Mesh Networks

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Mesh Networks

• A.k.a ad-hoc

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Definition

• A local area network that employs either a full
  mesh topology or partial mesh topology
• Full mesh topology- each node is connected
  directly to each of the others
• Partial mesh topology- some nodes are connected
  to all the others, but some of them are only
  connected to nodes with which they exchange the
  most data

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History

• Originally sponsored by the Department of Defense
  for military use
• Goal was to provide packet-switched network in
  mobile elements of a battlefield in an
  infra-structureless environment
• Used a combination of ALOHA and CSMA and distance
  vector routing

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Full Mesh Topology

• Every node has a circuit connecting it to every
  other node in the network
• Yields greatest redundancy, so if one node fails,
  network traffic can be redirected to any of the
  other nodes
• Usually reserved for backbone networks since it
  is very expensive

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A full mesh topology
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Partial Mesh Topology

• Some nodes are organized in a full mesh scheme
  but others are only connected to 1 or 2 in the
  network
• Common in peripheral networks connected to a full
  meshed backbone
• Less expensive to implement
• Yields less redundancy

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A partial mesh topology
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Wired mesh

• It is possible to have a fully wired mesh
  network, however this is very expensive
• Advantages
• Reliable
• Offers redundancy
• Disadvantages
• - Expensive- large number of cables and
  connections required

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Wireless Mesh

• Definition- a wireless co-operative communication
  infrastructure between multiple individual
  wireless tranceivers that have Ethernet
  capabilities
• Can either be centralized for highly scalable
  applications, or can be decentralized

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• Advantages
• Reliable- each node is connected to several
  others when a node fails its neighbors find
  other routes
• Scalable- capacity can be added simply by adding
  nodes
• Nodes act as repeaters to transmit data from
  nearby nodes to peers too far away to reach- this
  results in a network that can span large
  distances over rough terrain
• Each node only transmits as far as the next node

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• Gizmo truck

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How does it work?

• Data hops from one device to another until it
  reaches its destination
• Each device communicates its routing information
  to every device it connects with
• Each device then determines what to do with
  received data- pass it on or keep it

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Types of Protocols

• Pro-active- distribute routing tables to the
  network periodically to maintain fresh lists of
  destinations
• Disadvantages
• Wasted bandwidth for transmitting routing tables
• Maintains routes that will never be used
• Some algorithms never converge in large networks

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• Re-active- also known as On-Demand these
  protocols find routes on demand by flooding the
  network with Route Request packets
• Disadvantages
• Delays in finding routes
• Excessive flooding can lead to network clogging

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Example ADDV

• ADDV- Ad-hoc On-demand Distance Vector
• Establishes a route to a destination only on
  demand
• Contrast to the most popular pro-active protocols

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How does ADDV work?

• Network is silent until a connection is needed
• The network node that needs a connection
  broadcasts a connection request
• Other nodes forward the message and record the
  node they heard it from, creating temporary
  routes back to the needy node

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• When a node that already has a route to the
  desired node gets the message it sends a message
  back through the temporary route to the
  requesting node
• The needy node then uses the route with the least
  hops to connect

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Failures

• When a node fails, a routing error is passed back
  to the transmitting node and the process repeats
• Also, note that unused entries in the routing
  tables are recycled after a time, so unused paths
  are not kept

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Drawbacks

• More time to establish a connection
• Initial communication to establish a route is
  heavy

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• Hierarchical- network orders itself into a tree
  or other hierarchy and sends requests through the
  structure

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Example Order One Network Protocol

• The network orders itself into a tree
• Each node periodically sends hello to its
  neighbors
• Each neighbor tells how many neighbors and
  connections it has and who its mother node is
• Each node picks the node with the largest access
  to links to be its mother node
• When two nodes pick each other as mother nodes,
  that is the top of the tree

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Routing

• When a node needs a connection with another node
  and a route doesnt exist it sends a request to
  its mother node
• This node then forwards the message to its
  mother node and so on until the original node
  is connected at the root to the node it wanted
• Next the algorithm tries to cut corners to
  optimize the path
• Each node on the route floods its neighbors with
  routing requests
• When a faster route is found, the unused part of
  the previous route is erased and flooding ceases
  on that route

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Advantages

• Produces fairly good routes while reducing the
  number of messages required to keep the network
  connected
• Uses only small amounts of memory at each node
• The network has a reliable way to establish that
  a node is not in the network

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Disadvantages

• Central mother nodes have an extra burden
• Eventually ceases to be scalable
• Link propagation time establishes a limit on the
  speed the network can find its root
• May use more power and bandwidth than other
  link-state protocols

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Where is it going?

• What is the future of wireless ad-hoc?
• Automata