Capacity of Wireless Mesh Networks: Comparing Single-Radio, Dual-Radio, and Multi-Radio Networks

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Capacity of Wireless Mesh Networks Comparing
Single-Radio, Dual-Radio, and Multi-Radio Networks

• By Alan Applegate

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Introduction

• Common Topologies
• Bus Network
• Backbone
• All nodes hear all transmissions
• Star Network
• Central switch/router
• Nodes only hear transmissions intended for them
• Mesh
• All nodes logically connected to each other
• Connection to only one node required
• Packets forwarded through nodes to reach
  destination

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Introduction Contd

• Types of Mesh Networks
• Wired
• Physical connection of every node to every other
  node
• Wireless
• Ad-hoc
• Permanent infrastructure
• Shared
• Switched
• Single-Radio
• Dual-Radio
• Multi-Radio

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History of Mesh Networks

• 1st wireless mesh networks were mobile ad-hoc
• Wireless stations dynamically participated in a
  peer-to-peer network (i.e. mobile p2p)
• Mesh used because it allowed a node to
  participate in a network without needing to
  communicate with every other node
• Also very flexible

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Why We Need Wireless Mesh Networks

• Traditional approach of a collection of WiFi
  access points connected to a backhaul network is
  extremely expensive
• Wireless mesh ad-hoc networks are much more
  flexible
• Lower cost
• Wider coverage
• More reliable

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Why Wireless Mesh Works

• Wireless works best with LoS
• Permanent wireless infrastructure mesh systems
  can utilize forwarding capabilities of the mesh
  architecture to maneuver around physical
  obstacles
• Less expensive than using high-power signals to
  blast through obstructions as used with
  traditional point to multipoint systems
• Maintains LoS for best quality signal
• Redundancy Reliability
• High levels of frequency re-use between mesh
  links
• Increases overall system capacity
• Works well in dense urban areas

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Terminology

• Single-Radio (Shared Mesh Network)
• A wireless mesh network that uses a single-radio
  to communicate to all neighboring nodes
• Total B/W between all nodes
• Mesh Access Points (MAPs)
• Include both mesh interconnection links and
  client access

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Terminology

• Mesh Points (MPs)
• Mesh nodes used exclusively for forwarding
• Dual-Radio Shared MAP
• Uses separate access and mesh link radios
• Only the mesh link radio is shared
• B/W shared between mesh links and client access

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Terminology

• Multi-Radio (Switched Mesh Network)
• Uses multiple radios to communicate via dedicated
  mesh links to each neighboring node in the mesh
• All B/W of radio channel dedicated to neighboring
  nodes link
• Uses separate access and multiple mesh link radios

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Terminology

• Mesh Cluster
• The collection of mesh APs that home to a
  particular wired egress connection
• Mesh Portal
• The mesh point located at the egress connection

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Single-Radio Shared Wireless Mesh

• Description
• Each AP node acts as a regular AP that supports
  WiFi client access as well as forwarding traffic
  wirelessly to other mesh points
• Same radio used for access and wireless mesh
  links
• Omni-directional antenna

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Single-Radio Shared Wireless Mesh

• Pros
• Lowest cost deployment of a wireless mesh network
• Simplest implementation

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Single-Radio Shared Wireless Mesh

• Cons
• Every packet generated must be repeated on the
  same channel to send it to at least one
  neighboring AP until reaching the mesh portal
• Creates excessive traffic
• More APs More traffic dedicated to forwarding
• Very little channel capacity available to support
  users

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Single-Radio Shared Wireless Mesh

• Cons contd
• Capacity reduction as a result of forwarding is
  between 1/N and (1/2)N where N is equal to the
  number of mesh link hops.
• i.e. Capacity available to users decreases with
  each additional AP

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Single-Radio Shared Wireless Mesh

• Cons contd
• Use of 1/N or (1/2)N depends on a number of
  factors including topology, location of the mesh
  portal and interference domain
• Interference domain of nodes whose
  transmissions will be sensed by and hence block
  the transmission of other nodes.
• 1/N is the most optimistic, which is achieved via
  routing protocols that optimize forwarding and
  eliminate unnecessary transmissions.

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Single-Radio Shared Wireless Mesh

• All clients and mesh APs must operate on the same
  channel and use the 802.11 MAC protocol to
  control contention for the physical medium.
• Entire mesh acts like a single AP, all APs and
  clients must contend for a single channel

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Dual-Radio Shared Wireless Mesh

• Description
• Separate radios for client access and mesh links
• Operate _at_ different frequencies
• Typical configuration
• 2.4 Ghz client access
• 5 Ghz mesh link
• 802.11 MAC

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Dual-Radio Shared Wireless Mesh

• Pros
• Improved capacity and scalability over
  single-radio
• Client access not affected by mesh link
  forwarding

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Dual-Radio Shared Wireless Mesh

• Cons
• Mesh link contention still limits capacity
• Sometimes results in blocking other APs
• Results in reduced system capacity as the network
  grows
• Dual-radio systems are a significant improvement
  over single-radio mesh designs and provide for
  more potential growth of a mesh cluster

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Multi-Radio Switched Wireless Mesh

• Description
• Separates access and mesh links
• Multiple radios
• Typically uses directional antennas
• Creates a dedicated link between mesh points
• aka multiple point to point
• Based on 802.11a
• Operates _at_ unlicensed 5 Ghz band

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Multi-Radio Switched Wireless Mesh

• Pros
• Provides increased capacity by overcoming shared
  mesh limitations inherent to single and
  dual-radio mesh architectures
• No longer a shared network
• Individual mesh links have dedicated radio
  channel
• Very rich mesh topologies possible

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Multi-Radio Switched Wireless Mesh

• Pros contd
• Low contention
• Much higher performance than dual- or
  single-radio
• More capacity and scalability
• More nodes more capacity
• Capacity only limited by wired backhaul
• Co-existence eliminated
• Interference reduced
• Backhaul latency low and predictable
• Mesh link range increased (directional antennas)

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Conclusion

• The capacity of wireless mesh networks is
  directly affected by the shared network
  contention of the mesh links between mesh points
  used to forward packets
• Single-radio best for small mesh clusters at the
  edge of the network
• Dual-radio represents evolution in the growth of
  a mesh network

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Conclusion Contd

• Multi-radio separate wireless access and mesh
  links
• This eliminates in-channel mesh forwarding and
  shared mesh link contention
• Results in high capacity system that can scale to
  support large networks with broadband service for
  many users.