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Topology Control, Interference, and Throughput for Wireless Mesh Networks

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Topology Control, Interference, and Throughput for Wireless Mesh Networks

presented by Qin LIU

- Introduction
- Network Model
- Interference Model
- Power Adjustment
- Channel Assignment
- Future Work

- A wireless mesh network (WMN) is a multi-hop wireless network that consists of mesh clients and mesh routers.
- Mesh routers form the backbone of WMNs.
- Some of mesh routers are called gateway nodes and connected with a wired network.
- provide Internet access

- Reduction of installation costs
- Only a few mesh router have cabled connections to the wired network.

- Large-scale deployment
- WLAN: One hop communication has limited coverage.
- WMN: Multihop communication offers long distance communication through intermediate nodes.

- Reliability
- Redundant paths between a pair of nodes in a WMN increases communication reliability.

- Self-Management
- A WMN is a special ad hoc network.

- broadband home networking
- community and neighborhood networking
- enterprise networking
- metropolitan area networks
- transportation systems
- building automation
- health and medical systems
- security surveillance systems
- …

- Support for ad hoc networking, and capability of self-forming, self-healing, and self-organization
- Mobility dependence on the type of mesh nodes
- Multiple types of network access
- Dependence of power-consumption constraints on the type of mesh nodes
- Compatibility and interoperability with existing wireless networks
- Multi-channel multi-radio system

- There are multiple non-overlapping channels
- IEEE 802.11b/a standards offer 3 and 12 non-overlapping channels, respectively.

- Each node is equipped with multiple radios
- interference reduction
- communicate with more than one neighbor at the same time
- full duplex operation
- throughput improvement

- A topology consists of a set of nodes and links, and it describes the connectivity information of the network.
- Links in topology are the result of some controlled parameters, such as transmission power and channel assigned.
- A good topology is critical to network performance.
- too dense energy consumption & interference throughput
- too sparse long path, disconnected network

- Reducing energy consumption and interference may be conflicting goals. [Burkhart 2004]
- We focus on topology control for interference reduction.

- Topology control in WMNs includes two steps:
- Power adjustment
- Channel assignment

- Power adjustment
- Define the physical topology of network
- A link between two nodes if they are reachable via transmission power.

- Channel assignment
- Define the logical topology on the top of the physical topology
- A link between two nodes if they are reachable and use a common channel.

- V : A set of nodes, representing the wireless devices in the Euclidean plane.
- : the maximum transmission power of node v
- p(u, v): the least required energy to transmit a message from u to v
- G(V, E): network graph, any link e = (u, v) E if
- GP(V, EP): physical topology, EP E
GPis a subgraph of G

- C: # of channels
- Q(v): # of radios on node v, and typically Q(v) <C
- A(v): the set of channels assigned on v, |A(v)|=Q(v)
- GL(V, EL): logical topology, any logical link e = (u, v; k) EL iff (u, v) EP and k A(u) A(v)
- There may be multiple logical links between a pair of nodes in GL, andit is a multi-graph.

physical topology

network graph

logical topology

- Interference model specifies conditions where a signal can be successfully received.
- Physical Model
- transmission from u to v (SNR: signal-to-noise ratio, SS: signal strength)

- Protocol Model (transmission from u to v)
- p(u) p(u, v), and
- no other interfering transmitter w, d(w, v) (1 + )∙ d(u, v) ( > 0)

- Other Interference Models
- Transmitter Model (Tx-model)
- Transmitter-receiver Model (Tx-Rx model)

- IEEE 802.11 MAC protocol
- RTS-CTS
- Symmetrical communication: Both the sender and the receiver should be free from interference for a successful transmission.

- Interference reduction based on network topology only
- network planning
- MIN interference while keeping certain network properties, such as k-connectivity and spanner

- Interference reduction based on network topology and traffic flows between nodes
- network planning and routing
- MAX network throughput

- K-connectivity
- The k-connected graphcontains at least k independent paths between any pair of nodes.
- Two or more paths are independent if they none of them contains an inner node of another.

- The deletion of any set of less than k nodes in the k-connected graph still leaves a connected graph.

- The k-connected graphcontains at least k independent paths between any pair of nodes.
- Spanner
- stretch factor: distance stretch factor, energy stretch factor, hop stretch factor
- distance stretch factor
- dG(u, v) (resp. ) denotes the minimum distance between u and v in G (resp. GP)

- GP is a spanner of G if the stretch factor is within a constant.

- Reduce interference of all transmitting signals
- Link-based Interference Reduction
- define the interference of a link

- Node-based Interference Reduction
- define the interference of a node

- Minimize the node coverage interference
- Cov(e) = |{wV| d(u, w) d(u, v)}} { wV| d(v, w) d(v, u)}|
- # of nodes that are affected when the link (u, v) is active.
- The network interference is defined as the maximum (or total, average) node coverage in the physical topology.
- MST is the optimal solution when minimizing the maximum node coverage in a connected physical topology.

node coverage

- Minimize the link interference
- # of links interfered by the link (u, v) in GP
- This definition of interference has been proposed, but no work on minimizing such interference in physical topology control has been reported.

link interference

- Minimize the sender-based interference
- the transmission power of u:
- the interference of node u:
- # of nodes that receive signals transmitted by u
- Minimize the maximum sender-based interference while keeping the network k-connected or spanner.
- Mnimize the average sender-based interference in a connected topology (NP-hard?)

IS(v) = 4

IS(u) = 1

- Minimize the receiver-based interference
- the interference of node v:
- # of nodes that affects node v
- It is more realistic because interference occurs at the receiver instead of the sender.
- A -approximation algorithm has been proposed to MIN the maximum receiver-based interference while keeping the topology connected in a highway model.

IR(v) = 2

IR(u) = 2

- Efficient channel assignment can greatly reduce the interference effect of close-by transmissions.
- Categories of channel assignments
- static assignment
- dynamic assignment
- hybrid assignment

- Channel assignment only
- Combine channel assignment and routing

- Minimum Interference Survivable Topology Control
- assumption: same transmission range r, same interference range R,
interference disk Dua disk centered at u with radius R

- link interference: node x, y, u and v
such that d (u, v) r and d(x, y) r and

k A(u) A(v) A(x) A(y) and

xDuDv or yDuDv

e1 = (x, y; k) interferes with e2 = (u, v; k)

- link co-channel interference
I(e): # of links in GL that interfere with e

- topology interference:
- objective: Minimize I(GL) while keeping the network k-connected. (Np-hard)

- assumption: same transmission range r, same interference range R,

- Before a channel assignment is known, the actual interference of links are unknown.
- potential interference Do not consider channel.

- First get a k-connected structure with minimum potential interference from the physical topology.
- Then assign the least used channels nearby to links in the non-increasing order of potential interference.

Combine Channel Assignment & Routing

- Given traffic demand, there is a circular dependency between channel assignment and routing
- Routing link capacity channel assignment link’s expected load routing

- LP-based Routing and Channel Assignment
M. Alicherry, R. Bhatia, and L. Li, “Joint Channel Assignment and Routing for Throughput Optimization in Multi-radio Wireless Mesh Networks,” MOBICOM 2005.

- constrained maximum network flow problem

- Problem: Given one destination u0, and the traffic demand du of each node u, find the optimal channel assignment, routing and scheduling scheme that achieves the maximum throughput.
- Notations:
- Nu:set of nodes with the transmission range of u
- Nu: set of nodes that within the interference range of node u, and u Nu
- The system works in a periodical synchronized mode where each cycle contains T time slots.
is the binary variable, only if link (u, v) is active on channel k at time slot t

- Radio Constraint: at any time, a node can use at most Q(u)different channels to send packets.
- Interference Constraint (Schedulable Constraint): at any time, two interference links can not be active at the same channel.
- Sufficient condition:

AB interferes with CD and EF. When AB is active, CD and EF should keep silent.

But CD and EF do interfere with each other, and they can be activated at the same time.

the percentage usage of link (u, v) on channel k

the available bandwidth of (u, v) on channel k, where c is the bandwidth of each channel

Basic structure of LP

- Due to relaxation in LP, the channel assignment may not be feasible. Post-processing is needed to make channel assignment feasible.

- Which interference criterion is more proper?
- What is the appropriate optimizing objective?
- Many optimization problems of topology control are NP-hard so that efficient algorithms are valuable.
- especially for channel assignment

- Distributed algorithms for practical networks.
- Consider power adjustment and channel assignment together.
- Interference-aware routing
- QoS call admission
- QoS multicast call admission

Thanks!

Q & A