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Intro to Ad hoc Network | Part 2

This Part is in continuation of Intro to Ad hoc Network | Part 1

Bringing up an Ad hoc Network

Third node C joins the network with its beacon signal. Two scenarios are possible:
(i) A & B both try to determine if single hop communication is feasible
(ii) Only one of the nodes e.g. B tries to determine if single hop communication is feasible and establishes a connection

The distinct topology updates consisting of both address and the route updates are made in three nodes immediately.

In first scenario, all routes are direct i.e. A->B, B->C, and A->C (Lets assume bi-directional links)

In the second scenario, the routes are updated

  1. First between B & C,
  2. then between B & A,
  3. Then between B & C again confirming that A and C both can reach each other via B.

Topology Update Due to a Link Failure

❒ Mobility of nodes may cause link breakage requiring route updates
❒ Assume link between B & C breaks because of some reason
❒ Nodes A & C are still reachable via D and E
❒ So old route between A &C was A->B->C is to be replaced by A->D->E->C
❒ All five nodes are required to incorporate this change in their routing table
❍ This change will happen first in nodes B & C
❍ Then A & E
❍ Then D

Challenges in Ad hoc Mobile Networks (1)

❒ Host is no longer an end system – can also
be an acting intermediate system
❒ Changing the network topology over time
❒ Potentially frequent network partitions
❒ Every node can be mobile
❒ Limited power capacity
❒ Limited wireless bandwidth
❒ Presence of varying channel quality

❒ No centralized entity – distributed
❒ How to support routing?
❒ How to support channel access?
❒ How to deal with mobility?
❒ How to conserve power?
❒ How to use bandwidth efficiently?

Problems Facing Routing in Ad hoc Networks

❒ Routers are now moving.
❒ Link changes are happening quite often.
❍ Packet losses due to transmission errors.
❒ Event updates are sent often – a lot of control traffic.
❒ Routing table may not be able to, converge.
❒ Routing loop may exist.
❒ Current wired routing uses shortest path metric.

Problems facing channel access in Ad hoc Networks

❒ Distributed channel access, i.e. no fixed base station concept.
❒ Very hard to avoid packet collisions.
❒ Very hard to support QoS.
❒ Early work on packet radio is based on CSMA.

Problems of Mobility in Ad hoc

❒ Mobility affects signal transmission ->Affects communication
❒ Mobility affects channel access
❒ Mobility affects routing
❍ Mobility-induced route changes
❍ Mobility-induced packet losses
❒ Mobility affects multicasting
❒ Mobility affects application

Mobility in Ad hoc Networks

❒ Mobility patterns may be different
❍ people sitting at an airport lounge
❍ New York taxi cabs
❍ kids playing
❍ military movements
❍ personal area network
❒ Mobility characteristics
❍ speed
❍ predictability
• direction of movement
• pattern of movement
❍ uniformity (or lack thereof) of mobility characteristics among different nodes.

Problems of Power in Ad hoc

❒ Ad hoc devices come in many different forms
❒ Most of them battery powered
❒ Battery technology is not progressing as fast as memory or CPU technologies
❒ Wireless transmission, reception, retransmission, beaconing, consume power!
❒ Quest for power-efficient protocols
❒ Quest for better power management techniques.

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