|
|
|
|
Many envisioned applications for ad hoc
networks require one-to-many communication |
|
Multicast protocols are intended to
efficiently support such communication patterns |
|
Multicasting well researched in fixed
networks (i.e., the Internet), but host and router mobility cause problems |
|
MANET specific protocols are being
proposed |
|
MAODV: multicast extensions for AODV,
establishes shared tree |
|
ODMRP: new multicast protocol, based on
per-source mesh |
|
Goal: study and compare protocols to
identify possible avenues for improvement |
|
|
|
|
MAODV: Multicast Ad Hoc On-Demand
Distance Vector protocol |
|
First node to join a group becomes
leader |
|
Leader periodically broadcasts group
hello messages (including updated group sequence number) |
|
Multicast tree based on hard state,
nodes joining or leaving require action to reconfigure the tree |
|
Downstream nodes who detect link
failure will try to reconnect to tree |
|
In case of network partition, two trees
get established, after network partitions merge, multicast trees are merged
again as well. |
|
One node will receive two group hello
messages for some multicast group and will ask the leader with lower ID for
permission to reconnect and will do so by joining multicast group |
|
|
|
|
NS2 |
|
Widely used network simulator |
|
Simple physical model (free space
propagation plus two-way ground reflection) |
|
MAC: 802.11 RTS/CTS |
|
Provides support for node mobility,
unicast protocols such as DSR, AODV for ad hoc networks already implemented |
|
Simulation parameters |
|
1000 x 1000 meter area, 250 m radio
range, 2 Mbps link capacity |
|
50 nodes, 1 multicast group |
|
900 seconds simulation time |
|
Mobility model: Random Waypoint model,
0 seconds pause time, max speed between 1 m/s to 20 m/s |
|
Traffic: CBR (4 packets of 512 bytes
per second and sender) |
|
Parameters varied: Number of Senders,
Node Mobility, Group Size |
|
|
|
|
MAODV has poorer packet delivery ratio |
|
MAODV uses a shared tree for data
dissemination. If a single tree link breaks because of node movement, packet
collision, or congestion, destinations cannot receive packets |
|
ODMRP provides redundant routes with a
mesh topology and the chances of packet delivery to destinations remain high
even when the primary routes are unavailable |
|
ODMRP suffers from scalability issues
as the multicast group increases or the sender size increases |
|
ODMRP maintains per-source meshes
connecting receivers and senders. As the number of senders increase, periodic
Join Query packets increase, causing higher amounts of congestion and control
overhead |
|
MAODV uses a single multicast group
leader to send out periodic Hellos through the network. Increasing the number of senders has
minimal impact |
|
|
|
|
|
Study impact of traffic (not just CBR) |
|
Preliminary results show that MAODV is
more sensitive to traffic type |
|
Reduce flooding overhead inherent in
both protocols using pruning and dominant pruning |
|
MAODV: lower data delivery ratio, but
also lower overheads |
|
Improve tree maintenance by
pro-actively predicting link failure and triggering tree maintenance BEFORE
receivers get disconnected |
|
Results for unicast case show that link
breakages based on received signal power strength can accurately be predicted
and used to significantly reduce number of dropped packets |
|
Reduce packet drop rates by 30% - 45% |
|
Increase control messages by 19% - 43% |
|
Reduce packet latency by up to 25%
(though some increase possible at low mobility rates) |