Topology Control, Interference, and Throughput for Wireless Mesh Networks

1
Topology Control, Interference, and Throughput
for Wireless Mesh Networks

• presented by Qin LIU

2
Outline

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

3
Introduction

• 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

4
Architecture
5
Benefits

• 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.

6
Applications

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

7
Features

• 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

8
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

9
Topology Control in WMNs

• 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.

10
Topology Control in WMNs

• 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.

11
Network Model

• 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
• GP is a subgraph of G

12
Network Model

• C of channels
• Q(v) of radios on node v, and typically Q(v) lt
  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, and it is a multi-graph.

13
Example
physical topology
network graph
logical topology
14
Interference Model

• 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)

15
Interference Model

• 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) (? gt 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.

16
Classification of Interference Reduction Methods

• 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

17
Network Properties

• K-connectivity
• The k-connected graph contains 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.
• 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.

18
Power Adjustment

• 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

19
Link-based Interference Reduction

• Minimize the node coverage interference
• Cov(e) w?V d(u, w) ? d(u, v)? w?V 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
20
Link-based Interference Reduction

• 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
21
Node-based Interference Reduction

• 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
22
Node-based Interference Reduction

• 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
23
Channel Assignment

• 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

24
Channel Assignment Only

• Minimum Interference Survivable Topology Control
• assumption same transmission range r, same
  interference range R,
• interference disk Du a 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
• x ?Du?Dv or y ?Du?Dv
• 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)

25
A Heuristic Algorithm

• 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.

26
Combine Channel Assignment Routing

• Given traffic demand, there is a circular
  dependency between channel assignment and routing
• Routing? link capacity ? channel assignment ?
  links 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

27
LP-based Channel Assignment Routing

• 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
• N?u set of nodes that within the interference
  range of node u, and u ? N?u
• 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

28
LP-based Channel Assignment Routing

• 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.
29
LP Relaxation

• 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
30
LP Relaxation

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

31
Future Work

• 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

32

• Thanks!
• Q A