Chapter 16 Wireless Mesh networks

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Chapter 16Wireless Mesh networks
2
Overview

• Introduction
• Mesh network defined
• Benefits of wireless mesh
• Current issues and solutions
• Mesh deployment issues
• Conclusion

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Introduction

• Used by
• Municipalities to enterprises
• Telecom providers
• Public safety
• Military
• Popular do to
• Ease of deployment
• Increase in network capacity
• mobility

4
Mesh Network Defined

• A set of fully interconnected network nodes that
  support traffic flows between any two nodes over
  one or more paths or routes
• Wireless provides connectivity while in motion
• Biggest example is the internet

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Benefits of wireless mesh

• Deployment
• Each node maintains multiple connectivity
• Allows for multiple paths
• Cost
• Wired networks are more expensive
• Many nodes use the same access point
• Continuity of services in case of device failure
• Users expect this because of the fee they pay
• Scalability
• Easy and cost effective to add more capacity

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Examples of Deployment

• Small in home mesh networks
• T.v., audio systems
• Public safety
• Police, fire, emergency responses
• wiMAX or 3G access links

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Current issues and solutions

• Netowork structure
• Intra-mesh channel re-use
• Medium access contention
• Mesh routing and forwarding
• Mesh security
• Congestion control
• Voice over mesh
• Mesh network management

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Network structure

• Plays a big role in performance of voice services
• Determines delay encountered by voice packets
• Parking lot effect
• Is a function of the number of hops in a mesh and
  the peering ratio
• As seen the hop depth H is 3 and the peering
  ratio Rp is 2
• The load on the portal link is equal to 7 peer
  links
• Solutions for the parking lot effect include
  proper dimensioning of the links of mesh network
  and/or limiting access to the mesh so as to keep
  the offered load within the limits imposed by the
  capacity of the available RF links

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Intra-mesh Channel Re-use

• Links between mesh nodes may use one or more RF
  channels
• One RF channel for rapid peer acquisition and
  mesh formation
• May cause interference between links
• Re-use worsens parking lot effect
• The further from a portal, the more a packet has
  to compete for the channel
• In some cases nodes may not see other node
  transmissions
• This leads to interference and packet loss

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Intra-mesh Channel Re-use

• Due to exponential back off for MAC in IEEE
  802.11 protocol, packet loss may lead to
  significant delays per packet per link
• Solutions require increasing the isolation
  between links
• Three dimensions are available
• Space(directional antennas)
• Frequency(multiple RF channels)
• Coding(using orthogonal codes for mesh links)
• Which is best depends on requirements and
  conditions of a given network deployment

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Medium Access Contention

• IEEE 802.11 MAC is designed for bursty data
  traffic in unpredictable RF environments
• Channel state is re-established for every packet
• Done with listen-before-talk and collision
  avoidance through use of contention window
• Medium access is subject to jitter
• For voice and video services, QoS extensions have
  been added to the standard
• EDCA is an enhancement of the basic distributed
  access control mechanism with a priority bias for
  certain types of traffic like voice, video, and
  best effort data

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Mesh Routing and Forwarding

• Routing of packets has an important role in the
  service level offered by a mesh network.
• Route set-up is based on finding the most
  efficient path to the portal
• Once links are formed, path does not change
  except for node or link failure
• Mobile mesh networks do not have static links
• A lot of research efforts have been invested in
  finding a solution to routing in fixed and mobile
  mesh networks

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Mesh routing and Forwarding

• Spanning tree routing
• Networks consisting of wireless bridges naturally
  form tree-like structures
• Rooted in a portal node
• May be set up using a combination of parameters
• Such as hop count and link transit delay
• Monitor functions which detect link failure
  and/or portal loss, may be used to trigger link
  or tree rebuilding
• These structures can be simple and efficient

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Mesh Routing and Forwarding

• AODV Ad-hoc, On-demand Distance Vector protocol
• Fairly simple, but not as efficient as spanning
  tree
• When route is needed or needs repair, its
  originator floods the network with a request for
  a destination
• The latter replies with a unicast that is
  forwarded back to the originator
• Loss of a link triggers a route error message up
  and down the route
• The cost of flooding in AODV depends on the rate
  of change of the environment

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Mesh Routing and Forwarding

• OLSR Optimized Link State Routing protocol
• Is a pro-active routing protocol that uses the
  link state as a driving factor and includes a
  multicast capability
• A subset of nodes called multi-point relays,
  provides anchors for neighbor nodes
• Link state information distribution can remain
  local and multicasting is supported naturally
• Although simple, the implementation of OLSR
  requires many different control messages.
• More efficient than flooding based parents,
  overall efficiency is not the best for this
  protocol

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Mesh Routing and Forwarding

• HWMP Hybrid Wireless Mesh Protocol
• In 2005, the IEEE 802.11 working group started
  development of the wireless LAN standard with a
  layer 2 mesh protocol
• Includes simple tree building protocol to handle
  static mesh, and AODV elements to support mobile
  mesh
• This allows a mesh to use the most efficient
  routing protocol appropriate for a given
  deployment or application

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Mesh Routing and Forwarding

• Regardless of the network structure, there will
  be alternative routes to a given destination and
  each node has to decide which next hop to send an
  a given packet to
• Requires the use of a consistent set of metrics
• All nodes involved must share the same meaning
  and measurement of the metrics of a given link or
  path
• Path metrics can include
• Number of hops, airtime needed to reach
  destination, complex values that bring together a
  variety of parameters such as hop count, link
  load, and SNR

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Mesh Routing and Forwarding

• The best metric to use depends on the type of
  mesh network and the operational conditions
• Different types of service have different
  requirements
• Voice packets are short and must be delivered
  within certain time constraints
• In this case link reliability and hop count are
  important routing metrics
• Background data service packets may be large and
  timing is not important
• In this case a high data rate is more important
  than link reliability

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Mesh Security

• Security in wireless mesh is complicated because
  of absence of human users that can be used as
  trusted parties during network initialization
• Security functionality and the storage of
  cryptographic data cannot be fully relied upon
• Mesh nodes perform a number of functions, each of
  which has its own security concerns

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Mesh Security

• Discovery
• This function serves to detect other mesh nodes
  that belong to the same owner or administrative
  domain
• Security is limited to authentication of the
  information provided by other nodes
• Can be provided by means of public key ciphers
  that allow the verification of digital signatures
• This is done depending on operational needs, the
  tolerance for overhead and the available budget
• In commercial mesh, the discovery function is
  left unprotected
• Exposes the network to spoofing and DOS attacks

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Mesh Security

• Peer link establishment
• This function creates secure links between mesh
  nodes and in the process it validates the
  non-protected data that nodes obtain during
  discovery
• Secure link establishment requires
• Nodes are able to identify themselves
• They can be authenticated
• They are able to set up a cryptographic session
  with each other that protects the flow of data
  and management information between nodes
• Various means are available to secure link set-up
• A combination of symmetric key ciphers and
  asymmetric ciphers are used

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Mesh Security

• Peer link establishmentcontinued
• IEEE 802.11X protocol
• Client requests a connection
• This is done with a security server
• Validates the ID of a node and generates keys

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Mesh Security

• Routing
• Form paths through a network that is used to
  forward data and management information
• Interfering with route set-up causes loss of
  connectivity among nodes as well as loss or
  hiding of data
• Attacks against routing functions tend to result
  in DOS like effects
• Solutions
• Expanding routing protocols
• Tesla approach
• Secure the link before routing is initiated

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Mesh Security

• Forwarding
• The forwarding function delivers packets to their
  destination, either directly or via intermediate
  nodes that lie on a path known to include the
  destination
• Confidentiality and integrity are most important
• Symmetric key cryptography is preferred for
  efficiency
• Can either be hop-by-hop or end-to-end between
  the original node and the end destination
• If connected to other networks, end-to-end is
  impractical, therefore higher layer security
  solutions like IPSEC must be used

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Congestion Control

• Congestion occurs when a source produces more
  than its sink can handle
• Each link is a sink and all links that feed it
  are potential sources
• Consider each link as potential sink for traffic
• Each link carries traffic in both directions
• Must monitor each outgoing link, and using flow
  control messages

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Congestion Control

• Other forms of congestion control
• Rate limiting the traffic sources
• Pro-active form of source flow control
• Downside isinefficiency

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Voice over Mesh

• Factors that can affect the quality of voice over
  mesh
• Choice of codec
• Different requirements with respect to jitter
• Choosing a codec dynamically
• Handover
• Client link and intra mesh links may change
• Time depends on a number of factors
• Other causes of voice quality impairment
• Radio measurement traffic
• Location signaling
• Legacy 802.11 devices that share the channel
• Non-802.11 devices like microwave ovens and
  cordless phones, bluetooth

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Mesh Deployment Issues

• For fixed mesh networks, deployment can be a
  complex issue, even though the self organizing
  capability makes life a bit easier
• The main factors that drive a fixed mesh
  deployment are
• The service level to be offered in terms of bits
  per second per user and on which basis, best
  effort or guaranteed
• The user density people per area
• The available sites for mounting the access
  transceivers
• The available favorable sites for mounting
  intra-network transceivers
• The available RF frequency band and the number of
  channels, shared or otherwise