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Chapter 16 Wireless Mesh networks

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Title: Chapter 16 Wireless Mesh networks


1
Chapter 16Wireless Mesh networks
2
Overview
  • Introduction
  • Mesh network defined
  • Benefits of wireless mesh
  • Current issues and solutions
  • Mesh deployment issues
  • Conclusion

3
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

5
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

6
Examples of Deployment
  • Small in home mesh networks
  • T.v., audio systems
  • Public safety
  • Police, fire, emergency responses
  • wiMAX or 3G access links

7
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

8
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

9
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

10
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

11
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

12
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

13
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

14
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

15
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

16
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

17
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

18
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

19
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

20
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

21
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

22
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

23
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

24
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

25
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

26
Congestion Control
  • Other forms of congestion control
  • Rate limiting the traffic sources
  • Pro-active form of source flow control
  • Downside isinefficiency

27
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

28
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
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