Gugrajah_Yuvaan_ Ramesh_2003.pdf

Routing Protocols for Ad Hoc Networks Chapter 2
probing [Chen99]. Nodes maintain local state information about the delay (channel
delay, queuing delay, processing delay) of each outgoing link, residual bandwidth on
each link, and the cost of the link, which is lower for links that have been in
existence for longer. The basic idea behind ticket-based probing [Chen99] is to limit
the number of candidate paths and avoid flooding by using probe messages with a
limited number of so called tickets. Each ticket corresponds to' one path searching so
the maximum number of searched paths is bounded by the tickets issued from the
source. When an intermediate node receives a probe message it decides how to split
the tickets and where to forward the probe depending on its local state information.
When the destination node receives a probe message, a possible path from the source
to the destination is found.
More tickets are issued for the connections requiring higher requirements and the
intermediate nodes assign more tickets to the links with larger residual bandwidth
that satisfy the QoS requirements. Probes carrying "yellow" tickets prefer paths with
smaller delays and more residllal bandwidth in order to satisfy the given
requirements. Probes with "green" tickets attempt to maximize the probability of
finding a low-cost path, which mayor may not have larger delays and the required
bandwidth. Algorithms are presented [Chen99] to determine the optimum number of
each type of ticket to assign to a probe depending on the QoS requirements. In
general, the lower the constraints, the lower the number of tickets required.
The dynamic nature of ad hoc networks makes it difficult to provide QoS. The
unpredictable way in which the network changes coupled with the fact that the state
information is usually imprecise results in high overheads in attempting to provide
QoS and QoS provision can be impossible [Chen99] if the ad hoc network changes
too fast The proposed QoS routing protocols for ad hoc networks are therefore based
on the assumption that the nodes are very slow moving and topology changes occur
infrequent!y enough for the routing protocol to be successful.
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