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An ad hoc network is a collection of wireless mobile nodes dynamically
forming a temporary network without the use of existing network
infra-structure or centralized administration. Due to the limited
transmission range of wireless network interfaces, multiple network hops
may be needed for one node to exchange data with another across the
network. In such a network, each mobile node operate not only as a host
but also as a router, forwarding packets for other mobile nodes in the
network, that may not be within the direct reach wireless transmission
range of each other. Each node participates in an ad hoc routing protocol
that allows it to discover multi hop paths through the network to any
other node. The idea of an ad hoc network is sometimes also called an
infrastructure-less networking, since the mobile nodes in the network
dynamically establish routing among themselves to form their own network
on the fly.
Some examples of the possible use of ad hoc networks include students
using laptop computers to participate in an interactive lecture, business
associates sharing information during a meeting, soldiers relaying
information for situational awareness on the battlefield, and emergency
disaster relief personnel coordinating efforts after a hurricane or
earthquake.
A lot of work has already been done in the area of unicast routing in
ad hoc networks. These routing protocols can be broadly classified into
two categories:
- Table driven
- Source initiated (demand driven)
Table driven routing protocols attempt to maintain consistent, up to date
routing information from each node to every other node in the network.
These protocols require each node to maintain one or more tables to store
routing information, and they respond to changes in network topology by
propagating updates throughout the network in order to maintain a
consistent network view. The areas in which they differ are the number of
necessary routing related tables and the methods by which changes in
network structure are broadcast.
Some of such routing protocols are:
A different approach from table driven routing is source initiated on
demand routing. This type of routing creates routes only when desired by
the source node. When a node requires a route to a destination, it
initiates a route discovery process within the network. This process is
completed once a route is found or all possible route permutations have
been examined. Once a route has been established, it is maintained by a
route maintenance procedure until either the destination becomes
inaccessible along every path from the source or until the route is no
longer desired.
The following protocols fall in this category:
- Ad Hoc On Demand Routing Protocol
- Dynamic Source Routing
- Temporally Ordered Routing Algorithm
- Associativity Based Routing
- Signal Stability Routing
Currently most of the existing wireless network proposals are based in
a single hop, cellular model supported by a wired infrastructure. The main
design challenge in this environment are the support of QoS connections in
the wireless segment and the graceful hand off of mobile hosts from one
base station to another. Work has been done in this direction and has been
mainly focussed on
- multimedia traffic integration within wireless cell
- timely rerouting of backbone virtual circuits from one cell to
another with minimal disruption of service.
The problem of interconnecting to the wired backbone acquires a new
dimension when the wireless segment is multi hop. Namely one must
guarantee the QoS not only over a single hop, but over an entire wireless
multi hop path. In turn this requires the propagation of QoS information
within the network. Key to the support of QoS reporting is QoS routing,
which provides path QoS information at each source.
In terms of QoS, it is inadequate to consider QoS merely at the network
level without considering the underlying media access control layer.
Again, given the problems associated with the dynamics of nodes, hidden
terminals, and fluctuating link characteristics, supporting end to end QoS
is a nontrivial issue that requires in depth investigation. Currently,
there is a trend toward an adaptive QoS approach instead of the plain
resource reservation method with hard QoS guarantees. Little work has been
done in this area till now, but now there is an active interest in
providing QoS in a multi hop wireless environment.
The following requirements can be identified for a multi hop wireless
network which supports multimedia traffic and/or is inter connected to a
wired network backbone offering quality of service guarantee:
- VC, Bandwidth reservation
- QoS Routing
- Congestion Control
- Mobility
* Courtesy some students
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