Research Overview
I have been a PhD student with Professor J.J. Garcia-Luna-Aceves
since summer 2001. Together we have been working on various protocols to
improve multicasting in multi-hop mobile ad hoc networks (MANET's).
Background
Ad hoc networking has its origin in the packet radio program of the
70's. A ``mobile ad hoc network'' (MANET) is an autonomous system of
mobile routers
(and associated hosts) connected by wireless links--the union of which
form an arbitrary graph. The routers are free to move randomly and organize
themselves arbitrarily; thus, the network's wireless topology may change
rapidly and unpredictably. Such a network may operate in a stand-alone fashion,
or may be connected to the larger Internet.
There are many applications for MANET's. These include personal area networking
where the mobile nodes may be cell phones, laptops, etc. They also have a
good potential in military operations where the nodes may be soldiers, tanks,
or airplanes. In addition they could prove useful in civilian environments
as diverse as taxi cab networks, conference rooms, boats and ships, search
and rescue operations as well as policing and firefighting.
There are fundamental differences between the architecture of wired networks
and wireless networks. In addition to the obvious difference that nodes in
ad hoc networks are mobile, there are a number of less obvious but equally
important differences. First of all the bandwidth available is of the order
of 1 Mbps, an order of magnitude less than that of wired networks. Secondly,
all communication in a wireless network is broadcast, which means that broadcast
is no more expensive than unicast. Thirdly, mobile nodes have limited battery
power. And finally, wireless links are much more error prone compared to
wired links.
Multicasting is the model of communication where many nodes would like to
send packets to many receivers. The objective of a multicast routing protocol
for mobile ad hoc networks (MANET) is to support the dissemination of information
from a sender to all the receivers of a multicast group while trying to use
the available bandwidth efficiently in the presence of frequent topology
changes.
The kind of protocols that MANET's will implement will depend on the exact
applications for which MANET's will be used. Most of the applications
of MANET's we have described above need one to many or many to many communication.
This makes multicasting a very important feature in such networks. As a result,
it is important to have a multicasting protocol that provides a high packet
delivery ratio even in extreme conditions (e.g., high mobility and high traffic
load). It is equally important for such protocols to have a low overhead,
because bandwidth and battery power are extremely precious in these kinds
of networks.
My Contribution
In light of the importance of multicasting in MANET's as described above,
my research has focussed on the development of multicasting protocols which
provide a high packet delivery ratio and low overhead in a wide range of
simulation scenarios. This research has resulted in the development of the
following protocols :
ROMANT
Robust Multicasting in Ad hoc Networks using Trees (ROMANT) builds a tree
connecting all receivers together. This tree is shared by a
ll senders in order to disseminate data packets to all receivers. The construction
of the tree is initiated by the receivers. One version of ROMANT has been
accepted in the International Journal on Wirless and Mobile Computing(IJWMC)
2005. A softcopy of the paper can be accessed at http://www.cse.ucsc.edu/~ravindra/romantJournal.pdf
PUMA
The Protocol for Unified Multicasting through Announcements (PUMA) builds
meshes that connect all receivers together. The main difference between a
tree as constructed in ROMANT and a mesh as constructed in PUMA is that a
mesh provides multiple paths between senders and receivers whereas a tree
provides only a single path between senders and receivers. As mentioned before
MANET's have nodes which may be mobile and the wireless links are error prone.
As a result certain packets may not be delivered to receivers through a multicast
tree which provides only a single path between senders and receivers. Mesh
based protocols like PUMA which send packets from senders to receivers on
multiple paths may thus have a greater packet delivery ratio. However in
benign conditions (low mobility and traffic load which leads to lesser collisions)
sending packets is not needed and is wasteful. In such situations PUMA is
able to reduce the redundancy when not required. In other words PUMA adapts
the amount of redundancy in the network depending on need. One version of
PUMA has been published in the Proceedings of the 1st IEEE International
Conference on Mobile Ad-hoc and Sensor Systems (MASS) 2004. A softcopy of
the paper can be accessed at http://www.cse.ucsc.edu/~ravindra/puma.pdf.
An implementation of PUMA for Linux is also available.
MODA
A relatively recent development in the MANET's has been the development of
directional antennas. Using directional antennas transmissions can be made
in specified directions. Directional antennas can be used to transmit to
longer distances for the same energy or transmit to the same distance for
lesser energy. Another advantage of directional antennas is that simulataneous
transmissions that would not be possible in omnidirectional antennas due
to mutual interference can now take place in directional antennas as long
as the directions of the transmissions do not overlap. Multicasting Over
Directional Antennas is the first multicasting protocol for MANET's which
exploits the use of directional antennas in order to bring about lower overhead.
MODA utilizes the longer range of transmissions of directional antennas so
that fewer nodes are involved in the forwarding of data packets. Fewer nodes
forwarding the packet translates into a lower data packet overhead. This
paper is currently under submission. A softcopy can be provided on request.
CLAMMP
Another topic in MANET's which has recently generated quite a lot of interest
is the development of protocols which exploit multiple orthogonal channels
for transmission of data packets. The main advantage of using this approach
is that nodes in each other's vicinity can carry out simultaneous transmissions
as long as the transmissions are performed in different channels. Although
protocols like SSCH and MMAC exist for exploiting such functionality for
unicast flows no protocol existed prior to CLAMMP for exploiting multiple
channels to increase network capacity for multiple flows. CLAMMP (Cross Layered
Ad hoc Multiple channel Multicasting Protocol) is so called because it integrates
mac and routing functionality to provide maximum throughput for multicasting
flows. This paper is currently under submission. A softcopy can be provided
on request.