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Mesh Networks
A.k.a “ad-hoc”
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
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
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
A full mesh topology
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
A partial mesh topology
Wired mesh
It is possible to have a fully wired mesh
network, however this is very expensive
Advantages
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Reliable
Offers redundancy
Disadvantages
- Expensive- large number of cables and
connections required
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
Advantages
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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
Gizmo truck
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
Types of Protocols
Pro-active- distribute routing tables to the
network periodically to maintain fresh lists
of destinations
Disadvantages
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Wasted bandwidth for transmitting routing
tables
Maintains routes that will never be used
Some algorithms never converge in large
networks
Re-active- also known as On-Demand
these protocols find routes on demand by
flooding the network with Route Request
packets
Disadvantages
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Delays in finding routes
Excessive flooding can lead to network
clogging
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
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
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
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
Drawbacks
More time to establish a connection
Initial communication to establish a route
is heavy
Hierarchical- network orders itself into a
tree or other hierarchy and sends requests
through the structure
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
Routing
When a node needs a connection with another node and
a route doesn’t 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
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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
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
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
Where is it going?
What is the future of wireless ad-hoc?
Automata