility Management Topology Control Power Mgmt Medium Access Scheduling MAC Layer Physical Layer Power Adjustment Directional Beam-Forming Channels Selection (frequency/code) GPS Positioning & Synchronizing DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  22. Wireless Research in the Stack 1. Enhancement of IEEE 802.11 to • improve achievable throughput • mitigate the hidden/exposed terminal problems • provide QoS (IEEE 802.11e) 2. Contention resolution with the use of • different frequencies/codes • directional antennas • different power levels Group membership management over wireless Application Layer Transport Layer TCP UDP Multicast Routing Routing Network Layer Mobility Management Topology Control Power Mgmt Medium Access Scheduling MAC Layer Physical Layer Power Adjustment Directional Beam-Forming Channels Selection (frequency/code) GPS Positioning & Synchronizing DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  23. Wireless Research in the Stack Group membership management over wireless Application Layer MAC that takes advantage of • different frequencies/codes • directional antennas • different WFQs to achieve service differentiation among nodes in the two-hop neighborhood Transport Layer TCP UDP Multicast Routing Routing Network Layer Mobility Management Topology Control Power Mgmt Medium Access Scheduling MAC Layer Physical Layer Power Adjustment Directional Beam-Forming Channels Selection (frequency/code) GPS Positioning & Synchronizing DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  24. Wireless Research in the Stack Cross layer issues 1. Security 2. Dependability (fault tolerance, robustness) 3. Extensibility (hybrid networks) Group membership management over wireless Application Layer Transport Layer TCP UDP Multicast Routing Routing Network Layer Mobility Management Topology Control Power Mgmt Medium Access Scheduling MAC Layer Physical Layer Power Adjustment Directional Beam-Forming Channels Selection (frequency/code) GPS Positioning & Synchronizing DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  25. Problem 2: Throughput degradation as a result of inter-/intro- flow interference

  26. Interference  Flows that are routed along different paths within the interference range compete for the channel bandwidth, resulting in inter-flow interference.  Consecutive packets in a single flow may be spread over the route and interfere with each other, resulting in intra-flow interference. transmission range D Y Z A B C interference range Queue at A DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  27. Control Knobs for Mitigating Interference  To mitigate interference and maximize the network capacity, there are several control knobs:  Transmit power  topology control  Carrier sense threshold  trade-off between spatial reuse and interference level  Scheduling consecutive transmission for interference-free connections  Channel diversity  use of non-overlapping channels DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  28. Topology Control  Topology Control: Instead of transmitting with the maximal power, each node collaboratively reduces its transmission power to defines the network topology DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  29. Topology Control – Cross Layer Design View Cross Layer Design Dynamic Topology Control w.r.t. Network Traffic Network Layer Network Capacity Topology Control Effect of MAC- Layer Interference Network Lifetime MAC Layer Critical Power Analysis Incorporating Physical Layer Characteristics Physical Layer DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  30. Problem Formulation  Objective: To reduce power consumption, mitigate MAC interference, increase spatial reuse, while preserving network connectivity  Requirement: The algorithm/protocol should rely only on local information  Input: A set of wireless nodes in the view of a node in the 2/3-D plane  Output: The transmission power (range) for each node to form the appropriate neighbor relation DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  31. Example: Local Minimum Spanning Tree Algorithm w3 w2 w4 w1 Node u’s neighborhood w5 u w6 w7 • Node u only retains w1and w6 as its neighbors in the final topology. DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  32. Properties of LMST First localized topology control algorithm! Preserves connectivity. After removal of asymmetric links, all links are bi-directional and the connectivity is still preserved. The (logical) degree of any node is bounded by 6.     DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  33. Spatial Reuse Through Controlling CS Threshold  The “contending area” can also be adapted through tuning  the carrier-sensing threshold B D C A Signal Strength F E CS Threshold distance DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  34. How CS Threshold Controls Contending Area  Larger CS threshold leads to smaller contending area  Less nodes compete the channel in time  less collisions  Larger CS threshold also leads to higher interference  Transmission rate depends on Signal-to-Interference-Noise Ratio B D C A Signal Strength F E CS Threshold distance DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  35. Benefits of Smaller Contending Area  Reduced MAC overhead  Better utilization of each communication link  More spatial reuse  At the cost of higher interference level and lower transmission rate What is the optimal CS Threshold? How does it relate to the transmit power? UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Low rate links DEPARTMENT OF COMPUTER SCIENCE High rate links Jennifer Hou

  36. Exploring Spatial Diversity Through Scheduling  Find sets of nodes that result in the least inter flow interference using location information  Defer consecutive transmission for mitigating intro-flow interference  Interleave transmission to interference-free nodes. D Y Z A B C Case 1 Case 2  Questions: How to find interference-free connections DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  37. Determining Interference-free Sets of Nodes  Each mesh node measures signal strength to the other nodes and reports to gateway nodes  A gateway node constructs the coordinate system with the use of measured signal strengths.  Find sets of nodes that result in the least inter-flow interference wireless node A wireless node measures RSSs to Jennifer Hou other nodes DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN

  38. Problem 3: Multi-Radio, Multi-path Routing

  39. Routing Metrics  Routing protocols based on the hop count often choose long, lossy links that give low throughput.  Routing metrics that are more sophisticated than the hop count need to be used to locate paths with high throughput and low loss rates.  This boils down to the issue of how to exploit MAC/PHY characteristics to better quantify the quality of a route. DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  40. Example Routing Metric  Estimated Transmission Time (ETT) metric.  ETT predicts the total amount of time it would take to send a data packet along a route.  Each mesh node sends periodic broadcasts at each available 802.11b bit-rate.  ETT for a given link = expected time to successfully send a 1500- byte packet at that link’s highest-throughput bit-rate, including the time for the number of retransmissions predicted by the measured delivery probabilities. Highest-throughput bit-rate = bit-rate with highest (delivery prob. X bit-rate)  ETT for a route = sum of ETTs for each of the route’s links 1 t  t : e2e throughput ti: throughput of hops 1  it i DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  41. Multi-radio, Multi-Path Routing  There may be multiple Internet gateways.  The routing decision is complicated by one more dimension  which Internet gateway to connect.  Devices can be equipped with multiple interfaces, each of which can be assigned different channels. Access point DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  42. Multi-radio, Multi-path Routing  IEEE 802.11 has multiple channels  12 in IEEE 802.11a, 3 non- overlapping in IEEE 802.11b  The routing decision can be further complicated by assigning different channels to interfaces and finding high-throughput routes on different channels. Soekris net4521 device DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  43. Benefit of Using Single Interface and Multiple Channels  Multiple channels can be used to mitigate interference and enable simultaneous transmissions.  When used with single radio nodes network partition may result. DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  44. Benefit of Using Multiple Interfaces and Multiple Channels  With sufficient radios and sufficient channels, interference can be completely eliminated.  Channel assignment becomes crucial DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  45. Open, Transparent Device Driver for Cross Layer Design

  46. Urbana Champaign Wireless Community Networks • We are currently working with Champaign-Urbana Wireless Community Network to deploy a open, city-wide wireless community network. • Currently 14 wireless nodes are operational in downtown Urbana, and we expect to extend to 40 nodes providing full coverage of Champaign and Urbana. •Both a research testbed and a production network DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  47. Hardware The CUWiN rooftop router  Contains a Soekris Engineering net4526 single-board computer in a weatherproof of enclosure and an 802.11b/g radio with Atheros chipset.  Operates in 802.11b-standard IBSS mode, and uses 802.11b rates, 1, 2, 5.5, and 11 Mb/s.  Is equipped with a CUWiN software solution to defeat 802.11 IBSS network partitioning and powered by the power- over-Ethernet injector.  Can configure itself as either an Internet gateway or a client, depending on whether it detects a DHCP server on the Ethernet interface. Approximately $500 per node (including installation)   DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  48. CUWireless Software Architecture Configuration database Web UI dhcpd /etc dhcpselect zebra routevizd dhclient mDNS responder hslsd nsbridge Routing socket Kernel FIB Ethernet NIC Wireless NIC DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  49. Why a Transparent Device Driver is Needed  The attributes used to characterize a link are no longer fixed, but rely heavily on physical layer characteristics.   Need to export several physical layer characteristics to higher layer modules.  Several functional modules need to be implemented in the device driver in order to support, for example  Scheduling and power assignment for cross-layer design and optimization  Interference detection and mitigation  Channel behavior understanding and modeling Some of the new functionality may be at conflict with 802.11 functionality! DEPARTMENT OF COMPUTER SCIENCE UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN Jennifer Hou

  50. New Quagga Clients CUWireless Software Environment Channel behavior modeling Channel utilization optimization Zebra FIB Frame scheduling Kernel Modified Madwifi Interference Detection & mitigation Frame transport Power assignment & Parameter turning Measurement Hardware abstraction layer Wireless NIC UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN (Atheros Chipset) DEPARTMENT OF COMPUTER SCIENCE Jennifer Hou

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