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Scaling the Throughput of Wireless Mesh Networks via Physical Carrier Sensing and Two-Radio Multi-Channel Architecture

Scaling the Throughput of Wireless Mesh Networks via Physical Carrier Sensing and Two-Radio Multi-Channel Architecture. Jing Zhu*, Sumit Roy*, Xingang Guo**, and W. Steven Conner** *Department of Electrical Engineering U of Washington, Seattle, WA **Communications Technology Lab

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Scaling the Throughput of Wireless Mesh Networks via Physical Carrier Sensing and Two-Radio Multi-Channel Architecture

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  1. Scaling the Throughput of Wireless Mesh Networks via Physical Carrier Sensing and Two-Radio Multi-Channel Architecture Jing Zhu*, Sumit Roy*, Xingang Guo**, and W. Steven Conner** *Department of Electrical Engineering U of Washington, Seattle, WA **Communications Technology Lab Intel Corporation, Hillsboro, OR

  2. Outline of Presentation • Mesh Networks: Introduction, Architecture • Enhancing Aggregate (Network) Throughput • 1. Enhance spatial reuse via optimal physical carrier • sensing • 2. Multiple Orthogonal Channels (frequency reuse) • Channel Allocation with clustering • Multi-Radio, Multi-channel Architecture  Towards a soft-radio architecture for high-performance MESH

  3. Mesh Networks: Salient Features • Scalability for coverage • Single hop  Multi-hop (mesh) • Heterogeneous Nodes, Hierarchy • Mobile Clients, APs, SoftAPs (router) • Multiple PHY technologies • WiFi, WiMAX, UWB, … • Challenge for MAC in Mesh - Current MAC Protocols (e.g. 802.11) are not optimized for Mesh • low efficiency, poor fairness, … • Key Solution Approach: Spatial Reuse + Channel Reuse

  4. Example1: AP-MT Mesh–Enterprise • As # clients (laptops) increase, more APs are needed in the same area. • Available # orthogonal channels is very limited (3 or 8 in 11b/a)  increased multiple acccess interference.

  5. Example 2: Wireless AP-AP Mesh

  6. PHY Optimization: MIMO, Adaptive Coded Modulation, etc. MAC Optimization Frequency Plan: 3 (11b), 7 (11a), ? (11n) Topology Control Link Capacity How to scale a MESH? Our Focus = X X Network Throughput Spatial Reuse Frequency (Channel) Reuse

  7. Outline • CSMA/CA – the core of 802.11 MAC • Spatial Reuse and Physical Carrier Sensing • Implementation of PCS in OPNET: Simulation of Spatial Reuse • Enhance Physical Carrier Sensing Scheme • Optimal PCS threshold through tuning: PCS adaptation • Channel Reuse: Two-Radio Multi-Channel Clustering Architecture • Next-gen: Adaptive MAC Framework for Mesh

  8. CSMA/CA – basic 802.11 MAC • Carrier Sensing Multiple Access / Collision Avoidance • Physical Carrier Sensing (PCS) for Interference Avoidance • Binary Exponential Back-off (BEB) for Collision Avoidance • (Optional) RTS/CTS Handshaking • Advantages: • Asynchronous, Distributed, Simple • Disadvantages: • Low Spatial Reuse (due to Non-optimized PCS) • No QoS Support (due to pure contention-based access)

  9. Spatial Reuse • Multiple communications using the same channel/freq happen simultaneously at different locations w/o interfering each other • Received SINR Model: • Physical Carrier Sensing • A station samples the energy in the medium and initiates transmission only if the reading is below a threshold  threshold optimization

  10. A1 I1 … I2 Tx Rx R B1 B2 Hidden node Problem Revisited Hidden Node: A node that cannot hear the current transmission but will cause the failure of the transmission if it transmits. Any node outside of transmission range of Tx and Rx could be a hidden node, which cannot be prevented by using RTS/CTS!

  11. Hidden Nodes in a MESH • Multiple (group) of hidden nodes in a mesh • Accumulation of interferences • Impossible to identify due to the unknown number of contributors. • Instead of preventing all hidden nodes, the goal of the interference avoidance/mitigation is pro-actively avoiding the worst-case interference • Sensed energy during PCS is a good indicator of interference level on the coming transmission. • The lower the sensing threshold, the higher the received SNIR on average

  12. Effect of PCS threshold on Network Throughput • Has a great impact on the performance • PHY improvement does NOT necessarily mean proportional improvement at MAC • Optimal PCS threshold varies with data rates and topology • How to set the optimal carrier sensing threshold dynamically?

  13. Analytical estimate of end2end t’put: Observations: Near optimal results can be achieved by simply tuning the carrier sensing threshold without using RTS/CTS Comparison with analytical estimates (simulation is for 90-node Chain) [1] Xingang Guo, Sumit Roy, W. Steven Conner, "Spatial Reuse in Wireless Ad-hoc Networks," IEEE VTC 2003, Orlando, FL, October, 2003.

  14. Optimal PCS Threshold • Assumptions: • Co-location of receiver and transmitter • Homogenous links (same reception power) • Ignore background noise • Saturation traffic load • Result: • Optimal PCS Threshold ≈ 1/S0, where S0 is the SINR threshold for sustaining the maximum link throughput • S0 = 11dB, 14dB, 18dB, and 21dB for 802.11b 1Mbps, 2Mbps, 5.5Mbps, and 11Mbps, respectively.

  15. 10x10 Grid with Local Only Traffic and Homogenous Links

  16. Comparison of 1/S0 with the Simulation Optimal PCS threshold 1/S0(dB) Simulations match the theoretical estimates !

  17. Enterprise Network: AP-MT Mesh 3 Channels 16 / 30 / 72/ 110 APs per channel 11Mbps, So = 21dB 154 m x 154 m Office Path Loss Exponent =3

  18. Scale the Capacity of Enterprise AP Network 73% 60% 40% 28% • Network capacity is proportional to # of APs • The optimal PCS achieves best per-AP capacity

  19. Summary: Spatial-Reuse for a single-channel MESH • Spatial-Reuse – the key to improve the aggregate throughput of a single-channel mesh • links sufficiently separated can transmit simultaneously without interfering each other • Limitations: • Not effective for a small scale network, i.e. the required minimum separation distance could be high. • For example, >7 hops in a regular chain network with 802.11b 1Mbps and path loss exponent = 2. • Further Scaling the Throughput with Multiple Channels!

  20. Scaling the Throughput with Multiple Channels • Takes advantage of multiple channels (even multiple bands) • 8 orthogonal channels in 802.11 a • 3 orthogonal channels in 802.11 b • UWB, 802.11, and 802.16 • Channel Bonding (wider channel BW) is another alternative • Increases peak link rate but does not translate to proportional MAC throughput increase • Lack of backward compatibility: proprietary solution • Multi-channel Approaches – Our Choice • No change on channel BW • Use all available channels through the network • Key issues: channel allocation

  21. Feasible Multi-Channel Architectures • One-Radio Multi-Channel Approaches* • Efficient, but will require new MAC (hence not backwards compatible) • Still cannot do full-duplex transmission (e.g.difficult to conduct channel sensing consistently due to channel switching) • Control overhead – per-packet channel swtiching • Multi Radio: One Channel per NIC(Network Interface Card) ** • Simple to implement • Each NIC channel is fixed (i.e. comes hard-coded from manufacturer) • no negotiation required for channel selection • Fully compatible with legacy • But costly, will not scale (number of NICs = number of channels) • Our Approach: Two Radio Multi-Channel • Scale, i.e. number of NICs fixed at 2 • Backwards compatible • Assumptions: ad-hoc scenario, irregular but not random topology, homogenous traffic  No need to frequently update the channel allocation! *:Jiandong LI, Zygmunt J. Haas, and Min Sheng; ``Capacity Evaluation of Multi-Channel Multi-Hop Ad Hoc Networks ''; IEEE International Conference on Personal Wireless Communications, ICPWC 2002. **: A. Adya, P. Bahl, J. Padhye, A. Wolman, and L. Zhu, A Multi-Radio Unification Protocol for IEEE 802.11 Wireless Networks, Microsoft Research, Technical Report MSR-TR-2003-44, July, 2003.

  22. Two-Radio Based Network Cluster • Channel Allocation with Clustering • Each cluster is identified a common channel – i.e. all inter-cluster communications using the default (primary) radio • Intra-cluster communications on different channels using the secondary radio • Interference Mitigation • Interference among co-channel clusters is minimized through an efficient channel selection algorithm – MIX (min. interference channel select). • Interference within the cluster is prevented by Physical Carrier Sensing. • Legacy compatible: legacy APs connect to mesh via default radio.

  23. Framework • Semi-distributed clustering channel assignment + distributed MAC mechanisms (802.11 DCF) • Semi-distributed: channel on secondary radio is assigned by the local cluster-head within the cluster • Distributed: CSMA/CA MAC protocols • Default vs. Secondary Radio • Both radios are for data transmission • The secondary radio has no administrative functionality, such as association, authentication, etc. • The common channel on the default radio is determined a-priori. • Layer 3 (IP) routing between the nodes

  24. Distributed Highest Connection Clustering (HCC) Algorithm* • A node is elected as a clusterhead if it is the most highly connected (has the highest number of neighbor nodes) node of all its ``uncovered" neighbor nodes (in case of a tie, lowest ID (e.g. MAC address) prevails). • A node which has not elected its clusterhead is an “uncovered” node, otherwise it is a “covered” node. • A node which has already elected another node as its clusterhead gives up its role as a clusterhead. * M. Gerla and J.T.-C. Tsai, "Multicluster, mobile, multimedia radio network", ACM/Baltzer Journal of Wireless Networks. vol. 1, (no. 3), 1995, p. 255-265.

  25. Clustering Procedure • Step 1: All nodes have their neighbor list ready (every node should know its neighbors, how many) • Step 2: All nodes broadcast their own neighboring information, i.e., the number of neighbors, to its neighborhood. • Step 3: A node that has got such information from all its neighbors can decide its status (clusterhead or slave)

  26. MIX – Minimum Interference Channel Selection • On-Air energy estimation per channel • t0: estimation starting time • T: estimation period • Ei(t): on-air energy at time t on channel i • k: Selected Channel

  27. Forwarding Table (MAC Extension) 192.168.0.3 192.168.0.1 192.168.0.4 Cluster 1 Cluster 2 192.168.0.2 • An IP packet will be forwarded to default or Secondary MAC/PHY according to the forwarding table in the MAC Extension layer.

  28. Example – 10 x 10 Grid Cluster-Slave Cluster-Head • Transmission range = d • d: neighboring distance

  29. Simulation Topology • Random, Local, and Saturate Traffic • 10 x 10 Grid • 802.11 b 1Mbps • 3 orthogonal channels • Path Loss Exponent = 3 • Packet Size =1024 Bytes • Dash Circle: Cluster • Dark node: Cluster-Head

  30. Tracing One-Hop Aggregate Throughput • The new multi-channel and two radio architecture achieves 3X performance, compared to a traditional single-channel and single-radio mesh.

  31. Throughput Distribution • Location-dependent fairness problem : Links Ai experience worse interference environment than links Bi and Ci, leading to the oscillation of the throughput distribution. • Future Work: How Physical Carrier Sensing could mitigate the location dependent fairness problem?

  32. 200m x 200m 100 nodes Random Topology

  33. Performance Comparison in Random Topology a) Tracing Aggregate Throughput b) Throughput Distribution • Performance gain of aggregate throughput is almost 3x (10Mbps vs. 3.5Mbps)

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