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

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channel assignment strategies for multi radio wireless mesh networks issues and solutions

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

road map

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
next generation wireless communication

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
wireless mesh networks

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
current wireless mesh networks

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
current wireless mesh networks1

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
trade off between maximum connectivity and minimum interference

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
taxonomy o f ca schemes

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

fixed channel assignment schemes

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
fixed channel assignment schemes cca

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

varying channel assignment schemes vca

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

centralized c hannel assignment c hya

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
centralized c hannel assignment c hya1

CA

FIXED CA

HYBRID CA

DYNAMIC CA

D-HYA

CCA

VCA

LLP

IACA

C-HYA

CLICA

Centralized Channel Assignment: C-HYA
varying channel assignment schemes clica

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
varying channel assignment schemes clica1

CA

FIXED CA

HYBRID CA

DYNAMIC CA

D-HYA

CCA

VCA

LLP

IACA

C-HYA

CLICA

Varying Channel Assignment Schemes -CLICA
dynamic channel assignment schemes

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
distributed channel assignment schemes d hya

CA

FIXED CA

HYBRID CA

DYNAMIC CA

D-HYA

CCA

VCA

LLP

IACA

C-HYA

CLICA

Distributed Channel Assignment Schemes D-HYA
distributed channel assignment schemes d hya1

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
hybrid channel assignment schemes

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
hybrid channel assignment schemes llp

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
hybrid channel assignment schemes iaca

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
hybrid channel assignment schemes iaca1

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
mestic

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
mestic algorithm overview

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
mestic link load calculation

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
mestic algorithm

* 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

mestic algorithm illustration

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
ns 2 simulation test bed

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
ns 2 simulation rsults mestic vs c hya

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
merits

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
mestic some demerits

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