Channel Assignment Strategies for Multi-radio Wireless Mesh Networks : Issues and Solutions

1. Channel Assignment Strategies for Multi-radio Wireless Mesh Networks: Issues and Solutions HabibaSkalli, IMT Lucca Institute for Advanced Studies SamikGhosh and Sajal K. Das, The University of Texas at Arlington Luciano Lenzini, University of Pisa Presented by: SurendranNeelakantan

2. Next Generation Wireless Communication • Wireless Mesh Networks & Current trend • Trade off between Connectivity and Interference • Taxonomy of CA schemes • MesTiC - New Proposed Scheme • MesTiCalgorithm and its Illustration • MestiC Performance Evaluation • Comparison of CA Schemes • MsTiC Merits and Demerits • Conclusion. Road Map

3. Next-generation wireless mobile communications will be driven by converged networks of different technologies, such as • (3G/4G) mobile cellular systems • IEEE 802.11 (WiFi) based wireless local area networks(WLANs), • Emerging broadband wireless technologies such as IEEE 802.16 (WiMAX). Next Generation Wireless Communication

4. Wireless Mesh Networks are the key component of the converged networks. Properties of Wireless Mesh Networks are : • Fully wireless • Multi hope Ad hoc • Dedicated Mesh routers • Cost effective • Flexible • Configurable Wireless Mesh Networks

5. Current Wireless Mesh Networks are single channel and single radio Problem: • Reduced Capacity due to Interference. Solutions: • Modified MAC Protocol (Dynamic Channel switching) • Directional antennas Draw backs: • Practical deployment is not feasible with direction antennas • Dynamic Channel switching requires tight Time synchronization between the nodes B 1 1 1 A C 1 1 D Current Wireless Mesh Networks

6. Another cheap and Attractive Practical solution : • IEEE 802.11b/g and IEEE 802.11a standards provide 3 and 12 non overlapping (frequency) channels, respectively, which can be used simultaneously within a neighborhood (by assigning non overlapping channels to radios) Problem : • Effective Channel Assignment Problem • Interference • One common channel use to setup connectivity which reduces no. of available channel. Current Wireless Mesh networks

7. THE GOAL • Channel assignment (CA) in a multi-radio WMN environment consists of assigning channels to the radio interfaces in order to achieve efficient channel utilization and minimize interference • All 4 channel can be exploited • Only 3 channel can be assigned to radios • Maximum connectivity that can be achieved 4 available Channels 2 interface /node 1 Channel interface 4 available Channels 2 interface /node B B B 1 1 2 2 1 1 2 1 A C A C A C 2 4 3 1 3 1 D D D Single channel scenario Maximum connectivity Minimum Interference Trade-off between Maximum connectivity and minimum interference

8. In a fixed scheme the CA is almost constant, while in a dynamic scheme it is continuously updated to improve performance. A hybrid scheme applies a fixed scheme for some interfaces and a dynamic one for others. • CCA – Common Channel Assignment • VCA –Varying Channel Assignment • C-HYA Centralized – Hyacinth • CLICA – Connected Low Interference Channel Assignment • D-HYA -Distributed – Hyacinth • LLP –Link Layer Protocol • IACA –Interference Aware Channel Assignment • The problem of optimally assigning channels in an arbitrary mesh topology has been proven to be NP-hard based on its mapping to a graph-coloring problem CA FIXED CA HYBRID CA DYNAMIC CA D-HYA CCA VCA LLP IACA C-HYA CLICA Taxonomy of CA Schemes Centralized CA

9. Fixed channel assignment schemes assign channels to interfaceseither permanently or for long time intervals with respect to the interface switching time. This is further subdivided into: • Common Channel Assignment(CCA) • Varying Channel Assignment.(VCA) Interface = Radio = NIC A virtual link between two nodes is defined as a possible direct communication link between them CA FIXED CA HYBRID CA DYNAMIC CA D-HYA CCA VCA LLP IACA C-HYA CLICA Fixed Channel Assignment Schemes

10. This is the simplest scheme. • The radio interfaces of each node are all assigned the same set of channels. Pros: • Multiple channel Increased network throughput Cons: • Limited advantage when no. of available Channels much greater than Number of radios CA FIXED CA HYBRID CA DYNAMIC CA D-HYA CCA VCA LLP IACA C-HYA CLICA B B 1 2 A C A C Fixed Channel Assignment Schemes - CCA 1 2 D D 12 channels available (802.11a) 2 radios/node

11. In the VCA scheme, interfaces of different nodes may be assigned different sets of channels CA FIXED CA HYBRID CA DYNAMIC CA D-HYA CCA VCA LLP IACA C-HYA CLICA B B 1 2 A C Con: • The assignment of channels may lead to network partitions and topology changes that may increase the length of routes between the mesh nodes A C Varying Channel Assignment Schemes(VCA) D 3 4 D 12 channels available (802.11a) 2 radios/node

12. Based on Hyacinth- a multichannel wireless mesh network architecture • Traffic directed toward gateway nodes • This algorithm assigns channels for a known traffic load • It first estimates the total expected load on each virtual link • Then the channel assignment algorithm greedily assigns channel. • The algorithm starts with an initial estimation of the expected traffic load and iterates over both channel assignment and routing until the bandwidth allocated to each virtual link matches its expected load. Pros: It take into account Connectivity and traffic load • Cons: already assigned links have to be revisited, (ripple effect) thus increasing the time complexity of the scheme CA FIXED CA HYBRID CA DYNAMIC CA D-HYA CCA VCA LLP IACA C-HYA CLICA Centralized Channel Assignment: C-HYA

13. CA FIXED CA HYBRID CA DYNAMIC CA D-HYA CCA VCA LLP IACA C-HYA CLICA Centralized Channel Assignment: C-HYA

14. A Topology Control Approach • Connected Low Interference Channel Assignment (CLICA) • Use Greedy approach for CA • That computes the priority for each mesh node and assigns channels based on the connectivity graph and conflict graph Pros: Overcomes link revisits (Ripple Effect) Cons: It does not incorporate changing traffic load CA FIXED CA HYBRID CA DYNAMIC CA D-HYA CCA VCA LLP IACA C-HYA CLICA Varying Channel Assignment Schemes -CLICA

15. CA FIXED CA HYBRID CA DYNAMIC CA D-HYA CCA VCA LLP IACA C-HYA CLICA Varying Channel Assignment Schemes -CLICA

16. Any interface can be assigned any channel. • Interfaces can frequently switch from one channel to another • Each node switches channels synchronously in a pseudo- random sequence so that all neighbors meet periodically in the same channel Pros • Ability to switch an interface to any channel. • Offering the potential to use many channels with few interfaces Cons: • Channel Switching delays are high (typically on the order of milliseconds in commodity 802.11 wireless cards) • Need for coordination mechanisms like Slotted Seeded Channel Hopping (SSCH) for channel switching between nodes CA FIXED CA HYBRID CA DYNAMIC CA D-HYA CCA VCA LLP IACA C-HYA CLICA Dynamic Channel Assignment Schemes

17. CA FIXED CA HYBRID CA DYNAMIC CA D-HYA CCA VCA LLP IACA C-HYA CLICA Distributed Channel Assignment Schemes D-HYA

18. Based on Hyacinth, a multichannel wireless mesh network architecture Channel Assignment Scheme • Algorithm (D-HYA) builds on a Spanning tree structure with every gateway node (root) connected to internet and mesh nodes are the leafs • Neighbor-to-interface binding selects interface to connect with neighbors and eliminates dependency to avoid ripple effect • Interface-to-channel binding select channel to assign the interface • Pros: It takes Traffic into account when channels dynamically assign to the interfaces • No Link revisit (Ripple Effect) • Cons: The tree-topology make it difficult for multi-path routing in mesh networks CA FIXED CA HYBRID CA DYNAMIC CA D-HYA CCA VCA LLP IACA C-HYA CLICA Distributed Channel Assignment Schemes D-HYA

19. Apply a fixed assignment for some interfaces • Dynamic assignment for other interfaces • Pros: They allow for simple coordination algorithms with fixed assignment, • It has the flexibility of dynamic channel assignment CA FIXED CA HYBRID CA DYNAMIC CA D-HYA CCA VCA LLP IACA C-HYA CLICA Hybrid Channel Assignment Schemes

20. Link Layer Protocol for interface assignment (LLP) • Categorizes available interfaces into fixed and switchable interfaces • Switchable interfaces can be switched over short timescales among non-fixed channels based on the amount of data Traffic • By distributing fixed interfaces of different nodes on different channels, all channels can be used, while the switchable interface can be used to maintain connectivity • proposed scheme is based on (SSCH), Protocol • Cons: This Scheme does not take into account the traffic load in assigning the fixed channels CA FIXED CA HYBRID CA DYNAMIC CA D-HYA CCA VCA LLP IACA C-HYA CLICA Hybrid Channel Assignment Schemes -LLP

21. Interference-Aware Channel Assignment (IACA) • Dynamic and Centralized • Aimed at improving the capacity of the WMN backbone and minimizing interference • Based on multi-radio conflict graph (MCG) • where the vertices in the MCG represent edges between mesh radios • assigns one radio on each node to operate on a default common channel throughout the network. • computes interference and bandwidth estimates based on the number of interfering radios, where an interfering radio is a simultaneously operating radio that is visible to a mesh router but external to its network. CA FIXED CA HYBRID CA DYNAMIC CA D-HYA CCA VCA LLP IACA C-HYA CLICA Hybrid Channel Assignment Schemes -IACA

22. works on a rank-based strategy where the rank for every available channel is based on interference and load CA FIXED CA HYBRID CA DYNAMIC CA D-HYA CCA VCA LLP IACA C-HYA CLICA Multi-radio Conflict Graph Hybrid Channel Assignment Schemes -IACA

23. CA • MesTiC is Mesh based Traffic and Interference aware Channel assignment. • MesTiCis a fixed, rank-based, greedy algorithm for centralized channel assignment, which visits every node once, thereby eliminating any link revisit (ripple effect). • The rank of each node is computed on the basis of its link traffic characteristics, topological properties, and number of NICs on a node. • Topological connectivity is ensured by a common default channel deployed on a separate radio on each node, which can also be used for network management. • Fixed schemes alleviate the need for channel switching • Nodes with heavy load give more flexibility. • Common default channel for communication prevents flow interruption. FIXED CA HYBRID CA DYNAMIC CA D-HYA CCA VCA LLP IACA MesTiC C-HYA CLICA MesTic

24. The central idea behind MesTiCis to assign channels to the radios of a mesh node based on ranksassigned by an algorithm to the nodes. • Compute the aggregate traffic at a node based on the offered load of the mesh network • The distance of the node, measured as the minimum number of hops from the gateway node • And the number of radio interfaces available on a node • Gateway node is assigned the highest rank as it is expected to carry the most traffic. • The aggregate traffic flowing through a mesh node has an impact on the channel assignment strategy MesTic Algorithm -Overview

25. A n estimate of expected link load is based on the notion of link criticality .To compute initial expected link loads, we assume perfect load balancing across all acceptable paths between each communicating node pair. • Let’s call the number of acceptable paths between a pair of nodes(s,d), P(s,d) that pass a link l, Pl(s,d) .Then Expected load on the link l Ql is calculated using the equation • Ql • Where B(s,d) is the estimated load between the node pair (s,d) in the traffic profile MesTic Link Load Calculation

26. * Number of radios(node) Calculate fixed rank for every node Every node is visited in decreasing order or the rank (II.1) If two nodes have already been assigned a common channel by default there is a link between those nodes (II.2) If not , for every possible unassigned link, the one that carries the higher traffic is assigned first. In the following manner : (II2.a) if the node visited still has an assigned radio, the least used channel is assigned to one of its free radios and a link established with its neighbor (II.2.b)Otherwise if all visited node’s radio have already been assigned , the least used channel among those already assigned to its radios is assigned to the link. I. Order all the nodes according to rank II. Visit every node in the decreasing rank order II.1 Assign channel to the incident link ,if the node and one of its neighbors are assigned a channel in common MesTic Algorithm Else: Node has an unassigned incident link II.2 Pick the neighbor with whom the node has the higher traffic in the traffic matrix II.2a. Assign the radio to the least used channel in the vicinity If the node has an unassigned radio Else II.2b Assign the link a least used channel those already assigned to the nodes radios

27. Rank order: D(210) > A(170) > C(150) [Aggregate traffic] • Visit gateway Nod B first, Assign CH1 to link B-A (120) • Assign CH2 to B-D (90) (Next higher traffic, Unassigned radio). [II.2,II.2a] • Assign CH2 to B-C (Assigned radio ,least used channel, less traffic)[II.2b] • Assign CH3 to D-C (Least used = Previously unassigned)[II.2a] • Assign CH3 to D-A , (Assigned radio ,least used channel, less traffic )[II.2b] CH1 CH2 CH3 B 3 channels 2 interface/node 120 80 B CH1 90 CH2 A C A C CH2 70 50 CH3 D CH3 D Connectivity and Link traffic Channel assignment with MesTiC MesTic Algorithm- Illustration

28. Protocol USED : Dynamic Source Routing (DSR) • Channel Assignment Scheme : MesTiC and C-HYA • Number of radios on each node :3 • Non overlapping channels:12 • No. of mesh nodes :25 • The simulation was performed for 100 s for a given set of traffic profiles • ns-2 was configured to report the aggregate throughput obtained in the network • The experiments were conducted on the mesh network topology with channel assignments generated by MesTiC, and repeated for the channel assignments generated by C-HYA Ns-2 simulation Test Bed

29. Simulation stabilizes around 40 s from the start of the simulation run, after which MesTiCreports a sustained higher aggregate throughput for the mesh network. Ns-2 simulation RsultsMesTicVs C-HYA • Similarly, at the stable region, with MesTiCthere is enough bandwidth for a larger number of flows in the system, with an average value of 14 flows against an average of 9 flows in C-HYA

30. Comparison of CA Schemes

31. CA • Is a fixed centralized scheme that takes into account traffic load information while assigning channels to radio interfaces. • Give more bandwidth to links that need to support higher traffic. • Allocates least used frequency channel(shared by fewer number of nodes) to Links with higher traffic • Does not suffer from ripple effects and topology constraints Compared to C-HYA and D-HYA • Uses a default radio that creates a fixed topology and avoid the need for a mechanism to ensure connectivity, as is the case for CLICA FIXED CA HYBRID CA DYNAMIC CA D-HYA CCA VCA LLP IACA MesTiC C-HYA CLICA MesTiC Merits

32. The proposed scheme has been designed for a mesh network with a single gateway node. • The Impact of channel assignment schemes on the performance of routing protocols for wireless mesh networks not well studied. MestiC- Some Demerits

33. Thank you !