SPAN

1
SPAN

• An Energy Efficient Coordination Algorithm for
  Topology Maintenance in Ad Hoc Wireless Networks

2
Requirement

• Power saving technique that reduces energy
  consumption.
• With sufficient density of nodes, only a small
  number of them need be ON.

3
About Span

• Randomized algorithm where nodes make a local
  decisions on whether to sleep or to join the
  forwarding backbone as a coordinator
• Each nodes bases its decision on the estimate of
  how many of it neighbors will benefit from it
  being awake, and the amount of energy available
  to it.

4
Developments Practical View

• Progress on low power hardware design for mobile
  devices.
• Network interface may often be idle, this power
  could be saved by turning the radio off when not
  in use.
• In practice however this approach is not used, a
  node must arrange to turn its radio on not just
  to receive packets addressed to it, but also to
  participate in routing and control protocol.

5
Issues

• The requirement of co-operation between power
  saving and routing protocol is very narrow in
  case of multi hope ac Hoc network.
• Nodes must forward packets for each other.
• The main issue of span is coordination of power
  saving with routing in ad hoc wireless network.

6
Required characteristics

• It should allow as many nodes as possible to turn
  their radio receivers off most of the time, since
  even an idle receiver circuit can consume almost
  as much energy as an alive transmitter.
• It should forward packets between any source and
  destination with minimally more delay than if all
  nodes are awake. This implies that enough nodes
  must stay awake to form a connected backbone.

7
Contd.

• The backbone formed by the awake nodes should
  provide about as much total capacity as the
  original network else congestion may increase.
  This means that path that could operate without
  interference in the original network should be
  represented in the backbone.

8
Example.
9
Span

• Span, fulfils these requirements.
• Each nodes in the network makes a periodic, local
  decisions on whether to sleep or stay awake as a
  coordinator and participate in forwarding
  backbone topology.
• To preserve capacity, a node decides to volunteer
  to be coordinator if it discovers that two of its
  direct cannot communicate directly or through the
  existing coordinator.

10
Factors to be Coordinator

• Amount of battery remaining
• The number of pairs of neighbors it can connect
  together.
• It does all this using only local information,
  consequently scaling well with the number of
  nodes.

11
Span design

• Span adaptively elects coordinators from all
  nodes in the network.
• Span coordinators stay awake continuously and
  perform multi-hop packet routing within the ad
  hoc network.
• While other nodes remain in power saving mode
  and periodically check if they should wake up an
  become a coordinator.

12
Span design contd.

• Span achieves four goals.
• Ensures that enough coordinators are elected so
  that every node is in the radio range of atleast
  one coordinator.
• It rotates the coordinators in order to ensure
  that all nodes share the task of providing global
  connectivity

13
Span design contd.

• It attempts to minimize the number of nodes
  elected as coordinators, increasing the network
  lifetime, without suffering low of capacity.
• It elects coordinators using only local
  information in decentralized manner.

14
Span design contd.

• Span is proactive each node periodically
  broadcasts HELLO messages that contains the nodes
  status, its current coordinators and its current
  neighbors.
• These HELLO messages are same to those in
  proactive ad hoc routing protocols.

15
Span design contd.

• Each node maintains only a small amount of
  additional state
• its coordinators and coordinators of its
  neighbors
• And the list of neighbors normally found in
  routing tables.

16
Coordinator Announcement

• The following coordinator eligibility rule
  ensures that the network is covered with enough
  coordinators.
• If two neighbors of a non-coordinator node cannot
  reach each other either directly or via one or
  two nodes coordinators, the node should become a
  coordinator.

17
Contd.

• Multiple nodes discover the lack of coordinator
  at the same time and decide to be coordinators.
• Coordinators delay announcement with random back
  off delay.
• HELLO message are announced after the delay if no
  message is been received.

18
Coordinator Withdrawal

• Each coordinator periodically checks if it should
  withdraw as a coordinator. It should withdraw if
  every pair of its neighbors can reach other
  directly or via some other coordinators.
• Also the node being coordinator for some period
  of time withdraws if each pair of neighbors, even
  if they are not coordinators. This gives chance
  to others to be a coordinator.

19
Contd.

• The node continues to serve as a coordinator for
  short period of time after announcing its
  withdrawal.
• Allowing the routing protocol to use the old
  coordinator until new coordinator is elected.

20
Effect of mobility

• Degree of moblity does not significantly affect
  routing with Span coordinators

21
Network lifetime

• Network lifetime is increased with a factor of 2
  with use of Span