Improving Capacity and Flexibility of Wireless Mesh Networks by Interface Switching Yunxia Feng, Minglu Li and Min-You Wu Presented by: Yunxia Feng Dept. of Computer Science & Engineering Shanghai Jiao Tong University Shanghai, China

Outlines  Problem Description  Hybrid Channel Assignment Protocol (HCAP) The Interface Assignment Strategies Coordination Policy of HCAP The Channel Assignment Algorithm  Evaluation Results  Conclusions and Future Works

Problem Description Fig. 1 The I-WMN Architecture Gateways (GAs): The routers that connect to wired networks via wired links; SNs: The mesh routers within the transmission range of GAs; GNs: All of the mesh routers except GAs and SNs.

Problem Description (cont.) Interference in I-WMNs is serious  Heavy traffic loads;  Multiple hops between the source and destination nodes;  One interface can only fix on one channel at the same time. Possible Resolutions  Equipping each node with multiple interfaces; Fix interfaces on different channels. However, it is difficult to equip each node with the same number of interfaces as available channels.  Dynamic channel assignment (DCA) Co-channel interference is minimized; Improves the network capacity; Enhances the scalability of WMNs.

Components of Multi-interface DCA Interface assignment strategy:  Decides the interface switching mode of nodes; Coordination scheme:  Provides methods to implement communications between nodes under given interface assignment strategy.

Problems Induced by DCA Switching Overheads  switching delay Dependence Problems  Connection dependence Nodes cannot communicate with each other due to switching to different channels.  Channel dependence Changing the channel of a particular link may result in changing allocated channels of many other links in the network.

Hybrid Channel Assignment Protocol (HCAP) Premises  GAs distribute across the network, and any two GAs do not interfere with each other even if they are assigned the same channel;  One SN node can only establish one routing path to the GA nodes at a time;  HCAP adopts one of current synchronization protocols;  If each node is equipped with only two interfaces, then, A SN is within the transmission range of only one GA.  If each node is equipped with more than two interfaces, then A SN is within the transmission range of the same number of GAs as the number of its interface that adopt static strategy.

The Interface Assignment Strategies GA Nodes:  Static interface assignment strategy to avoid frequent interface switching. All the Other Nodes (SN and GN nodes):  Hybrid strategy to keep flexible fixed interface (FI): the interface that adopts static strategy; switching interface (SI): the interface that dynamically switch channels.

The Interface Assignment Strategies (cont.) Declaration & Definitions  The channel assigned to every FI is different from that of nodes within its interference range as possible.  The channel is referred to the node ’ s receiving channel (C Rec );  The corresponding interface is referred to its receiving interface (I Rec ).  Dynamic links: links between hybrid nodes  Static links: links between GA nodes and SN nodes within their transmission range.

Questions in Implementing HCAP Which interface should a C Rec be assigned to?  C Rec is assigned to the FI of GN nodes;  C Rec is assigned to the SI of SN nodes. Reasons  The FI of every SN node should fix on one channel of the GA within its transmission range.  Every SN node should fix its FI on one channel of the GA within its transmission range to communicate with the GA since none interface on GA nodes can switch interfaces dynamically.  The assigned channels of GAs should be shared among the fewest nodes to minimize the interference level on them due to the heaviest loads.

The sketch of HCAP (a) Network topology (b) Sketch of HCAP Fig. 2 A simple example of HCAP

Coordination Policy of HCAP Divide the time of hybrid nodes into slots, whereas several slots compose one Cycle. Connection dependency problem in HCAP  Suppose that B is transmitting packets to node G on link at present. If node C wants to send some packets to B, there is connection dependency problem. Resolution  Designate several slots in every Cycle for the SI of every SN node to fix on its C Rec. We denote these slots by Receiving Slots (RS) to differentiate with other slots. The number and positions of RSs in a Cycle are determined by the settings of each specific network.

Broadcast Problem Description:  There is no permanent links between hybrid nodes The FIs of hybrid nodes are assigned different channels, whereas their SIs switch to different channels dynamically. Resolution:  Take several slots from the left slots of every Cycle as Broadcast Slots (BS), during which the SI of all hybrid nodes switch to a predefined channel to only transfer broadcast packets. The number and positions of RSs in a Cycle are determined by the settings of each specific network.

Channel Assignment Algorithm Three Phases  In the first phase, the algorithm assigns channels to both interfaces of GA nodes.  In the second phase, the algorithm assigns channels to the FI of SN nodes.  At last, the algorithm assigns C Rec to hybrid nodes. Channel Assignment Sequence  The algorithm always assigns channel to the node that has the biggest η. If multiple nodes have the biggest η, they are assigned channels in a random order. This sorting gives higher priority for nodes that have more restrictions.

Default Simulations Settings A 36-node I-WMN, where each node is equipped with two half-duplex interfaces. The 36 nodes randomly distribute in a 6×6 square grid network, and the distance between two neighboring nodes equals to transmission range. Four nodes are designated as GA nodes and connected to the wired network. The ratio between interference range and transmission range is set to 2. The simulation period is between seconds. Utilize the DSR protocol for route selection.

Evaluation Results Fig.3 Throughput Comparison Fig. 4 Adaptation to Flow changes

Evaluation Results Fig. 5 Throughputs vs. traffic fairness Fig. 6 Impact of broadcast slot

Conclusions & Future Works We have presented a hybrid channel assignment protocol to improve the capacity and flexibility of WMNs. The protocol does not depend on prior information on the traffic loads, whereas it can adapt automatically to the changes in the network. Extensive simulations show that the protocol improves the network capacity and flexibility. It also achieves better per-flow fairness, whereas it keeps both interface switching overheads and coordination complexity under tolerable range. As future work, we will provide analyses results and describe how to the algorithm in details.

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